Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-% Cr low carbon ferritic steels

The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-% Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr-1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1-100 K min(-1). In addition to the accurate determination of Ac-1, Ac-3, M23C6, MX carbide dissolution and delta-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823-873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr-1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4-5 K min(-1) for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature M-s, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.