The Implementation of Hot Isostatically Pressed Powder Type 316L/304L Pressure Boundary Components in a PWR Plant

Abstract

Hot Isostatic Pressing (HIP) has been used since the 1980s to consolidate porosity in cast metal shapes and improve mechanical properties in conventional forgings and wrought components. The availability of high quality metal powders has made it possible to produce Near Net Shape (NNS) items and more complex geometry components that are fully dense and offer an attractive set of properties at reduced cost. Powder HIP manufacturing reduces initial material usage and subsequent machining costs. Metal powder production and HIP processing are automated methods, which also provide protection against forging route obsolescence. Setup costs are lower and batch sizes smaller. HIPped powder microstructures are isotropic and equiaxed, with fine grain sizes not normally achieved in heavy section components, which facilitates ultrasonic NDE examination. Inclusion contents are lower and of more benign geometry, easing fracture and safety case development. Although widely used in the off-shore oil industry in high integrity applications, particularly to reduce welded connections, in the nuclear industry interest has been limited. The quality of HIPped powder items can provide through life cost savings since there is greater assurance of structural integrity compared to welded or wrought components. In an extensive programme of testing, it was established that HIPped powder 316L and 304L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion resistance. HIPped powder items are now in service as pressure retaining components in PWR plant. Effort is now directed at widening the range of components for which the HIP process is appropriate focusing on reducing welds in the plant construction sequence. This is particularly relevant to pipework manufacture and assembly. The benefits of facilitating an ASME Code Case for Powder HIP are also being considered.