Electron Microscope Investigations on Welding-simulation Heat-treated Specimens of the Pressure Vessel Steel 22 NiMoCr 3 7 with respect to Stress-relief Cracking

Abstract

Microstructural investigations of the coarse grain structure of weld simulation specimens as well as of a real weldment were performed on the pressure vessel steel 22 NiMoCr 37 with respect to stress-relief embrittlement and/or cracking. The precipitates in the grains as well as at the grain boundaries were studied by transmission electron microscopy. Segregation of companion elements and trace elements at the prior austenite grain boundaries was investigated by Auger-Spectroscopy. The weld simulation specimens which were overheated and subsequently loaded in a short-time creep test reveal low creep rupture elongation values in the temperature range of stress-relief heat treatments associated with a precipitation stage of small, needle-like Mo2C-precipitates. The prior austenite grain boundaries are decorated with globular Fe3C-precipitates and display a segregation of the elements S, N, Sn, Sb and Mn. The microstructure of the real weldment corresponds to that of the weld simulation specimens. It is concluded that the creep ductility of the grain interior is reduced by precipitation hardening of Mo2C at stress-relief temperature. Thus creep is concentrated at the prior austenite grain boundaries, where stress-relief cracking is promoted by grain boundary carbides and the segregation of companion elements as well as trace elements.