Hot isostatic pressing (Hipping) is the consolidation and densification under high temperature and pressure of metal powder, housed within a canister that represents the final geometry desired. HIP components offer the potential for nett-shape (NS) or

near-nett-shape (NNS) component fabrication with high densification of the volume, resulting in a small metallurgical grain size and thus providing superior mechanical properties to cast or even forged components.

HIPped components have been, and still are used in the automotive, aerospace and medical industries, and some experience of HIPped components has been gained in nuclear applications

The HIP process is well-established and consists of five key stages:

1. Procurement of metal powder to desired specification. This is a fundamental quality phase in the overall fabrication route. The powder must be of the right quality, with metal particle size distribution and morphology well defined. Failure to define and attain the correct powder will result in a poor-quality HIPped product.

2. Canister modelling and fabrication, including application of deformation modelling to optimise NNS potential. This is the most labour-intensive phase of the fabrication route. Canisters are typically low-alloy steel in sheet-metal form, that are formed and welded manually. Robustness of the canister and its welds is critical — failure of the canister during the HIP cycle will result in a defective HIP cycle — so each canister undergoes extensive inspection. The adoption of the HIP process over the unit volumes presented by small reactors depends on the effective automation of canister manufacturing.

3. Loading of canister in to the HIP furnace. The canister is loaded with powder, vibrated to maximise powder-fill, evacuated then sealed prior to loading to the HIP furnace. A typical loaded HIP canister is shown in Figure 12.9.

4. HIP cycle stage — typically in excess of 96 MN/m2 (14 000 psi) and 1200 °C Cycle time, pressure and temperature are dependent upon powder material and canister geometry, but typically this phase in the process is a small number of hours.

5. Removal of canister post-HIP cycle, via acid pickle or machining. A typical finished HIP component is shown in Figure 12.10.

HIP furnaces are available worldwide, each with their own ‘working envelope’ limiting the physical size of component which can be HIPped. This has an implication for the nuclear sector where some candidate components such as large valve bodies or pressure vessels exceed even the largest HIP vessel commercially available. The largest HIP vessel in existence today is in Japan and has a working envelope of some 2 m in diameter and 4.2 m in length. Nevertheless design schemes do exist for HIP furnace working diameters in excess of 3.5 m which would suit a number of large nuclear-grade components.

The potential for HIPping of components to save on unit cost and lead-time is great, especially when compared with forged components. The most time-consuming phase of the HIPping cycle is the canister development, but a regular drum-beat of components offers the opportunity to drive-down cost through automation of the canister fabrication process. A further distinct advantage of the HIP process is the inherent repeatability and robustness in the material properties: large forged components are notoriously difficult to produce and will always contain defects and variations in grain structure from component to component. HIPped components will always have the same fine grain structure throughout the bulk of the component and possess isotropic mechanical properties.

HIPped components are yet to be realised in a civil nuclear arena, but experience of HIPped product in an industrial environment is being gained through the oil and gas sector where HIPped large-bore pipework has been introduced. In the marine sector there are further opportunities where HIPping has yet to be realised, and the UK’s naval programme is already using some HIPped components and has gained good credibility in this part of the nuclear sector.

The small reactor programme offers a huge opportunity to introduce new HIPped components to a new reactor design, which could align well with a regulatory code case approval scheme. Retrospective introduction of HIPped components is a further opportunity but this would involve the like-for-like replacement of components and the design would not change. HIPping for small reactors offers the chance to influence the design of the components to suit the HIPping process.