Mechanical Properties and Metallurgical Examination Results for a Batch of Powder Metallurgy - Hot Isostatically Pressed Low Alloy Steel Grade 508 4N

This paper presents the work conducted by Rolls-Royce to investigate the mechanical and metallurgical properties of a Low Alloy Steel (LAS) grade 508 alloy batch of material. The material was manufactured using the Powder Metallurgy – Hot Isostatic Pressing (PM-HIP) method. The LAS was an ASME 508 4N model alloy. Rolls-Royce has used PM-HIP extensively for the manufacture of nuclear grade components such as valves, piping and pump bowls, and is now investigating its use for the manufacture of LAS pressure vessels in order to provide an alternative sourcing route to forgings to reduce costs and manufacturing lead-times. A key part of this work is to assess whether mechanical properties can be achieved that meet the specification requirements and that are also comparable to the forged equivalent. In this regard, the toughness of the material is of particular interest, with it being extremely important for LAS pressure vessel applications. This paper reports, that for this particular batch of material, the Charpy toughness was significantly below the minimum room temperature specification requirement, but that the tensile properties were well above the minimum specification requirements for proof and ultimate tensile strength. The Charpy toughness at room temperature was only at 38% of the specification requirement, and only at 21% of forged equivalent material. The reason for the poor Charpy results is potentially attributed to prior austenite retention given the highly faceted nature of the fracture faces and the size of the facet faces. Oxides and other precipitates, such as nitrides, are not believed to have significantly contributed to the low Charpy values in this powder batch. This is because inclusion assessments against other powder HIP material, which had exhibited higher Charpy values but produced with higher oxygen content, did not show a significantly different variation in non-metallic inclusion count, and there was no clear evidence of prior particle boundaries being evident in the cross-sectional studies of the microstructure. Hence, it is hypothesised that increased prior austenite formation was due to the increased nitrogen levels in the gas atomised powder.