CHAPTER 1 INTRODUCTION Casting is a manufacturing process where solid melted, heated to proper temperature and is then poured into cavity or mold, which contains it in the proper shape during solidification. Thus in single step, simple or complex shape can be made from any metal that can be melted. The resulting product can have virtually any configuration the designer desires. Casting is a process which carries risk of failure occurrence during all the process of accomplishment of the finished product. Hence necessary action should be taken while manufacturing of cast product so that defect free parts are obtained. Mostly casting defects are concerned with process parameters. Hence one has to control the process parameter to achieve zero defect parts. For controlling process parameter one must have knowledge about effect of process parameter on casting and their influence on defect. To obtain this all knowledge about casting defect, their causes, and defect remedies one has to be analyze casting defects. Casting defect analysis is the process of finding root causes of occurrence of defects in the rejection of casting and taking necessary step to reduce the defects and to improve the casting yield. In this review paper an attempt has been made to provide all casting related defect with their causes and remedies. During the process of casting, there is always a chance where defect will occur. Minor defect can be adjusted easily but high rejected rates could lead to significant change at high cost. Therefore it is essential for die caster to have knowledge on the type of defect and be able to identify the exact root cause, and their remedies.
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The casting that takes place at HIMSONS is essentially sand casting. It is used to manufacture hydraulic lift housings of tractors of world renowned companies like MAHINDRA AND MAHINDRA and NEW HOLLAND. The foundry is 100x50 metric squares in area, which pertains to task specific casting. It was established in 1943 by 2 young entrepreneurs Mr. Nalin Shah & Mr. Bhupendra Shah with the aim of combining the art and science of the casting process.
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CASTING DEFECT CAN BE CLASSIFIED AS FOLLOWS1. Filling related defect 2. Shape related defect 3. Thermal defect 4. Defect by appearance These defects are explained as follows. 1. Filling related defects: a) Blowhole:Blowhole is a kind of cavities defect, which is also divided into pinhole and subsurface blowhole. Pinhole is very tiny hole. Subsurface blowhole only can be seen after machining. Gases entrapped by solidifying metal on the surface of the casting, which results in a rounded or oval blowhole as a cavity. Frequently associated with slag’s or oxides. The defects are nearly always located in the cope part of the mold in poorly vented pockets and undercuts.
1.(Blow Hole In Housing Of New Holland Tractor)
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b) Sand burning :Burning-on defect is also called as sand burning, which includes chemical burn-on, and metal penetration.
c) Sand inclusion :Sand inclusion and slag inclusion are also called as scab or blacking scab. They are inclusion defects. Looks like there are slag inside of metal castings.
2. (Sand Inclusion In Housing Of New Holland Tractor)
d) Gas porosity :The gas can be from trapped air, hydrogen dissolved in aluminum alloys, moisture from water based die lubricants or steam from cracked cooling lines.
3. (Gas Porosity In Housing Of New Holland Tractor) 4|Page
e) Misrun:Defect is a kind of incomplete casting defect, which causes the casting uncompleted. The edge of defect is round and smooth.
4. (Misrun In Housing Of New Holland Tractor)
2. Shape defects:a) Mismatch :Defect Mismatch in mold defect is because of the shifting molding flashes. It will cause the dislocation at parting line.
5. (Mismatch) 5|Page
b) Distortion or warp :Warped Casting—Distortion due to warp age is known as warp defect.
c) Flash defect :Flash can be described as any unwanted, excess metal which comes out of the die attached to the cavity or runner.
6. (Flash defect In Housing Of New Holland Tractor)
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3. Thermal defect:a) Cracks or tears:Cracks can appear in die castings from a number of causes. Some cracks are very obvious and can easily be seen with the naked eye. Other cracks are very difficult to see without magnification.
. 7. (Crack In Housing Of New Holland Tractor)
b) Shrinkage:Shrinkage defects occur when feed metal is not available to compensate for shrinkage as the metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects.
8. (Shrinkage) 7|Page
4. Defects by Appearance:a) Metallic projection:Joint flash or fins. Flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.
b) Cavities :Blowholes, pinholes, Smooth-walled cavities, essentially spherical, often not contacting the external casting surface (Blowholes). The defect can appear in all regions of the casting.
9. (Crack In Housing Of New Holland Tractor)
c) Discontinuities :Hot cracking. A crack often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.
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10. (Blowhole In Housing Of New Holland Tractor)
11. (Mold Breakdown In Housing Of New Holland Tractor)
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CHAPTER 2 DEFECTS ANALYSIS AND SELECTION OF DEFECT Defects
Rejected Quantity
Job Rejection %
Blowhole
123
1.38
Bad Mold
42
0.47
Shrinkage
31
0.34
Bad Core
19
0.21
Cores Broken
16
0.17
Mismatch
13
0.14
Hard
7
0.07
Swell
1
0.01
Others
3
0.003
To understand the concept, data for occurrence of defects are collected from HIMSONS for one month. From this data, occurrence chart has been prepared which further helps to identify occurrence major defects in castings. We found that maximum defect occur due to blow hole.
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BLOWHOLES IN SAND CASTING There are different types of defects produced in sand casting. A high proportion of casting defects are caused due to evolution of gases. One of the major casting defects caused due to gases is holes (gas holes). Gas holes are pinholes and blowholes. This designation belongs to size of the hole and not its origin. Blowhole is very prevalent cause of casting scrap. Figure shows schematic of blowholes, showing blowholes near core, surface blowholes and casting strewn with blowholes.
12. (Blowholes)
The blowholes are smooth walled cavities, essentially spherical, often not contacting the external casting surface. The largest cavities are often isolated. In specific cases, the casting surface can be strewn with blowholes. The interior walls of blowholes can be shiny, more or less oxidized or in case of cast iron can be covered with a thin layer of graphite . Figure shows some slag blowholes having smooth surface and slag accumulated on smooth surface. The blowholes are usually revealed by machining or by heavy shot blasting. The defect may take the form of well defined bubble shaped cavities beneath the surface of the casting. These forms of holes may arise from entrapment of more than one sort of gas during the course of mold filling and 11 | P a g e
solidification. It is important to know the origin of and reactions producing these gases, so that correct diagnosis and cure can be affected. These defects occur during and after mold filling as gases produced/given off by mold or core additives/binders and slag are entrapped and at the latter stages of solidification as dissolved gases are rejected from the liquid metal. The blowhole will be formed when the gas pressure exceeds the metal pressure in a localized area. The primary gases associated with blowholes are CO, CO2, H, and N. If the casting has developed a thick enough skin gases will be trapped in the subsurface. Magnesium treatment causes an increase in surface tension of the metal allowing gases to be trapped more readily.
13. (Blow Hole)
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TYPES OF BLOWHOLES:When the hot metal is poured inside the sand mold, sand and sand contents gets heated and large amount of gases are produced inside the casting. The main gas producing processes in the mold are Rejection of dissolved gases from the metal
Entrapment of core and mold gases evolved under pressure
Reaction of carbon in the metal with oxygen or oxides
The above classification of origin of gases leads to following cases of formation of blowholes :
Blowholes due to high gas content of the metal:They are also called as endogenous gas holes or blowholes. These holes are caused due to excessive gas content in the metal bath and rejection of dissolved gases during solidification. The gases involved in this defect are hydrogen and nitrogen. Both are soluble in liquid cast iron and relatively insoluble in solid iron. As casting solidifies the insoluble gas is rejected and produces holes between growing crystals. Blowholes from carbon monoxide may increase on size by diffusion of hydrogen or less often nitrogen.
Blowholes from mold or core gases They are also called as exogenous gas holes. These holes are caused due to excessive moisture in molds or cores, core binders which liberate large amount of gas, excessive amount of additives containing hydrocarbons and blacking and washes which tend to 13 | P a g e
liberate too much gas. If the gas which is evolved from molds and cores cannot freely escape, it may get trapped in the liquid metal. The bubbles formed remain in the casting during solidification. The gases producing these holes consist mainly of steam, coal gas and hydrocarbon gases from decomposition of organic core binders.
Carbon oxygen reaction holes:The gas holes in this group may appear in variety of forms, but the gas responsible is carbon monoxide, produced by the reaction of oxygen containing substances with the carbon present in the cast iron. Manganese sulfide in the oxide rich liquid slag allows the reaction to take place at lower temperatures and facilitates the entrapment of gas in solidifying metal.
Mechanical entrapment of gas:They are also called as exogenous gas holes or blowholes. These holes are caused due to insufficient evacuation of air and gas from mold cavity and insufficient mold or core permeability The blowhole formation is also affected by the parameters like pouring temperature, rate of pouring, slag inclusion, moisture and clay content of mold and sand, type of binder and type of additives used etc.
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CHAPTER 3 CAUSES AND REMEDIES OF BLOWHOLE CAUSES OF BLOWHOLES:1.Design / pattern equipment • Lack of venting and vent locations • Metal entering into vents because of poor core fit and lack of sealing • Poor design of gating system / slow pouring /metal turbulence, lack of slag removal. • Short risers, not enough metal pressure • Excess temperature loss in gating systems due to long and narrow runners and gates • Insufficient space between mold cavity and flask walls & bars. • Pouring metal against large flat & hard surfaces.
2.Process • Molding Sand – high moisture, low permeability, clay balls, poor mixing, and incorrect volatile content in green sand. • Too high a moisture for the clay level and compactability • Excessive binders, un-dried coatings, uncured resins in nobake sand and cores. • Blocked or metal-capped vents. • Excessive gases dissolved in metal • Excessive oxygen (rusty scrap), hydrogen due to excess aluminium and titanium in metal, or excess nitrogen. • Cold metal temperatures and wet refractories. 15 | P a g e
• Interrupted pouring, slow pouring, too high or too low temperatures • Short poured molds • Refractories not dried properly • Too high CE or carbon
3. Most probable causes • Excessive gas producing materials in metallic charge. • Excess binder in core and/or molding sand. • Slow pouring and colder metal – lack of fluidity. • Vents missing or not working in cores and molds. • Low carbon or carbon equivalent. • Lack of metal filtration and/ or slag removal.
Due to type of sand being used:Resin-bonded sand • Inadequate core venting • Excessive release of gas from core • Excessive moisture absorption by the cores • Low gas permeability of the core sand
Clay-bonded sand • Moisture content of sand too high, or water released too quickly • Gas permeability of the sand too low • Sand temperature too high • Bentonite content too high • Too much gas released from lustrous carbon producer
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14. (Blow Hole)
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REMEDIES FOR BLOWHOLES: Resin-bonded sand • Improve core venting, provide venting channels, ensure core prints are free of dressing • Reduce amounts of gas. Use slow-reacting binder. Reduce quantity of binder. Use a coarser sand if necessary. • Apply dressing to cores, thus slowing down the rate of heating and reducing gas pressure. • Dry out cores and store dry, thus reducing absorption of water and reducing gas pressure.
Clay-bonded sand • Reduce moisture content of sand. Improve conditioning of the sand. Reduce inert dust content. • Improve gas permeability. Endeavour to use coarser sand. Reduce bentonite and carbon carrier content. • Reduce sand temperature. Install a sand cooler if necessary. Increase sand quantity. •
Reduce bentonite
content.
Use bentonite
with
a high
montmorillonite content, high specific binding capacity and good thermal stability. • Use slow-reacting lustrous carbon producers or carbon carriers with higher capacity for producing lustrous carbon. In the last instance, the content of carbon carriers in the moulding sand can be reduced.
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Moulding plant • Reduce compaction of the moulds. Ensure more uniform mould compaction through better sand distribution.
Gating and pouring practice • Increase pouring temperature. Reduce the pouring rate as appropriate. • Increase metallostatic pressure by changing the gating system. If possible raise the cope flask. By provide intial spark during pouring we can remove entrap gases
15. (Sparking At Himsons Cast Pvt. Ltd)
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16. (Sparking At Himsons Cast Pvt. Ltd)
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MAIN FACTORS AFFECTING FOR BLOWHOLES GENERATION PERMEABILITY:During the solidification of a casting, large amounts of gases are to be expelled from the mould. The gases are those
which have been
absorbed by the metal in the furnace, air absorbed from the atmosphere and steam and other gases that are generated by the moulding and core sands. If these gases are not allowed to escape from the mould ,they would be trapped inside the casting and causes defects. The moulding sand should be sufficiently porous so that the gases are allowed to escape from the mould. This gas evolution capability of the moulding sand is termed as permeability.
Besides these specific properties, the moulding sand should also have collapsibility so that during the contraction of the solidified casting, it does not provide any resistance which may result in cracks in the casting .They should be reusable and should have good thermal conductivity so that heat from the casting is quickly transferred.
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17. (SIEVE ANALYSER)
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MOISTURE CONTENT:As explained earlier, moisture is an important element of the moulding sand as it affects Many properties. To test the moisture of a moulding of a moulding sand , a carefully weighed test sample of 50 g is dried at a temperature of 105 Celsius to 110 Celsius for 2 hours by which time all the moisture in the sand would have been evaporated. The sample is then weighed. The weight difference in grams when multiplied by two would give the percentage of moisture contained in the moulding sand.
Alternatively, a moisture teller can also be used for measuring the moisture content. In this the sand is dried by suspending the sample on a fine metallic screen and allowing hot air to flow through the sample. This method of drying completes the removal of moisture in a matter of minutes compared to 2 hours as in earlier method. Another moisture teller utilizes calcium carbide to measure the moisture content. A measured amount of calcium carbide in a container along with a separate cap consisting of measured quantity of moulding sand is kept in the moisture teller. Care has to be taken before closing the apparatus that carbide and sand do not come into contact. The apparatus is then shaken vigorously such that the following reaction takes place: CaC2 + 2 H2O = C2H2 + Ca(OH)2
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The acetylene coming out will be collected in the space above the sand rising the pressure .A pressure gauge connected to the apparatus would give directly the amount of acetylene generated, Which is proportional to the moisture present. It is possible to calibrate the pressure gauge to directly read the amount of moisture
18. (% of moisture tester)
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19. (dial indicator of % of moisture tester)
GREEN COMPRESSIBILITY STRENGTH:Green compression strength or simply green strength generally refers to the stress required to rupture the sand specimen under compressive loading. The sand specimen is taken out of the specimen tube and is immediately put on the strength-testing machine and the forces required to cause the compression failure is determined.The green strength of sands is generally in the rangs of 30 to 160 kPa.
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20. (GCS tester)
21. (compressing machine)
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The above graph shows the relation between green permeability number and moisture. The strength of moulding sand is also affected by grain size and shape as shown in graph. The GCS increases with mitigation of grain size.
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Escape of Gases:The internal cavities should be so designed as to permit the escape of gases evolving from the cores when the molten metal is poured in Internal cores, which are small and long are likely to pose difficulties in clearing also. Sand burned into core holes, and fins or viens, are very difficult to remove when they are hard to reach. Providing access holes or clean-out holes, make available additional core prints for support, to vent core gases as well as to permit the core sand to be removed.
For example, an unsatisfactory design is illustrated in fig. The gases accumulating in the upper part of the core from blow holes. This problem may be reduced by making small vent holes for the escape of gases, as shown in fig. The redesigned vaulted shape of the upper portion of the casting as in fig. Would be the best way to ensure the escape of gases through the top core print.
22. (ESCAPE PROVISION FOR CORE GASES)
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Another way of solving the problem is to change the position of the casting with reference to the parting plane .For example, in fig., the casting being in the core gases are not vented at all. But by bringing the casting into the drag as in fig.the core gases are properly vented.
23. (CORE GAS VENTING)
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Day
GCS
Moisture
1
1180
4.5
2
1165
4.4
3
1185
4.5
4
1190
4.5
5
1150
4.3
6
1000
4.4
7
980
4.5
8
985
4.3
9
1150
4.5
10
1095
4.4
12
1165
4.5
13
1160
4.3
14
1165
4.5
16
1160
4.4
17
1150
4.5
18
1150
4.4
21
990
4.5
22
935
4.6
23
995
4.5
24
1035
4.4
25
975
4.4
26
1050
4.5
27
1000
4.3
30
900
4.4
Moisture Vs Days 4.65 4.6 4.55 4.5 4.45
Moisture
4.4 4.35 4.3 4.25 0
5
10
15
20
25
30
35
Green Compressibility Strength Vs Days 1400 1200 1000 800 GCS 600 400 200 0 0
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5
10
15
20
25
30
35
SYNOPSIS: We have seen lot of defects that arises in and defect on which we have worked is Blowholes. We had looked for the root causes of it and the remedies that we can take. We found that it depends on many factors few of them is permeability, moisture content and on Green compressibility strength(GCS) . Firstly, the occurrence of blowhole depends on permeability greatly. As it is property that allows the gas to escape from mold.It should neither be more nor less. If it is more the casting becomes brittle and if it gas will be trapped so chances of blow hole increases. Furthermore, it also depends on the moisture content .If the moisture content is more the amount of the vapour formed due to evaporation will be more so the occurrence of blowholes is probable.
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BIBLIOGRAPHY “Review of Casting Defect Analysis to Initiate the Improvement Process”A.P.More, Dr.R.N.Baxi, Dr.S.B.Jaju Mechanical Engineering Department, G.H.Raisoni College of Engineering, Nagpur. 440016 (India) Process optimization as a tool in the analysis of steel casting defects By Dr.Hathibelagal Roshan, Chief Metallurgist Maynard Steel Casting Company Milwaukee, WI 53217 A.Ghosh, A.K.Malik, “Manufacturing Science”, East-West Press Pvt. Ltd, First Edition, Reprint 2001 www.moderncasting.com Nikitin, V. G. Gasporenbildung in Gußstücken unter Einwirkung des hydraulischen Schlages in der Gießform Litejnoe proizvodstvo 1976, P. 28 – 29 (Russian) Manufacturing Technology Volume 1 By PN RAO
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