Austenite Formation from Martensite in a 13Cr6Ni2Mo Supermartensitic Stainless Steel

Abstract

The influence of austenitization treatment of a 13Cr6Ni2Mo supermartensitic stainless steel (X2CrNiMoV13-5-2) on austenite formation during reheating and on the fraction of austenite retained after tempering treatment is measured and analyzed. The results show the formation of austenite in two stages. This is probably due to inhomogeneous distribution of the austenite-stabilizing elements Ni and Mn, resulting from their slow diffusion from martensite into austenite and carbide and nitride dissolution during the second, higher temperature, stage. A better homogenization of the material causes an increase in the transformation temperatures for the martensite-to-austenite transformation and a lower retained austenite fraction with less variability after tempering. Furthermore, the martensite-to-austenite transformation was found to be incomplete at the target temperature of 1223 K (950°C), which is influenced by the previous austenitization treatment and the heating rate. The activation energy for martensite-to-austenite transformation was determined by a modified Kissinger equation to be approximately 400 and 500 kJ/mol for the first and the second stages of transformation, respectively. Both values are much higher than the activation energy found during isothermal treatment in a previous study and are believed to be effective activation energies comprising the activation energies of both mechanisms involved, i.e., nucleation and growth.

Modeling Solid-State Phase Transformations of 13Cr-4Ni Steels in Welding Heat-Affected Zone

“…A resulting activation energy of 509.9 kJ/mol is calculated, which agrees with the value previously obtained value by Bojack et al [36] on a similar fully martensitic steel however tested at lower heating rates. Hereafter, this austenitic transformation activation energy is used to compute the JMAK model parameters.…”

“…This method has been proven effective to characterize the austenitization activation energy of steels from various continuous heating dilatometry experiments. [33,36] The proposed method consists in determining the temperature at which the maximum reaction rate occurs for dilatometry experiments conducted with different continuous heating rates. Mittemeijer et al, [33,34] made the assumption that the phase volume content is determined by a state variable b.…”

“…It can be seen in Figure 13 that for a heating rate of 1 C/s, the transformation rate is significantly different at the end of the transformation. According to the study of Bojack et al [36] on 13Cr6NiMo steel, this could result from a change of transformation mechanism during the austenitic transformation attributed to a mechanism of nickel and molybdenum partitioning. Thus, enriched regions of an austenite-stabilizing element would have a lower activation energy and would therefore transform first, which would lead to a two-stage complete austenitization transformation.…”

Temperature Dependent Phase Transformation Kinetics of Reverted Austenite during Tempering in 13Cr Supermartensitic Stainless Steel

“…The variation of Ni content demonstrated the presence of reversed austenite in the darker region. Therefore, it demonstrated that the reverted austenite was produced by the enrichment of the Ni element and creates ideal nucleation positions from the matrix in super 13Cr martensitic stainless steel [40]. The Ni element was partitioned into reverted austenite from surrounding martensite, and it has the action to decline the Ms point and improve the stability of reverted austenite [36,41].…”

The Effects of Hot Deformation Parameters on the Size of Dynamically Recrystallised Austenite Grains of HSLA Steel

“…Nevertheless, they are often observed with various factors of the final mechanical properties. The mechanical properties are strongly dependent on the fraction of reverted austenite which is very sensitive to the heat treatment [25][26][27][28][29].…”