Effect of Precipitates and Cold Working on the Reheat Cracking of STS 347 Austenitic Stainless Steel

Abstract

The aim of this study was to investigate the effect of precipitates and cold working on the reheat cracking of STS 347 austenitic stainless steel. Three different heat treatments to form NbC and M23C6 precipitates were applied to the STS 347 austenitic stainless steel and 10% pre-strain was additionally introduced to simulate cold working. The NbC and M23C6 precipitates were examined by SEM and XRD analysis, and creep rupture test was conducted under various temperature and stress conditions. The specimen having intergranular NbC precipitate prior to cold working had enhanced intergranular creep resistance and the tendency to reheat cracking decreased at temperature conditions between 650 oC and 750 oC, compared to the specimens processed by solution treatment or M23C6 formation treatment. This means that for STS 347 that has not been subjected to stabilization treatment, the cold working strain limits required by ASME’s boiler and pressure vessel code should be lowered, or if stress relief treatment is impractical, the stabilization heat treatment should be performed prior to cold working. Although the precipitation of M23C6 is not the major cause of reheat cracking, it should still be considered as a factor that increase the sensitivity of reheat cracking because M23C6 forms mostly at the grain boundary and grows rapidly, adversely affects creep ductility.