Dilution effects on weld metal microstructure and liquation cracking susceptibility of 304B4 SS joined using E309 electrode

AWS E309 is the recommended electrode for joining boron containing 304B4 stainless steel used for fabrication of neutron shields in nuclear industries because electrodes containing boron are neither easily available nor specified in the codes. Further, 309 weld metal contain high volume percent of delta ferrite sufficient to reduce the hot cracking susceptibility of this steel. During fabrication of neutron shields using this steel for intermediate heat exchanger (IHX) of prototype fast breeder reactor (PFBR), extensive hot cracking was observed in these welds. Hence a detailed study on susceptibility of different zones of 304B4 SS (∼1.3 wt.% boron) weld joint was carried out using spot varestraint tests on four sample variations: 304B4 base metal, 304B4 fusion zone, 309 weld metal diluted by 304B4 base metal and undiluted 309 weld metal. From the study, it is found that liquation cracking susceptibility of 309 weld metal diluted by base metal and having 0.2 - 0.48 wt.% boron to be the highest among all samples tested. The next most susceptible region for liquation cracking was 304B4 fusion zone (which is present as unmixed zone in the weld metal close to fusion boundary) and the susceptibility to cracking for other regions was found to be insignificant. The microstructural characterization and microchemical analysis of the representative specimens revealed that base metal dilution has a significant effect on the microstructures formed in the weld metal and its susceptibility to liquation cracking. Semi-quantitative EDS analysis revealed that composition of the eutectic borides in 304B4 fusion zone has been significantly modified on re melting. From the results obtained, it is possible to recommend solutions to overcome the hot cracking issues faced during welding of 304B4 SS using standard welding consumables for stainless steel that does not contain boron.