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tions which are not diffusionless . .Thesejransformations still involveihe ..change of the crystal structure by shear and thus a macroscopic shapechange, though they are not usually termed "martensitic."

The growth of ferrite plates in bainite is a good example of this. Adefinite macroscopic shape change accompanies the transformation (seeF ig. 6 -2 0). It is diffusionless as far as the iron and substitutional solute atomsare concerned, but the rate of growth of the plates is determned by therate of flow of the rapidly diffusing carbon atoms through the austenite3way from the growing ferrite. (Carbon diffuses roughly 10!2 times faster inaustenite at these temperatures than does iron or a substitutional solute.)The free-energy decrease for the precipitation of a carbon-poor ferrite fromaustenite is appreciably greater than for the transformation of the higher-carbon austenite martensitically. Thus in the bainite range the lattice trans-forms by shear at arate determned by the rate of removal of diffusion ofcarbon away from the region being transformed.

The growth of Widmanstatten plates at high temperature seems to bea continuation of this process since the orientation relationship between aand 'Y is the same for martensite, bainite, and Widmanstatten, although thehabit plane changes. There is also a surface relief involved in formng aWidmanstatten, whereas no surface relief is found with grain-boundaryallotriomorphs of ferrite formed at the same temperature. Thus, at highertemperatures a Widmanstatten appears to form from austenite by a dif-fusion-accompanied shear, while a allotriomorphs forrn behind an a'Y inter-face at which no shear occurs.' Still otti'H examp1es are found inthe coherentprecipitation of transition phases such as the hcp 'Y' phase from fcc AI-Agalloys as was discussed in Seco 6-12. Surface relief is found where theseparticles meet the surface. This implies some sort of cooperative movernentof the matrix atoms across the interface, though again the precipitationmust be accompanied by a local change in composition.

The point to be remembered is that martensitic transformations andprecipitation reactions usually are distinct and different reactions. However,at lower temperatures it is not uncommon to find precipitates whose forma-tion involves a shape change plus a composition change.

8-7. strength of rnartenslte?Commercially the highest volume of materials heat treated to give a mar-tensitic transformation are the low-alloy steels represented by the first set!See articles by Christian, Speich, and Kaufman, el al. in "Decomposition of Austeniteby Diffusional Processes," V. Zackay and H. Aaronson (eds.), Interscience Publishers,New York, 1962.2 M. Cohen, Trans. AIME, 224:638 (1962); W. Leslie and R. Sober, Trans. A.1I1. SocoMelals, 60:459(1967).

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344 transformations in metals

- - 1. 1001. 000

900

800

< Il 700Q : ;.x:.~ 600>.

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martensite 345-wh'ich the' carbn is in'~6Iubl-~This can and do~s ultimately -'ead to the

precipitation of a carbide. H00e~r;sfeel' ~artnsites have co-nsiderablestrength before the precipitation of carbide, so let us temporarily bypassthis obvious contribution to hardening and consider other contributions.The martensite in low-to-medium carbon low-alloy steels consists of veryfine needles or lath.! Each of these laths is a separately formed martensiteolate, with pairs of sets formng so that the shear strains accompanyingtransformation in each tend to cancel one another. The lattlce invariantshear of the transformation occurs by slip. The resulting martensite has adislocation density comparable to that of a highly deformed metal (1 011 to1 012 dislocationa/cm"), in addition to a mean plate or needle thickness ofabout 0.1 L . These lath boundaries and the tangle of dislocations provideformdable barriers to dislocation motion in and of themselves. One can'tproduce carbn-free martensites in pure iron that give a comparable struc-ture, but a structure simlar to this can be produced in iron by shock load-ing, and it has aflow strength of roughly 100 ,000 psi.? This is far short of the300,000 psi strength attainable in quenched steel but is still far above theroughly 1 0 ,000 psi flow strength of annealed iron.

The mechanisms by which carbon in solution in martensite contrib-utes to hardness are built around the fact that the carbon atoms strain theferrite lattice and this strain can be relieved by carbon atoms redistributingto relieve the strains set up by dislocations. This carbon redistribution setsup a substantial binding between the dislocations and the carbon atoms sothat a large stress is required to l1-o~vehe dislotations away. This bindingresults from segregation to the dislocation core region, as well as from alocal ordering in the stress field of the dislocation many atom diametersaway. This ordering or rearrangement well away from the core can be under-stood with the help of Fig. 8 -1 2, which shows the distortion along a (ID O ) axis+hat is set up by acarbon atom If the atom jumps to an adjoining interstitialposition, note that the axis of the distortion shifts to a (1 00 ) direction normalto that of the original position. The dislocations also set up preferentialdistortion along certain directions. The carbon interstitial can thus become"bound" to the dislocation by making only one jurnp.!

Carbon can and does diffuse in martensite at room temperature. Oneresult is the binding to dislocations. Another result is the formation ofcarbon atom clusters. The uniaxial or dipole distortion referred to aboveleads to an attraction between the atoms which gives clustering on the l00type planes. That is, the distortional energy of the carbon atoms is reducedif they cluster on acube plane so that their axes of distortion are all parallel.1 P. M, Kelly and J. Nutting, J. Irori Sieel Lnsi .. 197:199 (1961).2 W. C. Leslie and R. Sober, Trans. Am. Soco Meials, 60:459 (1967).3 G. Schoeck, H. Wiedersich, and A. Cochardt, Acta Mei., 3:533 (1955).

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346 transformations inJ T1etals

This. is in effecra ~Gezone "such as ~wasdiscussed in connection with theage hardening of alurninurn alloys i the last chapter.:

The effect of this early redistribution of carbon on the strength can bestudied by working with Fe-Ni-C alloys with an M, well below room temperature. For example, an Fe-25Ni-O.4C alloy has an M, "" -35C. If themartensite is formed by cooling to about -lOQoC, the carbon is completelyimmobilized at the temperature where martensite forms. If this steel isheated to QOCand held for afew hours, the flow stress at afew percent strainis increased substantially (see Fig. 8-17). At OC the carbon atoms makeroughlya jump a second, so the increase shown here results from carbonredistribution to form clusters and to pin dislocations.

The two contributions to the strength of martensite discussed above1 R. A. Johnson, Acla Met. , 15:513 (1967); J. Genin and P. Flinn, Trans. AIME, 242:14 19(1968).

Ni, appraximate weight percent31 27 23 19 15

320.000 I

'" 280.000~(l)

a.ee 240;000 R ,. . _-----~(2)"u~ ",- . . - -' " 200.000 /a. /

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seem_to be the main ones. O_neis a structurai contribution which is largelyindependent of carbon content; the other, strengthening due to the re-arrangement of carbon atoms after quenching. The magnitude of this latterterm increases roughly linearly with carbon content and in a martensite with0.4 % e can contribute roughly twice as much to the tensile strength of thesteel asthe dislocation substructure. The strength of low-carbon martensitesthus stems primarily from substructure, while rnediurn-carbon martensitesgain more of their hardening from carbon rearrangemenU

!- j i

8-8. tempering in plain carbon steelBefore a piece of steel quenched to martensite is put into use, it is firstheated (tempered) at some temperature above room temperature but wellbelow the A l to give it some ductility. When a piece of steel is quenched, theoutside cools and contracts while the inside is still hot and plastic. As thespecimen center cools, it is restrained from shrinking by the rigid coldoutside. In addition to the stresses set up in this way, a volume increaseof the order of 1 percent accompanies the austenite to martensite trans-formation. If the steel still has some plasticity at the M. temperature, thesestrains can be relieved by local plastic deformation. If the M. is so low thatthe flow stress is very high, these transformation strains set up stresses inthe piece which are near, or above, the fracture stress of the piece. If theyexceed the fracture stress of the-piece, it crack_s.This quench cracking canbe avoided by !Wing to a mlder quench or by redesigning the piece. Even ifno cracking occurs on quenching, the piece may crack on sitting at roomtemperature. Or, if a load is applied it will add to the high locked-in stressesin some areas and cracking at a low applied stress results. If the piece isheated to only 100e, the flow stress is reduced enough to allow local flow topartially relieve the stresses and thus avoid spontaneous cracking. Suchtempering increases the toughness of the alloy and doesn't reduce thehardness at all. Figure 8-18 indicates the decrease in hardness resultingfrom a l-hr tempering anneal at various temperatures. The dashed curveon the figure shows the variation in toughness with annealing temperature.Toughness is here measured by the energy adsorbed by a notched speci-men in breaking by impacto The as-quenched martensite is somewhat likeglass in that it is quite hard but adsorbs relatively little energy in fracturing.Tempering reduces this tendency to brittleness.

The iron-carbon martensite formed in steel is highly supersaturatedwith carbono The changes that occur in this finely deformed, supersaturatedalloy on heating are quite consistent with what has been covered earlier in1 W. C. Leslie and R. Saber, Trans. Am_ Soco Meiols, 60:459 (1967).