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.
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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
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
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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.
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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. It refers to cl 0.45 carbon ,steel which has been quenched in water from it temperature of 870°C. and then tempered at various temperatures up to 700OC. It will be seen that aa the tempering temperature increases, the strength of the steel is diminished while the ductility, as measured by the elongation or reduction of area increws. The uniform distribution of the carbide and the control of the hardness of the steel are nut the only beneficial result3 attained by heat-treatment. It is readily proved by examination under the microscope as well as by observation of fractured surfaces, that steel, like all metals, is crystalline, every bpecimen of nietal being composed of a large number of crystal gTains closely united togcther. Each grain possesses well-marked cleavage planes along which it will break when the specimen is fractured. Fhese cleavage planes show up as bright facets on the fractured surface of a pieoe
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 494 'IHE INSTI'ltJTlON OW AUTOMOBILE ENGINEERS. of steel, and it is for this reason that the appearance of the surfaoe gives an indication of the condition of the mraterial. The size of the crystal grains has a great influence on the behaviour of the material under stress; generally speaking the smaller the grain size the tougher will the mtmial be, and the greater the resistance it will offer to the spreading of a crack. It is clear that this must be so becatwe the cleavage planes of the
FIG.3.-EEect of Tempering on Physical Propertiea of Carbon 8tcrel. grains lie in all possible directions, and a crwk hm to change itrs direction every time it passes a boundary surface. Phe transformations which occur at the " critical rangc " enable us to control the size of the crystal grains and to produce in any piece of steel the fine grained structure which is neceseary to secure toughness in the material. In heating the steel up through the critical range the original crystal structure is completely broken
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 SUMB hOLEh ON THY, MBTALLURUY OF STEEL. 495 down during the change from ferrite to austenite, and fresh austenite crystals are fornid when the lint: D E F, Fig. 1, is passed. The crystaLs are at first exceedingly minute, but aa the temperature is increased they grow rapidly, and at temperatures above 1,OOO”C. sewh a. size at which they are visible to the dtsd eye on the etched surfme of the steel. On cooling down, the grain size reniainr unaltered and controls the distribution of the cementite precipitated in the critical range. In the medium arbon stecl the ferrite fornis along boundaries and cleavage planes of the aastenite grains and a coarse crystalline structure in the austenitic stage leads to a, coarse grained ferrite and pearlib structure. Similarly for high carbon steels the cementite separates out along the grain boundaries and forins a network enclosing tho pearlite grains. Suc11 a structure is nearly always found in &eel which ha9 been rolled or forged, and can be readily removed by heating the steel to a temperature slightly above the line D E F, Fig. I, and cooling in air. This operation is known as normali~ing,and by refining the structiire renders the steel much tougher and diminishes tlic liability of the material to failure under dternating stre Wliil~1 hc ctr:trse’ hwciiiro fornied at a fairly high temperature can be rcnioved by subsequent treatment, other changes which take place at higher temperatures and relyult in the “ burning ” of the steel cannot be removed by any known process short of re- melting it. These change3 we by no ineans fully understood, but generally spcaking they appear to result in the segregation of oxides arid other impurities at the crystal boundaries, and the wnsequent m eakening of the bond which holds the crystals together. The steel in this condition is very brittle and breakn along the crystal boundaries giving iisc tfJ a ~ieculisrdull crystal- line appearance in the fractured surface. In order to illustrate more clearly the changes which take place within the critical range, the micrti-p11citvgraphs reproduced in Figs. 4- 11, 1’1:itec SSVII. to XXS . hi&\e heen taken from pieces of steel of various carbon wnitenks specially treated to rliow up the tranqformation poinh. The lost pieces were heated up to a high teniperature and then 60 Jhd very shdy in a large slag ball. Tinder these circumstances very large crystds are formed, and the etched structure is visible to the naked eye One end of each test piece was then heated to the melting point, while the other waa kept cool by pouring water on it. The bars w0re then quickly
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 cwlwl SO that the part which had been heated above the t.rans- formation point wausprevented from re-crystdlising; the lines aor- responding to the transformation point are clearly made visible on the specimen. Fig. 4, Plate XXVII., blio\vs a test piece made from steel of about 0.25 per cent carbon. The lower past which was kept cool, shows the coarse structure of ferrite and pearlite which has not been altered in the second heating. At the lower tilansformation point the pearlite is changed into sorbite, the ferrite remaining un- changed, while at the upper transformation point the ferrite dis- appears, and no structure can be detected in the part which has been heated beyond this temperature. The two transformation points are shown under higher Inagni- fication in Fig. 5, Plate XXVII., and Fig. 6, Plate XXVIII. Fig. 5 shows the yoarlite changing into sorbite, while Fig. 6 is taken just below the upper transformation line and shows the ferrite dis- appearing and being absorbed into the surrounding sorbite. Fig. 7, Plate XXVIII., is a specimen of 0.75 carbon steel showing a very sharp pearlite change point. Fig. 8, Plate XXIX., at a higher nmgnificntion is taken just below the pearlite change point and shows the pearlite lamine breaking up into small globules before dkwlving. Figs. '3, 10 and 11, Plates XXIS. and XXX., are taken froin a steel of about 1.3 carbon content. The lower part consisk of pearlite and cementite ; at the lower transformation point the pearlite is changed into sorbite, the cementite remaining unaltered, while at the higher point the cementite is completely dissolved. It is well known that the niechanical properties of steel can be very considerably improved by the addition to it of small amounts of other metals such as nickel, chromium, manganese, etc., and the use of steels containing these elements is now of fundamental importance in automobile and aircraft construction. When the mount of the added elernent is small, say below 5 per cent, the constituents of the steel as seen under the microscope remain un- changed, the alloying mctals being dissolved either in the ferrite or in the carbide, or in both. Fhe temperatures at which the various transformations take place are, however, considerably modified, and while the equilibrium diagram retains the same general form lls that given in Fig. 1, the boundaries of the various areas are altered, and the heat-treatment required by the steel is correspondingly modified. Fhe temperature limits of the critical
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 SOME XOI'ES ON THE MElALI,IIHGY OF bIBEL. 497 range lnay be either rai'sed or lowered according to the iiature of the added element, hut most (Jf the elementns usually employed diminish the solubility of ihe carbidc and thus tend to shift the point F to tho left and narrow the pcnrlitc rang1. Apart, how- cy\'t>r, from their eitcct 011 the teiiiperainre ot transfurmatiun the added elernenti eucbrcisc anotiici inor(' iinportant effect in that they all have a tendency to retard tho niolecrilar change 111 the iron and the coiisequent precipitation of the carbide
We have already seen that thr object of heat-troatnient iq to prevent the separation of ihe ferrite and pearlite, and that tliii can only be done by cooling the steel 50 rapidly that the changc doer not take place until the steel has attained a comparatively low temperature. Phis wppreasiuii of the change can only take place if the velocity of cooling is greater than tlie rate at which thr cliange ilself takes place We may call this the "critical cooling
\ elocity " of the steel and its value dcterniines the nature of the quenching M hich is necessary to satisfactorily treat the steel Measurements of the critical cooling velocity for certain carbon stecls have been found to give values 1 xiging froin 100" to GOOo C per second, and except in the case of very thin bar, this velocity can only be obtained by \later yucnching, and even so with bar6 of considerable size tlie wntral parts of the bar \\ill cool down too +mly for the change to be properly retarded. Apart from in, Yct up by ~iicliiq~'id ~~Jiiig gn c rthe to distortion and cracking, and set a limit to the size of tlie articles which can be so treated.
Thc addition of ~~eillell~5such a3 iimnganew, nickel, ~~hioniiuiii, ctc ~liniinisliesthe. &tical coding velocity uf the .tee1 and thus makes the treatiiicnt cif large or clclicate Imt5 possiblc The critical velocity of ;L stcel cwntaining ti per ccnt (if chroniiciiu lias 1)cc.n clcterrnined by Carpenter and iuuiid tcr be 0.8" C. per second, or 5everal hundred times less than that of itraight carbon steel When thc pieces to be treated itre sniall this rate can readily be obtained by cooling in air, mid such a cteel can therefore be hardened by heating ~ipabove the transformation poiiil and allow- ing it to cool freely in air. 'These steels are consequently oalled " air hardening " steels, and can bc used n ith advantage for parts liable to distort in quenching. A low critical velocity combined with good physical properties can be obtiained by the use of nickel and chromium in combination, and a steel containing from 4 to 5
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 per cent of nickel and 1.5 per cent of chromium iN now largely used. Air hardening steel can be tempered in exactly the same way as previously described for carbon steel and a similar control over its physical properties is possible. Fig. 12 shows the effect of tempering on the strength and ductility of a nickel chrome steel air hardened from 820" C. Nickel can be alloyed with iron in all proportions, and in sinall
[email protected] of Tempering on Phgsicnl Properties of Air Hardening Steel. amounts increases tlie strength of steel without diminishing its ductility. The eftect of nickel on the tomlierstures of transforma- tion is very marked, tlw tempersturcs being progresqively lowered with increasing percentage of nickel, while thc limits of the critical range are also nrtrrbwcd. Fig. 13 shows the cffect of additions of nickel on the transforma.tions of a 0.2 carbon steel. It will be observed that there is a very marked differenoe between the critical temperatures on the heating and cooling curves. This
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 is a140 dearly slluwii in the heating autl cooling curves of Fig. 14, which refer to a 5 per cent nickel steel. The transformation on cooling does riot in this take place until a temperature of 46OOC. is reached. This fact niay be taken advantage of in the heat-treatment of nickel steels, a5 tlic steel may be allowed to cod domu before quenching, and thus the tendency of the materid to crack or distort may be very appreciably diminished. Thus, for example, the steel, of which the transformations aru shown in Fig. 14, codti be hardened by hating t(J 800°, cooling in the furnace or in air to 500" :ind then qwnching in oil.
Fru, l:~.--lM'ect of Siokel on Critical Pointe.
Na~~ganae.-The effect of Irmngancse is similar tu that of nickel, tho tempcrature of trailshimation being progressively lowered with increasing manganese ocintent; the lowering is more rapid than in the case of nickel, bu that with about 8 per cent of manganese the transformation point i+ hlow cttmosphcric temperature, and the steel is normally in tlir. aastenitc condition and is therefore noii-magnetic Chromium.-The ~ffect of clironiiuni on the transformation tempcrature is cornparatively sinall. Tho lower limit correspond- ing tc, thc linr G I<, Fig 1, is raiwd while that of the upper
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 600 THE IKBTlI'UTION OF AUTOMOt%ILEPNGLNEEHI. limit is lowered, the critical range being thus considerably nar- rowed. @'he effect of chromium in diminishing the critical cooling velocity is, on the other hand, very great, and it is therefore an essential constituent of air hardening steels. Chromium lias also a marked effect on the pliysioal properties of the steel, increasing the strength and diminishing the ductility, and for this reason is generally employed in combination with nickel or vanadium.
5 % Nickel Steel
rime Per Deoree FIG. 14.-Heating and Cooling Curve of Nickel Steel.
A few words are necessary on tho casting of steel ingots. The liquid steel from the furnam is ptrured into a ciast-iron mould, and on coming into contact with the cold sides of the mould, the steel begins to freeze, crystallites of solid steel forming on the sides of the mould, as the nm~cools down these grow inwasds and join together until the whole of the ingot is frozen. Referring again to Fig. 1, we see that solidification will begin when the
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 temperature has falben to that given by the line A C, and be com- plete when the temperature corresponding to A B is reached. Thus a 1 per cent carbon steel begins to solidify at 144O0C , and is completely frozen at 1190°C. During this time the corn- position of the crystals is continuously changing. The first crop of crystals that are thrown down contain a smaller peroentage of carbon than thoso that form toward? the end of the prooess. This difference of composition will also apply with regard to any impurities such as sulphur and phosphorus, which tend to lower the melting point of the steel. These variations in composition are to some extent wiped out by diffusion during the subsequent cwooling and working of the steel, but generally spfaking thcre will be a considerably grelatei. percentage in the oentre and upper part of the ingot, which arc the la,st to freeze, and these parts of the ingot have to be rejected The contraction of the steel on freezing and the liberation of gases dissolved in the molten fiteel tend to produce cavities in the ingots, and careful regulation of the conditions of casting and of the shape of the ingot are neoessary in order that thaw cavitiea and blow holei may be reduced to a minimum, and as fiar 'CLS possible gathered together in tho part of the ingot which is to be rejected. Fig. 15 shows the appearance of a section of an ingot in which these conditions have been ful- filled. The impurities are collected into thc central part of the top of the ingot, and the gasea have collected into a large cavity known as the "pipe." If the ingot be mopped by cutting along a line A B all the defective steel will be removed and the part rolled down will be free from flaws or inclusions. The casting of the ingot is the first step in the long series of operations which lead to the production of the finished automobile part, and all them operations must be carefully and scientifically carried out if the finished product is to give satisfaction under service conditions. In particular, the oomposition of the steel must be aarefully selected for the service which the part will be called upon to perform, and the heat-treatment must be adjusted to the composi- tion and to the nature of the service. We have already shown how the critioal range of the steel is altered by variations in its composition, and it is quite impossible to give the steel a satis- factory treatment unless its composition is accurately known, and it is, therefore, important that the steel user should know the composition of every consignment he receives
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 A hrge &are of the trouble nc)n being experienced with sted is to be laid at the door of the storekcepers, both at the steel makers mid at the manufacturers. The preqent scarcity of steel -- --
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I.'rn. Ij.-8Section of Ingot. put.3 a constant temptation before the storekeeper to pwout any steel he may have on hand without taking the trouble to find out if it is suitable for the purpose for which it is to b,e used, while the fa& that maay storekeepers have little idea of the importanoe
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 SOME NOTBE ON THE LIETALLIJRQY OP BIEEL. 503 of the composition of the steel often leads to the mixing up of different consignments with the result that all trace of the origin of the steel is lost, and c.arefu1 heat-treatment produces the most irregular results. In the case of steel which is to be used for highly stressed parts it is absolutely essential that the bars produced from each cast be kept separate, and that the steel throughout its passage through store and shops be labelled and kept in such a way that. it will be possible for the cast analysis to be produced and the correct treatment of the steel obtained. It is not possible within the scope of this paper to go fully into tho question of the physical properties of steel. Generally speak- ing, a wrought iron with practioally no carbon content has an ultimate tensile breaking stress of about 18 tons per sq. in., and its the carbon increases up to 0'8 or 0.89 per cent the ultimate stress rises, if the steel is suitably treated, to some 40 or 50 tons per sq. in. with a corresponding reduction in elongation. Gene- rally speaking, structural steels used for buildings are ordered to an ultimate breaking stress of 28 to 33 tons per sq. in. with a yield of somo 20 tons per sq. in., and elongation of about 25 per wnt. This specifiaation can be well complied with by a steel containing from 0.1 to 0.3 per mt of' loarbon. For automobile work large quantities of straight carbon steel, having a carbon content of from 0'3 to 0'4 per cent, is in demand. Yhe ultimate stress in this caae is from 30 to 40 tons per sq. in. with an elongation of from 20 to 25 per cent. The introduction of nickel chrome and similar elements has the effect of enabling the tensile to be greatly increased, so that, for example, steel with 0'3 per cent of carbon, 4 per cent of nickel, 1 per cent of chrome, and not more than 0'04 per cent of sulphur and phosphorus, 0'3 per cent of manganese, 0.1 to 0.2 per cent of silioon, would give from 60 to 70 tens ultimate breaking stress, from 50 to 60 tons yield, 18 to 20 per cent elonga- tion, with suitable heat-treatment. Figs. 3 and 12, which have been previously referred to, are illustrations of the various physical properties which could be abtained from given steels by varyiw heat-treatment. As regards case-hardening steels, advantage is here taken of the very hard condition, in fact glass-hard condition, obtained by quenching a steel with a high mrbon content, whereby great wear rosisting properties are obtained. If the whole of a case-hardened part were made from such a steel it would have little or no power
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 504 '1HE 1NSrITUTlON OF AUTOhlOl~Il~l~EhOlNI4EKS. of resisting stress, therefore, the core 01' inner portion of the case-hardened part must contain a very small peroentagc of carbon, tso that when the quenching operation is carried out the core will not bc hardened, but will remain ductile. This is effected by taking a steel with very low carbon content, and with cei-tain regard to other chemical properties, heating it in contact with carbon which, given suitable temperature and time, will penetrate a soft steel and form an outer layer or skin containing a very high percentage of carbon Ctase-hardening steels forms a special sub- ject, hardly within the scope of the paper. There is another point which is of interest tu the machine shop, and that is the rbugh machining qualitiw of steel of different chemical compositions. Consultations between the steel mlakers md the automubile manufacturers ai-e essential where high physical results, coupled with good machining qualities, are in- dispensable. Finally, caFe, oo-operation and a knowledge of tlie material used, and the purposes for which it is required, will done lelad to ~llCCWS In order to illustrate and eniphiasise the points set ouc above the following noies on failures which have occurred in War service have been Compiled. They have been selded from a number of reports of examinations oE broken park3 of motor lorrias made for the Director of Transport. In all thirty specimens were examined, and in twenty-three cases the examination enabled the cauw of failure to be determined with reasonable certainty, while in the other seven cases either the material was found to be perfectly satisfactory or the defectr noticed did not appear to be sufficiently serious to have led to the failure of the part. It will perhaps be of inkrest to classify the cases of failure acoording to the nature of the defects rwealed by examination. These defects can be grouped under the following heads:- 1. Defectiw Composition.4his group comprising cases in which the material employed is not suitable for the service which the part will be oalled upon to stand, and also caws of impurity in the metal. 2. Locul Defects.-caused by the presence of slag and similar inclusions in the metal. 3. Defects poducred during Maw&rtirre, -such as forging and quenching cracks.
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4. Defective heat-tredment,-of the material. 5. Defects in the Nunufacture of Case-hardened Parts.-Several of the specimens show combinations of these defects, and amlysing the twenty-three cas5 we get thc following classification:- Composition ...... 2 Composition and treatment ...... 5 Inclusions ...... 1 Inclusions and treatment ...... 1 Forging and quenching cracks ...... 2 Haat-treatment ...... 9 Case-hardening treatment ...... 3 A glance at this list will show whiat an important part defective treatment has played in the failure of the parts in question.
DEFECTIVECOMPOSITION. 1. Worm &%uft from Motor Lorry.-fI'his shaft had broken across in the threaded part, a crack having started from the root of the screw-thread. Micro-examination showed the presence of a large number of sulphide inclusions accompanied by it strongly banded structure. Excess of sulphur wm confirmod by analysis which showed 0'075 per cent. 2. Broken Steering Arm from Motor Lorry.-A photograph of this specimen is shown in Fig. 16, Plate XXXI. ; the coarse crystal- line appearance of the fracture will be observed. Mim-examina- tion ehoived that the steering arm was made of very low carbon &I, and had been badly overheated, the ferrite grains mnging up to 1.5 mm. diameter. When the section was deeply etohed the structure wen in Fig. 17, Plate XXXI., was observed. This appear- ance is asociated with high phosphorus content, and this was confirmed by analysis which showed 0.140 per mnt. The high percentage of phosphorus together with the overhcat- ing has produced the exmwively coarse structure of the material and led to failure of the part. 3. Back Axle rShaft broken in Castellated Portion.-A large number of them shafts had broken in service. The appearance of a typioal fracture is shown in Fig. 18, Plate XXXII. Micro-esa- mination of the shaft indicated exoessive carbon content, and this wa~confirmed by analysis which gave:- Carbon ...... 0.600 Nickel ...... 1.421 Chrome ...... 1.026 HAGNALL-15'1 In. KK
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 506 THE 1NSTlTUTlON OF AUTVMOUlLE ENGINEERS.
The high carbon content renders the shaft liable to fail by fatigue when subject to severely localised stresses such as am set up in a castehted shaft. The majority of the shafta were in the normalised condition, but one of the shafts had apparently been quenched from a high temperature, and not subsequently tempered. This shaft gave a Brine11 figure of 419, and its micro-structure is 8hown in Fig. 19, Plate XXXII. 4. Pins from the Wheel of a Caterpillar l‘sactor.--%veral oasw were inet with in which failure was due to the USB of tool steel in par& subject to ,shock, the she1 being hardened by quenching to give a wearing surface. Fig. 20, Plate XXXIII., which is 11. photograph of a broken pin from a caterpillar tractor, is t,ypiCal of the cases that occur. Such a part should have been made of low carbon steel case-hardened. The micro-structure is shown in Fig. 21, Plate XXXIII., and shows thc inartensitic structure of quenched high carbon steel.
FAILURESCAUSED BY THE PRESENCE OF INCLUSIO~~S 5. Brokm Steering Arm.4he micro-photograph Fig. 22, Plate XXXIV., shows a section through a large streak of slag running along a bring arm which broke in mrvice, and i an ommple of large inclusions which are occaaionslly met with. Fig. 23, Plate XXXIV., which is from a section through a broken ci.zlnkshaft, is a more typical case of comparatively small inclusions which often lead to failure by the development of fatigue cracks starting from an inclusion which happens to occur at a highly stresed point of the shaft.
FAILURESDUE TO DEFECTIVEHEAT-TREATMENT.-MILD STEEL. 6. Brokm Bolt from. Differential Casing.4he micro-photo- graph Fig. 24, Plate XXXV., shows the structure of the material of this bolt, and illustrah the effect of low annealing temperatum on low carbon steel. By heating such a steel to a temperature of from 670’0. to 7OO0C., i.e., just below the lower change point, the pearlite coalesces into granules giving rise to the ‘I granulap ’’ or “divorced” pearlite ahown in the photograph. In this condi- tion the steel is very soft and has a very low yield point. 7. Broken Steering Am.-Fig. 25, Plate XXXV., ,shows the ,ctructure observed in this part, and is B good example of the coarse
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structure produced in medium wbon steel by overhwting. Tho arm haa been forged at a high temperature, and the coarse structum produood has not been removed by subsequent treatment. @he steel in this condition is very brittle, and quitc? unsuitable for we in a part liable to shock.
ALLOYSTEELS. 8. Brokm Crank&aft.-Cl!his is qn axample of an important part having been put into service in the untrmtod condition. Fig. 26, Plats XXXVI., shows the structure seen in a section near the fractum, and iq typical of 4cc.l which has hem forged, and not subsequently treated. 9. Front Axle.-A number of ipecimrii+ uero received which had broken after being repaired in the liold. In nrarly ievcry case this waq due to the smiths linving quenc5ed the put aftor heating up for straightening or some siiuilas opoi=ttion. -4s in themajority of oases the park dealt with are made in medium carbon steel, this treatment renders them extremely hard and brittle Fig. 27, Plate XXXVI., md Fig. 28, Plate XXXVII., refer to -1 front axIe which had broken after such a repair. Big. 27 shoma the structure in the part of the ~xlenot affected by tho troat- merit: and indicates a steel of from 0.5 to 0.6 carbon in the ammaled condition. Fig. 28 shows a mdion of the broken end, and mill be seen to consist entirely of marteniite, the .feel in this part of the axle being de:id h:i,rt?
~:ISE-IIdRDENED PARTS. 10. Brokau Di8erential Pinioia.-This is it11 example of ex- cassivct carburisation; the cam, containing :ti1 excesw of oarbide which is men in the micro-photograph E'ig. 29, Plate XXXVII., as' a network of cementite, is very hard and brittle, and in servica the martensite grains work loose and fall out pToducing a coarae jagged surface often seen in worn-out gears. 11. Pin from Unikersal Joint.4he commonest fault of cilss- hardened articles is brittleness of the core produced by incorrect treatment after carburising. In the double quenching treatment which should be given to aU case-hardened parts which are subject to heavy stress, it is essential that the temperature of the first quenohing should exoeed tho upper critical point of the core, while KX2
Downloaded from pau.sagepub.com at Purdue University Libraries on June 4, 2016 508 'IHE IKS I'I'I'UTION OF AUTOMOBILE ENGINEERS. thi> teiuperaturc of tlie 5eCOlld quenching should be below this limit but above tlie traiisl'orination point of the case, and this can only be attained by very careful regulation of tho furnace tern- pcrdture. Fig. SO, Plate XXXVIII., i5 a section of a pin from a iiniversal joint in which the tempcrstui.e before quenching was below the upper critical point of the core, and shows area of clear ferrite due to incomplete diffusion of tlie carbon, the structure corresponding to that shown in Fig. 6, Plate XXVIII.
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Fu;. 4.-Test piece showing change points of 0.2 Carbon Steel.
18’1~. ~.-IIOR~CTchange point of 0.2 Onrhon Steel. 300 mngnifications.
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I7ro. 6.--TJpper change point 0.2 Cnrbon Steel. 300 magniticationa.
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FIG.I).--'l'eet piece showing change points of hyper-entectoid Steel.
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FIG.lO.-Chauge points of hyper-entectoid Steel. 20 magnifications.
Fra. 11.-Lower change point of hyper-entectoid Steel. 300 magnifications.
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FIG.16. -Broken Steering Arm from Motor Lorry.
Fra. 17.--BrOken Steering Ann. 300 magnifioationa.
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Fis. 18. ---Back Axle Shtrft.
FIO.19.-Structure of Back Axle Shaft. 300 magnificationn.
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FIG.2O.-Pin from Caterpillar Tractor.
FIG.2 1 .--Pin from Caterpillar Tractor. 300 magnifications.
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FIG.88. -Slag Streak in broken Steering Arm. 800 magnifications.
FIG.83.--81ag in broken CIniikshaft. 300 magnifications.
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FIG.%.-Broken Bolt eliowing Granular Pearlite. 300 magnifications.
FIG.2tj.--Coarse Struuture of broken Steering Aim. 150 magnifications.
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FIG.26.-Brokm Crankshaft (Untreated Alloy Steel). 300 magnificatioim
FIG.!27.--Uroken Front Axle. 300 magnifications.
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FIG.PX.--Brokeli Front Axle. 300 magnifications.
FIG.29.-Csse of differential Pinion showing Free Cementite. 300 magnifications.
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FIG.30.-COre of Pin of Universal Joint. 300 magnifications.
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