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SOME NOTE8 on THE] Metallultgy of STEEL SOME NOTE8 ON THE] METALLUltGY OF STEEL. A Paper read by Brig.-General BA~NALL-WILD,C.M.G., before a SpeciaZ Meeting qf London Graduates held on Ma-y loth, 1918, at the Worlts of Messrs. Nupier Motors, Ltd, Acton. Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 80218 NOTES ON THE METAI,LUHC;\ OF STEEI.. 4 89 BY BRIQ.-GENEXAI,BAONALL-WILD, C.M.G. (PASTPBESIDENT). L ii the basii €or such a large number of parts required in automobile construction that a brio€ study of the elementary problcnir connected with it< use is of value to all engineers con- nected vith ili(\ intiii<try Thesc notes arr not intende:l fc~rThe use of the metallurgiata in control of physical and chemical labora- tories whom the majority of firms now employ, but for thuse in charge of machining and other operations, to draw their attention t,o what may happen mhcn bteci is abiised and, above all, to point out to them tlic neccwitr for cnrefiil w,rekee1)ing JVitli~~utdif- fercntiation betn eeii the ,:ii*ious steels succcisful heat-treatment is impossible, and tlie higher grades of steel are practically useless unless heat-treated. In order to explain tlie nature of the changes which take plme in steel during heat-treatment it will be iiece ry to refer briefly to the iron-carbon eyuilibriiiiii diagram, Fig. 1, arranged so as to show the different states of the stoel under varying condikions of temperature. The vertical scale represents the temperature and tlie horizontal scale the percentage of carbon in the steel. The whole diagram is divided up into a. number of areas each ooveriw certain ranges of temperature and of clarbon content; the boun- daries of each of these araas show the limits within which each con- Btituent of the steel can exist in the equilibrium condition. Phe fundamental constituents of steel are iron and carbide of iron, the latter being a chemical compound containing 6'2 per cent of carbon. Phis compound when crystallised out so as to be recognised under the microscope is known aa cementite, and fre- quently occurs in the forin of long needle-shaped crystals standing out white under the usual acid etching, see Fig. 11, Plate XXX. In low and medium carbon ste8elthe carbide does not occur in this form, but separates out in the form of thin plate- lwween layers RAQNALL-WILD. 1" Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 490 THE INUTITUTIOH OF AUTOMORI3,E ENOINEEKR. of iron, presenting the appearance shown in Fig. 8, Plate XXIX. The proportion of the two constituents is fairly constant, being about 1 of carbide to 6 of iron, corresponding to a carbon content of 0'89 per cent, and the resulting structure is knovn as pearlite. In the annealed condition, which may for practical purposes be regarded ,w a state of equilibrium, low carbon ste,el consists of grains of iron and of pearlite in the proportion corresponding to its carbon content, the amount of pearlite increasing with the carbon content until a steel of 0'89 per cant is reached, this particulclr steel consisting entirely of pearlite. P L rpu?u Aurtenrtr + Cemenite Cementrte t P carlit e PIQ.1 .-Iron-Carbon Equilibrium Diagram. When the steel contains a percentage of carbon higher than this figure the exws carbide separates out as cementite and the steel is a mixture of pearlite and cementite. In Fig. 1, the upper line A C represents the beginning of the solidification of the steel. Above this line the steel is completely liquid and the slope of the line shows that the melting point is lowered by the addition of carbon. The solidifioafon does not take place at one temperature, but the transition from the liquid to the solid state is spread over a range of temperature represented Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 SOME NOTE6 ON ‘I‘HE XIE’I’A1,LURffY OF STEEL. 49 1 by the area A C B. The part of the diagram below A B bouncled by the linw A B F E D represents the state of solid sohtion; Over the whole range of temperature represented by this mathe carbide is completely dissolved in the iron. On passing the line D E F B the carbide and the iron begin to separate out and the line G K marks the completion of the separation. We have thus three well-marked states in which the steel can exist, first the liquid state with the carbide dissolved in the iron, then the solid solution state in which the carbide is still completely dissolved, and finally below 70OoC. we have the iron and carbide separated out, the steel being simply a mixture of these constituents. This falling out of solution of the carbide is due to a molecular change which takes place in the iron at this temperature, a change which is marked by an alteration in most of its physical properties and especially in those connected with magnetic quality. Whe change in the properties of the metal is so great that wo are justified in regarding the iron at the higher temperature ag being a different material, and to mark this differenoe it is now usual to refer to it as austenite, while iron as it exists at ordinary temperature is referred to as ferrite. From the point of view of heat-treatment the imporbant point is that the carbide is fully soluble in the austenite but is quite insoluble in the ferrite and is, therefore, thrown out of solution as the steel changes from one state to the other in cooling. The precipitation of the carbide takes place between the tem- peratures represented by the linee D E F B and G K, and this interval is known QS the “critical range” of the steel. A know- ledge of these temperature limits is of vital importance in the heat-treatment of steel, and their determination is necessary before ib successful heat-treatment‘ can be worked out for any particular steel. This determination is readily made by taking advantage of the fact that the molecular changes in the iron are accompanied by absorption and evolution of hat. Whese give rise to variations in t,he rate of heating and cooling, and the transformation points are marked by sharp changes in the curve showing the time rate of heating and cooling of a specimen of the steel. A typical heating and cooling curve fur a medium carbon steel is shown in Fig. 2. Phe vertical scale represents the temperature of the specimen, while the horizontal scale shows the time taken in heating up through 1°C. Whcn the specimens are hmeated up uniformly in an electric furnaoe the time per degree is at fimt I12 Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 fairly constant, and the curve is a straight vertical line, but when a trinpemturc of 720°C. is reached, as the line G F is passed, tho curve swings sharply to the right, indicating that the rate of heating is diminished. As the furnace is supplying heat at a uniform rate this means that the specimen during the solution of Fxa. Z.-Heating and Cooling Curve 0.41 per cent Carbon 8teel. the carbide is absorbing heat, and this abwiytion is continued until all the pearlite is dissolved. On cooling, the reverse change takes place, and heat is given out as the carbido is precipitated. CChe temperature of the specimen is measured by a thermocouple, and the limits of the critical range are thus detormined. Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 SOME NOTP.3 ON THE IlETA1,LURQY OF f3TEEL. 49 3 rl'he solution aiid precipitation of the oarbide of iron takes a oertain time, and on this very vital point the whole possibility of the heat-treatment of steel depends. It will be noted that Fig. 1 is referred to as the equilzbrium diagram, to indicate that it shows the state of the steel whcn cooled sufficientJy slowly to reach the equilibrium condition. If, on the other hand, the steel is cooled quickly by plunging in oil or water, the precipitation of cementite is prevented, and the carbon remains uniformly distributed in the iron even at ordinary temperatures. It is not possible with Btraight carbon steel to carry out the cooling quickly enough to supprew the molecular clinnge of the iron, and thus retain the steel in the austenitic condition The effect of the rapid cooling is to cause the change to be retarded and to take place at a much lower temperature. Undei these circurnstsnres the oarbide is unable tLl coalesce inh pearlite layer5 and a transition product known as martensite is fornied This product is hard and brittle, and is unfit for structural uie. It is, however, very unstable and on re-heating the steel it breaks donn and the carbide gathers together into very fine grained pearlite By varying the temperature of re-heating any desired 3t:tte of hardnesq between that of the martensib formed by direct quenching and that of a state almort as soft as ir derived from annealing is obtained. This operation of re-heating the quenched steel is known as tempering, and the control thus obtained over the hardness of the steel is one of the great advantages uf heat-treatment. Fig. 3 shows the efk'ect of tempering on the strength and ductility of steel.
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