Manufacture and stress rupture properties of hipped austenitic-ferritic transition joints

Diffusion welding by hot isostatic pressing (HIP) has been used to produce dissimilar metal weldments between martensitic 9% chromium steel P92 and austenitic steel 1.4910 (Fe17Cr13Ni3MoN). Considering the results of optical metallography of the joints, it was found that the optimum HIP conditions for good bonding were 1200°C at 150MPa pressure for a duration of 1h. Stress rupture tests were carried at 600 and 650°C, which showed that good bonding could be achieved, and that the stress rupture strengths were close to those of the P92 parent material. Some joints were produced using a nickel alloy insert between the two steels. It was found that the nickel alloy insert had a deleterious effect on the rupture strength due to the formation of continuous carbide layers at the interface between the nickel alloy and the martensitic steel. This led to early failure of the joints in stress rupture tests. Further work was carried out to assess the effect of thermal cycling on the integrity of the joints. The HIP diffusion weldments withstood 180 thermal cycles between 650 and 20°C without failure.