In Situ Thermo-magnetic Investigation of the Austenitic Phase During Tempering of a 13Cr6Ni2Mo Supermartensitic Stainless Steel

Austenite Formation from Martensite in a 13Cr6Ni2Mo Supermartensitic Stainless Steel

“…Due to the lower concentration of austenite-stabilizing elements in the increased fraction of austenite on tempering, fresh martensite forms during cooling for tempering temperatures exceeding 913 K (640°C) leading to a decrease in retained austenite fraction, as discussed in previous work. [24] The samples previously austenitized at 1223 K (950°C) for 0.5 hour show a scatter in the austenite fraction of up to 12 vol pct. On the other hand, an austenitization treatment at 1323 K (1050°C) for 30.5 hours leads to results showing a greater reproducibility and also a clear peak of the austenite fraction on tempering at 913 K (640°C).…”

“…Moreover, the activation energies for both stages are almost twice as high as the activation energy of 233 kJ/mol for austenite formation during tempering at temperatures between 858 K and 968 K (585°C and 695°C) for 4 hours of samples previously austenitized at 1323 K (1050°C) for 30.5 hours. [24] B.…”

“…The fractions of retained austenite at room temperature, measured immediately after cooling from tempering between 888 K and 928 K (615°C to 655°C) for 4 hours, are shown for both previous austenitization treatments in Figure 12. The data as published in [24] of samples previously austenitized at 1323 K (1050°C) for 30.5 hours are shown for comparison. With increasing tempering temperature, the austenite fractions are increasing until a tempering temperature of 913 K (640°C).…”

“…The activation energies for the first stage are around 170 kJ/mol higher than the activation energy of 233 kJ/mol, determined for austenite formation from martensite during isothermal treatment of the 13Cr6Ni2Mo SMSS. [24] A different value of the activation energy implies that a different mechanism is governing the transformation rate. From the tempering experiments presented in our previous work, [24] it is assumed that the growth of austenite in the 13Cr6Ni2Mo SMSS is governed by the diffusion of substitutional austenite-stabilizing elements.…”

“…[24] A different value of the activation energy implies that a different mechanism is governing the transformation rate. From the tempering experiments presented in our previous work, [24] it is assumed that the growth of austenite in the 13Cr6Ni2Mo SMSS is governed by the diffusion of substitutional austenite-stabilizing elements. The influence of nucleation on the overall activation energy is more complex.…”