Effect of PWHT on the dissolution of δ-ferrite in the welded joint of 12Cr–1Mo steels for steam turbines

Abstract

In this research work, the optimal parameters of post welding heat treatment (PWHT) for 12Cr–1Mo Steel in the welding condition are determined. The PWHT aims to decrease δ-ferrite percentage in the interface of the heat-affected zone (HAZ) and partially melted zone (PMZ). The parameters of post welding heat treatment (time, temperature, and cooling media) are determined through statistical analysis of a full factorial design. These considered δ-ferrite percentage in the HAZ-PMZ interface as the response variable, as well as microhardness in the critical welding regions. Subsequently, a predictive equation of δ-ferrite percentage in the HAZ-PMZ interface was obtained after the post welding heat treatment using non-linear regression coefficients given by full factorial design. The interaction between time and cooling media was essential to improve the PWHT. Also, a high temperature in the PWHT produced a total decrease of δ-ferrite. These mechanisms were associated with the dissolution of forming elements of the primary δ -ferrite, such as Cr and Mo. The PWHT optimal parameters to eliminate δ-ferrite were the temperature of 860 °C with the following combinations of treatment time and cooling media: 1 h-air and 2 h-oil.