Characteristics of negative creep aging and its microstructure-oriented tensile behavior

Abstract

UNS S17400 steel was creep-aged under a constant stress of 300 MPa at 480 degrees C and 600 degrees C for 1, 3, and 5 h, and its microstructural phase evolution was investigated. During the initial creep aging at 480 degrees C for 15 min, negative creep strain occurred because of the stress-assisted precipitation, which resulted in Cu-rich precipitates impeding the movement of dislocations. By contrast, the sample creep-aged at 600 degrees C had a typical creep-strain curve comprising a primary creep stage and secondary steady-state stage. The creep stress accelerated the growth of Cu-rich precipitates and austenite gamma phase reversion, which were clearly observed in samples creep-aged at 600 degrees C. The reverted gamma phase formed due to the stress-assisted aging at 480 degrees C for 3 h, as well as at 600 degrees C due to the elevated temperature aging. The samples with the longer creep aging times decreased in tensile yield strength but increased in strain hardening rate. Strain hardening after strain softening was observed in the sample creepaged at 600 degrees C for 5 h, which had the highest strain hardening rate. Strain hardening was attributed to the dislocations becoming entangled with nanoscale Cu-rich precipitates, the formation of microtwins, and the presence of deformation-induced martensite.