The HIP process uses a containment vessel to subject a component to high temperature and pressure. An inert gas is used to prevent a chemical reaction in the material. The most commonly used gas is argon, although the choice of metal can also reduce the negative effects of chemical reactions. Commonly-used metals include mild steel, stainless steel and nickel.
As the chamber is heated the pressure inside the pressure vessel increases, although gas pumping can also be used to increase the pressure level. The pressure is applied from all directions, which is why the process is ‘isostatic,’ leading to a thermal treatment under high pressures to consolidate the materials.
Because of the isostatic nature of the pressurising medium, greater flexibility with regard to sample shape is achieved through this process and, because of plastic deformation at the interface, surface preparation need not be so stringent.
It may be possible to alleviate some of the coefficient of thermal expansion mismatch problems associated with bonding dissimilar materials, since component design is more flexible. Therefore the design can be arranged so that a compressive stress is obtained at the interface on cooling.
The critical aspect of this technique is that the interface must be isolated from the gaseous pressure medium. This is typically achieved by encapsulating the component. There are three main types of encapsulation:
- direct sealing of the circumference of the contact area between the two parts;
- placement of a sleeve of material around the contact area between the two parts;
- full, or partial encapsulation of the entire component.
After HIPing, this encapsulating material is normally machined off.
The HIP production technique offers a range of benefits, including improved efficiencies, lower costs and the ability to create parts that cannot be achieved by other means. The advantages of HIP include:
1. Improved Design Freedom:
The HIP process can produce components that are unobtainable using other means. This includes designs with complex geometries, while high performance requirements can be met without the need for welded joints.
2. Reduced Welding:
This process reduces the requirement for costly and critical welded joints. This not only helps improve the potential integrity of the part but also saves time and money on welding and weld inspection operations.
3. Material Cost Saving:
Near net shape production techniques like HIP are cost effective in the use of materials when compared to machining. Reducing the need for machining saves time as well as materials, reducing expenditure and the environmental impact of waste materials.
4. Improving Mechanical Properties:
Because of the fine and uniform grain size associated with HIP metals, parts created with this method have the same properties in all directions. This includes reaching theoretical density with limited grain growth that may not be possible with conventional, pressureless sintering.
5. Less Hydrogen Induced Stress Cracking (HISC):
HIP virtually eliminates the chances of HISC due to the fine grain structure of HIPed parts.
6. Narrow Scatter Band:
The scatter in mechanical properties is lower than that found with forging and casting. HIP does not produce the same variations in mechanical properties as there are fewer uncontrollable parameters during the production process.
7. Improved Ultrasonic Inspection:
Inner defects can be located more easily with HIPed parts. This is due to the finer grain structure that produces less interfering static than the coarser grain structures found with other traditional processes.
HIPed parts can include layers of different steel grades, while cladding offers an additional dimension to the freedom of design.
Despite the many advantages of HIP, there are still a few drawbacks associated with the process:
- Low Production Quantities: The process is only suitable for smaller production runs of less than around 10,000 per annum.
- Cost: While HIP is cheaper than many subtractive processes due to the lack of material wastage, it is costlier than many other powder manufacturing methods due to the slower processing speed and need for expendable tooling.
HIP components are used in applications that have high performance demands, including those within the aerospace, oil and gas, chemical and nuclear industries.
Hot isostatic pressing can be used to process castings by turning metal powders into compact solids. To achieve this, the inert gas is applied at between 7,350 psi (50.7 MPa) and 45,000 psi (310 MPa), although 15,000 psi (100 MPa) is the most common. The process soak temperatures vary according to the metal being used, with aluminium castings using 900°F (482°C) and nickel-based superalloys requiring heats of 2,400°F (1,320°C).
The simultaneous application of heat and pressure removes internal voids and microporosity via a combination of creep, plastic deformation and diffusion bonding. This process can be used to consolidate metal powders, ceramic composites and metal cladding as well as reducing microshrinkage and improving fatigue resistance. As well as being used to fabricate metal matrix composites, HIP can be used as a post-processing step for additive manufacturing.
Another application for the HIP technique is the processing of radioactive waste. The waste is packed into a thin-walled metal canister and the absorbed gases are removed using high heats. The remaining material is then compressed using argon gas, shrinking the steel canisters and minimising the space required for transport and disposal.
Hot isostatic pressing uses high temperatures and pressures to create parts and products using a range of different ceramic and metal materials. The process involves placing powdered materials into a metal mould in the shape of the finished item. This mould is sealed and air is removed from inside with a vacuum pump.
Heating in a hot isostatic press joins the powders as the internal pressure increases through the use of an external gas pressure. This process has found a range of applications across industry, from creating full density, bespoke parts to manufacturing super alloys and other metals used in aerospace and other industries.