Microstructural evolution and mechanical property of a Ni-Fe-based weld metal during long-term exposure at 650 °C and 700 °C

A newly developed Ni-Fe-based weld metal has been researched for the microstructural and mechanical evolutions during prolonged thermal exposure using scanning electron microscopy and transmission electron microscopy. The M23C6-carbides precipitated at grain boundaries and around the primary (Nb, Ti)C phases in the Ni-Fe-based weld metal during prolonged exposure at 650 °C and 700 °C. The Cr, Mo-rich sigma(σ) and Cr-rich α-Cr formed as platelets and mainly located in the interdendritic areas during the exposure. σ phase had the orientation relationships of [001]γ//[112̅]σ,(2̅20)γ//(1̅10)σ and (2̅2̅0)γ//(111)σ with the γ matrix. Higher (Al + Ti) content accelerated the precipitation of these Cr-rich phases and resulted in a significant deterioration of tensile ductility after 5000 h at 700 °C. Precipitating and coarsening of spherical γ′ dominated the tensile strength evolution and the optimized γ′ radius with the best tensile strength was estimated to be 21-25 nm. (Al + Ti) content had no obvious effect on the γ′ coarsening rate but it affected the γ′ particle density Ns in two sides: On the one hand, higher (Al + Ti) content increased the Ns, which increased the 0.2% yield strength (Rp0.2)and tensile strength ( Rm). On the other hand, higher (Al + Ti) content increased the amount of σ phases which induced the γ′ envelopes to form and resulted in the decrease of Ns, and it would decreased the Rp0.2 and Rm.