Strain Induced Hardening of Advanced Austenitic Stainless Steels Evaluation of Creep Properties

Cold working and bending of tubes are fabrication processes in the manufacture of boilers for power generation. However, the associated strain induced hardening of austenitic stainless steel can have an adverse impact on creep ductility, potentially resulting in failure after short times in operation. This issue is recognized in boiler and pressure vessel design codes such as ASME I, PG 19, which contain guidelines for maximum levels of strain, depending on material and component type, above which solution treating is required. However, there is some industry concern that this limit is high and boiler manufacturers may impose lower limits before solution treatment is required. The creep ductility of four austenitic stainless steels with prior strain levels of 12% and 15% was reviewed. For both strain levels, the materials were ranked as TP310HCbN, XA704, TX304HB and Sanicro 25 in terms of increasing ductility. Solution annealing recovered the creep ductility in all the materials to elongation levels exceeding 10% and with the exception of Sanicro 25, may be required for strains to 12% and 15%, to ensure the materials had sufficient creep ductility. It is suggested that the guidelines for austenitic stainless steels containing Cb, V and N in ASME I PG 19 be reviewed as reduced strain limits would help to reduce the incidence of strain induced precipitation hardening (SIPH) failures.