Deterioration of weldability of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W

Summary

The purpose of this study is to clarify the mechanism of weld cracking in the HAZ of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W during repair. The results obtained may be summarised as follows:

1. The results obtained in butt welding cracking tests using as-cast specimens and aged specimens (aged for 247 Msec) suggest that ductility-dip cracking occurs in the HAZ of the aged specimens and that this cracking mainly propagates through the microconstituents precipitated at the dendrite grain boundaries.

2. The results obtained in the elevated temperature high-speed tensile tests of aged specimens and as-cast specimens in the temperature range 673–1473 K suggest that the aged specimens sustain an especially heavy loss of ductility in the temperature range below 773 K.

3. The results obtained in the elevated temperature high-speed tensile tests and small-sized U-groove restraint cracking tests at 773 K using aged specimens and specimens aged at 1323 K suggest that the hot ductility of the base metal strongly affects the weld cracking susceptibility.

4. The results obtained in the investigations of identification of the microconstituents in specimens aged at 1323 K and the changes in their deformation suggest that, among the eutectic NbC microconstituent and massive M₇C₃ microconstituent found in the as-cast specimens, the M₇C₃ changes into M₂₃C₆ and the eutectic NbC changes into theη phase (Nb₃Ni₂Si) as a new microconstituent under long-term ageing. These microconstituents are simultaneously formed in a network-like pattern at the dendrite grain boundaries.

5. The weld cracking affecting the aged specimens is considered to be due to the fact that the deformation of the dendrite grains is restrained by the microconstituents covering the dendrite grain boundaries in a network-like pattern, that the dendrite matrix has a reduced deformability, and notably that cracking is initiated from the more cracking-prone η phase to propagate through the M₂₃C₆.