Microstructure and high temperature tensile properties of narrow gap braze joint between X-40 and IN738

Abstract

In the braze repair of gas turbine components, substituting the parent metal with dissimilar metals can improve the mechanical properties of the refurbished parts. In this study, the microstructure and high temperature tensile properties of brazed joints made of X-40 (the parent metal) and IN738 (the substitute metal), were evaluated in the narrow gap as brazed and heat treated conditions. The resulting joint contained primary γ-Ni phase, discrete refractory element rich carbides/carboborides and eutectic phases. The carbides and eutectic phases exhibited Vickers hardness values several times higher than those of IN738 and γ-Ni phase particles. The high temperature yield strength of the as brazed joints was also greater than that of the X-40 base metal, but the ductility was significantly lower. The fracture surfaces revealed that cracking occurred in both intergranular (among the γ-Ni grains) and transgranular modes. Crack initiation was found to be associated with discrete carbides/carboborides and eutectic phases. It was observed that post-braze heat treatment at 950°C for 120 h improved the high temperature tensile ductility due to a reduction of carbides and eutectic phases in the brazed joint. Increasing the heat treatment time reduced the size and the amount of carbides and eutectic phases and contributed to an increase in transgranular fracture. However, extending the heat treatment time up to 840 h reduced the ductility due to oxidation damage in the braze joint.