Effects of heat treatment processes on microstructure and creep properties of a high nitrogen 15Cr-15Ni austenitic heat resistant stainless steel

Conventional thermo-mechanical treatment (CTMT) and modified thermo-mechanical treatment (MTMT) process were applied for manufacturing a high nitrogen niobium-stabilized 15Cr-15Ni austenitic alloy. CTMT process consists of 5h of solution treatment at 1270°C followed by water quenching and subsequent aging at 820°C for 50h. MTMT process differs from CTMT process in hot plastic deformation performed immediately after the solution treatment at 1270°C and longer aging time. Microstructure and creep properties of the steel obtained by both processing routes were investigated. Creep rupture tests at 750°C showed double increase in rupture time brought about by MTMT process. Examination of crept microstructure by transmission electron microscopy revealed that the improved creep properties in MTMT process were mainly due to improved distribution uniformity of fine nano-sized carbonitride precipitates in the austenitic matrix and that MTMT process has no effects on the number density and distribution of copper precipitates present in the steel. However, the creep ductility in MTMT process drastically reduced comparing to CTMT process. The higher density of grain boundaries due to finer grain recrystallized microstructures and the formation of higher volume fraction of coarser M₂₃C₆ precipitates at the boundaries are believed to be the main reason for the lower creep ductility in MTMT process.