Microstructure and creep properties of graded transition joints between nickel-based alloys and martensitic heat-resistant steels

Interfacial failure can significantly reduce creep lifetime of dissimilar metal welds (DMWs), bringing great economic losses and safety issues to power plants. The fundamental reason for interfacial failure is steep gradient of chemical compositions between different metals. Many researchers have made great efforts to develop graded transition joints (GTJs) aiming to eliminate the steep gradient, but the results of creep tests were rarely seen. The goal of the present work was to determine whether GTJs could effectively prolong creep life compared with conventional DMWs, and further to clarify the reason. In our study, a GTJ between creep-resistant steels and nickel-based alloys was fabricated by a dual-wire TIG method. It was shown that the gradients of chemical compositions and hardness were significantly reduced over a longer distance of ∼17 mm than that of ∼100 μm in conventional DMWs, and microstructures varied from full martensite to mixed martensite + austenite, and finally to full austenite. The formation of dual-phase region composed of martensite and austenite was due to the segregation behavior of Cr, Ni, and Mo to inter-dendritic regions during solidification. Creep tests showed that creep life of the GTJ was much shorter than conventional DMWs, and two different failure modes were found, one of which failed in COST E HAZ (creep life 790 h), and the other in the nickel-rich martensite zone of transition region (creep life 435 h). The failure in COST E HAZ was the combined results of creep, plastic deformation and oxidation, and the failure in nickel-rich martensite zone was due to localized austenization induced shrinkage during creep tests. Nickel-rich martensite was not observed after post weld heat treatment (PWHT) due to the sluggish kinetics of nickel atoms diffusion, while creep tests provided enough time for nickel atoms to diffuse into austenite, which transformed into martensite with nickel enriched after creep tests. Based on the results, it is concluded that the GTJ might not be a feasible solution to extend creep life of DMWs at least in the current situation, due to the inevitable occurrence of nickel-rich martensite.