Microstructural mapping in closed die forging process of superalloy Nimonic 80a valve head

Abstract

Optical microscopy, XRD, TEM and FEM were used to study the microstructral map in the alloy in the upsetting and closed die forging processes. The maps of hardness, grain size, dislocation and the secondary phase γ′ in the upset and closed die forged nimonic 80a alloy valve head were investigated. It is found that the metals flowed from the asymmetric central axis outwards in the upsetting and forging process. The flowing model of metals together with the thermal field led to a heterogeneous distribution of grain size, dislocation configuration and the secondary phase γ′. It is also necessary to mention that the largest grain size, the lowest density of dislocation and secondary γ′ phase only occurred in the transitional region between the valve head and valve stem, which may weaken the strength in this region.