Opening of the Peter G. Winchell symposium on the tempering of steel
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temperature, and it is already changed before the first measurements can be made. He neatly circumvented this difficulty by designing a series of iron-carbon-nickel alloys having the same Ms temperature, but well below room temperature ( - 3 5 ~ in this way, the specimens could be conveniently prepared in the austenitic condition and then subcooled to produce virgin martensites for measurement or testing at subzero temperatures without any inadvertent aging. That procedure has now led to a stream of investigations around the world on the aging and tempering of initially virgin ferrous martensites. However, in this approach, the virgin martensites are invariably associated with retained austenite. Peter was the first to establish a direct method for extrapolating bulk properties back to zero retained austenite, or up to 100 pct martensite. By systematic treatments, he was able to plot bulk measurements made at subzero temperatures as a function of the volume percent of martensite, thus making the aforementioned extrapolations feasible. But on aging and testing over a range of subzero temperatures, Peter noted that aging sets in even at temperatures as low as - 6 0 ~ For example, Figure 1 illustrates how the hardness at - 1 9 5 ~ varies with aging temperature for Fe-C-Ni alloys with increasing carbon content.] This was the first time it was clearly demonstrated that aging processes in virgin martensites can start at surprisingly low subzero temperatures, and that such aging causes an increase in hardness, superimposed on the hardness of the virgin martensite. Also, this age hardening reaches its maximum at or somewhat above room temperature and, therefore, plays a role in the observed strength of as-hardened
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Fig. l - - H a r d n e s s at -195 ~ of iron-carbon-nickel martensites after aging for 3 h at temperatures plotted. VOLUME 14A, JUNE 1983--991
steels when typically quenched t o room temperature. This aging increment is now attributed to the diffusion-dependent clustering of carbon atoms within the martensite, prior to any carbide precipitation which might otherwise be characteristic of the conventional first stage of tempering. All of these phenomena, treated in Winchell's early MIT paper, have provided a firm foundation for much of the subsequent research on the aging and tempering of ferrous martensites up to the present time; it will certainly show through in several of the papers to be presented during this Symposium. Another milestone was reached in the 1980 Winchell paper 2 with P.C. Chen on "Martensitic Lattice Changes during Tempering." That outstanding contribution could easily hav
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