Strengthening mechanisms and creep rupture behavior of advanced austenitic heat resistant steel SA-213 S31035 for A-USC power plants

Abstract A newly advanced heat-resistant steel SA-213 S31035 (22Cr25Ni3W3CuCoNbN), is currently the strongest commercially available austenitic heat resistant steel and has been recognized as one of the main candidate materials for A-USC plants operating at 700 °C. Up to now, the reason for which has excellent creep performance is not clear yet. In this paper, the S31035 steels were crept at 700 °C with applied stresses in the range of 140–220 MPa up to 15627 h, and during creep the microstructure evolution including the precipitation of second phases as well as their strengthening mechanisms were deeply investigated. Creep tests results show that S31035 steel has much higher creep strength than S30432 steel, with extrapolated creep rupture strength of about 118.0 MPa at 700 °C for 50,000 h. Furthermore, it has great rupture ductility with an area reduction of more than 50%. The excellent creep strength of S31035 steel was produced by the combined precipitation strengthening of nano-scaled Cu-rich phase, needle-shaped Laves phase, and secondary Z phase. The nano-scaled Cu-rich phase was very stable during long-time creep and acted as the dominant precipitation strengthening. Unlike the coarse and granular Laves phase in ferritic P92 steel (9CrW), the spindly needle-shaped Laves phase in austenitic S31035 steel could effectively impede the moving of dislocations and greatly increase the creep strength. And the coarsening of Laves phase in width was slight but in length was apparent, which was conducive to the strengthening effect. The fine secondary Z phase also showed relatively slow coarsening rate, contributed an important part of strengthening enhancement. The precipitation of detrimental σ phase is effectively suppressed through the increased content of Ni and the addition of Co. Besides, the grain-boundary sliding and grain rotation occurred during creep. Under high stresses (≥170 MPa) and short-time creep, intragranular cracks dominated the creep failure. The cracks preferred to produce inside the grains with higher Taylor factor which are hard to deform. Under relatively lower stresses (≤140 MPa), intergranular cracks dominated the creep failure and cracks mainly generate on the grain boundaries between the grains with greatly different deformations. The texture //RD formed during creep, suppressing the proceeding plastic deform and improving the creep resistance. Under lower stress and long-time creep, a new texture //RD occurred, which improves the creep rupture ductility of the S31035 steel.