A method that applies an equally high pressure from all directions with an inert gas at high temperature, to densify powder or eliminate closed internal pores in castings and sintered bodies. Abbreviated HIP.
Japanese name: 熱間静水圧プレス
Hot isostatic pressing appears in 1 practice question across 1 topic, including Materials processing and joining.
HIP is a treatment in which parts, or capsules filled with powder, are placed in a pressure vessel and an equal pressure is applied from all directions at high temperature, using an inert gas such as argon as the pressure medium. Pressures of about 100 to 200 MPa are used, with the temperature chosen for each material below its melting point but high enough for diffusion and creep to proceed, held for several hours. Examples are temperatures in the 900s°C for titanium alloys and around 1100 to 1200°C for Ni-based superalloys. Closed internal pores collapse by plastic deformation, creep and diffusion of the surrounding material, and the facing surfaces bond by diffusion and disappear. Because the pressure is equal from all directions, the shape of the part hardly changes.
HIP can only close pores that are closed, that is, not connected to the surface. Gas from the pressure medium enters open pores and balances the pressure inside and out. Sintered bodies are therefore treated after the relative density exceeds roughly 92 to 95% and the pores have closed, and there is also a method that carries out sintering and HIP in succession in one furnace (sinter-HIP). When making parts directly from powder, the powder is packed into a metal capsule, which is evacuated and sealed and then consolidated as a whole. Densifying powder with a packing fraction of 65% to full density shrinks it by about 13% in length, so the capsule shape is designed to allow for this shrinkage.
Isostatic pressing also includes cold isostatic pressing (CIP), which compacts powder in a rubber mold at room temperature using a liquid as the pressure medium. CIP is a forming step that makes the density of the compact uniform, with densification done later by sintering, whereas HIP treats at high temperature and densifies close to full density on the spot. Hot pressing, which applies pressure in one direction to material in a graphite or similar die, uses simpler equipment, but shapes are limited to simple ones such as disks and plates, and density tends to vary with direction. The strength of HIP is that it applies equal pressure even to parts of complex shape.
HIP is widely used to remove internal shrinkage cavities and pores in cast parts for aircraft engines and so reduce scatter in fatigue strength, and it has become indispensable as a post-treatment for parts made by metal additive manufacturing. It is also used in PM-HIP, which makes large near-net-shape parts from powder, and in HIP bonding, which joins different materials by diffusion. However, inclusions such as oxides and pores with oxidized inner surfaces do not bond well even when collapsed and can remain as defects. Because the material is held at high temperature, grain growth and changes in precipitates also occur, so conditions are set together with the heat treatment after HIP.
Answer: C. At high temperature, an inert gas such as argon is used as the pressure medium to apply an equally high pressure from all directions
In HIP the part is placed in a pressure vessel and, while heated to high temperature, an equal pressure of about 100 MPa or more is applied from all directions by an inert gas such as argon. Closed internal pores collapse and disappear by creep and diffusion, raising density and fatigue strength without greatly changing the shape. It is also used to eliminate pores in turbine-blade castings and in metal parts made by additive manufacturing. Open pores connected to the surface do not collapse because gas enters them, so the part is first sintered until the pores close, or the powder is sealed in a metal container (capsule) for treatment. Using a rubber mold at room temperature is cold isostatic pressing (CIP), and pressing in one direction while passing current is a method such as spark plasma sintering.
Both apply equal pressure from all directions, but HIP uses an inert gas at high temperature as the pressure medium and goes as far as densification, while CIP uses water or oil at room temperature as the pressure medium and forms powder in a rubber mold uniformly. Compacts made by CIP are then densified by sintering.
Closed pores mostly collapse to near full density. However, open pores connected to the surface do not collapse, and pores with oxidized inner surfaces may not bond. If gas such as argon was trapped inside pores, they can also swell again when the part is later reheated.
Pressures of about 100 to 200 MPa are common, about 1000 to 2000 times atmospheric pressure. If the pressure is too low, pores take a long time to collapse, and if the temperature is too low, the material deforms with difficulty, so temperature, pressure and time are combined for each material.
In HIP the part is placed in a pressure vessel and, while heated to high temperature, an equal pressure of about 100 MPa or more is applied from all directions by an inert gas such as argon.
Hot isostatic pressing (HIP) is used to eliminate closed po…Creep Sintering Carbon equivalent Extrusion Heat-affected zone Investment casting Sensitization Shrinkage cavity Springback