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 ~artensitic transformations, there has been considerable debate about the factors responsib1e for promoting twinning in the martensite structure. It has been suggested that M 5 plays an important role-~lowering the Ms results in an increase in the probability of internal twinning. 1 -3 The f\ 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 M 5 temperature.It is well known that the Ms temperature is strongly dependent on the composition of the parent phase, austenite. Izumiyama et a1. 5have shown the effect of individual alloying elements upon the Ms temperature for iron-based binary alloys. Their results show that Al, 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 -2 -including carbon are in complete solution). These formulae are shown in Table I, 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, a result confirmed by several recent investigations. 13 -14 Moreover, cobalt is included only in one formula, that of Carapella. The coefficient for cobalt is rather high, compared to that for nickel or chromium. But the results of Izumiyama et a 1 . , Figure 1 Rowland and Lyle, and Nehrenberg, the coefficient for silicon is shown to be negative. For this investigation, the effect of silicon is considered to be equivalent to that of molybdenum.. Therefore, a term of -7.5 Si has been included in some of the equations.

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Transmission electron microscopy, and microhardness testing were used to examine the as-quenched structure and mechanical properties of a series of rapidly solidified (R.S.) iron-tungsten-carbon-alloys ranging from 6 to 23 percent tungsten with a constant W:C atomic ratio of 2:1, and Tl high speed tool steel. The R.S. iron-tungsten-carbon alloys were found to exhibit a significant change in microstructure and hardness as the tungsten and carbon content was increased. The change in morphology was from lath martensite in the lower tungsten alloys to a solidification structure of ~ferrite cells surrounded by austenite and M 6 C carbide, in the higher tungsten alloys. A model is proposed to explain the morphological change. In addition, the tempering behaviors of R.S. Fe-6.3wt%W-0.2lwt%C, Fe-23wt%W-0.75wt%C and Tl high speed tool steel were examined and compared to those observed for the conventional solution-treated and quenched alloys. A discussion is also included on the microstructural dependence upon cooling rate of R.S. high speed tool steels.

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