Abstract
The purpose of this study was to make clear the mechanism of weld cracking in HAZ of long term used HP heat-resistant cast steel containing Nb, Mo and W when this steel was repaired.According to observation of fracture surface of weld cracking, a crack was regarded as a ductility-dip cracking. Moreover a crack propagated in microconstituents on dendrite boundaries.The amount, the structure and the precipitating site of microconstituents changed during aging at the temperature of 1323 K, which was the average temperature of materials during operating. On the other hand, the hot ductility at 773 K of these aged materials was evaluated using the Gleeble tester. It declined with the lapse of aging time. Comparing the change of microstructure with the change of hot ductility at 773 K, the decline of hot ductility was related to η phase (Nb3Ni2Si) and M23C6 precipitating like a network on dendrite boundaries for long aging. As a result, the decline of hot ductility of the base metal promoted weld crack in HAZ.
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