mining and metallurgy - 1945 - ferrous physical metallurgy...
TRANSCRIPT
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
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,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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