Influence of stress relief annealing on the microstructure and properties of GX12CrMoVNbN9-1(GP91) cast steel

The paper presents an effect of stress relief annealing, applied to casts after the repair by welding, on the microstructure and mechanical properties of quenched and tempered martensitic GX12CrMoVNbN9 – 1 cast steel (called GP91). The test pieces being the subject of research were taken out from a test coupon. Heat treatment of GP91 cast steel was carried out at the parameters of temperature and time appropriate for the treatment of multi-ton steel casts, while stress relief annealing was performed at the temperatures of 730 and 750ºC. After quenching and tempering GP91 cast steel was characterized by the microstructure of high-tempered martensite with numerous precipitations of carbides of diverse size. Mechanical properties of the investigated cast steel after heat treatment fulfilled the standard requirements. Stress relief annealing contributes to the processes of recovery and recrystallization of the matrix as well as the privileged precipitation of M₂₃C₆ carbides on grain boundaries. Changes in the microstructure of the examined cast steel cause deterioration in mechanical properties – the higher the temperature of stress relief annealing, the greater the deterioration.