Graded grains make finer parts - hot isostatic pressing
The materials and manufacturing benefits of hot isostatic pressing of hardfacing and stainless steel powders for pressurised water reactor components is described by Barry Burdett, a materials specialist at Rolls-Royce Plc Raynesway in Derby, UK.
Hot isostatic pressing (HIP) has been used for many years to consolidate porosity in cast metal shapes and improve mechanical properties. When applied to fine metal powders, it is possible to produce near net-shape items and more complex geometric components that are fully dense and offer an attractive set of properties at a reduced cost. Hot isostatic pressed (HIPped) powder microstructures are isotropic and equiaxed, with uniformly fine grain sizes not normally achieved in heavy section components, which facilitate ultrasonic examination. Improved features to accommodate non-destructive evaluation 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, for example, in the offshore oil industry where it is well established in high integrity applications, particularly in place of welded joints. However, 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 article presents a materials perspective in Rolls-Royce on the use of HIPped metal powders since the 1990s in pressurised water reactors (PWR), 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 tonnes in weight, have equivalent or slightly better strength, toughness and corrosion resistance compared to their forged or cast counterparts. This data is now part of a submission to American standards organisation ASME for inclusion in the Pressure Vessel Code.
High quality components
Cost and flexibility in production are key factors in the energy supply industry, where components are now sourced from an international market. This is particularly evident in the areas of welding and forging. Weld hardfacing, in particular, has always been a skilled process and inherently more prone to manufacturing defects. Oxy-acetylene overlays have been widely used in nuclear valves, but the skill base and training opportunities for new technicians have diminished. Powder consumable techniques such as plasma transferred arc (PTA) hardfacing have improved quality and reproducibility, but these are complex pieces of kit that are not often used, making them expensive to qualify and maintain.
Pressure vessel procurement requires small numbers of high quality components – this area has also seen a marked reduction in capability, with a much smaller forging supply base and loss of experienced companies. Hot isostatic pressing powder processing offers an alternative manufacturing route with lower set-up costs that do not require complex or expensive tooling. The starting material for the powder HIP process is normally a high quality gasatomised metal powder. Other powder manufacturing technologies are available, such as water atomisation and mechanical milling, however, particles produced by gas atomisation are uniform and round with low levels of oxygen (< 200ppm) and other impurities. Because the particles are formed very rapidly during gas atomisation, there is little segregation and these powders have predictable HIP behaviours with regards to flow and shrinkage.
A good range of powder suppliers exists to meet the quality requirements for the HIP process. Each powder particle has a similar composition to the next one, and the component retains a fine grain size during the HIP compaction process, even in thick section items. Final component shape is achieved by making a thin steel can of the required dimensions to contain the powder. An allowance of ~30% must be made to accommodate the volume shrinkage that occurs during the process. Complex structures can be made this way, incorporating features not possible using conventional casting or forging methods. Welds can be removed from the design, simplifying construction and subsequent nondestructive evaluation (NDE) requirements – a real advantage, as welds are a major source of residual stress, driving fatigue and stress corrosion mechanisms.
Austenitic and duplex stainless steels are the dominant materials in this industry. Hot isostatic pressing consolidation vessels have been previously limited to one metre diameter by three metres high, but new European capacity at one and a half metres diameter is available, and a new facility in Japan measures two metres in width. This greatly increases the range of shapes that can be processed, and is beginning to change the thinking on the potential range of components that could be manufactured in this way.
Hardfacing manufacturing development
Stellite materials are preferred for wear protection in light water nuclear environments, as they have good wear and corrosion properties. The oxy-acetylene welding process degrades the corrosion resistance of the cobalt alloy deposit through carbon pickup. While there is every confidence that these components were perfectly sound when they were manufactured and passed dye-penetrant inspection, the gross porosity evident in the valve seat profile (image, above right) is believed to be the result of selective attack of the hardfacing matrix over many years in the field, adding cobalt to the circuit inventory. This is undesirable as cobalt activates in a neutron flux, creating a radiation hazard to plant operators and maintainers.
The move to powder processing was originally made with the deployment of the plasma transfer arc hardfacing process, using a powder consumable. While this prevented carbon pickup, there were limitations on the type of component geometry that could be tackled. The powder size distribution used for PTA (45–150mm) was modified to improve HIP consolidation. The powder process produces significantly less carburisation of the heat-affected zone and substrate. Since the introduction of the HIP powder consolidation and direct HIP bonding of hard seats into components, there have been no reported incidences of valve leakage or premature failure due to degraded Stellite microstructures. There is now a drive to move to solid-state joining for the majority of hardfacing applications.
The benefit of the HIP powder process on stainless steel microstructures is that the HIPped material has a finer grain size and fewer inclusions, which are round rather than linear. A range of component and pipework geometries in Type 316L and Type 304L has been assessed. A full destructive examination of a recent valve body preform has confirmed the properties determined on an initial prolongation and again shows excellent mechanical properties compared to the design requirements (see table above) and forging equivalents. Average grain size in the component was ASTM No. 6. The next application will be a component of about seven tonnes in weight. Demonstrating satisfactory performance in HIPped items by assessing prolongations will be incorporated in future component applications. The results from destructive assessment of HIPped assemblies have shown the uniformity of properties across different parts and thicknesses of the component. This provides even better confidence than a similar approach on forged components, which are known to exhibit material property variations through the section thickness.
HIP powder processing, initially of hardfacing alloy powders and more recently, austenitic stainless steel materials, has been extensively evaluated through laboratory and prototype component testing. The overwhelming body of mechanical and corrosion test data indicates that HIP powder processing of these materials can be used to produce high-integrity components with additional benefits in manufacture, reduced set-up and minimum order costs. An ASME Code Case for powder HIP is currently being progressed. There is now sufficient manufacturing experience and test data to support this.