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

Abstract

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 A c1, A c3, M23C6, MX carbide dissolution and δ-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 Ms, 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.

Heterogeneous austenite grain growth in martensitic 9cr steel: Coupled influence of initial metallurgical state and heating rate

“…Figure 7 shows the dilatometric curves that account for the transformation to austenite during heating of the T91 and T9 steels when the heating rate is settled at 1, 30 and 50 ºC/s and the initial metallurgical state is the as-received one. As expected, the higher the heating rates, the more the temperatures A c1 and A c3 are shifted to higher values [26][27][28][29][30][31] . At the same time, for the 30 and 50 ºC/s cases, there is a clear change of slope in the intercritical portion of the curve at temperatures in the region of 900-925 ºC, suggesting that the transformation to austenite proceeds with two distinct regimes.…”

“…This implies, in turn, to assume that the only precipitated phase to be found in the as-quenched condition of the T91 steel is constituted by MXtype carbides or mixed carbonitrides (M = Cr, Nb, V; X = C, N). This assumption follows from the fact that the M 23 C 6 carbides existing in the as-tempered state are dissolved during austenite holding and will not precipitate during quenching and also by noting that the solvus temperature of MX-type carbides or carbonitrides is significantly higher than that of M 23 C 6 carbides [31]. Since the T9 steel cannot form MX-type carbonitrides, there will be no carbides present in its as-quenched condition.…”

Effect of Heat Treatment on Microstructure and Hardness of Grade 91 Steel

“…DSC was used to measure the heat absorbed or liberated during heating or cooling associated with carbide precipitation and other phase changes. Ganesh et al [10] were able to find the phase transformation temperatures including temperature ranges for dissolution of various precipitates in different low carbon steels.…”

Non-instantaneous growth characteristics of martensitic transformation in high Cr ferritic creep-resistant steel

“…As the predominant microstructure of high Cr ferritic creep-resistant steels, martensite formed during continuous cooling after austenization has also been studied extensively [9][10][11][12]. However, the model for nucleation and growth of martensite in high Cr ferritic steels is rarely mentioned.…”