An examination of the validity of existing empirical formulae for the calculation of m s temperature
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An Examination of the Validity of Existing Empirical Formulae for the Calculation of Ms Temperature C.Y. KUNG and J. J. RAYMENT
When examining phase transformations in ferrous systems, it is often desirable to have prior knowledge of the martensite start temperature (Ms). In the area of martensitic c. Y.KUNG and J. J. RAYMENTare both Staff Scientists at Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript submitted September 1, 1981. 328--VOLUME 13A, FEBRUARY 1982
transformations, there has been considerable debate about the factors responsible for promoting twinning in the martensite structure. It has been suggested that Ms plays an important r o l e - - l o w e r i n g the Ms results in an increase in the probability of internal twinning) '2'3 The Ms temperature has also been shown to have a direct effect on the toughness of steels. Quenched steels which have a high Ms are usually dislocated packet martensites and also have been shown to undergo significant autotempering, 4 both factors being beneficial to toughness. On a more basic level, a knowledge of the Ms temperature can be useful, to ensure that a heat treatment and subsequent kinetic study of bainitic transformations occurs in a temperature range above the Ms temperature. It is well known that the Ms temperature is strongly dependent on the composition of the parent phase, austenite. Izumiyama et al s have shown the effect of individual alloying elements upon the Ms temperature for iron-based binary alloys. Their results show that A1, Ti, V, and Co effectively increase the Ms temperature, whereas Nb, Cu, Cr, Mo, Ni, C, and N decrease the Ms temperature. There have been several formulae which have attempted to relate the Ms temperature with alloy composition, (with the assumption that all alloying elements including carbon are in complete solution). These formulae are shown in Table 1,6-12 with minor modifications shown in parentheses, which are described later. In these formulae, cobalt is the only element included which slightly increases the Ms temperature,
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