Effects of W on creep behaviors of novel Nb-bearing high nitrogen austenitic heat-resistant cast steels at 1000 °C

Abstract

Recently, there is an urgent demand from automotive industries to develop new austenitic heat-resistant cast steels that withstand exhaust gas temperatures as high as 1000 °C. In this study, the effect of W additions on the creep behavior of a series of Nb-bearing high nitrogen austenitic heat-resistant cast steels was investigated at 1000 °C and 50 MPa. Microstructures before and after creep rupture tests were carefully characterized to illustrate the microstructural evolution during creep deformation. Microstructural analyses revealed that the W addition did not affect the formation of primary Nb(C,N), whereas significantly promoted the precipitation of primary (Cr, W)-rich phases, particularly those cellular structures along grain boundaries. The W solid solution strengthening and the secondary precipitation strengthening of submicron-scale (Cr, W)-rich phase improved the creep resistance of the alloys through increasing the hardness of γ-austenite and pinning the dislocations. However, the primary cellular (Cr, W)-rich structures significantly accelerated the nucleation and propagation of creep cracks along grain boundaries, thereby increasing the creep rate and decreasing the creep life.