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REPORT OF THE INTERNSHIP
PEOPLES STEEL MLL LIMITED.
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SUBMITTED BY:
NAME AM E UNIVERSITY
ROL ROLL NUM NUMBER
WASEEM KHAN (grp.leader) SALMAN SAFDAR SAHER ABBAS
MY-062 MY-045
NED-UET, KARACHI MEHR AN UET, JAMSHORO JAMSHORO
07MT51
AYATULLAH QURESHI FAHAD JABBAR
DUR DURATIO ATION N OF INTE INTERNS RNSHI HIP P:
NED-UET, KARACHI
08MT14 08MT66
MEHR AN UET, JAMSHORO JAMSHORO MEHR AN UET, JAMSHORO JAMSHORO
21 JUNE– 5th JULY, 2010
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HISTORY OF PSM The project to set up PSM was originally planned in late 60's with the technical collaboration of M/s. Nissho Iwai Company of Japan. It was designed to produce a wide variety of simple alloy steels in small volumes in a variety of shapes such as billets, bars, sheets, plates, castings and forgings. To keep pace with the emerging technology, PSM was modernized in 1989 with the technical assistance from Voest Alpine Industrial Services (VAIS). The melt shop was completely revamped by adding Ladle Furnaces, degassing unit, Continuous Caster, bottom poured Ingot Casting and ElectroSlag Remelting (ESR) facilities.
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INTRODUCTION: Peoples Steel Mills (PSM) Limited is the only facility of its kind in Pakistan engaged in the development and production of alloys and special steels. The plant is located about 20 kilo meter north of Karachi on Manghopir road; its various facilities are spread over 254,960 sq. meters. Modern electric melting and refining, degassing, continuous casting, bottom poured ingot casting and electro-slag remelting techniques are being used at PSM to produce high quality alloys which are in compliance with BÖhler, American, British, German, and / or Japanese standards. Hammer and rota forging and hot rolling facilities are being used to produce forgings, plates and bars in various sizes. A modern materials' testing facility equipped with the latest equipment and well trained staff to perform accurate analysis of its products also exist at PSM.At PSM meticulous efforts are made to ascertain the quality of each product throughout its production cycle.PSM is committed to supply alloys and after sales services to the complete satisfaction of its valued customers.The purpose of making this report is to share our experience and knowledge which we gained from the internship of PEOPLES STEEL MILL LIMITED (PSM). PSM is basically divided into two main sections; one is the production section and the other is quality assurance department (qad).The production department holds the control of manufacturing of their different products (billets, blooms, slabs, plates, sheets, etc.) of different grades of steel. The process starts from the scrap yard then the scrap goes to electric arc furnace, ladle furnace, VOD, continuous casting or ingot casting, electro slag remelting, Qshop, N-Shop, P-Shops, R-Shops, and finally, after all the processes, to the quality inspection department.The laboratory checking is necessary for good quality of steel, for that, after each single process PSM assure its quality by QAD laboratories. These labs are equipped with different machines and equipment which inspect the material on microscopic and macroscopic level on online basis. Moreover, QAD can also check the mechanical properties of the material by different machines. These machines include; Brinell hardness tester, Rockwell hardness tester, tensile test machine, and charpy and izod impact testing machines, etc. PSM have following laboratories in the QAD section: Metallography, photography, mechanical testing, spectrometry, rapid analyzer, atomic absorption lab, and wet chemical analysis etc.All the information and knowledge which we get from PSM is submerged in this report. Moreover, we are also thankful to PSM for the generous help in increasing our practical knowledge in the field and also to the engineers, workers, and every department of PSM which gave their precious time to us and provide as much knowledge as they can, and also to chairman of metallurgical engineering department and our class advisor for selecting us for this knowledgeable internship.
PRODUCTION PROCESS: The raw material (scrap) is brought to the electric arc furnace. After that it goes to the ladle furnace where the addition of elements is done (if required). The molten material is than put into the vacuum oxygen decarburization plant for final purification. After this process the material is either proceeds to the continuous casting or to the ingot casting.From continuous casting it goes
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to the bar rolling mill and from ingot casting the material is transferred to the forging shops, blooming & slabbing mill or to the radial forging. The flow chart of the process is given below.
PROCESS FLOW DIAGRAM:
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SCRAP YARD: Scrap is a term used to describe recyclable materials left over from every manner of product consumption, such as parts of vehicles, building supplies, and surplus materials. The word scrap means small fragments of discarded item. It is 2 nd largest source of iron in steel making after hot metal.It is basically iron or steel, that is either generated within mill (home or return scrap e.g. slab crops, pit scrap ,cold iron or home scrap),or purchased scrap. Electric arc furnace melt shops generally use 100% of total charge scrap either purchase(buildup heat charge) or return scrap (remelt heat charge and use normally for high alloy or special steels).The important properties of scrap are its bulk density(600-2000kg/m3),metallic yield, size, shape. Scrap segregation my conduct to control trace elements like cu, sn, zn etc
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There is a scrap yard in the corner of melt shop where the scrap steel and other raw materials is stored. This is the longitudinal section having capacity of around 30,000 tons of scrap storage approx. The steel scrap from the port and from local markets via dumpers and the other raw
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materials from central store comes here in 1 ton bags. This section is equipped with 3 overhead cranes namely: •
Overhead magnetic crane
•
Morris (Pakistan).
•
IHI (Japanese)
Overhead magnetic crane:
This crane consists of an electromagnet which is used for charging the scrap to the bucket. It has a loading capacity of 5 tons. Morris Pakistan:
Scarp yard is equipped with two16 tons Morris cranes one crane can perform the charging of bucket by two methods i.e. by electromagnet and by grabbing jaws. It has loading capacity of 16 tons. The grab jaws used for light and long pieces of scraps and for non magnetic materials. The second crane can perform charging only by jaw grabbing. The grabs are used for the light and shattered scrap like structural steel rods. IHI (Japanese):
The fourth crane is the IHI which is Japanese branded, it has a loading capacity of 16 tons. It is used for charging the bucket. Scrap yard is also equipped with 9 charging buckets with different sizes, capacities and tare weights. It made up of mild steel. Their loading capacities are ranging between 6 - 6.5 tons. The buckets with their tare weight are given below: Names
Tare weight (kg)
A
6540
B
6480
C
6535
D
6585
E
6380
F
6535
G
2740 (small bucket)
H
6240
I
6125
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RAW MATERIALS FOR STEEL MAKING: Steel scrap: PSM totally depends upon the steel scrap. Two standard types of scrap HMS and HMS-1 are imported from Dubai and England and also some ratio from local markets. HMS (heavy melting scrap) includes the high density metals bolts, guarders, machine parts and forged parts etc. This type of scrap is generally small and attractive having normal size of 2x2.It has yield of 85%. While the HMS-1 is the light density steel scrap consists of large pieces like sheet cuttings and pellet size steel balls, etc. The yield is 95%. The main markets are Dubai and England from where steel scrap is imported. Ferro- alloys: The Ferro-alloys used for setting the composition of the steel throughout the process of refining comes here from central scrap yard in 1 ton bags.
Others: Other raw material includes the limestone, coke breeze, fluorspar, and carbon powder.
Raw materials: 1-FLUXES: 1-
Limestone
2-
Fluorspar
3-
Coke breeze
4-
Carbon powder
5-
coal
6-
Silica sand
7-
Bauxite
8-
MgO
9-
Dolomite
10-
Lime-CaO
2-OXIDI ZING AGENTS: 1-
Liquid oxygen gas
11 2-
Air
3-
Iron ore
4-
Mill scale
3-DEOXIDI ZING AGENTS: 1-
Carbon powder
2-
Ferro Silicon
3-
Aluminum
4-
CaSi
5-
CaAl
6-
SiMn & SiCr
4-REFRACTORIES 1-MgO 2-MgCO3. 3-Dolomite-Ca.Mg.(CO3) 4-Fireclay-SiO2. 5-Carbon-Magnesia 6-Chrome-Magnesia 7-Ramming Castables 8-Gunning Materials 9- Cements.
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10-Others 5-UTILITIES 1-Electricity 2-Industri al water 3-Drinking water 4-R.O. water 5-Natural gas 6-Injecting fuels 7-Liquid fuels 8-Liquid N2 9-Liquid Ar gas 10-steam 6- ALLOYING ADDITION: 1-Ferro-Silicon 2-Ferro-Silico-Maganase 3-Ferro-Manganase 4-Ferro-Molybdenum 5-Ferro-Vanadium 6-Ferro-Sulfur 7-Ferro-phosphorous 8-Ferro-Titanium 9-Ferro-Tungsten 10-Ferro-Chrome 11-Ferro-Silico-Chrome 12-Ferro-Boron 13-Ferro-Niobium
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14-Ferro-Columbium 15-Nickel Metal 16-Cobalt Metal 17-Manganase Metal 18-Copper Metal
PRODUCTS OF PSM: CAPABILITY
SIZE/SPECIFICATIONS
Melting and Refining 15 ton heat through Electric Arc Furnace and Ladle Refining 15/12 Alloy steels including stainless, high speed ton heat through VD/VOD. steels and hot & cold working tool steels Electro-Slag Re-melting of 250-600 mm dia (max.) ingot
Casting Ingot Casting
500 kg to 15 tons ingots depending on steel grade and shape.
Continuous Casting
110, 125, 150, 230 mm square billets and blooms.
Casting (Foundry)
2 kg to 10 ton depending on Box or Pit Moulding
Rolling Plate Rolling
Thickness : Min : 3.0 mm Width : 1000 to 1350 mm
Bar / Flat Rolling
Round : 5.5 to150 mm Square: 50 to 100 mm Flat : 4.8 x 40 to 20 x 120 mm
Forging Hammer Forging
Die forging : up to 10 kg Free forging : up to 2000 kg
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Rotary Forging
50 to 200 mm ruling section
Finish Type
As Rolled As Forged Heat Treated Machined Peeled Cold Drawn
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SHOP RETURN: Here the rejected steel from the mill is dumped for usage of reheating with settled composition. The metals of different grades are dumped in different boxes. The following are the metals which were placed there:
Bin#
Scrap
Grades
Composition %
1.
Tool steel
K-107,k-110,k-105
Cr < 10.5, w ( 1.5-0.5)
2.
Unalloyed
Ssa-211,aisi1040
Cr < 1 , Mn 1.5
3.
Austenitic
Aisi 316,3162
Cr < 25, Mo ,305, Ni 20
4.
Austenitic without mo
Aisi 304,304 l
Cr < 30,ni < 2.5
5.
Ferrite S.S.
Aisi 304,420 Ni
C , 0.5, Cr 10-17
6.
Cr. Mo.V alloyed
W-302,v-354
Cr 10, mo (.15-3), v (0.5-3)
7.
Special steel
-
-
8.
Marraging steel
W-725
Mo< 5, Ni < 20, co < 15
9.
Mn. Steel
-
-
10.
Low alloyed cr.ni
19cn5
Cr 0.2-2, Ni < 5
11.
Mild steel
-
Co 0.25
ELECTRIC ARC FURNACE: The steel scrap comes directly from port to the yard via truck. While the other raw materials like limestone, fluorspar, coke breeze, carbon powder and Ferro-alloys from the central store comes here in one ton bags. The material is charged in the bucket through cranes in the size 4x4 if greater than cut down by gas cutter and then bucket is weighted in the weight bridge. After weighing, the bucket goes to charge the EAF.PSM is equipped with 2 electric arc furnaces having capacity of 15 tons with lift and swinging type roof. The charge is melted through three 12 inch diameter graphite electrodes whose consumption rate is 2.5 kg/ton. Furnace is lined with basic magnesia and a 6 inch dolomite brick with the working lining of carbon magnesia bricks containing 8-12% carbon and rest is the magnesia.
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THEORY Steel is not any specific product. It is essentially a malleable alloy of iron and one or more of other element like C, Cr, Si, V, W or almost any active element (other than gas) from the periodic chart of element. Steel may contain many other elements such as Al, Mn, S, P, O etc which are not added specially for any specific purpose but are inevitably present. These are known as impurities in steel. We should have to eliminate or reduce down to the required level according to specification. Construction: EAF consist of metallic shell of varying diameter depending upon furnace capacity. Carbon magnesite (Refractoriness 1700 °C) refractory is used in back lining or safety lining while magnesia carbon is used as working lining, which is facing high temperature. It consists of three electrodes with water cooling system, Electrode made up of Carbon Graphite (98% Pure Carbon). Its melting point is 3450 °C.
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Figures: Schematic Diagrams of Electric Arc Furnace
Function: It is the Oxidation process the main purpose of EAF is to melt the charging materials (feed) and dephosphorisation (at temperature ranges 1550 to 1630 °C). Partially alloying can be done in EAF. Before starting EAF following parameter should be inspected: • Check cooling system • Movement of electrode, roof, door. • Titling mechanism • Refractory lining 1. Bank 2. Slag line 3. Tap hole 4. Door – Sill • Electrode holder • Air pressure
Working Operations: The electric arc furnace operates as a batch melting process producing batches of molten steel known "heats". The electric arc furnace operating cycle is called the tap-to-tap cycle and is made up of the following. 1) Furnace charging 2) Melting and Refining 3) De-slagging 4) Tapping 5) Furnace Preparation or Furnace turn-around
1) Charging: The first step in any tap-to-tap cycle is "charging”. Bottom drop bucket is used in EAF. The charge is dropped out from the roof in EAF. The distribution of the charge in the furnace is quite important factor. A certain amount of light scrap at the bottom permits easy melting of the charge
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with minimum of damage to the bottom. As a thumb rule 40% having 40% medium and 20% light scrap are incorporated to charging.
2) Melting: Melting is done through arcing, the electrodes are lowered manually, the ARC is struck and the electrodes are put on automatic control. The arcing gap between the electrode and the charge is regulated by automatic control, which raise or lower the electrode and maintaining desired arc gap (or arc length) by maintaining constant arc voltage. Maximum voltage we supply 530 Volts and current is first stage (before melt down is approximately 13000 A) and in second stage (after melt down is 9000 A) so the heat generate is greater than 3000 °C due to this high temperature we use carbon graphite electrode (M.P 3450 °C) we can not use the cupper electrode because of it M.P 1084 °C. During melting oxygen lancing is carried out of pressure at about 4 to 6 bar so all the charge is liquidus, and form a slag layer of black color oxygen lancing in arc furnace has the following advantages. • Utilization factor of power is low. • Although cheap high carbon Ferro-alloys are used to large extent, costly low carbon Ferroalloys are still needed for final adjustment and in terms of cost it is by no means negligible • Oxygen lancing develops high bath temperature The black color shows the high percentage of phosphorous and other elements and indicate that slag is acetic in nature so we have to increase its basicity by addition of lime (CaO) and O 2 lancing to convert into basic because of basic process of EAF. Oxygen react with carbon produces Carbon mono oxide or Carbon di oxide and ejected through chimney and this process is called decarborization and also Oxygen react to phosphorous and produce phosphorous penta oxide and lime react with phosphorous Penta Oxide produce calcium phosphate this process is called dephosphorisation. The compound calcium phosphate is stable at temperature 1550 to 1630 °C because at high temperature it is a stable and broken and phosphorous is again down in molten metal (bath). The lime is work as a flux to bring down the softening point of the gangue material so reduce the viscosity of slag and to decrease the activity of some components to make it stable in the slag phase. Carbon powder is also used to produce foaming slag.
3) Deslagging: De-slagging operations are carried out to remove impurities from the furnace. During melting and refining operations, some of the undesirable materials within the bath are oxidized and enter the slag phase. It is advantageous to remove as much phosphorus into the slag as early in the heat as possible (i.e. while the bath temperature is still low). The furnace is tilted backwards and slag is poured out of the furnace through the slag door with the help of rebble (which is water cooled and made up of mild steel). Removal of the slag eliminates the possibility of phosphorus reversion. During slag foaming operations, carbon may be injected into the slag where it will reduce FeO to metallic iron and in the process produce carbon monoxide which helps foam the slag. If the high phosphorus slag has not been removed prior to this operation, phosphorus reversion will occur. During slag foaming, slag may overflow the sill level in the EAF and flow out of the slag door. The following table shows the typical constituents of an EAF slag :
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Component
Source
Composition Range
CaO
Charged
40 - 60 %
SiO2
Oxidation product
5 - 15 %
FeO
Oxidation product
10 - 30 %
MgO
Charged as dolomite
3-8%
CaF2
Charged - slag fluidizer
MnO
Oxidation product
S
Absorbed from steel
P
Oxidation product
2 - 5%
Before tapping ladle should be inspected: • • • • •
Check refractory lining Free board Purging plug Nozzle point If Ladle does not warm then pre-heated.
4) Tapping: The molten metal is poured into the teeming ladle and this ladle is transferred to the ladle furnace. 5) Furnace Turn-around: Furnace turn-around is the period following completion of tapping until the furnace is recharged for the next heat. During this period, the electrodes and roof are raised and the furnace lining is inspected for refractory damage. If necessary, repairs are made to the hearth, slag-line, tap-hole and spout. In the case of a bottom-tapping furnace, the taphole is filled with sand. Repairs to the furnace are made using gunned refractories or mud slingers. In most modern furnaces, the increased use of water-cooled panels has reduced the amount of patching or "fettling" required between heats. Many operations now switch out the furnace bottom on a regular basis (2 to 6 weeks) and perform the hearth maintenance off-line. This reduces the power-off time for the EAF and maximizes furnace productivity. Furnace turn-around time is generally the largest dead time (i.e. power off) period in the tap-to-tap cycle. With advances in furnace practices this has been reduced from 20 minutes to less than 5 minutes in some newer operations.
PROCESS: The furnace is operated max at 10,000 amperes with voltage of 440 volts and 5.2 mw of power. Furnace is firstly preheated at 300-400c for 8 hrs then it is charged in 2-5 turns depending upon the type of scrap. Each charge is firstly melted partially by some arcing by lowering the electrodes, so that other charges can be done to make total of 16 ton of charge, because the height of furnace is small. The melting is done in around 1 hr by simultaneous arcing through 3 electrodes which are controlled by automatic system with lowers and raises the electrodes. The
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air gap is so that must form make
maintained resistance in order to arcs. The temperature of arc is about 40004500c which makes the temperature of around 1700c. The slag is prepared by adding limestone for the removal of impurities in the form of oxides. Since high alloy and special steel are made, so for this double slag practice is used at EAF. First oxidizing slag is produced by oxygen lancing which also decreases the fuel consumption. The steel sample is then sending for chemical analysis. When chemical report comes the required alloy addition is done and if any composition is done wrongly or there is inherited problem in composition which can neither be settled then cocktail method is applied in the EAF, then steel is desulphurized and dephosphorized at around 1600 c, as phosphorus made its oxide p2o5 and goes to slag but it has tendency to come back to metal which is dangerous for the steel so for this extra oxygen is given by adding iron ore (hematite). The main reactions are stated below:
CHEMISTRY 2Fe
+
O2
-------
FeO
FeO
+
Si
-------
Fe + SiO2 (slag)
FeO
+ Mn
-------
Fe + MnO (slag)
FeO
+P
-------
Fe + P2O5 (slag)
2C
+ O2
-------
2CO ▲
Fe
+ S
-------
Fes
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FeS
+ CaO
-------
FeO
+ CaS
FeO
+C
-------
Fe
+ CO ▲
After slag is prepared it is removed from slag door or working door. The fluidity of slag is controlled by coke breeze and fluorspar which decreases and increases the fluidity of slag respectively. The molten metal is then tapped out by tilting the furnace at 45 degrees through tapping door into the 16 tons preheated ladles and shifted to the ladle furnace.
L ADLE FURNACE: PSM has 2 ladle furnaces which consist of a heavy mild steel shell of 30mm and 3 electrodes of 8 inch diameter. The electrode guide on roof is water cooled in order to prevent damage. The furnace normally works at around 500-600 amps with 400 Volt-Ampere of power the ladle is lined with the following layers of refractory: 1. Microtherm or mica tiles or asbestos fibers (6mm) 2. Alumina based insulation bricks (32 mm) 3. Chrome tile (32mm) 4. Carbon coated magnesia bricks (working lining) (150mm) The ladle with the help of crane comes towards furnace then crane put that ladle on the rails then rails moves towards the furnace then furnace roof is placed on top of ladle then the electrodes are lowered and arcing is given to increase the temperature for further processing, the molten metal is continuously purged with argon gas at 2 bar pressure from bottom to homogenous the steel. Then reducing with high temperature environment is required for desulphurizing for this reducing slag is prepared by adding some lime, fluorspar and coke breeze, the slag so form covers the molten bath which reduces the oxidation by restricts the atmospheric oxygen to enter into it. After desulphurization the sample is then send for chemical analysis. The temperature is noted after time to time by using thermocouples of k type (Al-Tg). The chemical report comes in 10 minutes then according to the needs the Ferro alloys are added in order to set the chemistry of required grade by using the formula: % comp = amount required – obtained amount x total amount Purity Construction: Its construction is similar to EAF but in this furnace the electrode is used having 8-inch dia of the same length 1.5m and the molten metal, which has tapped into the teeming ladle, is transferred here for refining processing. Ladle furnace is consist of two refractory lining Magnesia Tile and Carbon Block (having weight 4Kg) This ladle having nozzle point and purging plug point and a free board depending upon the vacuum degassing process for example if we have to do VD process so 300 mm free board is required and in VOD 800 mm is required. With
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the help of purging plug point the N2 or may be Ar gas is supplied to homogenize the molten metal (bath). Configurations include swing roof, fixed roof, swing electrode and dual station LMF facilities combined with vacuum degasser stations where necessary. These designs have incorporated alloy and flux material handling systems ranging from basic configurations to completely automated systems. The water cooling system of the ladle furnace unit consist of two closed circuit that circuit chemically purified water and an open circuit that cools the water form closed circuit . Cooling is provided for the current supply system, the roof and the system of the transformer .The water quality requirement high. Design includes:
Provide a power source capable of holding and increasing the temperature of steel in the ladle. Systems designs aim at a 7° F temperature rise after stabilization. Provide an inert gas stirring system to insure homogeneity of the temperature and chemistry of liquid steel. Provide a method to increase the slag cover deep enough to bury the arc and maximize the transfer of heat into the liquid steel while minimizing the transfer of heat into the ladle of ladle furnace refractory’s or water-cooled parts. Ladle furnace systems including electric power supply, operator control pulpits, PLC based automation system, inert gas stirrings, ladles, ladle transfer cars, additives charging and injection systems, deducting and auxiliary equipment such as the electrode make-up stations. Each heating station is equipped with units for enhanced ladle metallurgy: Gas stirring system for alloy and temperature homogeneity Alloy and flux addition systems including cored wire Temperature, carbon and oxygen measurement lances The fume evacuation system is designed to avoid the passage of air over a bare steel surface inside the ladle furnace. Evacuation systems handle surplus fumes leaving the furnace, either from the roof gap or from the electrode holes.
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Function: The main purpose of LF is to refining and desulphurization at temperature ranges 16301750 °C. Before starting lf following parameter should be inspected: • Check cooling system Movement of electrode and roof • Electrode holder •
PROCESS: Teeming Ladle is placed in the pit of ladle furnace and inserts the gas pipe in the purging plug then through automatic machine control the roof of the furnace is fixed over the ladle. Supply the current with the help of electrode of 10000 A with the purging of N 2 / Ar gas of 2 bar pressure. Purging is only done to homogenize the bath. Taking out the sample with the help of sample spoon. During refining bath samples are periodically taken out to assess the progress of refining lime (CaO) and spar (CaF) may be added during refining to keep the slag in shape once the temperature are at the desired label the heat is ready to be deoxidized. Bath temperature is measured with the help of thermocouple. Fe-Mn and Fe-Si etc do preliminary Deoxidation. Many operators block the heat by dipping the electrodes in the bath. This practice improves cleanliness of the bath. Putting coke breeze on the slag can form strongly reducing carbide slag. It forms carbide as: CaO + 3C → Ca C2 + CO Which is quite effective in desulphurising the bath. The Lime to coke ratio may be 6:1 to 12:1. Slag without coke to with little of coke is while in color and is called lime slag. And that with high percentage of coke is gray in color and is know as carbide slag. Carbide slag tends to recarburise the bath and it is not good for making low carbon heat. For heat containing less than 0.15% C reducing slag is make by the addition of lime, Fe-Si and Al. No Coke is added for it would recarburise the bath. After reducing refining is over, the bath is fully deoxidized by the addition of Fe-Si and Al. The final alloying addition may now be added based on the analysis of the bath. The metal can be held to pick up temperature without loosing any alloying addition. The process is therefore better to make any type of alloy steel because we have to increase the temperature for further process.
PURPOSE OF LADLE FURNACE: After setting the chemistry the ladle is shifted to the vacuum oxygen degasser for further purification and gases removal.
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The following are the main purposes of using ladle furnace: 1. Max desulphurization 2. To adjust final chemical composition 3. To attain required temperature for VOD and continous casting. 4. To get actual weight of steel 5. Proper deoxidation 6. To increase production rate 7. To save fuel CHEMISTRY: The following reactions take place: FeS + CaO
------
FeO + CaS (slag)
FeO + C
------
Fe
+ CO ▲
VACCUM DEGASSING & VACCUM CARBON DECARBURIZ ATION & VACCUM OXYGEN DEOXIDATION (VD,VAD,VOD): The ladle is put on ladle stand and the vacuum is created in the vacuum cover placed on top of the ladle and sealed with fireclay lumps. The pressure of the chambers is first lowered from atmospheric pressure to 170 torr by using 4 ejectors controlled with water pump of 55 KW which sucks all the moisture and air from the vacuum chamber then final pressure is reduced to 3.7 torr. Continous purging of argon gas is ensured at 1.2 - 5 nm3/hr to homogenize the molten steel. By this process the gases like o2, n2, h2 and so2,etc are removed because of the property that all gases goes from the higher concentration level to lower conc. Level, so the gases collected in the vacuum chamber. If the carbon content is greater than vacuum oxygen decarburizing is done at 14 bars by which carbon reduces. The chemical result which was taken at lf reached here if there is still problem in chemistry then Ferro alloys are added here. The required temperature is attained which is required for I.C. or C.C. If temperature is greater then a piece of solid steel is inserted in calculated amount and if is lower then oxygen is lanced which increases the temperature. Then all the gases are removed by sucking and the pressure is the released. Now the molten steel is send for casting.
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Ladle Degassing (VD, Tank Degassing) In the Tank Degassing method the ladle with molten steel is placed into a vacuum chamber. The ladle is equipped with a porous refractory plug mounted in the ladle bottom. Through the plug argon is supplied during vacuum treatment. There is an addition hopper with vacuum lock on the chamber cover. The hopper is used for adding alloying elements and/or slag components. The reaction [C] + [O] = {CO} starting in the steel under vacuum conditions causes stirring, which is additionally intensified by argon blown through the bottom porous plug. Intensive stirring of the melt and the slag results in deep desulfurization of the steel. Desulfurizing slags possessing high sulfur solubility are used in this process. Argon and CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous nitrogen. Because the gasses dissolved in steel beyond a certain limit are undesirable because their presence finally leads to several defect in steel product the vacuum degassing are meant to produce steel having low gas contents and inclusions either in relatively or very small amount at a reasonable cost or in large quantities at considerable expenses degassing. Vacuum degassing was originally meant to reduce hydrogen from steel that were prone to hairline cracking and hence the name. It was soon realized that deoxidization of steel ,by way of carbon and oxygen reaction cloud also be cleanliness since the product of deoxidization ,begin gas escapes out of the melts. The present day degassing plant may involve one or more of the following functions.
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FUCNTIONS OF VACCUM DEGASSING: 1. 2. 3. 4.
To remove the hydrogen from the steel . To improve cleanliness by removing oxygen in the form of CO gas. To reduce steel of a very low carbon content (beow 0.03%). To bring about desulphurization of steel by reagent carried along with an inert gas like argon bubbled through the bath. 5. To ensure better control of chemical composition of steel by adding the requisite amount of adding under vacuum.
PROCESS: Vacuum treatment consists of exposing liquid steel to a low-pressure environment (ranges 760-3.7 tor) this decreasing the pressure with the help of different steam ejectors Ladle from L.F is taken to V.D through crane and placed at V.D plant. Through purging plug, argon gas is purged then temperature is measured. Now the process of V.D or V.O.D (mostly for Stainless Steel) is applied on molten metal. In this process suction hood comes over the ladle and creates vacuum by reducing . As low-pressure gases like O 2, H2 & N2 exist in molten metal, when we reduce the pressure gases pressure increases and they come above molten metal surface. Three condensers for water showering and water cooled pipes are used; some gases are released in atmosphere while some come out from ladle in combination with water.For V.O.D, oxygen lancing is applied for decarburization to remove excess carbon. All oxides dissociate forming CO with Carbon. Now temperature is measured again. Adding Coke breeze to maintain carbon does alloying adjustment. Sometimes silica is also added. Rice husk is used to protect temperature or atmosphere reactions by forming layer above metal surface. Then finally the ladle is shifted to casting plant (either C.C or I.C).
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CONTINOUS CASTING: THEORY: Continuous casting may be defined as teeming of liquid metal in a short mould with a false bottom through which partially solidified ingot is continuously withdraw at the same rate at which metal is poured in the mould. In continuous casting, the molten steel from the steelmaking operation or ladle metallurgy step is cast directly into semifinished shapes (slabs, blooms, and billets). Continuous casting represents a tremendous savings in time, labor, energy, and capital. By casting the steel directly into semifinished shapes, the following steps are eliminated: ingot teeming, stripping, and transfer; soaking pits; and primary rolling. Continuous casting also increases yield and product quality. PROCEDURE:
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The continuous casting process, molten steel flows from a ladle, through a tundish into the mold. The tundish holds enough metal to provide a continuous flow to the mold, even during an exchange of ladles, which are supplied periodically from the steelmaking process. The tundish can also serve as a refining vessel to float out detrimental inclusions into the slag layer. If solid inclusion particles are allowed to remain in the product, then surface defects such as “slivers” may form during subsequent rolling operations, or they may cause local internal stress concentration, which lowers the fatigue life. To produce higher quality product, the liquid steel must be protected from exposure to air by a slag cover over the liquid surface in each vessel and by using ceramic nozzles between vessels. If not, then oxygen in the air will react to form detrimental oxide inclusions in the steel. Once in the mold, the molten steel freezes against the water-cooled walls of a bottomless copper mold to form a solid shell. The mold is oscillated vertically in order to discourage sticking of the shell to the mold walls. Drive rolls lower in the machine continuously withdraw the shell from the mold at a rate or “casting speed” that matches the flow of incoming metal, so the process ideally runs in steady state. The liquid flow rate is controlled by restricting the opening in the nozzle according to the signal fed back from a level sensor in the mold. The most critical part of the process is the initial solidification at the meniscus, found at the junction where the top of the shell meets the mold, and the liquid surface. This is where the surface of the final product is created, and defects such as surface cracks can form, if problems such as level fluctuations occur. To avoid this, oil or mold slag is added to the steel meniscus, which flows into the gap between the mold and shell. In addition to lubricating the contact, a mold slag layer protects the steel from air, provides thermal insulation, and absorbs inclusions. Water or air mist sprays cool the surface of the strand between the support rolls. The spray flow rates are adjusted to control the strand surface temperature with minimal reheating until the molten core is solid. After enough metal has solidified like a conventional casting onto its head, the dummy bar is then slowly withdrawn down through the continuous casting machine and steady state conditions evolve. The process then operates continuously for a period of one hour to several weeks, when the molten steel supply is stopped and the process must be restarted. After VOD the teeming ladle comes to 3 strands continous caster. The C.C. plant mainly consists of a liquid metal reservoir and distribution system (a tundish), a controlled vibrating stage consists of 3 water cooled and .01% chromed pure cu moulds each with radioactive level detector, electromagnetic stirrer, secondary water cooling showers zones, mechanically operated hydraulic and pneumatic based roller systems, connected at each strands to pull the casted billets, radioactive bar positioning detector, a dummy bar, shearing equipment and run out table to cooling beds. First the dummy bar is fixed at specific height in the mould the sides of the mould is then packed by asbestos rope then steel rods and nail chips are inserted in order to start nucleation process, then tundish is set on the mould holes on vibrating stage then through shroud manipulator high alumina shroud is placed on ladle nozzle through which the molten metal from ladle pours into tundish to prevent oxidation. The tundish is lined with high alumina bricks and is preheated at 1100 – 1200c for 5 hrs. Molten metal goes to moulds through 2 SES (submerged entry shroud) made with high alumina coated with zirconia. The speed of the molten metal is controlled by stopper inside the tundish with is operated by programmable logical control system. The timings are set according to the grade and size of the billet. Some rough ideas are: for 110x110mm size 2 meter/min, 125x125mm 2.2 meter/min, and 150 x 150 mm 2.1 meter/min and for 230x230 mm .9 meter/min. The vibration produces by the stage prevents the metal to stick with walls and also casting powder is also put for same. Then dummy bar starts to move downwards and the electro magnetic stirrer is activated in order to prevent columnar grain formations. Then metal starts to come from bottom side in a form of billets then after casting it is sheared from dummy bar and further sheared, by shearing machine according to customer demands and then heat
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number is tagged by tagging machine. Moreover for different grades of steel PSM paint different color codes. Liquid Steel Transfer There are two steps involved in transferring liquid steel from the ladle to the molds. First, the steel must be transferred (or teemed) from the ladle to the tundish. Next, the steel is transferred from the tundish to the molds. Tundish-to-mold steel flow regulation occurs through orifice devices of various designs: slide gates, stopper rods, or metering nozzles, the latter controlled by tundish steel level adjustment. Tundish Overview The shape of the tundish is typically rectangular, but delta and "T" shapes are also common. Nozzles are located along its bottom to distribute liquid steel to the molds. The tundish also serves several other key functions: •
Enhances oxide inclusion separation
•
Provides a continuous flow of liquid steel to the mold during ladle exchanges
Maintains a steady metal height above the nozzles to the molds, thereby keeping steel flow constant and hence casting speed constant as well (for an open-pouring metering system). •
•
Provides more stable stream patterns to the mold(s)
Mold The main function of the mold is to establish a solid shell sufficient in strength to contain its liquid core upon entry into the secondary spray cooling zone. Key product elements are shape, shell thickness, uniform shell temperature distribution, defect-free internal and surface quality with minimal porosity, and few non-metallic inclusions.The mold is basically an open-ended box structure, containing a water-cooled inner lining fabricated from a high purity copper alloy. Mold water transfers heat from the solidifying shell. The working surface of the copper face is often plated with chromium or nickel to provide a harder working surface, and to avoid copper pickup on the surface of the cast strand, which can facilitate surface cracks on the prod uct.Mold heat transfer is both critical and complex. Mathematical and computer modeling are typically utilized in developing a greater understanding of mold thermal conditions, and to aid in proper design and operating practices. Heat transfer is generally considered as a series of thermal resistances as follows: • • • •
Heat transfer through the solidifying shell Heat transfer from the steel shell surface to the copper mold outer surface Heat transfer through the copper mold Heat transfer from the copper mold inner surface to the mold cooling water
PURPOSE OF ELECTRO MAGNETIC STIRRER:
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1. The mold is oscillated vertically in order to discourage sticking of the shell to the mold walls. 2. To homogenizes the alloying elements completely during flow of molten metal, inorder to prevent macro segregation at the centre of cast product.
SOLIDIFICATION AT MOLD: Once in the mold, the molten steel freezes against the water-cooled walls of a bottomless copper mold to form a solid shell. Drive rolls lower in the machine continuously withdraw the shell from the mold at a rate or “casting speed” that matches the flow of incoming metal, so the process ideally runs in steady state. The liquid flow rate is controlled by restricting the opening in the nozzle according to the signal fed back from a level sensor in the mold. Below mold exit, the thin solidified shell (6-20 mm thick) acts as a container to support the remaining liquid, which makes up the interior of the strand. Water or air mist sprays cool the surface of the strand between the support rolls. The spray flow rates are adjusted to control the strand surface temperature with minimal reheating until the molten core is solid. After the center is completely solid (at the “metallurgical length” of the caster, which is 10 - 40m) the strand is cut with oxyacetylene torches into slabs or billets of any desired length. Mold Oscillation Mold oscillation is necessary to minimize friction and sticking of the solidifying shell, and avoid shell tearing, and liquid steel breakouts, which can wreak havoc on equipment and machine downtime due to clean up and repairs. Friction between the shell and mold is reduced through the use of mold lubricants such as oils or powdered fluxes. Oscillation is achieved either hydraulically or via motor-driven cams or levers which support and reciprocate (or oscillate) the mold.Mold oscillating cycles vary in frequency, stroke and pattern. However, a common approach is to employ what is called "negative strip", a stroke pattern in which the downward stroke of the cycle enables the mold to move down faster than the section withdrawal speed. This enables compressive stresses to develop in the shell that increase its strength by sealing surface fissures and porosity. Secondary Cooling Typically, the secondary cooling system is comprised of a series of zones, each responsible for a segment of controlled cooling of the solidifying strand as it progresses through the machine. The sprayed medium is either water or a combination of air and water.
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32
33
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INGOT CASTING: After steel is refined, the molten steel is tapped into refractory lined ladles. The steel is then teemed (poured) into a series of ingot molds. After the ingots solidify, the ingot molds are stripped and the ingots are placed in soaking pits for heating and to equalize the internal and external temperature. Following the soak, the ingots are hot rolled in large primary rolling mills to produce slabs, blooms, or billets depending on the final products desired. Ingots range in size from several hundred pounds for specialty steels to 300 tons for large forging applications. Slab ingots range in weight from 10 to 40 tons - averaging about 20 tons. Ingot shape and weight are selected to meet the specific requirements of the product to be made. ingots are manufactured by the freezing of a molten liquid (known as the melt) in a mold. The manufacture of ingots has several aims. Firstly, the mold is designed to completely solidify and form an appropriate grain structure required for later processing, as the structure formed by the freezing melt controls the physical properties of the material. Secondly, the shape and size of the mold is designed to allow for ease of ingot handling and downstream processing. Finally the mold is designed to minimize melt wastage and aid ejection of the ingot, as losing either melt or ingot increases manufacturing costs of finished products. After steel is properly refined by EAF, lf and VOD, the molten metal comes to the ingot casting hall by teeming ladles. Some alloying materials and deoxidizers may be added during tapping of steel. The teeming ladle has a slip door mechanism in the bottom equipped with a nozzle by which steel is poured in controlled manner. The ladle is moved by
35 overhead crane to a pouring platform where the steel is then poured into 40mm dia trumpet for bottom pouring and directly into moulds for top pouring casting of the desired dimensions. The bottom pouring ingot casting has many advantages like greater numbers of moulds can be filled at a time, smooth movement of molten steel into moulds, prevent formation of deformed bodies due to splashes, etc. Different casting powders are used for lubrication. The anti-piping powder is placed inside mould in bottom to prevent formation of piping and feeder head of composite material is placed on top of mould to keep the top surface molten so piping can easily be removed from top.
Types of ingots:
Flat:
Square:
Octagonal:
Weight
Height
F1 2ton
1300 mm
F1 1600kg
1300 mm
F1 1200kg
1100 mm
F1 1000kg
1000 mm
F1 800kg
1000 mm
F1 600kg
800 mm
Sq 1250kg
1450 mm
Sq 1100kg
1100 mm
Sq 700kg
930 mm
Oct 14 ton
1700 mm
Oct 11 ton
1625 mm
Oct 8 ton
1500 mm
Oct 5 ton
1400 mm
Oct 2 ton
Round:
r 2000kg
2240 mm
r 600kg
2240 mm
Casting powder composition (percentage): Powder
Low carbon
Medium carbon
High carbon
C
0.1
5
14
SiO2
52
38
35
CaO
33
34
35
MgO
4
7
20
36 Fe2O3
1
3
7
NO2
2
6
8
K 2O
2
6
8
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Ingot In got casting of steel
CASTING STRUCT STR UCTURE URE OF STEEL ST EEL:: The physical structure of a crystalline material is largely determined by the method of cooling and precipitation of the molten metal. During the pouring process, metal in contact with the ingot walls rapidly cools and forms either a columnar structure, or possibly a "chill zone" of equiaxed dendrites, dendrites, depending upon the liquid being cooled and the cooling rate of the mold.
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INGOT INGOT CASTING ASTING & CONTINO CONTINOUS US CASTI CASTING NG DEFECTS: 1. Piping 2. Seg Segregati ation 3. The The crys crysta tall seg segre rega gati tion on 4. Ingo Ingott seg segre rega gati tion on 5. Ingo Ingott def defeects: cts: 6. Dull we welding 7. Doubl oublee skin kins: 8. Rip Ripped ped sur surfa face ce 9. Teem eeming la laps 10. Slag patches patches and and brick inclusi inclusion on 11. 11. Boot Bootle leg g 12. 12. Fin Fins 13. Crocod Crocodile ile skins skins 14. Skin Skin holes holes 15. 15. Poro Porosi sity ty 16. 16. Bow Bow hole holess
39 17. Pitted Pitted surfa surfaces ces 18. 18. Crac Cracks ks 19. Center Center defects defects of the the ingot ingot 20. Flakes Flakes in stee steell 21. Non –metall –metallic ic inclusion inclusionss
INGOT INGOT CASTING ASTING & CONTINO CONTINOUS US CASTI CASTING NG DEFECTS INSPECTION: This section is an important part of quality production here all the products of continous casting i.e. billets and blooms and of the ingot casting c asting are carefully examine piece by piece by the experts who examine each work piece thoroughly, and if any defect found then he mark that piece and send some samples to the QAD for macro etching testing for internal defects examinations. The following are the defects which are found on continous casting products: 1. Long Longit itud udin inal al corn corner er crac cracks ks 2. Tran Transv sver erse se cor corne nerr crac cracks ks 3. Tran Transv sver erse se crac cracks ks 4. Long Longit itud udin inal al crac cracks ks 5. Star Star crac crack ks in in SS SS 6. Depression 7. Osci Oscill llat atio ion n mar marks ks 8. Subc Subcut utan aneo eous us blo blow w hole holess 9. Macr Macro o incl inclus usio ion ns 10. Spits (overlappi (overlapping) ng) 11. Transvers Transversal al depressio depressions ns 12. 12. Scra Scraps ps 13. Cross Cross sectio sections ns
ELEC ELECTRO TRO SLAG REME REMELTI LTING NG::
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Electro slag Remelting (ESR) is a process of remelting a consumable electrode utilizing the heat generated by an electric current passing through a molten slag between the electrode and the solidifying ingot. The consumable electrods are as cast or forged cylindrical parts made of an alloy to be remelt. An electro slag remelting process process (ESR) starts when the lower tip of a consumable electrode is immersed into a pool of molten slag. The premelted slag possessing electrical conductivity is located on the water-cooled mold base connected to a power supply. The electric current (commonly AC) passing through the the slag keeps it at high temperature, which is about 360ºF (200ºC) higher than the melting point of the remelted metal. The electrode tip is heated by the hot slag and starts to melt forming droplets of liquid metal, which disconnect from the electrode and sink through the slag layer. The slag composition is based on calcium fluoride (CaF 2), lime (CaO) and alumina (Al 2O3). The slag composition provides the following properties: • • • • •
Melting point lower than that of the remelted alloy; Required level ov viscosity; Required level of electrical conductivity; High solubility of sulfur ; Capability to adsorb non-metallic inclusions. inclusions.
The molten steel in form of both liquid film on the electrode tip and descending droplets contacts with the slag and get refined due to desulfurization and removal of non-metallic inclusions (sulfides and oxides). oxides). The droplets enters the molten steel pool, bottom of which is progressively solidifying. solidifying. The water-cooled copper mold provides relatively high gradient of temperature resulting in high solidification rate. Solidification front is moving upwards (unidirectional solidification) forming sound homogeneous metal structure. The ingot has a good surface quality due to a thin slag film covering it. Advantages of Electroslag of Electroslag Remelting • • • • • • • •
Deep desulfurization; Refining non-metallic inclusions; inclusions; Homogeneous Distribution of non-metallic inclusions Fine Grain structure; No Shrinkage defects; Low macrosegregation; Good surface quality; Controllable process.
Applications of Electroslag of Electroslag Remelting •
•
Superalloys (nickel (nickel base alloys possessing high strength, Creep resistance and oxidation resistance at high temperatures. Large Forging ingots. Fabrication of large steel ingots requires technologies providing high purity steel with low level of macrosegregation and shrinkage porosity. The technology of electroslag remelting (ESR) meets these demands. Ingots up to 200 metric tons are produced by electroslag remelting utilizing simultaneous remelting of
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• • • • •
tens of electrodes. The ingots are used for manufacturing heavy parts for electric power generating units (turbine shafts, generator rotor shafts). Rolling mill rolls. Tool and die steels. Stainless steels. Titanium alloys for aerospace applications. Titanium scrap remelting. Titnanium machining turnings and foundry scrap may be recycled by ESR providing cost effective secondary titanium.
ESR process description
In ESR process the steel ingot produced by the primary refining form the electrode to be drip melted into water cooled copper mould. Esr is carried out in open atmosphere and slag layer used act as a resistor between the electrode and the which is responsible for melting the electrode the slag used as a refining agent. The product of these process is exceptionally suited the production of forging of high alloy steel. A method of controlling contamination of, and alloy variations in the remelted product of an electro-slag refining (ESR) or remelting process. More particularly, this invention is directed to a slag forming flux, to the timing and/or manner in which
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the flux is utilized during such method, and to a slag deoxidizer. The controls are achieved by the sequential additions of a (1) flux of predominantly CaF 2 /Al2 O3 with the balance a mixture of SiO2 and MnO, and (2) a non-aluminum containing slag deoxidizer such as CaSi. Preferably, the flux is added in two portions about one-third at start-up with the remainder after the start-up portion has melted to form a slag blanket. As a preferred alternative, the total amount of SiO2 /MnO of the flux is added with the start-up portion and the remainder portion comprises essentially only CaF 2 /Al2 O3. Metallurgical electro slag remelting process: Due to the superheated slag that is continuously in touch with the electrode tip, a liquid film of metal forms at the electrode tip. As the developing droplets pass through theslag, the metal is cleaned of non-metallic impurities which are removed by chemicalreaction with the slag or by physical flotation to the top of the molten pool Theremaining inclusions in ESR are very small in size and evenly distributed in the remelted ingot.Slags for ESR are usually based on calcium fluoride (CaF2), lime (CaO) and alumina (Al2O3). Magnesia (MgO), titania (TiO2) and silica (SiO2) may also be added, depending on the alloy to be remelted. To perform its intended functions, the slag must Have some well-defined properties, such as •Its melting point must be lower than that of the metal to be remelted; •It must be electrically efficient • Its composition should be selected to ensure the desired chemical reactions • It must have suitable viscosity at remelting temperature. In spite of directional dendritic solidification, various defects, such as the formation of tree ring patterns and freckles, can occur in remelted ingots. Reasons for the occurrence of these defects are the same as in VAR. It is important to note that white spots normally do not occur in an ESR ingot. The dendrite skeletons or small broken pieces from the electrode must pass the superheated slag and have enough time to become molten before they reach the solidification front. This prevents white spots.The ingot surface covered by a thin slag skin needs no conditioning prior to forging. Electrodes for remelting can be used in the as-cast condition.
ESR PRODUCTS: ESR is the most sophisticated refining technology of PSM for making high grade steels with high purity for specific purposes. The round ingots r-600 and r-2000 whose dia is around 390mm and 220 mm respectively are used for this process. Their capability is of producing 315 – 720 mm dia ingots. Four types of cu moulds are used in ESR: Mould size
Electrode size
335 mm
3 m (R-600)
43 410 mm
4 m (R-600)
620 mm
3 ½ m (R-2000)
750 mm
3 m (R-2000)
ESR CONSTRUCTION: The furnace consists of the following parts: Loading arm Electrode arm Mould assembly. Trolley or base unit Loading arm change the position of the electrode while electrode arm holds the electrode during the process. Mould assembly consists of a high density water cooled pure copper mould with special alloy jacket to hold water. And cover insulated with glass wool. The base unit trolley consists of 3 main parts the mild steel base plate, the copper plate and the mild steel starter bridge for ingot sitting. The movement required during the process is controlled by a hydraulic clamp system First the feeder head portion is cut down by steel cutter then it is weld with non consumable stainless steel pin whose tip is made up of ms for emergency bases if metal is over melted. Then the cu base is filled with alumina powder in order to protect base from arc damages. Then a round of packet paper is placed on center in which mild steel chips, aluminum tablets and powdered slag is filled which serves two purposes: initialization of arc by its resistance and to start nucleation. Then the electrode which is the impure metal ingot which is preheated at 600c for ½ hr minimum is placed inside the furnace then by the adjustment of stage the electrode is settled on that base plate. Then insulation cover is placed on top of furnace and a bucket of granulated universal slag which is a mixture of CaO, caf2 and alumina is then added in which CaO refines the metal, caf2 controls the fluidity and alumina produces resistance. Then heating is started by applying current. During refining melting and solidification occur simultaneously, the slag used in this process is artificially made slag whose composition is as: 33% CaO, 33% Al & 33% CaF2. In the initial the al tablets are inserted which act as deoxidizers. The whole process is of about 8-10 hrs. After refining the metal found in casted form in cu mould.
Q-Shop (bar rolling mill): Rolling can be defined as: The application of compressive forces to reduce the cross sectional area through work rollers is called rolling.
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•
Hot Rolling
•
Initial breakdown of an ingot or a continuously cast slab is achieved by hot rolling. Mechanical strength is improved and porosity is reduced. The worked metal tends to oxidize leading to scaling which results in a poor surface finish and loss of precise dimensions. A hot rolled product is often pickled to remove scale, and further rolled cold to ensure a good surface finish and optimize the mechanical properties for a given application.
FLOW
SHEET OF BAR ROLLING MILL
BILLETS ↓↓↓↓
Pre- heating Furnace ↓↓↓↓
Roughing Mill ↓↓↓↓
Hot Shearing ↓↓↓↓
Finishing mill
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INPUT OF Q-SHOP Continuous casting products, 1-sq billet (110x110mm )size. 2-sq billet
(125x125mm)size.
3-sq bloom (230x230mm) size. Flat and rolled bars are made in this shop. The input material for this shop is billets and blooms from continuous casting machine. The billets and blooms from C.C. go to the material bank, where the material is stored, and then supplied to the desired shop with the appropriate size. The cutting of the material is also done there. The billets and blooms after cutting to the desired size go to the Q-Shop where further processing is done. Q-Shop is equipped with the following machines: •
Reheating furnace
•
Roughing mill
•
Finishing mill
•
Cooling bed
•
Shearing machine
•
Annealing furnace
•
Straightening machine
•
Quality inspection
Reheating furnace: In bar rolling mill pusher type reheating furnace is used and are gas fired furnace. The material is first heated into the reheating furnace normally up to 1250°C to 1300°C. This temperature differs with every grade of steel. The furnace capacity is about 6 tons per hour. The furnace is about 40 ft long and is the pusher type furnace. The pressure of 14.8 -15 Psi is maintained in the furnace so that the atmospheric air doesn’t the furnace. The refractory material of the furnace is high alumina bricks and silicate board. The furnace comprises of 3 parts; pre-heating zone, soaking zone, and ejector. The material is pre-heated in the pre-heated zone. This zone consist 4 burners and its length is about 4 meter. The material is preheated with the time rate of 0.5 min per mm. Then it is pushed into the soaking zone. In this zone the material is hold for the time rate of 25mm per hrs. So that the temperature of the surface and the core will be homogenize. At a time 32 billets of 110 x 110mm can be stored in the soaking zone. After the soaking of material then it is ejected to the entry table through the ejector.
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PUSHER TYPE REHETING FURNACE Roughing mill(medium mill): Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls .The rotating rolls perform two main functions: 1-Pull the work into the gap between them by friction between workpart and rolls 2-Simultaneously squeeze the work to reduce cross section. In bar rolling mill three high roughing mill is used and the rollers are made of cast iron, cast steel, and chilled cast iron .From the entry table the work piece is put into the 3 high reversing roughing mills. The rollers of the roughing mill is of high carbon steel with 0.1% C. the rollers rotate with the help of an electric motor of 7.5 KV. The direction of roller (forward or backward) is controlled by pinion stand. The maximum dia that the roller can roll is 480 mm. After getting the approx. size, the work piece is transferred to the finishing mill. It should be noted that reduction should be > 6.
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Finishing mill: Finishing mill consists of 5 cross country type rolling machines, which are named as F1, F2, F3, F4, & F5. The first four are 3 high rolling machine, while the F5 is 2 high rolling machine. The roller of F5 is of chilled cast iron because its surface finishing is good. The minimum temperature for this process is 750 C. From the F5 the final shape and thickness of bars is gained. Then the work piece is transferred to the cooling bed with the help of roller table. The material is then cooled at the cooling bed for about half an hour.
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Shearing machine: For getting the desired length of the bar there is a shearing machine which works on the hydraulic force. Then the material is heat treated if required in annealing furnace.
heat treatment furnaces: Q-shop contains 2 heat treatment furnaces, used for heat treatment of the bars as per requirement of customer. The furnaces are trolley type. The fuel used in these furnaces is natural gas. The capacity of the furnaces is 15 tons. There lengths are 7 meter each and each has 10 burners. The refractory of the furnace is of high alumina bricks.
Straightening machine: Q-shop has a roller type motorized straightening machine. This machine is used to straighten the bars. Some time when the temperature of the work piece decrease during the rolling processes the bar gets the wave shape and to reduce the waves straightening machine is used. Then the material is transferred to the quality control department.
Quality inspection: After the above processes material goes for quality check department. The defects in the material are detected there. Following are the defects which can come in the bars.
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•
Scratches
•
Fin
•
Guide mark
•
Seam
•
Folding
•
Roll mark
•
Shifting / misalignment of rolls
•
Ovality
•
Cracks
•
Pits
•
Chips formation
•
Excessive scale formation
•
Overlapping
Product sizes: Round bars :
deformed bars:
Dia
12, 14, 16… 57, 60 mm
Length
4 to 6 meter
dia
8, 10, 12, 16, 18 mm
Flat bar: Size
6 X 40 mm to 20 X 120 mm
Length
4 to 6 meter
N-SHOP (BLOOMING & SLABING): INPUT OF N-SHOP: 1- Flat ingot. 2- Square ingot.
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3- Bloom (cast and rolled). 4- Billet (150 x 150 mm). For the formation of plates PSM have N-shop. The raw material for this shop is the slabs from ingot casting. PSM also imports slabs from Pakistan Steel Mill, Ukraine, and England. This mill comprises of a reheating furnace, a 2 high reversible rolling machine, a hot shearing machine, cooling bed, and a leveler.
Reheating furnace: The reheating furnace is of pusher type. It is comprises of three zones: •
Top zone
•
Bottom zone
•
Soaking zone
The furnace capacity is 9 ton per hour and have 18 burners. Dumpers are used to maintain the pressure of the furnace. The source of fuel in this furnace is natural gas. The consumption rate of the natural gas is about 820NM 3 . The highest temperature of the furnace is 1300°C. After heating, the material is transferred to the reversible rolling mill.
Rolling mill: In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed ascold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process and cold rolling processes the most tonnage out of all cold working processes.There is a 2 high reversible rolling mill with a motor power of 1000 K Watt. The diameter of the rollers is 820 mm and the length is 1800 mm. Both rollers rotate in opposite direction. Bars and various structural shapes are made through this mill by successive passes in forward and backward directions. The rolls are made of chilled cast iron. The work piece is entered into the gap between the rollers. Since the top roll is moveable, it can be move for small or big thickness. The rolling is carried out at temperature above 0.7 Tm (Tm is the melting temperature of the work piece).
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Shearing section: After rolling, the material is then transferred to the shearing machine, where the required size of the plates is gained by hot shearing it with the shearing machine. The billets and blooms are sectioned by gas cutting. Then the material is transferred to cooling bed for cooling it.
N-SHOP Products: Plate: Thickness
10 mm (min)
Width
1000 to 1350 mm
Length
4 to 6 meter
FORGING SHOP (R1, R2 & R3): Forging: Forging is the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 170 metric tons. Forged parts usually require further processing to achieve a finished part. Forging is the process by which metal is heated and is shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press. Forging refines the grain structure and improves physical properties of the metal. With proper design, the grain flow can be oriented in the direction of principal stresses encountered in actual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation.
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Physical properties (such as strength, ductility and toughness) are much better in a forging than in the base metal, which has, crystals randomly oriented .
Introduction: R1, R2, and R3 shops are the forging shops. Forging is done at austenitic temperature the purpose of forging is to control the following parameters; Properties mechanical; the strength of the material is improved. Micro structure; the coarse crystal structure of the cast ingot becomes fine. Shaping; homogenous mass of steel of desired size and shape can be obtained.
The impurities of the different grades of the steels can be minimized as inherited through different casting process. These impurities include: •
Segregation
•
Porosity
•
Piping
•
Inclusion
•
Blow holes of minimum depth
•
Pin holes
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Forging ratio: Forging ratio = input cross section area / output cross section area Forging ratio is the important parameter during forging; on this basis we design the input size and shape of the stock for forging. For carbon steel and low and medium alloy steel is kept 6 while for the high alloy steel and those grades which have critical use like SS is kept 8.
Forging action: First the stock is transformed into the bar or block then it is heated to austenitic range allow easy of metal for shaping purpose. Three major forging actions for shaping are; Drawn out; increasing the length while reducing the crass section area. Upset; increasing the cross section area while reducing the length. Squeezed; it is done in closed impression dies causing the multi-directional flow of metal so
that the metal acquires the shape of the die cavity
Cold vs. hot forging:
–
1-Hot or warm forging due to the significant deformation and the need to reduce strength and increase ductility of work metal
-
2-Cold forging advantage is increased strength that results from strain hardening There are four types of forging process in all the R-shops 1. Free forging. 2. Die forging. 3. Hammer forging.
4. Press forging.
Free forging: Air is used to lift the anvil through which hammer is attached and then hammer is released it fall with gravity action and it is called gravity drop forging or free forging. Free forging is done for making round bar. The operating pressure for air- drop forging = 7 bar. It is done by gravity dropped hammer .
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Open die forging: Open die forging is the hot working of steel between flat or countered dies. Open die forging is done for giving particular shape, the forging products include steam turbine rotors, hydroelectric shafts, ring and disc type components, work rolls, sleeves, die blocks, and forged blooms, billets, and bars, etc can be made by this technique(a) Open-die forging - work is compressed between two flat dies, allowing metal to flow laterally without constraint. (b)Impression-die forging - die surfaces contain a cavity or impression that is imparted to workpart, thus constraining metal flow - flash is created.
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Hammer forging: The metal to forge is pre-heated then it is placed over the anvil and hammered to obtain the desired shape of the metal.
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Press forging: Press forging is an operation characterized by the process of deformation which consists of a lot of heating and cooling. During the process, the material is slowly condensed into a shape by increasing pressure. There are two dies; one stationary and one pushed towards the other, which compresses the part. Press forging is variation of drop-hammer forging. Unlike drophammer forging, press forges work slowly by applying continuous pressure or force. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot Cold press forging is done on cold annealed steel and hot press forging is done on a large armored plate. The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete workpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. The press forging is done by forging the hammer to the pre-heated stock and then it is press by the same hammer. The stock is placed over the base of circular shape. It can be rotated so that the forging can be done uniformly. Pressing force = 1700 ton Operating pressure = 250 bar There are different mechanisms for press forging;
1-Hydraulic press.
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2-Pneumatic press .
The difference is just the operating pressure is applied to the ram is given through hydraulic system where as in pneumatic press it is given through the pneumatic system.
Types of hammer: Different hammers of different types are used in PSM. The hammer is selected according to the force required for shaping of the metal and alloy. The lists of them are mentioned below; Die forging hammer 500 kg 2 hammer 3 ton
1 hammer
0.25 ton
1 hammer
Free forging hammer 1 ton 2 ton 3.5 ton The difference between the free forging hammer and die forging hammer is that Die forging hammers have a guiders which help in avoiding miss alignment of the hammer resulting in the folding of bending of the die forged product, thus the exact force is applied to the stock in uniform direction which govern better finished or semifinished product. Counterblow hammer 6 ton It is gravity drop hammer but its both dies are movable. Counter blow hammer is advantage that the impact doesn’t transfer to the body because both dies are moving. I.e. Action and reaction Mechanical press These presses are used for trimming and notching purpose. 50 ton 100 ton 200 ton 250 ton
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Free forging press 1700 ton (This press forging hammer is made of the steel grade Din 1.2344)
R-1 SHOP: Input CC square billets. Rolled bar from p-shop Radial forged bar from P-2 shop The input diameter of the bar for both type of bars are 80, 60, & 130. Equipment Three Pre-heating furnaces and one annealing furnace and all are gas fired furnace Capacity = 40-50 Kg Capacity=100 Kg Capacity=200 Kg Capacity=400Kg (annealing) Free forging hammer
0.5
ton
1
ton
3.15
ton
Counterblows die forging hammer
6 ton Band saw (for cutting purpose)
Processes in R-1 shop The processes done in the R-1 shop are: Free forging (for pinion shaft, etc) Die forging (gear blanks) Trimming and notching.
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Trimming The excess metal which remains around parting lines or around other edges after previous operation is removed by this process. The die forged product is placed over the hollow anvil, the cavity is desired shape. It is then pressed and the trimmed product is fall down through the cavity the excess material is left around the anvil . In R-1 shop one Trimming press of 400 ton is used to remove trimming. Notching In this process the small indentations in the edges of the die forged product are removed. Final finishing is done after notching and trimming because of bending due to the miss-alignment of the ram. In R-1 shop two notching press is used one is of 300 ton and the other is of 200 ton.
Gears are also made from the forging although they can be made from machining but forging have following advantages: • Mass production •
Material loss is reduced
•
It is economical
•
The efficiency or life the gear is increased
Output of die forging Gear blanks and grade bars, etc Output of free forging Round shafts and discs Output of press forging Heavy (12 ton) round shafts and rings
R-2 SHOP: Input Cast ingot Sq 700 Sq 1250 Oct 2000 ESR ingot
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R 400 R 600 R 750 Equipment Box cooling furnace Manipulator, 1.5 ton 3 furnaces of 2 ton One is Batch type (electric furnace given by KRL) Two Gas fired furnace. Neutral refractory is used For gas fired furnace .The fuel used in the furnace is the mixture of gas and air with the proportion of 1; 10.
Process Free forging Die forging
Heat treatment Then the heat treatment of the steel grade is done. The heat treatment cycle and methods is adopted according to the grade of the steel, mechanical properties desired, the application of the finished product and the requirement of the customer heat treatment process includes: •
Annealing
•
Normalizing
•
Hardening
•
Tempering
Normalizing is generally recommended after the forging because due to the forging the structure is become banded type structure.
R-3 SHOP: Input Cast ingots
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SQ 1250 Oct 2000, 3000, 2500, 5000,8000,12000 ESR ingot R 400 R 600 R 750
Equipment 3 furnaces
Capacity 40 ton, 3 ton, and 2 ton One 40 ton capacity furnaces Dimensions = 6 x 3 No. Of burners = 6 No. Of thermocouples = 3.
It has three zones; each zone has two burners and a thermocouple Hot air is blow into the furnace and it was caught by the exhaust and again recycled and reused.
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1700 ton press forging machine
2 Manipulators
Ring rolling machine: Ring rolling is a hot forming process that produces seamless rings varying in size from a few inches in diameter, and weighing less than one pound, to over 25 feet in diameter and face heights approaching 10 feet. The process and equipment are similar in principle to rolling mills
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used for plate. In both processes, the metal is rolled between two rolls, which move toward each other to form a continuously reducing gap. In ring rolling, the rolls are of different diameters. It begins with a hollow circular perform that has been upset and pierced, similar to performs used for ring forging, The perform is placed over the idler or mandrel roll, which is forced toward the drive roll. The drive roll rotates continuously, reducing the wall thickness, imparting the desired shape to the cross section, and increasing the diameter .For larger rings, the mill may also have radial oriented or "pinch" rolls which control the height of the ring. They also help to maintain squareness and alignment with virtually no axial growth. In some cases, such rolls can reduce the height as much as required. They are not, however, generally used to roll contours on either top or bottom surfaces. Thickness-to-height ratios normally range from 16:1 to 1:16, and special equipment can extend these ranges. Ring rolling produces seamless rings with forged properties, which results in optimum mechanical properties, and predictable and efficient machinability. Tooling cost is low, set-up time is fast, rolled sections require little or no machining. The process is also highly material efficient. The perform typically utilizes up to 95% of the starting billet. Material losses come from the hole punched in the perform, oxidation in medium to large size rings, and any required machining.
Band saw for cutting purpose. Cutting oil is used as a coolant in order to overcome the friction. Cutting oil is mixture of oil and the water in the proportion, 1 lit oil + 4 lit water .
3 trimming machine
After press forging it is then go for heat treatment .i.e. annealing. Normalizing, hardening, tempering. Heat treatment is done in the two batch type furnaces whose capacity is 4 ton and 2 ton. Then it goes for final inspection
Defects •
Folding
•
Internal crack
•
Surface crack
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•
Banded micro structure
•
Dimensions exceed tolerance
•
Flow lines
•
Cold shuts
•
Scales
•
Unfilled section
•
Rupture fiber
P-SHOP: P-1 shop (sheet rolling mill) N-shop plates are used as a raw material for the sheet making in P-1 shop. Plates are hot rolled through 2 high reversing mills. 1.5 - 8 mm sheet can be made by hot rolling and for the size from 1mm to 1.5mm sheets are cold rolled. The shop is equipped with the following machines •
Reheating furnaces
•
Two high rolling mill 1-Roughing mill 2- Finishing mill
•
Shearing machine
•
Annealing furnace
•
Leveler
Reheating furnaces Plates are heated up to austenitic temperature to get the single phase (homogeneity). For this purpose P-1 shop is equipped with two walking beam type furnaces. One is use for primary heating whereas other for secondary heating. The furnaces are divided into three zones: 1-Charging zone
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2- Pre-heating zone 3-Soaking zone Reheating temperature differs with every grade of steel but normally it is about 1200°C. The length of the furnaces is about 14 meters and about 18 burners are placed with in the furnaces. Natural gas is used as a fuel in these furnaces. The capacity of furnaces is about 2 tons per hours. Rolling Mill Hot rolling is a metalworking process that occurs above the recrystallization temperature of the
material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (122 to 212 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling. Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and work pieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams and H-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high-temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface. [4] Dimensional tolerances are usually 2 to 5% of the overall dimension. P-1 shop has 2 rolling mills: Roughing mill
The inputs for the roughing mill are the plates of thickness 10-12 mm made in N-shop. Roughing mill is 2 high non-reversible mill, in which plates are reduced to the size of about 5-8 mm thickness. The diameter of the rollers is 780 mm each. After achieving the rough thickness of the sheet it is then passed to the secondary reheating furnace.
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Finishing mill This is also a 2 high non-reversible rolling mill. The sheets from the secondary reheating furnace pass through this mill to get the final thickness of the sheets. The minimum thickness of 1.5 mm can be achieved through this mill.
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Shearing machine After getting final products from finishing mill, the cutting of sheets in required size is necessary. This is done by the shearing machine . Heat treatment furnace P-1 shop has one heat treatment furnace. It is a gas fired furnace. After shearing, if the particular grade requires heat treatment then sheets are sending to this furnace. Normally annealing is done in this furnace. The capacity of this furnace is 2 tons. Leveler machine After the treatment the sheets passes through the leveler, if required. The rollers of the leveler are arranged in alternate order through which it is easy to minimize the waves in the sheets.
P-2 shop: P-2 shop can be divided into following four parts: 1. Heat treatment
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2. Forging section 3. Finishing section 4. Cold rolling This shop is equipped with a reheating furnace, an automatic radial forging machine, a hot abrasive cutting machine, a cold rolling mill (4 high), and 4 heat treatment furnaces. The finishing section contains; 2 peeling machines and one straightening machine. Reheating furnace P-2 shop has a walking beam type furnace. The furnace is gas fired in which the input is heated up to 1250°C. The furnace has three zones, charging zone, preheating zone, and soaking zone. The billets, blooms, round bars etc are charged in the furnace. The charge is then preheated and after that hold in the furnace for soaking. Soaking is done to homogenize the temperature of the surface & centre of the input so as to minimize the cracks/defects in the forging. Radial forging machine : RADIAL FORGING is forming with four dies arranged in one plane that can act on the piece simultaneously. This action eliminates the spreading observed i n open-die forging and restricts expansion of the material primarily to the axial direction of the bar. The four-die configuration imposes predominantly compressive strains on the part, reduces the propensity for surface cracking, and prevents internal defects from opening and growing. Radial forging was first conceived in Austria in 1946, and the first four-hammer machine was built in the 1960s. Today there are hundreds of machines worldwide in a number of different sizes and configurations. In primary metals production, where the speed of production and the control of microstructure must be maintained, the radial forge produces bars and billets from ingots or intermediate cogs. (See the article “Practical Aspects of Converting Ingot to Billet” in this Volume.) Rounds, squares, flats, or other axisymmetric bars can also be produced with a radial forge. Coupled with mandrels, tubing and other hollows can also be produced on the radial forge. In part production, radial forging is used to form axial symmetric solids or hollows with complex external and/or internal contours. These parts can be forged to very close tolerances very efficiently. Radial forging is performed with four hammers automatic machine. In the radial forging machine, however, a work piece is forged at the same time by four hammers arranged in one plane. The maximum forging force that each die can exert is 30MN. A radial press contracts the circumference of the work piece equally and simultaneously. This causes the entire surface of the work piece to be under compressive stresses. These stresses prevent the formation of surface cracks and improve existing defects.
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Current application includes round and square bars from ingots and blooms. Moreover the shafts and axels for railway and trucks can also be made through this machine.The four hammer radial forging machine is basically a short stroke mechanical press. The stroke of the forging connecting rods is initiated through eccentric shafts. Eccentric shafts are drive by motors through a drive gear. The forging connecting rods can be in their stoke position so that the round, square or rectangular cross-sections can be forged. The work piece axis is always maintained on the centre of the forging hammers. The manipulator of the machine moves only in longitudinal direction for exact guidance. Chuck head slides on the machine bed. During round cross sections forging, the chuck head rotates the work piece in cycle with the forging hammer that is the rotary movement stops while the hammers are in contact with the work piece. The indexing position of the chuck head spindle is required for squares, rectangular or octagonal inputs can be set automatically. advantages Due to the fact that it is an automatic machine and contains four hammers, its production rate is higher than other forging machine and it consumes less energy. Moreover the work piece forged is very close to the tolerance range. Input for radial forging Bloom 230 mm square Billet
150 mm square
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Round bars
50 - 220 mm
Octagon
100 - 230 mm
Forging dies Different dies are being used for different shape. The following are the some of the sizes of forging dies: Flat forging dies
Ø 82, 84, 111, 115, 25 X 55 mm
DH 127.5
Ø 85
DH 137.5 mm
Ø 65
DH 132.5 mm
Ø 75
DH 145 mm
Ø 50
Hot abrasive cutting machine After completing the forging process from LFM, the work piece then goes to hot abrasive cutting to be cut into the desired size. The cutting blades are made of hard material mostly it is made up of silicon carbide or tungsten carbide. Cold rolling mill Cold rolling occurs with the metal below its recrystallization temperature (usually at room
temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass. Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Quarter-hard is defined by its ability to be bent back onto itself along the grain boundary without breaking. Half-hard can be bent 90°, while full-hard can only be bent 45°, with the bend radius approximately equal to the material thickness. Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield-point phenomenon (by preventing Luder bands from forming in later processing). [6] It is also used to breakup the spangles in galvanized steel Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
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Cold rolling is required when we need to make the plates of thickness below 1.5 mm. The minimum thickness of the plates formed in hot rolling mill in P-1 Shop is 1.5 mm. For further reduction cold rolling is required. In P-2 shop there is a 4 high non reversible cold rolling mill for this purpose. The minimum thickness that we can get by this mill is 1 mm.
Heat treatment furnaces P-2 shop contains 4 heat treatment furnaces for the processes of annealing, normalizing, tampering, quenching, hardening etc., which are as follows: Austria furnace This shop contains an Austria furnace which has a capacity of 15 metric tons. There are 14 burners placed in this furnace. The furnace is divided into three zones due to the placement of different thermocouples. It is a gas fired bogie type furnace. China furnace The capacity of this furnace is 10 metric tons. It is also a gas fired bogie type furnace. It has 6 burners, 2 burners in each zone. Top head furnace This furnace has a capacity of 2 tons. This is an electrically fired furnace. The maximum temperature of 1100°C can be achieved in this furnace. Japanese furnace P-2 shop also contains a Japanese furnace which is a gas type furnace and the capacity of this furnace is about 2 tons. Refractory of the furnaces The refractory lining of the above four furnaces is produced by glass wool. Glass wool is a advanced refractory material and it can be heated and cooled rapidly.
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Finishing section The finishing section of P-2 shop is composed of a straightening machine and two peeling machines. Straightening machine The machine is used if the forged piece is not straight that is the piece is bend or in waves like shape. This machine contains rollers arranged in alternate order (one by one on top and bottom) with the help of which the material straighten. Peeling machines There are 2 peeling machines in P-2 shop which is used for the finishing of the forged material. After the forging and heat treatment processes there are some scales on the work piece, to remove these peeling machines are used.
LABORATORY: QUALITY ASSURANCE AND DEVELOPMENT DEPARTMENT (QAD) : Quality assurance department in PSM is responsible to ensure the desirable quality of incoming raw materials and outgoing finished products. On the analysis systems are available to measure the chemical composition of alloy steels undergoing various processing sequences. The cleanliness level of the material is checked using conventional Metallographic techniques. The micro structural developments following various treatments are characterized to establish structure property relationship. TESTING FACILITIES QAD provides online inspection for production in PSM and however also provides commercial testing facilities. QAD lab is also equipped with modern and conventional equipments and methods, the details of which is stated below 1. Wet chemical testing 2. Spectrophotometer 3. Atomic absorption photometer 4. Optical emission spectrometer 5. Rapid analyzer 6. Rock well hardness tester 7. Brinell hardness tester
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8. Micro vicker hardness tester 9. Shore hardness tester 10. Optical microscope 11. Photography unit 12. Bend test machine 13. Impact testing machine 14. Tensile testing machine 15. Mechanical workshop 16. Muffle furnaces. Sample Preparation: There are two types of sample: 1) Online sample: This comes from M-shop during production for the composition
analysis. This is in the form of cup whose minimum dia should be 20mm. the surface of this sample is grind to give sharp results . 2) Non-online sample: This comes after casting to check the mechanical properties
of the material. The sample is formed into different shapes according to the type of testing required.
CHEMICAL ANALYSIS Spectrometry: There are two spectrometers, one is for Ferro-alloys and other is for non-ferrous alloys. Emission spectroscopy is a spectroscopic technique which examines the wavelengths of
photons emitted by atoms or molecules during their transition from an excited state to a lower energy state. In this lab the purpose of spectroscope is to check the chemical composition of metals. The diameter of the chamber of spectroscope is 18mm, and electrode is made up of graphite. The distance between the electrode and sample is 3mm. By applying the voltage the energy OF 50 KW is stored in the capacitor, this energy helps to produce the spark between the sample and graphite electrode. The temperature of the spark is about 10,000K. These light
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waves energy then passes to the polarizer to align the scattered waves. Then a dispersion medium is provided to isolate the wavelength. After that, they strike with sensors. These sensors are in the form of phototubes there are 24 to 28 photo tubes for different element ,which are made up of different metals, for example for manganese there is a photo tube of made of manganese and for silicon there is a photo tube of silicon. These tubes convert the waves into the voltage and with the help of voltage the % composition of different elements in the material is obtained. The result is obtained in only 20seconds. Rapid analyzers: There are three different rapid analyzers are used. These analyzers work on the basis of IR detection.
Carbon and Sulfur analyzer: The range of this analyzer is 0.6 ppm to 6% carbon and 0.6ppm to 0.35% sulfur. The sample for this analyzer is formed into chips by drilling.
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The carbon and sulfur analyzer has one heating chamber and one IR detector chamber. In heating chamber an induction crucible furnace is used for the melting of the sample. The temperature of the melt in this furnace is set to 1200-1300C. To accelerate the melting operation tungsten particles are inserted in the crucible made of Porcelain. Pure oxygen gas supplied at a pressure of 3-4 bar. It first passes through the filter tube in the heating chamber which contains the layers of NaOH, MgCl2 and glass wool. After filter the oxygen passes through the IR detector where the intensity of light passed is detected. The oxygen then comes back to furnace chamber and reacts with the carbon and sulfur of the melted sample and carbon monoxide and sulfur trioxide is formed. The IR detector has a reagent tube which contains copper oxide as an catalyst to covert carbon monoxide into carbon dioxide and sulfur trioxide into sulfur dioxide. These gases finally pass through IR and the intensity of light passed is noted which then give the result in the form of % composition of carbon and sulfur. Oxygen and Nitrogen analyzer:
The sample is prepared in the form of pins. To melt the sample the principle of conduction is followed. Sample is placed in the graphite crucible which meets with the electrode placed inside the IR detector. The conduction takes place between the graphite and the electrode which helps to melt the sample. The maximum temperature of 1600C is maintained for the melt. Here the Helium gas is used as a carrier gas and follows the same functions as oxygen in carbon and sulfur detector. There is also a filtration system. The first filter tube contains molecular sieves, the second contains the layers of cotton, charcoal, glass wool and schutze reagent (converts carbon monoxide into carbon dioxide) and the third filter has layers of glass wool, NaOH and MgCl2. The results are obtained in ppm. Hydrogen analyzer: This contain thermal conductivity cell in place of IR detector and by comparing the thermal
conductivity of hydrogen gas from sample, we get the result. The argon gas is used as a carrier gas and the principle of conduction is followed for the melting of sample. The result is obtained in ppm.
Wet chemical analysis There is a facility of chemical testing for specific elements in steels and other Ferro alloys, refractory, and non-ferrous alloys by adopting ASTM and JAPANESE method. The wet chemical analysis for mild steel:
The main elements for analysis in steels are Manganese, Sulfur, Phosphorus, Carbon, and Silicon. First of all the steel is dissolved in aqua regia For silicon: silicon is precipitated by •
SULPHURIC ACID METHOD
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•
DRYING MRTHOD
•
PERCHLORIC ACID METHOD (Here we are using perchloric method because the ppts of silicon are formed in gelatinous form).
Precipitates of silicon are filtered and washed with 1% HCL then again washed with water to remove perchloric acid. After washing, the silver nitrate is added to check the presence of chloride. Now silicon ppts are taken in a platinum crucible and dried through burner and then ignite in a furnace at 1000 C to get pure silica. The silica then obtained in powder form. It is then cooled and weight through weight balance. The crucible containing silica is placed on alumina bricks and hydro fluoric acid is added to volatile the silica then red hot the crucible to remove remaining acid. It is then cooled and weighted again. The difference in weight before and after Hydro fluoric acid treatment is noted. For manganese: Sodium bismuthate (NaBrO3) method:
Sodium bismuthate oxidizes the Mn at the oxidation state of +7 which is the unstable state of Mn. Ferrous sulphate is added to reduce it at a state of +2 and producing purple color. Excess ferrous is removed by potassium dichromate . Spectrophotometer method:
colored solution of Mn is formed by adding POTASSIUM PER IODIDE in the presence of Sulfuric acid then Mn +7 purple color is formed and then the direct reading is obtained though spectrophotometer . A
For phosphorus: Spectrophotometer method: A blue color solution of phosphorus is made with molybdenum and hydrogen sulphate by adding Ammonia hepta molybdate and Hydrogen sulphate. The blue color complex is formed at 90C and then analyzes the solution through spectrophotometer. Volumetric method: Phosphorus is precipitated by ammonia hepta molybdate giving yellow ppts of phospho molybdate which is then dissolved in 0.1% NaOH. The excess NaOH is back titrate by 0.1% HNO3. Atomic absorption spectrometry: In atomic absorption spectroscopy, the composition of element is calibrated by a standard solution of different elements like Mn, Cr, Ni, and Cu.
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The atomic absorption spectrometer consists of lamps of different elements which are used as a light source. The burning chamber contains a capillary tube which sucks the prepared solution. The solution is then atomized and isolated through manipulator. When the light source from the lamp strike the atom of same element as that of lamp, the wavelengths are detected and the composition in ppm is obtained.
MECHANICAL TESTING: The facility of mechanical testing includes:
Tensile testing: Both destructive and non-destructive techniques are used to characterise materials for their mechanical and physical properties. Universal tensile testing machines, up to a maximum of 60 tonnes capacity, are available. There is a Universal Testing Machine used for tensile testing. The sample for the testing is made according to ASTM E8 standard. The gauge length of 50mm or 100mm is marked on the sample. The sample is then placed between the lower and upper jaws of the machine and the load is applied until the fracture takes place. The maximum force of 600KN can be applied. The final length is then noted which is used to calculate the % elongation in the material. The graphical result in the form of stress-strain diagram is also obtained.
Impact testing: Impact tests are designed o measure the resistance to failure of a material to a suddenly applied force. The test measures the impact energy, or the energy absorbed prior to fracture. The most common methods of measuring impact energy are the Charpy test and Izod Test. The charpy test machine and izod test machine both are available in mechanical testing laboratory.
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CHARPY TEST: Charpy test specimens normally measure 55x10x10mm and have a notch machined across one of the larger faces. The notches may be: •
V-notch – A V-shaped notch, 2mm deep, with 45° angle.
•
U-notch – A 5mm deep notch with 1mm radius at the base of the notch.
The Charpy test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is fixed in place at both ends and the striker impacts the test piece immediately behind a machined notch. At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated based on the height to which the striker would have risen, if no test specimen was in place, and this compared to the height to which the striker actually rises. Tough materials absorb a lot of energy, whilst brittle materials tend to absorb very little energy prior to fracture.
IZOD TEST:
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Izod test sample usually have a V-notch cut into them, although specimens with no notch as also used on occasion. The Izod test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is clamped vertically with the notch facing the striker. The striker swings downwards impacting the test piece at the bottom of its swing. Tests are often performed at different temperatures to more closely simulate the actual service conditions. In the case of low temperature tests, specimens may are kept in a freezer until their temperature has equilibrated. They are then immediately removed and tested within seconds of removal from the freezer. At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated based on the height to which the striker would have risen, if no test specimen was in place, and this compared to the height to which the striker actually rises. Tough materials absorb a lot of energy, whilst brittle materials tend to absorb very little energy prior to fracture. Bend testing: There is also a bend testing machine in mechanical testing laboratory. Bend tests for ductility provide a simple way to evaluate the quality of materials by their ability to resist cracking or other surface irregularities during one continuous bend. Bend testing determines the ductility or the strength of a material by bending the material over a given radius. Following the bend, the sample is inspected for cracks on the outer surface. Bend testing provides insight into the modulus of elasticity and the bending strength of a material.
Torsional testing: A torsion test can be conducted on most materials to determine the torsional properties of the material. These properties include but are not limited to: • • • • •
Modulus of elasticity in shear Yield shear strength Ultimate shear strength Modulus of rupture in shear Ductility
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While they are not the same, they are analogous to properties that can be determined during a tensile test. In fact, the "torque versus angle" diagram looks very similar to a "stress versus strain" curve that might be generated by a tensile test. Torsion tests can be performed by applying only a rotational motion or by applying both axial (tension or compression) and torsional forces.
Hardness testing: Hardness testers in PSM include: Brinell hardness tester, Rockwell hardness tester, and Vickers hardness tester 1-The Brinell hardness test method consists of indenting the test material with a 10
mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation. 2-The Rockwell hardness test method consists of indenting the test material with a
diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load usually 10 kgf. When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position. While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration. When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration. The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number. 3-The Vickers hardness test method consists of indenting the test material with a
diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated.
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The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.
Rockwell tester vicker tester
brinell tester
METALLOGRAPHY: Metallography consists of the microscopic study of the structural characteristics of a metal or an alloy and general study of metal and their behavior with particular reference to their micro structure and macro structure. A Metallurgical Microscope helps us to determine: •
Grain size and shape
•
Size, shape and distribution of various phase and inclusion
•
Mechanical and thermal treatments of the alloys
PREPARATION OF SPECIMEN the sample is prepared as the specimen to study its microstructure by using various metallographic techniques. SELECTION OF SPECIMEN The selection of specimen for microscopic study may be very important if a failure is to be investigated; the specimen should be chosen as close as possible to the area of failure and should be compared with one taken from the normal section .
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SECTIONING / CUTTING OF THE SPECIMEN Sectioning is performed to remove a suitable size for subsequent mounting and polishing. Since the intended plane of polishing is usually determined by the sectioning operation. Abrasive cut off machine is used for metallographic specimen cutting. The ability of a metallurgist is to diagnose the desired problem in the metals or to get required features out of a metallic specimen depends strongly upon a well polished cutting practice.
MOUNTING OF THE SPECIMEN There are basically two main purposes of mounting. 1. PRIMARY PURPOSE 2. SECONDARY PURPOSE
Hot mounting
cold
mounting PRIMARY PURPOSE The primary purpose of mounting is for convenience in handling specimens of difficult shapes or sizes during the subsequent steps of preparation and examination.
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SECONDARY PURPOSE The secondary purpose is to protect and preserves extreme edges of surfaces defects during preparation. Thickness of the mount should be sufficient to enable the operator to hold the mount firmly during grinding and polishing thereby to prevent a rocking motion and to maintain a flat surface. Molding materials which is used in PSM are Bakelite and acrylic. •
Pre heating time for mounting =8 min
•
Heating time =20 min
•
Cooling time for mounting=10 min
•
Pressure= 60KN
GRINDING OF THE SPECIMEN Grinding is done using rotating discs covered with silicon carbide/ emery paper and water. There a number of grades of paper, with 120, 180,220,240,400,500, 600 grains of Silicon carbide per square inch. 180 grades therefore represent the coarsest particles and this is the grade to begin the grinding operation. Continue grinding until all the blemishes have been removed, the sample surface is flat, and all the scratches are in a single orientation. Wash the sample in water and move to the next grade, orienting the scratches from the previous grade normal to the rotation direction. This makes it easy to see when the coarser scratches have all been removed. After the final grinding operation, wash the sample in water followed by alcohol and dry it before moving to the polishers.
POLISHING OF THE SPECIMEN Polishing is the final step in production a surface that is flat, scratch free, and mirror like in appearance. Such a surface is necessary for subsequent accurate metallographic interpretation, both qualitative and quantitative. The choice of a proper polishing cloth depends upon the particular material being polished and the purpose of the metallographic study.
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There are basically two types of polishing •
intermediate polishing
•
fine polishing
INTERMEDIATE POLISHING: After mounting, the specimen is polished on a series of emery papers containing successively finer abrasive. The polishing compound used for intermediate polishing is alumina (Al2o3) powder (with a particle size of 0.05 microns) placed on a cloth-covered surface of a rotating polishing wheel. FINE POLISHING: The time consumed and the successes of fine polishing depend largely upon the care that was exercised during the previous polishing steps. The polishing compound used for fine polishing is diamond powder (particle size about 6 microns) carried in a paste that is oil soluble is placed on a cloth covered rotating wheel. Distilled water is used as a lubricant.
ETCHING OF THE SPECIMEN The purpose of etching is to make visible the many structure characteristic of metal or an alloy. The process must be such that the various parts of the microstructure may be clearly differentiated. This is accomplished by use of an appropriate reagent which subjects the polished surface to chemical action. Etching is done either by a) Immersing the polished surface (of the specimen) in the etching reagent (Nital) b) Rubbing the polished surface gently with a cotton swab wetted with the Etching Reagent After etching, the specimen is again washed thoroughly and dried. Now the specimen can be studied under the microscope.
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MACRO ETCHING: It is the process for finding the granular structure, such as, chill grains, columns grains and Equiv.-axed grains. And also different casting defect determine, horizontal and vertical cracks, below holes, pin holes, piping, inclusion etc
micro examination: the specimen is then finally tested under optical microscope for the detailed microstructural studies of the material for getting the actual morphology and the detailed properties by the grain structures.
THE END
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