Measuring the flu.idity 0/ slag. Photo by Bethlehem Steel CQ.

in StuJies 01 HarJenabi/ity, Graphitization, Embritt/ement,

By Francis M. Walters, Jr.Division of Physical Metallurgy, Naval Research Laboratory;

Chairman, Committee on Metallography and Heat-Treatment,Iron and Steel Division, A.I.M.E.

Progress ReporteJ

anJ Di/atometry

I N spite' of the war and the preoccu­pation of many physical metallur­gists with work on secret or confi­

dential problems, definite progress wasmade during 1944 in our understandingof the behavior of steel and of the ef·fect of alloying elements. During thefirst two years of the war there wasmuch concern over the possible un­availability of certain alloying ele­ments; this period saw the developmentof the N.E. (National Emergency)steels. However, the measures takeneased the alloy situation and currently,highly practical problems under studyare the graphitization of low-carbonsteel and weld-cracking, while investi­gations of more theoretical interesthave concerned hardenability, isother­mal reactions, the effect of hydrogenand the interpretation of the notched­bar impact test.

HardenabilityMethods of measuring hardenability

and the effect of alloying elements onthis most important property of steelhave 'continued to hold the interest of

This paper repre8ents only the personalopinions of the author, and in no wayreflects the official attitude of the U. S.Navy. Published with permission ofNavy Dept.

FEBRUARY, 1945

many metallurgists. Although the eud­quench bar developed by Jominy andBoegehold is suitable for most types ofsteel {or which hardenability is an es­sential engineering characteristic, theneed has been felt for a means of deter­mining hardenability that is greater orless than that readily measurable onthe standard bar. Steels of low hard­enability are of interest because a fairinverse correlation between hardenabil·ity and weldability has been found: ameasurement of hardenability may in­dicate the welding technique (current,electrod.e size, rate of travel of the arc)which is required to make a successfulweld; or such a measurement mayshow that a particular heat of steelshould not be welded at all unless pre­heat or postheat can be used. Harden­ability under rates of cooling slowerthan those obtainable with the standardend·quench bar is of interest in thestudy of air-hardening steels for applica­tions that require low thermal gradientson cooling in order to avoid quenchcracks.

That hardenability can be predictedqualitatively from isothermal transfor-

mation diagrams IS to be expected.Such diagrams have shown that thesecondary maximum· hardness mea­sured on end-quench bars of somesteels is real and not due to experimen­tal error. The isothermal transforma­tion products at 900 deg. F. of such asteel as N.E. 9650 are harder thanthose formed at 800 deg. F. This sec­ondary maximum of hardness may beattributed to the formation of acicularferrite. Alloy and carbon segregationhave both been shown to have a realeffect on the hardness contour of theend-quenched bar and may prove ofreal value in the determination of solu­tion-treatment cycles.

The end-quench test has been shownof practical value in determining theaustenizing time and temperature nec­essary to meet hardenability require­ments for steels in various prior struc­tures resulting from such treatmentsas annealing, normalizing, and quench­ing from various temperatures.

Recent investigations on the effectof alloying elements appear to confirmthe Grossmann principle that each al­loying element increases the size of

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Complex carbides and hardenabilityRecent work appears to give a clue bide (Fe,Mo) "C. instead of the simpler

in Tegard to the mechanism by which cementite and to the necessity for thean alloying element increases harden- diffusion of molybdenum. A renewedability. It has been shown for pure study of the thermodynamics of aus­iron-molybdenum-carbon alloys wit h tenite decomposition and carbide {or­about 0.80 per cent carbon that during mation may explain why half a perisothermal transformation in the range cent of molybdenum prevents the for·1300 deg. F. to 1200 deg. F. the car- mation of cementite.bide formed is not cementite but a Other interesting items from thiscompIex face-centered cubic carbide work on molybden um steels are: the(Fe,Mo) "C•. This unit cell contains ll6 diffusion rate of molybdenum is muchatoms and the structure is that of one higher in ferrite than in austenite atof the carbides of chromium and of the same temperature, and isothermalmanganese. At !300 deg. F. this car- reaction as well as quenching and tem­bide forms to the exclusion of cementite pering leaves a fairly large propor­and may contain as little as 3.5 per tion of molybdenum in the ferrite.cent molybdenum in alloys containing When quenched, the molybdenum steelsonly 0.50 per cent molybdenum. On re- contained only cementite but on tem­acting at llOO deg. F. both cementite pering long enough, the comfllex car­and (Fe,Mo) "C. are formed, the rela- bide (Fe,Mo) "C. was formed at thetive amounts depending on the molyb- expense of the cementite.denum content. It is rather surprising The results of this investigation onthat so little of a third element can molybdenum steels, as well as recentchange the crystal structure of the work on the carbides formed on tem­carbide of the eutectoid. Inasmuch as pering chromium steels and on thethe alloys studied were carefully distribution of carbon between titaniumhomogenized, the high molybdenum and iron in steels should stimulate notcontent of the carbide suggests diffu- only the study of the carbides formedsion of molybdenum as well as of car- by alloying elements on isothermal re­bon during the formation of pearlite. action and tempering at various tem­The effect of molybdenum on harden- peratures after quenching but also theability may be ascribed to the longer determination of diffusion rates of vari­time required to build the complex car· ous alloying elements in ferrite as well

round which may be ha-rdened through­out on quenching by a percentagewhich depends on the amount of thealloying element present and which isindependent of the other alloying ele­ments in the steel except, perhaps,when the steel contains two or moreelements that form stable carbides. Ingeneral, the specific effect of the ele­ments reported by Grossmann has beenconfirmed by later investigators. Tohis list have been added titanium andzirconium, elements which like vana­dium, combine readily with carbon,oxygen, and nitrogen and are used inrelatively small amounts so that it isdifficult to assess their effect on hard­enability. Deviations from calculatedhardenability are often to be expectedfor the usual methods of chemicalanalysis do not distinguish betweenthat part of the total alloy additionthat is effective in increasing harden­ability and that part which decreases itthrough the fixation of carbon orthrough the formation of particleswhich act as nuclei for the formationof pearlite.

In his work on calculated harden­ability, Grossmann arrived at the idealcritical diameters of the steels studiedby the examination of quenched roundsof suitable diameter. Other investiga­tors have used the end-quench bar and

rr!lde use ,)f the eurve developed byGrossmann to convert hardenability asmeasured hom the quenched end ofthe bar to ideal critical diameter. Thisconversion Gurve may not be quite ac­curate for the lower range of harden­ability, which, although perhaps un­important practically, is of consider­able interest in the experimental studyof the effect of carbon and the alloy­ing elements on hardenability. Somedifficulty has been found in determin­ing the "half-hardness" which Gross­mann uses as the criterion for completehardening. The hardness of a structurethat is 50 p~ cent martensite is rela­tively easy to determine with low-alloysteels in which the other 50 per cent isferrite and pearlite. High-alloy steelsmay have structures, such as bainiteformed at intermediate temperatures,resulting in a higher hardness at the 50per cent martensite zone than thatfound in the low-alloy steels of thesame carbon content. The effect ofsmall amounts of high temperaturetransformation products on the physi­cal properties of quenched armor andspecial heat-treated steels has madenecessary hardenability investigationsbased on a structure of 100 per centmartensite rather than the commonlyaccepted criterion of 50 per centmartensite.

as in austenite. Most of the work oncarbides goes back to pre-X-ray daysand the heat-treatments employed weredesigned to produce large carbides re­sistant to chemical attack rather thanto determine what happened under con­ditions of practical importance or theo­retical interest. It is to be suspectedthat the results of such investigationswill modify the glib and easy classifi­cation of alloying elements into "fer­rite strengtheners" and "carbide form­ers."

GraphitizationThe complete failure about eighteen

months ago of a carbon-molybdenumsteel steam pipe adjacent to a weldcaused considerable alarm. This fail­ure, which occurred after 5Vz years'service at a temperature of 935 deg. F.,was due to the formation of a nearlycontinuous layer of graphite about VBin. from the weld. Examination of otherpipes in service showed graphitizationwhich, however, was well enough dis­persed so as not to cause failure.Out of the intensive studies conductedon this problem a few facts haveemerged; molybdenum steel does notgraphitize as rapidly as carbon steel,tensile stress increases the rate ofgraphitization, and steels killed withhalf a pound or less of aluminum perton are not likely to give trouble. Cer­tain observations on the effect of heat­treatment on the tendency towardgraphitization have led to the sugges­tion that complex carbides are morestable in the range 850 to 1000 deg. F.than is cementite. It has been sug­gested that graphitization could be pre­vented by using an alloyed iron with­out carbon, by tying up the carbon bya heat-treatment which would form acomplex molybdenum carbide, or byusing an element that would form aneven more stable carbide at the tem·perature at which the pipe is to beused. Future specifications will prob­ably set a limit of half a pound ofaluminum per ton as the prescribed de­oxidation practice to be used in theproduction of high-temperature steampipe.

HydrogenHydrogen continues to serve as the

standard explanation for any still un­explained phenomenon and in many in­stances sufficient evidence has been ad­duced to make the argument convinc­ing. Iron-manganese alloys whichtransform to alpha have been shown tobe brittle when containing hydrogen.This may explain the lack of ductilityobserved by Hadfield in his investiga­tion of iron-manganese alloys and theimprovement in ductility recently found

88 MINING AND METALLURGY

,to result from tempering such alloys at1000 deg. F. The low ductility is par­ticularly noticeable in those alloys thatform alpha at low temperature withresultant microcracks. Actual or in­cipient fissures are usually necessaryaccessories to the crimes of which hy­drogen is accused. Certain investiga­tors have made a good case for theresponsibility of hydrogen in weld­cracking. The hydrogen content of weldmetal produced with different types ofelectrodes has been related to the ten­dency toward cracking. The explana­tion involves such data as the differ­ences in permeability and solubility ofhydrogen in austenite and ferrite. Theincrease in ductility of a welded struc­ture with lapse of time between weld­ing and testing has been ascribed tohydrogen although relief of micro­stresses caused by the formation ofmartensite in the heat-affected zonesurrounding the weld bead seems anequally good reason, for experimentalevidence indicates little choiee betweenthe hydrogen explanation and themicro-stress theory.

BainiteFurther evidence has been secured

in regard to the nature of the bainitereaction in hypoeutectoid steels. Dila­tometric, X-ray diffraction, and micro­scopic studies lead to the" conclusionthat during isothermal reaction a rap­id movement of carbon occurs whichresults in regions of austenite withhigher and lower carbon than average,the low-carbon austenite transformingat the reaction temperature into a su­persaturated ferrite by a mechanismsimilar to that by which martensite isformed. The degree of completion ofthis bainite reaction is a function ofthe transformation temperature, thelower the temperature the greater theamount of austenite decomposing tobainite. This part of the reaction iscompleted in a relatively short timebut the remaining austenite is remark­ably stable.

Dilatometry

A symposium on recent develop­ments in diIatometric analysis high­lighted the versatility of this metallo­graphic method. The five papers of this

. symposium covered such diverse appli­cations as determination of the amountof austenite transformed at subatmos­pheric temperature, determination ofthe critical cooling- rates of low-carbonsteels; measurement of the coefficientsof expansion of nonferrous alloys andglasses, observation of the aging ofaluminum alloys, and the growth ofcast iron. Heating and cooling ratesemployed in these various types of

F'EBRUARY, 1945

ANNUAL REVIEW tSS~

dilatometers ranged from a few de­grees an hour to several hundred de­grees a second. Dilatometric observa·tions at various rates of cooling, com­bined with the microscopic examina­tion of the structures resulting fromsuch rates of cooling, should do muchto advance our understanding of thebehavior of steel when cooled continu­ously from the austenizing tempera­tures. The devices for indicating lengthchanges included the interferrometerand a special type of electronic tube,and methods of observation rangedfrom the visual to' the photographicrecording of 'tn oscillograph trace.

Cast ste.ls

Studies on cast steels have concernedthe effect of composition on harden­ability and the effect of heat-treatmenton tensile properties. Hardenabilitymeasurements showed that the effect ofalloying elements on steels was thesame in the as-cast condition in smallsections as it was after forging andthat the hardenability could be calcu­lated using Grossmann's factors. In theinvestigation of the effect of heat-treat­ment, 25 cast steels were each givensome forty heat-treatments and therelationship between the tensile strengthof each steel and the per cent elonga­tion was found to be a straight linewithin the limits of exper,imental error,

Tensile· testingI1wchine in thelaboratorv ofBattelle Me­moriallmtitute.

a result at variance with similar dataon wrought steels which indicate acurvilinear relationship.

Impact resistance

Papers on notched-bar impact testshave ranged from the experimental.dealing with observations on the effecto[ type and curvature of notch, thebreadth of specimen, temperature oftesting, and the volume of strainedmetal, to a theoretical discussion on thecauses of brittle failure, including theequivalence of decreasing the temper­atllre and raising the strain rate.

SummaryAlthough we have gone a long way

ih the development of a philosophy ofsteel, unsolved problems remain to oc­cupy the research minded. What is the'cause of temper brittleness? Why dosome steels show poor impact resistanceat higher temperatl~Te than others?How valid are the generalizations nowcurrent: "The maximum hardness towhich a steel can be quenched dependson the carbon content alone" and "Theproperties of a steel are determined byits microstructure, and alloying ele­ments are a means of securing thoseproperties in a given piece under givenconditions of heat-treatment"? The an­swers to some of these questions willno doubt be sought in the eoming year.

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