Anomalous creep behaviour of 316 stainless steel at 550°C

The results offifteen constant-load creep tests at 550°C, with nominal stresses in the range 200 to 360 MPa and with test durations ofup to 14 000 h, are presented. The usual primary, secondary and tertiary creep behaviour was exhibited for nominal stresses greater than about 330 MPa. At lower stresses, ‘renewed’ primary and secondary creep regions were observed. The renewed secondary creep strain rates were found to be about an order of magnitude greater than the initial secondary creep strain rates. The results indicate that the occurrence ofthe renewed primary and secondary creep regions is associated with time-dependent exposure to a temperature of 550°C. The presence or magnitude of the prior stress level does not appear to have any significant effect.

The results are relevant to design procedures because extrapolation of short duration or high stress data to long-term design lifetimes is often required. Unless the possibility ofthe occurrence ofrenewed primary and secondary creep is taken into account, gross errors in strain predictions could occur.