Mechanism of hot cracking, especially ductility-dip cracking in HAZ during repair welding was investigated using modified HP-type heat-resistant cast alloys. The change in hot ductility was evaluated with the reduction of area at 773 K by the Gleeble test. The hot ductility of each alloy was almost at the level of 20 percent in as-cast situation, while decreased with aging and fell down below about 8 percent after service exposure or long term aging at elevated temperature. Microscopic observation revealed that the brittle fracture occurred in service exposed alloys, and that cracks initiated and propagated preferentially through microconstituents. Hot ductility was decreased remarkably with increasing the amount of microconstituents on dendritic boundary, and with proceeding the phase transformation of microconstituents as NbC->#eta# phase or M_(23)C_6-> G phase. It was deduced that the hot ductility would be deteriorated by the stress concentration in microconstituents and/or the plastic constraint on dendritic grain due to the formation of continuous network of microconstituents surrounding the dendritic boundary. It follows that ductility-dip cracking susceptibility during repair welding was enhanced by the reduction of hot ductility.
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