Effects of vacuum annealing powder de-oxygen process on nickel-based alloy and transition joint made by hot isostatic pressing

Powder quality, especially oxygen level in powder, directly affects the metallic products fabricated by powder metallurgy hot isostatic pressing (PM-HIP). For Ni based alloys, a slightly elevated oxygen level can significantly reduce the strength and toughness. We investigated vacuum annealing powder deoxygen process and its effects on Ni Alloy 600M used as PM-HIP material and HIP transition joint material. The initial oxygen level is 168ppm. For the purpose of transition joint, two joint configurations, including SA508 low alloy steel to 316L stainless steel and Grade 91 ferritic martensitic steel to 316H stainless steel. Intergranular carbide formation due to inter-diffusion between high-carbon low alloy steel to low-carbon austenitic alloys leads to toughness reduction. Prior particle boundary (PPB) oxides, at very low level, also affects the HIP consolidation of Ni alloy transition. The study confirmed that the feasibility of using vacuum annealing to reduce oxygen level in powder and improve the quality of PM-HIP Ni-based alloy. However, more complex changes in mechanical properties were observed in the transition joint interfaces. The mechanistic causes are discussed.