Powder metallurgy: investigation of metallurgical and technological aspects and potential applications for critical components of turbomachineries

The application of powder metallurgy (PM) technologies to the manufacturing of Oil & Gas turbomachineries’ components was investigated in the course of research collaboration with the Material and Processes Engineering Department of General Electric Oil & Gas (Italy). The thesis focused on the study of the pressure-assisted Hot Isostatic Pressing technology for the processing of the corrosion resistant Ni-base alloy N07626. The densification behaviour of the N07626 metal powder in condition of pressure assisted sintering was investigated by experiments conducted on a small scale by uniaxial hot pressing condition using a Spark Plasma Sintering (SPS) machine in the aim of extending the result to the initial stage of densification of HIP. The SPS exepriments demonstrated that the densification rate is strongly affected by the process temperature and it is less sensitive to the variation of applied pressure. The microstructure and mechanical properties of full-dense HIPped N07626 alloy, produced according to a fixed proprietary cycle and several experimental deviations were analyzed. The microstructure was studied by Optical Metallography, Scanning Electron Microscopy, Energy Dispersed X-Ray Spectroscopy and Electron Backscatter Diffraction. The mechanical properties of the alloy were assessed by tensile testing, conventional and instrumented Charpy V-Notch testing, JIC fracture toughness tests and fatigue crack growth rate testing. The tensile and impact toughness properties resulted sensitive to the local accumulation of oxygen in Oxygen Affected Zones (OAZs), that leads to a ductile to brittle transition in the impact toughness of the material. Two models for formation of OAZs were proposed based on the phase transformation and the oxidation/reduction reactions taking place in the HIP. The mechanical properties were discussed on the base of the microstructure of the Prior Particle Boundaries (PPBs) interface, focusing of the phase transformation products, represented by a thin layer of submicrometric oxides and carbides. The fracture mode was explained by the analogy with models of ductile micro-mechanisms of void nucleation and coalescence and with fracture models of particulate reinforced metal-matrix-composite. The Charpy impact toughness and the fracture toughness were correlated to the oxygen concentration and to the density of inclusions. The fatigue crack propagation behavior was discussed focusing on the effect of clustering of inclusions on the crack propagation path. A relation between the Paris slope with the impact toughness was found. Finally the increase of processing temperature (HIP and heat treatment) was found significanty beneficial for the toughness. This effect was investigate by grain-size analysis and was proposed to be related to a reduction of density of PPBs inclusions.