A new empirical formula for the calculation of MS 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 temperature of martensite transformation) temperatures are extrapolated to pure iron from binary Fe–C, Fe–Ni, Fe–Cr, Fe–Mn, Fe–Co alloys, anomalous temperatures, e.g. 520, 680, 700–750, 800–900°C, or even as high as the equilibrium γ→α temperature of 912°C, are obtained. In order to shed light on these conflicting results, a new empirical formula MS(°C)=795−25,000C1−45Mn−35V(Nb+Zr+Ti)−30Cr−20Ni−16Mo−8W−5Si+6Co+15Al525−350(C2−0.005)−45Mn−35V(Nb+Zr+Ti)−30Cr−20Ni−16Mo−8W−5Si+6Co+15Al,whereC1<0.005,0.005≤C2<0.02 is proposed to predict the MS temperature of pure iron and super-low carbon alloy steels. The effect of steel composition on the MS temperature is discussed. The validity of this new equation together with 11 other existing empirical formulae for the calculation of MS temperature in pure iron and super-low carbon alloy steels is examined. For more than 80% of super-low carbon alloy steels, the MS temperatures can be predicted by this new formula to within ±25°C. The new formula indicates an MS temperature of 795°C for a steel without carbon or other alloying elements (“pure” iron). This is a better approximation to the experimental result of 750°C than for other formulae when the cooling rates exceed 35,000°C/s, and the carbon content is lower than 0.0017 wt.%.