Glossary

Hot isostatic pressing (HIP) is a densification and defect-healing process where a metal (or ceramic) part is placed in a high-pressure vessel and exposed simultaneously to high temperature and isostatic (equal in all directions) gas pressure—most commonly argon—so internal voids close and diffusion bonding occurs at particle/grain boundaries. A classic way to say it is: HIP uses heat + uniform gas pressure to drive plastic flow, creep, and diffusion that eliminate porosity and improve integrity without melting the part.

In an industrial HIP cycle, parts (or sealed “cans” of powder) are loaded into the vessel, the chamber is pressurized with inert gas, and the load is heated to a temperature below the solidus but high enough that the material can deform/diffuse under pressure. Modern systems can run up to roughly 2000°C and pressures on the order of 2000 bar (~200 MPa) depending on equipment and material system.

Where HIP really earns its keep is in quality-critical metalwork. For castings, HIP is used to eliminate internal shrinkage and microporosity, which can substantially improve fatigue performance and reliability. For powder metallurgy, HIP can consolidate powder into fully dense components (often by sealing powder in a can, evacuating it, then HIP’ing it—commonly called PM-HIP). For additive manufacturing, HIP is a common post-process step to reduce part porosity and improve performance for fatigue-critical applications—though the benefit depends on the as-built defect population (HIP can’t magically “erase” every defect type if the build quality is poor).

Historically, HIP was developed in the mid-1950s at Battelle Memorial Institute (often cited as 1955) and was originally described as gas-pressure bonding, first used for diffusion bonding/cladding work tied to early nuclear applications. ASME even recognizes the “First Hot Isostatic Processing Vessels” as a historical engineering landmark, reflecting how significant the pressure-vessel technology was to making HIP practical and scalable.