Abstract
Hot Isostatic Pressing (HIPing) has been used by RollsRoyce to successfully manufacture nuclear pressure boundary components such as valves, piping, and pump casings. The majority of these components have been manufactured from stainless steels, typically 316L. There is also considered to be potential benefit from the HIP manufacture of other plant materials, including the large pressure vessel Low Alloy Steel (LAS) materials such as ASME SA-508. The advantages would include cost and lead time reductions, material quality, uniformity and inspectability. Applying the HIP process to LAS presents particular challenges compared to stainless steels. This is due to the propensity of LAS for oxygen pick-up, either in the powder manufacturing stage or subsequent can filling operations. The potential for oxide formation on the powder particles presents the risk of the material properties being adversely affected, particularly the fracture toughness which is extremely important in relation to the structural integrity of nuclear pressure vessels. This paper presents work conducted to assess the feasibility of achieving the required LAS material properties by HIPing to support nuclear pressure vessel manufacture. It presents tensile and Charpy impact toughness results, and makes comparisons with forged equivalent material. The LAS investigated was an ASME SA-508 Grade 4N model alloy. The paper shows that in all cases the HIPed material achieved higher tensile property values than the forged equivalent material. Although meeting the Rolls-Royce Charpy impact toughness target requirements, the HIP LAS Charpy Impact toughness values were lower than for forged equivalent material. From microstructural analyses conducted, it is considered that oxide decoration at the powder Prior Particle Boundaries (PPBs) is the main reason for the toughness difference to forged equivalent material. It is therefore proposed that in order to increase the Charpy impact toughness values to a level consistent with forged material, the oxygen level of the HIPed material will need to be reduced from the level achieved in this work, i.e. by either reducing the initial powder oxygen level or reducing further oxygen pick-up in the can filling operations, or both.
