Mechanistic study of hot cracking and development of new resistant Ni-based high-Cr alloy weld metal

Abstract
Alloys 52/152 are commonly used as weld metals of 30% Cr-Ni based alloy in the world PWRs. However, these alloys show high susceptibility to ductility-dip cracking (DDC) as one kind of hot cracking. In order to improve the DDC susceptibility of Alloys 52/152, the effect of the chemical composition on DDC susceptibility of 30% Cr-Ni based alloys was studied on 18 laboratory melted heats by many kinds of hot cracking testing, comparing with a commercial heat of Alloy 52. Hot cracking susceptibility based on the Longi-Varestraint test and On-cooling hot ductility test seems to be appropriate for the evaluation of solidification and liquation cracking susceptibility, but not so suitable for the evaluation of DDC. In case of strain-to-fracture (STF) test, DDC occurred in the temperature range of 900 to 1,000oC. However, this temperature range is different from the ductility-dip cracking temperature range obtained by high temperature tensile testing.
The authors studied a novel thermal cycling test under tensile loading of the round-notched bar type specimen, using Gleeble test machine. In this kind of testing the mechanism of the DDC is thought to be different from the earlier observations and hypotheses. The DDC initiation and fracture temperatures were measured in the temperature range of 1000 to 1150oC for the 30% Cr-Ni based alloys. The DDC susceptibility was markedly improved by the addition of C and Nb contents.