Microstructural changes in weld metal during isothermal process

摘要

This paper describes microstructural observations of weld metals sampled from actual equipment and weld metals taken from samples with different Bi contents to investigate changes in weld metal microstructures and to determine whether or not the addition of Bi exerts any effect on the production of precipitates as an embrittlement factor when stainless steel weld metals operate in high-temperature environments. The results obtained may be summarized as follows.1 Most of the ferrite phase of weld metal sampled from actual equipment, through being exposed during operation to high temperatures near 873K in the long term, is transformed into the #sigma# phase, carbides (M_(23)C_6, M_7C_3) and austenite.2 Among the #sigma# phase and carbides precipitated in the ferrite contained in weld metal sampled from actual equipment, the #sigma# phase has a block-like morphology, being precipitated at the grain boundaries and trans-granularly inside the ferrite grains. The carbides (M_(23)C_6, M_7C_3) are basically continuously precipitated at the #sigma#/#gamma# grain boundaries.3 The results obtained in the investigation of #sigma# phase precipitation behaviour when Bi-free and Bi-containing weld metals were aged at 873-1073K suggest that Bi addition has little effect on #sigma# phase precipitation.4 The reheat cracking susceptibility of the Bi-containing weld metal shows a more pronounced change than that of the Bi-free weld metal. #sigma# phase precipitation is therefore not considered to be an important factor controlling the reheat cracking susceptibility.5 With an increasing Bi content, the oxygen content of the weld metal increases and the contents of inclusions accordingly increase, and the thus induced increase in transgranular hardness is assumed to be a factor heightening the reheat cracking susceptibility.