Effect of precipitates on the hot embrittlement of 11Cr–3Co–3W martensitic heat resistant steel for turbine high temperature stage blades in ultra-supercritical power plants

Abstract

Abstract The hot ductility of as-cast 11Cr–3Co–3W martensitic heat resistant steel was studied by tensile tests in the temperature range from 900\u202f°C to 1200\u202f°C. A serious hot embrittlement was observed at 1150–1200\u202f°C, appearing as a whole intergranular fracture. And the ductility fully recovered with deformation temperature decreasing to 1100\u202f°C and below, appearing as a dimple fracture. The fracture appearance, microstructure and precipitates were analyzed to investigate the hot embrittlement mechanism. The phase transformation of precipitates and their effect on hot embrittlement were mainly discussed. Results show that the hot embrittlement is caused by the precipitation of (Cr, Fe, W)-rich sigma phase, which precipitates and forms flaky structure at grain boundaries of austenite and delta ferrite at 1150\u202f°C and above in the experimental steel. The flaky structure composed by brittle sigma phase seriously destroys the bonding of grain boundaries and causes intergranular cracks. The recovery of ductility with deformation temperature decreasing to 1100\u202f°C is attributed to the phase transformation of precipitates from sigma phase to M6C-type carbides. Flaky-distributed sigma phase is unstable at 1100\u202f°C and below and tends to transform to isolated-distributed M6C-type carbides, which have less harmful effect on ductility.