The effect of heat treatment on the embrittlement of dissimilar welded joints

In oil and gas transportation systems, austenitic nickel alloy filler metals are deposited on low alloy steel forgings, to form dissimilar welded joints. An example is the joint between forged hubs and linepipes used in subsea operations. A buttering technique is used, allowing the steel forging to be post weld heat treated (PWHT) before an offshore closure weld is carried out. Many years of successful service has been achieved; however high profile failures have occurred within the fusion zone of the ferritic forging and nickel alloy buttering, due to hydrogen ingress. To investigate the embrittlement mechanisms, a programme of microstructural characterisation, environmental performance testing, strain measurements, and fractography was conducted. Two forging materials were compared: i) F22, a 2.25wt% chromium, low alloy steel, and ii) 8630, a low alloy forging steel with a higher carbon content than the F22. The forgings were buttered with Alloy 625; an austenitic weld metal. The effect of PWHT on microstructure evolution and fracture resistance was studied by applying heat treatments to the as-welded specimens. Slow strain rate notched bend tests, within a simulated marine environment, were conducted to rank the resistance to fracture. Strain evolution during PWHT was measured using neutron diffraction. Microstructural examination of the dissimilar welds clarified a tri-modal split in the mechanical properties between as-welded, heat treated, and ‘over-aged’ samples. Modelling of carbon diffusion across the fusion boundary revealed the extent of phase precipitation during PWHT. In the as-welded samples interfacial failure was dominant due to disbonding at the fusion boundary, whilst in ‘over-aged’ samples, failure occurred by linking cracks at M7C3 precipitates. A competitive failure mechanism was proposed: the optimal heat-treatment of dissimilar joints can be predicted by examining the competing mechanisms of heat-affected zone tempering, residual stress reduction, and diffusion-controlled precipitate formation.