HIPping (Hot Isostatic Pressing)

Hot isostatic pressing — commonly referred to as HIP or HIPping — is a post-casting densification process that eliminates internal porosity and shrinkage defects in metal castings by simultaneously applying high temperature and high isostatic pressure in an inert gas atmosphere. For investment castings used in aerospace, defense, and other demanding applications, HIP is one of the most powerful tools available for improving casting integrity, mechanical properties, and fatigue performance 

Engineered Precision Casting Company utilizes NADCAP approved sources to perform this specialized process.

What is Hot Isostatic Pressing?

The HIP process subjects castings to a combination of elevated temperature and high isostatic pressure — typically in the range of 15,000 psi— applied uniformly from all directions through a high-pressure inert gas, most commonly argon. The combination of elevated temperature — typically 85 to 95 percent of the alloy's solidus temperature — and uniform high pressure causes internal voids, microporosity, and shrinkage cavities to close and metallurgically bond, producing a fully dense microstructure essentially free of internal defects.

The isostatic nature of the applied pressure is critical to the process. Unlike uniaxial pressing, which applies force in a single direction and can distort the part geometry, isostatic pressing applies equal pressure uniformly from all directions simultaneously. This means that the external dimensions and surface geometry of the casting are not significantly affected by the HIP process — the part retains its shape while internal defects are eliminated. For investment castings with complex geometries and tight dimensional tolerances, this characteristic makes HIP uniquely suitable as a densification treatment that improves internal integrity without compromising the dimensional precision that investment casting provides.

Mechanical Property Improvements from HIP

The elimination of internal porosity through HIP produces measurable and significant improvements in the mechanical properties of investment castings, particularly in fatigue performance and dynamic loading resistance — the properties most sensitive to the presence of internal defects.

Fatigue life is the mechanical property most dramatically improved by HIP. Internal pores act as stress concentrators that initiate fatigue cracks under cyclic loading, and the elimination of these initiation sites by HIP can increase the fatigue life of investment castings by factors of two to ten or more compared to non-HIPped castings of the same alloy and geometry. For aerospace and defense components subject to high-cycle fatigue loading — including structural brackets, actuator components, engine hardware, and landing gear fittings — this improvement in fatigue performance is the primary driver for specifying HIP.

Tensile ductility and elongation improve substantially after HIP as the internal voids that limit plastic deformation are closed. Elongation values in HIPped castings typically approach or meet the minimum values specified for wrought material in the same alloy, a significant improvement over non-HIPped castings where elongation is limited by void-initiated fracture. This improvement in ductility is particularly important for castings in precipitation hardening stainless steels and high-strength alloy steels, where the combination of high strength and limited ductility makes the material most sensitive to the presence of internal defects.

Ultimate tensile strength typically shows a modest improvement after HIP, as the elimination of internal voids reduces the effective stress concentration that limits the load-carrying capacity of the casting cross-section. While the improvement in tensile strength is generally less dramatic than the improvement in fatigue life and ductility, the combined effect of improved strength, ductility, and fatigue performance produces a casting with a significantly more favorable combination of mechanical properties than the non-HIPped condition.

Learn More About Hot Isostatic Pressing for Investment Castings

Engineered Precision Casting Company provides HIPping and other secondary services to provide complete investment casting solutions.