HIGH-TEMPERATURE MECHANICAL BEHAVIOR OF POWDER METALLURGY – HOT ISOSTATIC PRESSED 316H STAINLESS STEEL

Powder metallurgy (PM) – hot isostatic pressing (HIP) is a manufacturing method that uses high temperature and pressure to consolidate metallic powders into near-net shape components. Benefits of the PM-HIP process can allow for reducing post-processing fabrication steps, such as welding/machining; lower production costs; and decreased component procurement lead times. These reasons have caused PM-HIP to be considered as a fabrication method for high-temperature nuclear reactors. However, PM-HIP is not a qualified process within the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section III, Division 5 for high-temperature reactor construction. The purpose of this work was to evaluate the adequacy of 316H stainless steel (SS) fabricated by PM-HIP for high-temperature, nuclear service. The PM-HIP 316H SS compositions investigated were within the American Society for Testing of Materials specification for wrought Type 316H stainless steel. The PM-HIP heats were directly compared to a wrought-product counterpart heat, which is approved for use in ASME BPVC Sec. III, Division 5. The PM-HIP 316H composition was and is of interest because it is qualified for Section III, Division 5 Class A construction for use at elevated temperatures. To evaluate the material properties, low-cycle fatigue and creep-fatigue tests were performed on each heat. These test results were combined with optical metallography and scanning electron microscopy to understand variations in failure mechanisms. Mechanical tests showed that low cycle fatigue performance between the PM-HIP and wrought 316H SS heats were similar. However, the creep-fatigue testing showed that the PM-HIP material failed at less than half the number of cycles to failure compared to the wrought-product form. Based on microstructural analysis, it is thought that the oxide particles inherent with the powder-based process may be the cause of reduced creep-fatigue performance. Because of this and published literature relating oxygen content to reduced Charpy V-notch toughness, PM-HIP material at different overall oxygen concentrations were analyzed and compared. Lowering the total oxygen content to values as low as currently, commercially viable resulted in negligible changes to creep-fatigue performance.