Lesson 9.3ShapeChapter 9 · Powders and ceramics

Hot isostatic pressing (HIP)

Squeeze with hot argon from all sides to close pores

Hot isostatic pressing (HIP) puts finished or nearly finished parts in a pressure vessel and squeezes them with hot argon gas. The pressure pushes equally from every direction (“isostatic”), so hidden pores collapse and weld shut while the shape stays the same. Jet-engine turbine blades and 3D-printed titanium implants are routinely treated this way.

Photo: Quintus Technologies, CC BY-SA 4.0, via Wikimedia Commons

Hot isostatic pressing · vessel cross-section, pores closing, one cycleThe vessel is drawn cut in half; the inset zooms into one part, and the small graph sketches one cycle. Circled numbers match the steps.

How it works

Load and seal. Parts go in on a fixture, and the vessel is closed with thick steel plugs; in big units a heavy frame holds the plugs in place. The load can be castings, 3D-printed parts, or metal powder sealed in a steel can.
Fill with argon. Air is pumped out, and a compressor pumps in argon, an inert gas that does not react with hot metal.
Heat and pressurize. A furnace inside a heat shield brings the parts to about 900–1,250 °C for steels, titanium and nickel alloys (around 500 °C for aluminum). Heating the trapped gas raises its pressure to typically 100–200 MPa, about 1,000–2,000 times atmospheric pressure.
Hold. For typically 2–4 hours, the hot metal is soft enough to creep (flow very slowly) into its pores. The pore walls touch and bond by diffusion (atoms drifting across), leaving solid metal. Pores open to the surface cannot close, because the gas gets into them.
Cool and unload. The furnace cools, the argon is recovered for reuse, and the parts come out with the same shape at essentially full density. Powder cans are then removed by machining or acid.

Key facts

Materials: Castings and 3D-printed parts in titanium, nickel alloys, steels and aluminum; metal powders in sealed cans; technical ceramics
Typical parts: Turbine blades, medical implants, valve bodies, tool-steel blocks, printed metal parts
Part size: From small implants to parts of several tonnes; the biggest vessels hold loads around 2 m across
Precision: Shape is kept; parts shrink only by their pore volume, usually a tiny fraction of a percent for castings
Surface finish: Left as it was: HIP does not touch the surface
Batch size it suits: Any: one part or a full furnace load, often run by specialist HIP service companies
Tooling and machine cost: No part-specific tooling; machines cost from hundreds of thousands of dollars (lab units) to many millions (production units)
Speed: One cycle typically takes 6–12 hours including heating and cooling; less in units with rapid cooling

Strengths and limits

Strengths

Removes internal pores, reaching essentially 100% density
Large gains in fatigue life (resistance to repeated loads) and reliability
No part-specific tooling: any shape works if it fits the vessel
Can turn powder into a solid part, or bond two metals together

Limits

Cannot close pores that connect to the surface
Long, costly cycles, usually an extra step after casting or printing
Very expensive machines, so most companies use a service provider
Powder cans add cost and must be machined or dissolved away

Where it's used

AerospaceCast turbine blades and vanes, structural engine castings
MedicalHip and knee implants in cast or 3D-printed titanium and cobalt-chrome
Additive manufacturingStandard post-treatment for printed titanium and nickel-alloy parts
Oil, gas and energyValve bodies, pump parts and manifolds made from powder by HIP
ToolingPowder tool steels and high-speed steels with a fine, even structure

How to recognize it

HIP leaves no visible mark: the part looks exactly like the casting or print it was.
The proof is inside: a cut and polished section, or an X-ray, shows no pores.
Large parts made from powder by HIP show machined surfaces all over, because the steel can was cut away; drawings and certificates often say “HIPed”.

Compare with

Powder metallurgy9.1presses cold powder in a die from two directions and leaves pores; HIP presses hot from all sides and removes them.Spark plasma sintering9.4also combines heat and pressure, but presses along one axis in a graphite die, in minutes instead of hours.Investment casting1.2often comes just before HIP: cast turbine blades are HIPed to close shrinkage pores.

Watch it

Everyday Metallurgy · Why heat plus equal pressure closes internal pores

Quintus Technologies · The vessel, furnace and argon cycle, from a machine maker

Hiperbaric · An animation of one full heat, pressure and cooling cycle

TWI Ltd · How powder sealed in a can becomes a solid, near-final part

See real photos

Wikipedia: Hot isostatic pressing — history, typical pressures, temperatures and uses; few photos
Wikimedia Commons photo search

Remember

HIP squeezes parts with hot argon from all sides at once, so the shape is kept.
It closes internal pores, giving essentially full density and much longer fatigue life.
It is a long, costly batch step, used where a part must not fail.