Hot Isostatic Pressing (HIP) of type 316L austenitic stainless steel powder has been an established manufacturing practice for more than twenty five years within the oil and gas sectors and more recently in the naval defence industry. The successful ASME Code Case approval (N-834) has facilitated the manufacture of 316L components via Powder Metallurgy HIP (PM/HIP) for the civil nuclear sector. However, a number of issues have tended to hinder the uptake of PM/HIP as an alternative viable manufacturing route for both castings and forgings.
Firstly, the powder specification for 316L and HIP processing parameters has typically been left to the discretion of the manufacturers. As such, the finer details of HIP product specification require greater clarity and definition for optimum performance/reproducibility. Secondly, comparison of historical data for 316L PM/HIP has shown variation in the Charpy impact toughness performance. These differences have been attributed to the oxygen content of the atomised powder, with greater oxygen contents yielding product with reduced impact properties.
Based on these factors, a systematic study of the current state of the art of 316L commercial powder production, encapsulation/consolidation and selected HIP parameters was undertaken in collaboration with the Electric Power Research Institute (EPRI). A 316L powder specification was developed that primarily limited the oxygen content of the powder to under 130ppm. This lower oxygen limit reflects the improvements that commercial powder suppliers have been making over the past decade to ensure greater powder cleanliness.
The test programme generated a significant body of test data based on 3 × 3 × 3 matrix of: powder supply, HIP service provider and HIP sustain times. The results were excellent across the full range of variables studied with all test billets passing the specification requirements of ASTM A988 and additional imposed requirements.
Very consistent 316L material properties were produced for billets manufactured via differing HIP service providers across the comprehensive destructive test programme. This demonstrates the robustness and uniformity of the PM/HIP supply chain in producing 316L material of the requisite quality. In addition, no significant difference in material properties was noted for material pressed between 2–8 hours hold time, suggesting that the HIP process window is large with respect to hold time.
Of significant note was that material produced with one powder yielded material with consistently the highest strengths and Charpy impact toughness. This has been attributed to chemical composition of the powder, which featured both a low oxygen and also a high nitrogen content.