Relationship between grain boundary structure and intergranular corrosion in heat-affected zone of type 304 stainless steel weldments

Grain boundary carbide precipitation and intergranular corrosion in the weld heat-affected zone of type 304 austenitic stainless steels were investigated using optical, scanning and transmission electron microscopy to discuss effects of grain boundary atomic structure on precipitation and corrosion from a cyrstallographic viewpoint. In the grain boundary precipitation region, the frequency of grain boundary carbide precipitation had a maximum near the coarse-grained region and decreased gradually with the distance from there to the unaffected base metal region. The frequency increased with weld heat-input. The Strauss test of 304 steel weld HAZ showed that all the cracked grain boundaries after the bend test had carbide precipitates and that the frequency of the cracked boundaries was roughly proportional to the frequency of grain boundary precipitation. Electron channelling pattern analyses in SEM showed that grain boundaries with ordered atomic structures were hardly etched after the 10% oxalic acid etching test. Kikuchi line analyses in TEM showed that grain boundaries with ordered atomic structures had no carbide precipitates. These facts have suggested that grain boundary carbide precipitation and corrosion in stainless steel weld HAZ depend sensitively on the crystallographic character of the grain boundaries.