Cold Work Effect on Creep Rupture Strength of Austenitic Boiler Steels

In order to clarify the effect of cold work, warm work at working temperatures of up to 400°C and chemical compositions on the creep rupture strength of austenitic steels used for boiler tubing and high temperature support structures, long-term creep rupture tests were carried out on typical 18Cr-8Ni system steels consisting of TP304H, TP316H, TP321H and TP347H grade tubes and of TP321 plates. The long-term (100,000 hours) creep rupture strength of these steels was evaluated in terms of working ratio and Ni-equivalent. It was consequently clarified that creep rupture strength was substantially reduced in the cold-worked TP321 and TP321H materials, although warm-work resulted in less work-induced deterioration. It was also found creep rupture strength was enhanced by the higher Ni-eq in 18Cr-8Ni austenitic steels, and that the combined conditions of working ratio and Ni-eq govern the creep rupture strength criteria of weaker or stronger than as-received strength. Additionally the effect of cold work on the creep rupture strength and ductility of recently developed creep-strength enhanced 23Cr austenitic stainless steel (a candidate material for the hot end of superheaters in ultra-high temperature fossil-fired power plants) was considered. The strength of cold worked 23Cr austenitic steel was observed to fall below the as-received strength at stresses within about 120MPa, while re-solution annealing recovered the creep strength level to the as-received strength across the entire stress region.