Does Hot Isostatic Pressing (HIP) Change Casting Dimensions? A Detailed Explanation

Does HIP Affect the Dimensions of the Casting?

Hot Isostatic Pressing (HIP) can cause minimal, predictable dimensional changes in a casting, but it is fundamentally a near-net-shape process that does not typically result in significant or uncontrolled distortion. The primary effect is a slight, uniform volumetric shrinkage, which is a direct and intentional consequence of densification.

The Mechanism of Dimensional Change

During the HIP cycle, the combination of high temperature and isostatic gas pressure collapses and eliminates internal porosity. As these voids and microshrinkage cavities are permanently closed, the material consolidates, leading to a slight overall reduction in volume. This shrinkage is generally isotropic (uniform in all directions) due to the nature of the isostatic pressure. For a typical vacuum investment casting, the linear shrinkage from HIP is typically in the range of 0.1% to 0.5%, depending on the initial porosity level and the specific superalloy used.

Comparison to Other Processes

This minimal shrinkage is far less disruptive than the dimensional changes caused by other metalworking processes. For example:

• Forging: Involves massive plastic deformation, drastically changing the shape and dimensions of the initial billet or preform.

• Machining: A subtractive process that intentionally removes significant material to achieve final dimensions.

In contrast, HIP preserves the intricate geometry of the original casting. A complex single crystal turbine blade will retain its airfoil profile and internal cooling passages, simply becoming slightly smaller and fully dense.

Manufacturing Considerations and Best Practices

Because the dimensional change is predictable, it can be proactively compensated for during the design and tooling phase. For high-precision components destined for aerospace and aviation, the initial casting pattern is often intentionally oversized to account for the post-HIP shrinkage. This ensures the final part meets dimensional specifications after densification. Following HIP, components almost always undergo final superalloy CNC machining on critical interfaces to achieve tight tolerances and surface finishes. This machining step removes a minimal amount of stock because the HIP process has already established the near-final geometry.

In summary, while HIP does cause a slight and predictable reduction in size, it is not considered a process that distorts or alters the fundamental shape of a casting. Its ability to densify a component while maintaining geometric integrity is one of its key advantages, making it an essential step for producing high-integrity, leak-free components for industries like power generation and oil and gas.