A new empirical formula for the calculation of M-S temperatures in pure iron and super-low carbon alloy steels

It has often been observed that when the experimentally determined Ms (the upper limit of the martensite transformation, or the practical start temper ature of martensite transformation) temperatures are extrapolated to pure i ron from binary Fe-C, Fe-Ni, Fe-Cr, Fe-Mn, Fe-Co alloys, anomalous temperat ures, e.g. 520, 680, 700-750, 800-900 degreesC, or even as high as the equi librium gamma - alpha temperature of 912 degreesC, are obtained. In order t o shed light on these conflicting results, a new empirical formula M-s (degreesC) = (795 - 25,000C(1) - 45Mn - 35V(Nb + Zr + Ti) - 30Cr - 20Ni - 16Mo - 8W - 5Si + 6Co + 15Al 525 - 350(C-2 - 0.005) - 45Mn - 35V(Nb +Zr + Ti) - 30Cr - 20Ni - 16Mo - 8W - 5Si + 6Co + 15Al, where C-1 < 0.005, 0.00 5 less than or equal to C-2 < 0.02 is proposed to predict the M-S temperature of pure iron and super-low carbo n alloy steels. The effect of steel composition on the M-S temperature is d iscussed. The validity of this new equation together with 11 other existing L empirical formulae for the calculation of M-S temperature in pure iron a nd super-low carbon alloy steels is examined. For more than 80% of super-lo w carbon alloy steels, the M-S temperatures can be predicted by this new fo rmula to within +/- 25 degreesC. The new formula indicates an M-S temperatu re of 795 degreesC for a steel without carbon or other alloying elements (" pure" iron). This is a better approximation to the experimental result of 7 50 degreesC than for other formulae when the cooling rates exceed 35,000 de greesC/s, and the carbon content is lower than 0.0017 wt.%. (C) 2001 Elsevi er Science B.V. All rights reserved.