REED'S GENERAL ENGINEERING KNOWLEDGE FOR MARINE ENGINEERS LESLIE JACKSON CEng, FRINA Extra First Class Engineers' Certificate THOMAS D MORTON CEng, Extra First Class Class Engineers' Engineers' C ertificate
COLES NAUTICAL London
Published by Coles Nautical an imprint of A C Black Publishers Ltd 36 Soho Square, London 3QY www.adlardcoles.com
PREFACE Copyright
Coles Nautical 2003
First edition published by Thomas Reed Publications 1966 Second edition 197 Reprinted 1974, 1976 Third edition 1978 Reprinted 1979, 1984 Fourth edition 1986 Reprinted 1990, 1994, 1995, 1997, 1998, 1999, 200 Reprinted by Coles Nautical 2003, 2006, 2008, 2009 ISBN
36-8264-9
All rights reserved. No part of this publication may be reproduced in any form or by any means graphic, electronic or mechanical, including photocopying, recording, taping or information storage and retrieval systems - without the prior permission in writing of the publishers. A CIP catalogue record for this book is available from the
Library.
This book is produced using paper that is made from wood grown in managed, sustainable forests. It is natural, renewable and recyclable. The logging and manufacturing processes conform to the environmental regulations regulations of the country of origin. Printed and bound in Great by the MPG Books Group While all reasonable care has been taken in the publication of this book, the publisher takes no responsibility for the use of the methods or products described in the book.
Note:
The object of this book is primarily to prepare student s for the Certificates of of Competency of the Department of Transport in the subject of Gen eral Engineering Knowledge. Knowledge. It also covers the syllabus for Engineer Cadet courses in the subject. The text is intended to cover the ground work required for the examinations. Th e syllabus and principles involved involved are virtually the same for all examinations but question s set in the Class Class One require the most detailed answer. The book is not to be considered as a close detail reference work but rather as a specific examination guide, in particular most of the sketches are intended as direct applications to the examination requirements. If further knowledge from an interest aspect is required the student. is advised advised t o consult a specialist text book, lubrication; stabilisers, metallurgy, etc., as the range of modern marine practice has superseded the times whereby whereby all the subject can be accurately presented in one volume. The best method of study is to read carefully through each chapter, practising sketchwork, and when the principles have been mastered to attempt the few examples at the end of each chapter. Finally, the miscellaneous questions at the end of the book should be worked through. The best preparation for any examinations is the work on examples, this is difficult in the subject of Engineering Knowledge as no model answer is available, no r indeed any on e text book to cover all the possible possible questions. As a guide it is suggested that the student finds his information first and then attempts each question in the book in tur n, basing his answer on a good descriptive sketch sketch and writing occupying about one side of A4 in 20 minutes. In order to keep as closely abreast as possible to the latest examination questions the book has been extensively revised. The Department of Transport publish publish examination question question papers and have given permission to reproduce questions from them. L. JACKSON T. D. MORTON
Published by Coles Nautical an imprint of A C Black Publishers Ltd 36 Soho Square, London 3QY www.adlardcoles.com
PREFACE Copyright
Coles Nautical 2003
First edition published by Thomas Reed Publications 1966 Second edition 197 Reprinted 1974, 1976 Third edition 1978 Reprinted 1979, 1984 Fourth edition 1986 Reprinted 1990, 1994, 1995, 1997, 1998, 1999, 200 Reprinted by Coles Nautical 2003, 2006, 2008, 2009 ISBN
36-8264-9
All rights reserved. No part of this publication may be reproduced in any form or by any means graphic, electronic or mechanical, including photocopying, recording, taping or information storage and retrieval systems - without the prior permission in writing of the publishers. A CIP catalogue record for this book is available from the
Library.
This book is produced using paper that is made from wood grown in managed, sustainable forests. It is natural, renewable and recyclable. The logging and manufacturing processes conform to the environmental regulations regulations of the country of origin. Printed and bound in Great by the MPG Books Group While all reasonable care has been taken in the publication of this book, the publisher takes no responsibility for the use of the methods or products described in the book.
Note:
The object of this book is primarily to prepare student s for the Certificates of of Competency of the Department of Transport in the subject of Gen eral Engineering Knowledge. Knowledge. It also covers the syllabus for Engineer Cadet courses in the subject. The text is intended to cover the ground work required for the examinations. Th e syllabus and principles involved involved are virtually the same for all examinations but question s set in the Class Class One require the most detailed answer. The book is not to be considered as a close detail reference work but rather as a specific examination guide, in particular most of the sketches are intended as direct applications to the examination requirements. If further knowledge from an interest aspect is required the student. is advised advised t o consult a specialist text book, lubrication; stabilisers, metallurgy, etc., as the range of modern marine practice has superseded the times whereby whereby all the subject can be accurately presented in one volume. The best method of study is to read carefully through each chapter, practising sketchwork, and when the principles have been mastered to attempt the few examples at the end of each chapter. Finally, the miscellaneous questions at the end of the book should be worked through. The best preparation for any examinations is the work on examples, this is difficult in the subject of Engineering Knowledge as no model answer is available, no r indeed any on e text book to cover all the possible possible questions. As a guide it is suggested that the student finds his information first and then attempts each question in the book in tur n, basing his answer on a good descriptive sketch sketch and writing occupying about one side of A4 in 20 minutes. In order to keep as closely abreast as possible to the latest examination questions the book has been extensively revised. The Department of Transport publish publish examination question question papers and have given permission to reproduce questions from them. L. JACKSON T. D. MORTON
CONTENTS Materials Manufacture of iron and steel processes. Cast iron. Simple metal lurgy of steel and cast iron. Proper ties of materials ductility, hard ness, etc. Testing of materials tensile, hardness, impact, etc. Nondestructive tests. Treatment of metals hardening, tempering, annealing, etc. Forming of metals casting, forging, etc. Elements in irons and steels. Effects of alloying elements. Non-ferrous metals. Nonmetallic materials. Table of properties and uses of various metals. Welding electric arc processes, preparation, faults. Soldering and brazing. Gas cutting .
CHAPTE R
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CHAPTE R
1 46
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- Fuel Technology Liquid fuels - petroleum, distilla tion, refining. Testing of liquid fuels and oils - density, viscosity, point, calorific value, etc. Combust ion of fuel - combustibles, hydro carbons, flame temperature, addi tives. Analysis of flue gases recorders, Clean Air Act, dissociation, heat balance. Combustion equipment - burners, air registers, fuel system, viscosity control. Gaseous fuels - compati bility, LNG and LPG, toxic
2
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vapours, explosive vapours, tests
CHAPTER
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Boilers and Ancillaries -Safety valves - types, materials, adjustment, testing. Water level indicators - direct, remote. Other boiler mountings - soot blowers,
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47
91
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emergency operation. Rules relating to steering gears. Ship stabiliser electric control, hydraulic actua tion, fin detail, etc. Auto control block diagrams, steering, ......................... stabilisation. .. .......
feed check valves. Boilers waste heat. Cochran. Scotch boiler, con -
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valve. scale, treatment. Evaporating and distilling plants flash evaporator, drinking
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175 210
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C HAPTER
Shafting - Alignment - general, in ship, in
6
shops (crankshaft and bedplate), telescope, overall, pilgrim wire. Crankshaft deflections data, bearing adjustments. Shafting stresses calculations, inter mediate, thrust, crank and propeller shafts. Shafting rules shafts, liners, bush and bolts. Propeller shaft and sterntube water and oil types, withdrawable stern gear, propeller bearing type, roller bearing design. Controllable pitch propeller. Shafting ancillaries torsionmeter, dynamometer, thrust block, ball and roller bearings. Simple balancing revolving masses, inertia forces. Simple vibration transverse, axial, torsional, dampers. ............... .......
4
Corrosion, Boiler Water Water - Treatment and Tests
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Other corrosion topics fretting, pitting, fatigue. Boiler corrosion values. electro-chemical action.
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water treatment coagulants, deaeration. Treatment for laid up boilers. Boiler water tests alkalinity, chlorinity, hardness, etc.
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141 141 1 74
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5
Gears - Steering Telemeter (transducer) systems -
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hydraulic transmitter, bypass valve, receiver. Telemotor fluid, charging,
C HAP T E R
7
Refrigeration - Basic principles - phase changes. Refrigerants - properties. Freon. The vapour compression system operating cycle, faults, thermo dynamic cycles, intermediate liquid cooling, critical temperature. Com pressor reciprocating (veebloc), rotary, centrifugal, screw, lubricant. Heat exchangers condenser, evaporator, heat transfer, liquid level control. Direct expansion automatic valves,
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21 1 255
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control. Absorption type. Brine circuits properties, battery system, ice making, hold ventilation. Air conditioning basic principles, circuit, heat pump, dehumidifier. Insulation, heat transfer.
Sewage and sludge systems. Pipe arrangements and fittings bilge, ballast, rules.
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350
403
Laval, self cleaning. Lubrication fundamentals, additives. Bearings journal. Definitions pitting, scuffing, oxidation, etc. Lubricating oil tests. Bearing corrosion. Grease.
404
439
Instrumentation and Control Instruments sensors and measur ing elements for temperature, pressure, level, flow etc. Calibration. Telemetering display, scanning, data logging, terminology. Components; amplifier, transducer. Signal media. Control theory terminology, closed loop system. Actions; proportional, integral, derivative. Pneumatic P and I+D controllers. Electric electronic P + I + D controller. Control systems diaphragm valve, electric telegraph, fluid temperature control, automatic boiler control, unattended machinery spaces (UMS), bridge control I C engine.
440
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CHAPTER
and Oil - Lubrication Purification Gravitation separation. Filtration methods - types of filter, stream line, filter coalescers, oil module (fuel and lubricating oil). Clarification and separation - disc and bowl centrifuges. Sharples,
CHAPTER 10
256
300
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8
- Fire and Safety Principle of fire. Fire
prevention and precautions. Types of fire and methods of extinguishing. Fire detection methods patrols, alarm circuits, detector types. Critical analysis of fire extinguishing mediums mediums water, steam, foam, Fire extinguishers (foam) types. Foam sprea ding installations. installations. Fire extinguishers types. flooding systems. Inert gas installations. installations. Water spray systems. Merchant Shippng (Fire Appliance) Rules extract. Breathing apparatus.
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CHAPTER
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301
349
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Pumps and Pumping - Systems Types of pumps reciprocating, centrifugal, axial, screw gear, water ring. Central priming system. Emergency bilge pump. Comparison of pumps suction lift (head), cavitation, super cavitation. Associated equipment and systems heat exchangers (tube and plate), central cooling systems, modular systems, domestic water supply and purification, hydrophore systems. Prevention of pollution of the sea by oil Oil in Navigable Waters Act, oily-water separators. Injectors and Ejectors.
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- Management Management
CHAPTER 12
processes.
General
473
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industrial management of divisions, planning, production, personnel, development etc. Further terminology, queueing theory. IDP. M. OR. Some practical applications, critical path analysis, planned maintenance, replacement policy, ship maintenance costs, optimal maintenance policy, co -ordination. On -ship management shipping company structure, administration. Report writing English usage, examination requirements, speci men question and answer, test examples technique. ....................
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CHAPTER
MATERIALS
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SPECIMEN EXAMINATION QUESTIONS Class 3 Miscellaneous Specimen Paper Class 2 Miscellaneous Specimen Paper Class 1 Miscellane ous Specimen Paper
MANUFACTURE OF IRON AND STEEL 474
-490 Iron ores are the basic material used in the manufact ure of the various steels and irons in present use. In its natural state iron ore may contain many impurities and vary considerably in iron content. Some of the more important iron ores are: Hematite 30 to 65% iron content approximately. (2) Magnetite 60 to 70% iron content approximately. Iron ores are not usually fed direct into the blast furnace in the natural or as mined condition, they are prepared first. The preparation may consist of some form of concentrating process washing out the earthy matter) followed by a crushing, screening and sintering process. Crushing produces even sized lumps and dust or fines. The fines are separated from the lumps by screening and then they are mixed with coal or tar dust and sintered. Sintering causes agglomeration of the fines and coal dust, and also causes removal of some of the volatiles. The sinter along with the unsintered ore is fed into the blast furnace as part of the charge (or burden), the remainder of the charge is principally coke which serves as a fuel and limestone which serves as a flux. Preparation of the iron ores in this way leads to a distinct saving in fuel and a greater rate of iron production. In the blast fu rnace the charge is subjected to intense heat, the highest temperature is normally just above the pressurised air entry points being about 1800°C. The following are some of the reactions which take place in a blast furnace: At bottom, Carbon + Oxygen Carbon Dioxide. (2) At middle, Carbon Dioxide + Carbon Carbon Monoxide.
-
-
=
=
(3)
Iron Oxide + Carbon Monoxide
Iron + Carbon Dioxide.
From (3) the iron which is produced from this oxidation reduction action is a spongy mass which gradually falls to the furnace bottom, melting as it falls and taking into solution carbon, sulphur, manganese, etc. as it goes. The molten iron is collected in the hearth of the furnace, with the slag floating upon its surface. Tapping of the furnace takes place about every six hours, the slag being tapped more frequently. When tapped the molten iron runs from the furnace through sand channels into sand pig beds (hence pig iron) or it is led into tubs, which are used to supply the iron in the molten condition to converters or Open Hearth furnaces for steel manufacture. Pig iron is very brittle and has little use, an analysis of a sample is given below.
-
,
=
Combined Carb on Graphite Silicon
-
0.5% 3.4% 2.6%
Manganese Phosphorus Sulphur
0.5% 0.03% 0.02%
Open Hearth Process In this process a broad shallow furnace is used to support the charge of pig iron and scrap steel. Pig iron content of the charge may constitute 25% to 75% of the total, which may vary in mass depending upon furnace capacity between 10 to 50 tonnes. Scrap steel is added to reduce melting time if starting from cold. Fuel employed in this process may be cnrichcd blast furnace
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6
Constituent
Metal
Slag
Carbon Silicon Sulphur Phosphorus Manganese Silica Iron oxide Alumina Manganous oxide Lime Magnesia Phosphorus Sulphur
1.1 0.04 0.4 19.5 5.6 1.2 8.7 50.0 5.0 9.0 0.2
TABLE
1.1
3
MATERIALS
REED'S GENERAL ENGINEERING KNOWLEDGE
20 hours later Finished Steel% 0.55 0.1 0.03 0.03 0.6
gas (blast furnace gas may contain 30% CO after cleaning) which melts the charge by burning across its surface. Reduction of carbon content is achieved by oxidation, this may be assisted by adding a pure iron oxide ore to the charge. are reduced either by oxidation or absorption in the slag. At frequent intervals samples of the charge are taken for analysis and when the desired result is obtained the furnace is tapped. Analysis of metal and slag in a basic open hearth furnace. (See Table 1.1) Bessemer Process In this steel making process a blast of air is blown through a charge of molten pig iron contained in a Bessemer converter. The refining sequence can be followed by observing the appearance of the flames discharging from the converter, since the air will bring about oxidation of the carbon, etc. After pouring the charge, a mixture of iron, carbon (usually in the form of coke) and manganese is added to adjust the carbon content, etc., of the steel. The principal difference between Open Hearth and Bessemer steels of similar carbon content is brought about by the higher nitrogen content in the Bessemer steel and is also partly due to the higher degree of oxidation with this process. This leads to a greater tendency for embrittlement of the steel due to ageing in the finished product. Typical nitrogen contents are: Bessemer steel 0.015% approximately, Open Hearth steel approximately Modern Processes Various modern steel making processes have been developed and put into use, some extensively. These include the L.D., Kaldo, Rotor and Spray processes. The L.D. method of steel manufacture the letters are the initials of twin towns in Austria, Linz and Donawitz uses a converter similar in shape to the old Bessemer, and mounted on trunnions to enable it to be swung into a variety of desired positions. Fig. 1.1 is a diagrammatic arrangement of the L.D. converter. Scrap metal and molten iron, from the blast furnace, would be fed into the converter which would then be turned to the vertical position after charging. A water -cooled oxygen lance would t hen be lowered into the converter and oxygen at a pressure of up to 11 bar approximately, would be injected at high speed into the molten iron causing oxidation. After refining, the lance is
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REED'S G E N E R A L E N G I N E E R I N G KNOWLEDGE
withdrawn and the converter is first tilted to the metal pouring position and finally to the slag pouring position. If the metal is of low phospho rus content oxygen only is used, if however, it is high in phosphorus, powdered lime is injected with the oxygen and the blow is in two parts, the process being interrupted in order to remove the high phosphorus content slag. The Kaldo and Rotor processes have not found the same popularity as the L.D., even though they are similar in that they use oxygen for refining. They both use converters which are rotated and the process is slower and more expensive. B.I.S.R.A. the British Iron and Steel Research Association) have developed a process in which the molten iron running from the blast furnace is subjected to jets of high speed oxygen that spray the metal into a container. gives rapid
refining since the oxygen and the metal intimately mix. T he main advantages with this system are that the intermediate stage of carrying the molten metal from the blast furnace to steel -making plant is eliminated, and incrcnsed. Open Hearth furnaces by the fitting of oxygen lances in th eir roo fs. This speeds up steel production and the process is becoming and more similar to the L.D. process. Eventually open hearth will be superceded. Acid and Basic Processes When pig iron is refined by oxidation a slag is produced. Depending upon the nature of the slag one of two types of processes is employed. If the slag is siliceous it is the acid process, if it is high in lime content the basic process is used. Hence the furnace lining which is in contact with the slag is made of siliceous material o r basic material according to the nat ure of the slag. Thus avoiding the reaction: ACID + BASE SALT + WATER. Low phosphorus pig irons are usually rich in silicon, this produces an acid slag, silica charged, which would react with a basic lining, hence silica bricks are used, which are acidic. High phosphorus pig iron requires an excess of lime added to it in order to remove the phosphorus. The slag formed will be rich in lime which is a basic subtance that would react with a silica brick lining. Hence a basic lining must be used oxidised dolomite (carbonates of lime and magnesia). Both acid and basic processes can be operated in the Open Hearth, Bessemer, L.D., and Electric Arc furnaces, etc. =
retractable lance
CAST IRON Cast iron is produced by remelting pig iron in a cupola (a small type of blast furnace) wherein the composition of the iron is suitably adjusted. The fluidity of this material makes it suitable for casting; other properties include; machinability, wear resistant, high compressive strength. SIMPLE METALLURGY OF STEEL AND CAST IRON
L.D. PROCESS Fig. 1.1
Carbon can exist in two states, crystalline and non-crystalline. In the former state, diamond and graphite, the latter is pure carbon. Pure iron (ferrite) is soft and ductile with considerable strength, when carbon is added to the iron it combines with it to
6
MATERIALS
REED'S G ENER AL E N G I N E E R I N G KNOW LEDG E
form a hard brittle compound. This compound of iron and carbon called iron carbide or cementite lies side by side with ferrite in laminations to form a structure called pearlite, s o called because of its mother of pearl appearance. As more carbon is added to the iron, more iron carbide and hence more pearlite is formed, with a reduction in the amount of free ferrite. When the carbon content is approximately 0.9% the free ferrite no longer exists and the whole structure is composed of pearlite alone. Further increases in carbon to the iron produces free iron carbide with pearlite reduction.
,
free
,
cooling rate. or malleable cast iron is composed of pearlite and graphite and can be easily machined. Pearlite and cementite gives white cast iron which is brittle and difficult to machine and hence is not normally encountered in Marine work. The following diagram (Fig. 1.2) analyses the above in diagramm atic form. PROPERTIES OF MATERIALS The choice of a material for use as an engineering component depends upon the conditions under which it will be employed.
.
carbon steel
0
0 3
05
car bon
17
6.7
MICROSTRUCTURE VARIATION WITH INCREASING CARBON CONTENT Fig 1.2 The steel range at approximately 2% carbon content the cast iron range commences. Carbon for cast iron may vary from 2% to 4%. This carbon may be present in either the form of cementite or graphite (combined or free carbon) depending upon certain factors one of which is the
carbon
steel
DIAGRAM SHOWING EFFECT UPON MECHANICAL PROPERTIES BY INCREASE IN CARBON CONTENT Fig. 1.3
8
REED'S
GENERAL ENGINEERING
KNOWLEDGE
Conditions could be simple or complex and hence in choosing, the engineer requires some guidance. This guidance is invariably in the form of a material's mechanical properties and those of principal interest are as follows: Ductility:
Is that property of a material which enables it to be drawn easily into wire form. The percentage elongation and contraction of area, as determined fro m a tensile test are a good practical measure of ductility.
Brittleness:
Could therefore be defined as lack of ductility.
Malleability: Is a property similar to ductility. If a material can be easily beaten or rolled into plate form it is said to be malleable.
MATERIALS
the component erosive, corrosive, fatigue, stresses, thermal, shock etc. (2) shape and method of manufacture casting, forging, machining, drawing etc. (3) repairability, can it be brazed, welded, metal-locked etc. (4) cost. Hence for a ship-side valve, sea water suction: (1) working conditions: corrosive, erosive, little variation in temperature, relatively low stresses, possibility of impact. Material required should be hard, corrosion resistant with a relatively high impact value. (2) shape and method of manufacture: relatively intricate shape, would most probably be cast. Material could be spheroidal graphitic cast iron, cast steel or phosphor bronze. Taken in order, they are increasingly expensive, easier to repair, increasing in corrosion resistance and impact value. TESTING OF MATERIALS
Elasticity:
If all the strain in a stressed material disappears upon removal of the stress the material is elastic.
Destructive and non-destructive tests are carried out upon materials to determine their suitability for use in engineering.
Plasticity:
If none of the strain in a stressed material disappears upon removal of the stress the material is plastic.
Hardness:
A material's resistance to erosion or wear will indicate the hardness of th e material.
Strength:
The greater the load which can be carried the stronger the material.
Tensile Test This test is carried out to ascertain the and ductility of a material. A simple tensile testing machine is shown in Fig. 1.4. The specimen is held in self aligning grips and is subjected to a gradually increasing tensile load, the beam must be maintained in a float ing condition by movement of the jockey weight as the oil pressure to the straining cylinder is increased. An extensometer fitted across the specimen gives extension readings as the load is applied. Modern, compact, tensile testing machines using mainly hydraulic means ar e more complex and difficult to reproduce f or examination purposes. For this reason the authors have retained this simple machine. With values of load with respect to extension the nom inal stress-strain curve ca n be drawn, the actual stress-strain curve is drawn for comparison purposes on the same diagram. The difference is due to the fact that the values of stress in the nominal diagram are calculated using the original cross sectional area of the specimen when in actual fact the cross sectioned area of the specimen is reducing as the specimen is extended. Specimens may be round or rectangular in cross section, the gauge length being formed by reducing the cross section of the centre portion of the specimen. This reduction must be gradual as rapid changes of section can affect the result. The relation,
Toughness: A material's ability to sustain variable load conditions without failure is a measure of a material's toughness or tenacity. Materials could be strong and yet brittle but a material which is tough has strength and resilience.
Other properties that may have to be considered depending upon the use of the material include; corrosion resistance, electrical conductivity, the rmal conductivity. Questions are often asked about the properties, advantages and disadvantages of materials for particular components, ship-side valve, safety valve spring etc. A method of tackling such a problem could be to (1) consider working conditions for
MATERIALS REED'S GENERAL
gauge length to cross sectional area of specimen, is important, otherwise varying values of percentage elongation may result for the same material. A formula attempting to standardise this relationship in the U.K. is; gauge length
=
Cross sectional area.
In the tensile test the specimen is broken, after breakage the broken ends are fitted together and the distance between reference marks and the smallest diameter are measured. Maximum load and load at yield are also determined. From these foregoing values the following are calculated: Percentage elongation
Final length original length Original length -
=
Percentage contraction of area Ultimate tensile stress
Yield stress
=
=
=
Original area final area x Original area
-
loo
Maximum load Original cross-sectional area
Yield load Original cross-sectional area
Percentage elongation and percentage contraction of area are measures of a materials ductility. Ultimate tensile stress is a measure of a materials strengt h. Yield stress gives indication of departure from an approximate linear relationship between stress and strain. It is the stress which will produce some permanent set in the material when tubes are expanded.
gauge length
round or rectanaular fracture
Factor of Safety this is defined as the ra tio of working stress allowed to ultimate stress, hence:
-
actual stress-strain
Factor of Safety
and is always greater than unity.
=
proof stress
0
parallel ferrous metal
metal
C
tensile
or of gauge length
NOMINAL STRESS-STRAIN DIAGRAM Fig. 1.4
Components which are subjected to fatigue and corrosion fatigue conditions are given higher factors of safety than those subjected to static loading tail end shafts 12 or above, boiler stays about 7 to 8. Hooke's law states that stress is proportional to strain if the material is stressed within the elastic limit. Stress Strain or Stress Strain x a constant =
MATERIAL S
REED'S GENERAL ENGINEE RING KNOWLEDGE
The constant is given the symbol E and is called Young's modulus or the modulus of elasticity.
.
Stress Strain
13
to wear. There are numerous tests that can be employed to determine hardness, only two will be described. handwheel
bevel
The modulus of elasticity of a material is an indication of stiffness and resilience. As E increases then stiffness increases. By way of a simple explanation, we could consider two identical simply-supported beams, one of cast iron, the other of steel, each carrying a central load W. The deflection of a beam loaded in this way is given by screw
Where 6 Where L Where Where E
deflection of beam under the load W. length of the beam. second moment of area of section. modulus of elasticity of the material.
Since the beams are identical
x
1 E a constant.
for steel is greater than E for cast iron, hence, 6 for steel is less than for cast iron. Hence, steel is stiffer than cast iron. For this reason as well as strength, less steel is required in a structure than cast iron. E
0.1% Proof Stress
For non-ferrous metals an d some alloy steels no definite yield point is exhibited in a tensile test (see Fig. 1.4). In this case the 0.1% proof stress may be used for purposes of comparison between metals. With reference to the graph (Fig. 1.4) a point A is determined and a line AB is drawn parallel to the lower portion of t he curve. Where line AB cuts the curve the stress at that point is read from the graph. This stress is called the 0.1 proof stress. the stress required to give a permanent set of approximately 0.1% of the gauge length. Hardness Test
The hardness of a material determines basically its resistance
table
BRINELL HARDNESS TESTING MACHINE Fig. 1.5 Brinell Test: This test consists of indenting the surface of a metal
by means of a 10 mm diameter hardened steel ball under load . The Brinell number is a function of the load applied and th e area of indentation, thus: in newtons Area of indentation in mm2 Only the diameter of the indentation is required and this is determined by a low powered microscope with a sliding scale. Tables have been compiled t o avoid unnecessary calculations in ascertaining the hardness numeral. Loads normally employed are 30,000 N for steels, 10,000 N for copper and brasses and 5,000 N for aluminium. Duration of application o f the load is usually 15 seconds. (Industry is still using the old system of calculating Brinell numbers, load in of indentation in mm2. Hence, their Brinell numbers will be less by a factor of 10.)
14
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MATE RIALS
G E NERAL E NG INEER ING KNO WLEDG E
Pyr amid Test: The surface of the metal under test is indented by a diamond square-based pyramid and the Vickers pyramid number (VPN) is determined by dividing the area of indentation into the load applied. This test is also suitable for extremely hard materials, giving accurate results, whereas the Brinell test's reliability is doubtful above 6,000 Brinell. Table 1.2 gives some typical values. Material Brass Mild steel Grey cast iron White cast iron
Number
V.P.N.
600
600
1300
1300 2050 4370
4150
TABLE 1.2
15
Impact Test This test is useful for determining differences in materials due to heat treatment, working and casting, that would not be otherwise indicated by the tensile test. I t does not give accurately a measure of a material's resistance to impact. A notched test piece is gripped in a vice and is fractured by means of a swinging hammer (Fig. 1.6). After the specimen is fractured the hammer arm engages with a pointer which is carried for the remainder of the swing of the arm. At the completion of the hammer's swing the pointer is disengaged and the reading indicated by the pointer is th e energy given up by the hammer in fracturing the specimen. Usually three such tests are carried out upon the same specimen and the average energy to fracture is the impact value. By notching the specimen the impact value is t o some extend a measure of the material's notch brittleness or ability to retard crack propagation. From the practical standpoint this may be clarified to some extent: Where changes of section occur in loaded materials shafts, bolts, etc.) stress concentration occurs and the foregoing test measures the materials resistance to failure at these discontinuities. Table 1.3 gives some typical IZOD values for different materials, considerable variation in IZOD values can be achieved by suitable treatment and alteration in composition. IZOD Value (Joules) Stainless steel 0.5 Mn steel S.G. iron (annealed) Grey cast iron
up to 3
Turbine blades. General purpose mild steel Camshafts, gear wheels. Cylinders, valves.
TABLE 1.3
square
2 m m deep
IZOD IMPACT MACHINE Fig. 1.6
Charpy V Notch, using a different hammer and vice arrangement, th e IZOD machine can be converted into a Charp y V Notch machine where the specimen is placed horizontally upon two parallel stops between which the hammer swings and breaks the specimen. The advantage to be gained by this method is that the specimens can be very quickly set up in the machine. Hence
