Results of further X-ray structural investigations of the iron–carbon and iron–nitrogen systems and of related interstitial alloys

Abstract

Starting with a hardened structure consisting of tetragonal martensite and retained austenite, high-carbon steels undergo three structural changes during low-temperature tempering (see Antia, Fletcher & Cohen, 1944), each stage being accompanied by characteristic changes in hardness and other physical properties. It is generally agreed that the second stage is the transformation of retained austenite, but although the first and third stages are associated with the decomposition of martensite into the ultimate aggre- gate of ferrite and cementite, their exact natures have been in dispute (Honda & Nishiyama, 1932; Hggg, 1934). Honda & Nishiyama considered the first stage to be the transformation of tetragonal to cubic martensite. H/~gg's view is supported by more recent workers (Antia et al. 1944; Arbusov & Kurdjumov, 1941; Kurdjumov & Lyssak, 1947) who conclude that the first stage of tempering in- volves the formation of ferrite and a precipitated phase which is not cementite but which is too finely dispersed to yield an X-ray diffraction pattern. From a martensite specimen tempered at 200 ° C., Heidenreich, Sturkey & Woods (1946 a, b) obtained an electron-diffraction pattern corresponding to e-iron nitride (F%N) or some phase iso- morpheus with it. Because of the very small nitrogen con- centration of the steel used, it was suggested (Jack, 1946, 1948b) that the hexagonal 'FeaN' was possibly an e-iron carbonitride. New X-ray observations now give direct evidence that the loss of tetragonality of martensite during the first tempering stage is due to the precipitation of a close-packed hexagonal iron carbide which it is proposed should be named e-iron carbide, or e-Fe3C, because of its structural similarity with e-Fe3N, e-Iron carbide is formed as a coherent transitional phase in which the Laue condi- tion for X-ray reflexion is obeyed preferentially in a direction normal to the (101)lattice planes. The unit-cell dimensions a=2.73, 0=4.33A., c/a=1.58, are similar to those given by tteidenreich et al. for their 'Fe3N '. The spacing of the (101) planes is almost identical with that of the (101) planes of martensite, so that a simple orientation relationship between the two phases is prob- able. The hexagonal carbide for:ms the carbon-rich extreme of the series of close-packed hexagonal e-iron carbonitrides British Iron and Steel Research Association. t Formerly Senior Scientific Officer, B.I.S.R.A. Now at the ChemistryDepartment, King's College, Newcastle-upon-Tyne 1, Englaad. * Paper MG/C/50/50 of the Metal Physics Committee, (Jack, 1948 b) at their lower interstitial-atom concentration limit, Fea(C,N ). A similar iron carbide, Fe2C, has been described by Hofer, Cohn & Peebles (1949) as the carbon- rich extreme of the same carbonitride series at its upper interstitial-atom concentration limit, F%(C, N). During the third stage of steel tempering, e-iron carbide transforms to give a fine dispersion of very thin platelets of strained and slightly distorted cementite, still coherent with the matrix and with the plane of the platelet parallel with the (001) lattice plane of the cementite structure. The observed diffraction pattern is markedly different from that of crystalline cementite since the Laue condition is fulfilled completely only in directions parallel with the (001) planes and is relaxed in the [001] direction. Included among the strongest reflexions are the previously un- identified reflexions observed on X-ray photographs of tempered martensite by Arbusov & Kurdjumov ( 1941) and ascribed by them to a new iron carbide. With increasing tempering times, or at higher temper- atures, the gradual growth and recrystallization of the distorted eementite platelets are accompanied by loss of coherency and reduction in lattice strain. The proposed structural changes are correlated with observed changes in hardness and specific volume of steels during the three tempering stages, and are also supported by electron- microscope studies, the results of which were published (Trotter & McLean, 1949) after the completion of the present investigation.