Effect of post-weld heat treatment on fusion boundary microstructure in dissimilar metal welds for subsea service

Microstructural features at the fusion boundary in dissimilar metal welds significantly influence the susceptibility to local hydrogen embrittlement under subsea service conditions. Two subsea dissimilar metal welds using nickel-based filler metal on low alloy steel substrate were studied in this work in order to investigate the nature and microstructural evolution of the dissimilar interface during post-weld heat treatment (PWHT), and to determine resistance to hydrogen embrittlement. The delayed hydrogen cracking test (DHCT) was utilized to determine hydrogen-assisted cracking (HAC) susceptibility. Thermokinetic modeling was conducted to study phase formation and carbon diffusion across the dissimilar interface. Diffusion calculations incorporated initial compositional gradients after welding and the nonmoving phase boundary during PWHT. Resistance to HAC was in good agreement with previously obtained DHCT results for the tested dissimilar metal welds. Tempered martensite in the heat-affected zone of the steel substrate was observed as a function of PWHT temperature. Fresh martensite with high local hardness formed during cooling in highly diluted weld metal regions. The cell model incorporated in DICTRA effectively predicted differences in carbon concentration profiles across the dissimilar interface and type of carbide precipitation as a function of PWHT procedure and steel substrate.