Effect of heating rate on martensite to austenite transformation kinetics in supermartensitic stainless steel weld deposit

Abstract

The supermartensitic stainless steels (SMSS) have a better combination of weldability, toughness and corrosion resistance than conventional martensitic and duplex stainless steels. Nevertheless, for both base material and weld deposit, heat treatments (inter and sub critical tempering) are required to control the resultant microstructure. A tempered martensite microstructure with a high content of reformed austenite, obtained after a post-weld heat treatment, improves the properties of SMSS weld deposits. The comprehension of austenite formation during post-weld heat treatment in weld metal deposits is a key aspect to predict the final properties. In the present work, the transformation kinetics of martensite into austenite in SMSS all-weld metal deposit was studied by dilatometry, considering the effect of the heating rate (1, 10, and 100 °C/s). In addition, the austenitic transformation kinetic was modeled by a sigmoidal equation. The increase of the heating rate increases Ac1 and Ac3, and reduces the range of the transformation (Ac3 – Ac1) from 120 to 80 ºC. The maximum transformation rate reached values of 0.023, 0.018, and 0.019 1/°C for 1, 10, and 100 °C/s, respectively. Thus, the maximum transformation rate was reached for the slowest heating rate. Nevertheless, the average transformation rate increased as a function of the heating rate. This could be associated with a change in the transformation mechanism.