Hot Isostatic Pressing (HIP) has been used for many years to consolidate porosity in cast metal shapes to improve mechanical properties. When applied to fine metal powders, it is 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.
NNS items from powder deliver cost savings by reducing initial material usage and subsequent machining costs. Powder production and HIP processing are automated methods, which provide protection against forging route obsolescence. Setup costs are lower and batch sizes smaller, particularly well suited to small numbers of high integrity components.
HIPped powder microstructures are isotropic and equiaxed, with uniformly fine grain sizes not normally achieved in heavy section components, which facilitates ultrasonic NDE examination. Improved features to facilitate NDE are readily incorporated into the HIP assembly. Inclusion contents are lower and of more benign geometry, easing fracture assessment.
Use of the technology has grown, particularly in the off-shore oil industry where it is already established in high integrity applications, particularly in place of welded joints. Take-up in the more conservative nuclear industry has been slow. 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.
This paper presents a materials perspective on the way in which Rolls-Royce has gained experience with HIPped powders since the 1990s, initially with hardfacing materials to minimise welding defects and provide a robust manufacturing route. Building on this database, we have now established that HIPped powder 316L/304L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion resistance to their forged or cast counterparts. This data is now part of a submission to ASME for inclusion in the Pressure Vessel Code.
Rolls-Royce now has a robust methodology in place to develop safety cases and is extending the number of applications in pressure boundary components on current and future classes of equipment. A strength-in-depth argument has been endorsed by external approval organisations and is supporting current submarine build programmes. Other applications in the growing civil nuclear market are now under consideration.