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What is Hot Isostatic Pressing (HIP)?
HIP, Hot Isostatic Pressing, is one of material processing methods, which compresses materials by applying high temperature of several hundreds to 2000 °C and isostatic pressure of several tens to 200MPa at the same time. Argon is the most commonly used pressure medium.
Hot pressing is very similar to HIP. Milling, forging, extrusion also apply high temperature and pressure, but not isostatic pressure unlike HIP.
Difference between HIP and Hot Pressing
HIP applies isostatic pressure to materials using gas pressure, while hot pressing applies only uniaxial pressure.
To explain the difference of HIP and hot pressing clearly, suppose that HIP or hot pressing is applied to Material A (metal with pores inside) and Material B (metal with uneven ends).
In case of HIP, Material A, as shown in Figure 1, will contract keeping its initial shape until pores inside disappear, and bond together due to diffusion effects. On the other hand, Material B undergoes no shape change at all because uniform pressure is applied to the uneven edges.
In case of hot pressing, the same phenomena as the case of HIP occur to Material A, which is shown in Figure 2. Material B, however, can't keep its initial uneven shape because pressure is applied only to the convex portions. Both Material A and Material B will have different final shapes after hot pressing depending on shapes of a mold and a punch used. Fabrication of large products and moldings under high temperature might be difficult because of ununiformity due to friction force with a mold and constraints due to temperature and dimensions during the deformation.
Compared to hot pressing, HIP can provide material shapes not much different from the initial one after pressure. A material even after changing its shape can keep its initial shape, and will be relatively less restricted by processing of products. By making full use of these features, HIP has been applied in various fields.
Pressure Medium Gas (Argon Gas) under High Pressure
Argon gas at 1000°C and under pressure of 98MPa is likely to cause intense convection due to low density and coefficients of viscosity (30% and 15% of water, respectively), and high coefficients of thermal expansion. Therefore, heat transfer coefficients of HIP equipment become higher than that of an ordinary electronic furnace.
HIP Application
HIP is applied in a wide range of fields as follows:
- Pressure sintering of powder
- Diffusion bonding of different types of materials
- Removal of residual pores in sintered items
- Removal of inner defects of castings
- Rejuvenation of parts damaged by fatigue or creep
- High pressure impregnated carbonization method
| Applied Technology | Practical Application | Material under Study | |
|---|---|---|---|
| 1 | Pressure sintering of powder | PM high speed steel (jigs), Ni-based superalloys (engine turbine discs), Ti alloys (aircraft components), Cr (target) | Ti-Ni alloy, amorphous metal, Si3N4, SiC |
| 2 | Diffusion bonding(Production of composites) | Nuclear fuel assemblies (nuclear reactors), B fiber-Al alloy composites (space shuttle struts), various corrosion/abrasion-resistant alloy composite parts (valves for corrosive gas, mill roll, cylinders for injection molding machines etc.) | SiC-Al alloy composites, Nb3Sn-copper, Si3N4-stainless steel |
| 3 | Removal of residual pores in sintered items | Cemented carbide (jigs), Al203(cutting tools), Al203-TiC (cutting tools), soft ferrites (magnetic head), Si3N4 (bearings and ceramics structure) | SiC, PSZ, ZnSe |
| 4 | Removal of inner defects of castings | Al alloys, Ni-base superalloys (jet engine turbine blades), Ti alloys (aircraft components), 17-4PH stainless steel | |
| 5 | Rejuvenation of parts damaged by fatigue or creep | Superalloy precision castings (gas turbine blades) | |
| 6 | High pressure impregnated carbonization method | Carbon composites |
HIP Treatment
Materials need various treatment depending on the situations. The most typical methods include 'Capsule Method' and 'Capsule Free Method'.
'Capsule Method', as shown in the right figure, is to carry out HIP after enclosing powder or a body molded from powder in a gastight capsule and evacuating the capsule.
This 'Capsule Method' can provide high densification even for materials that are difficult to sinter by ordinary sintering techniques. Therefore, it is most commonly adopted as pressure sintering process of powder materials. It's also used for diffusion bonding of different types of materials or high pressure impregnated carbonization methods.
The following Table gives a summary of main materials for capsule free methods and HIP treatment temperature/pressure.
| Material | Temperature | Pressure |
|---|---|---|
| Powder high-speed steel | 1,000 to 1,200°C | to 100MPa |
| Ni base alloy | 1,170 to 1,280°C | 100 to 150MPa |
| Ti alloy (Ti-6Al-4V) | 880 to 960°C | to 100MPa |
| Cr | 1,200 to 1,300°C | to 100MPa |
| Cu Alloy | 500 to 900°C | to 100MPa |
| Al Alloy | 350 to 500°C | to 100MPa |
| Cemented carbide (WC-Co) | 1,300 to 1,350°C | 30 to 100MPa |
| Ti Ba O3 | 1,000 to 1,200°C | to 100MPa |
| PZT | 950 to 1,150°C | to 100MPa |
| Ni-Zn-ferrite | 1,050 to 1,180°C | to 100MPa |
| Mn-Zn-ferrite | 1,180 to 1,250°C | to 100MPa |
| Al2O3 | 1,350 to 1,450°C | to 100MPa |
| Y-PSZ (yttria partially-stabilized zirconia) | 1,350 to 1,500°C | to 100MPa |
| Si3N4-Al2O3-Y2O3 | 1,700 to 1,800°C | to 100MPa |
| SiC | 1,950 to 1,050°C | 100 to 200MPa |
Meanwhile, if the pores inside the material are isolated, closed off, and not connected to the surface of the material, these pores might be squashed and eliminated by HIP treatment. On the other hand, open pores that connect with the surface of the material will not be squashed even after HIP treatment. Therefore, HIP treatment to the material with the closed pores can provide high densification of the entire material.
Such materials require no capsules for HIP treatment, which is called 'Capsule Free Method'. This is used for removal of residual pores on sintered items, removal of inner defects of castings, and rejuvenation of parts damaged by fatigue or creep.
HIP Effects
HIP treatment to castings can improve creep fracture lifetime by 1.3 to 3.5 times, elongation, and contraction, as shown in the following Table, depending on the types of alloys.
| Alloy | State | Test Conditions | Lifetime | Elongation | Contraction | |
|---|---|---|---|---|---|---|
| Temperature | Stress | |||||
| IN738 | Casting | 1,253K | 152MPa | 68.4x103sec | 11.8% | 20.0% |
| Casting+HIP | 1,253K | 152MPa | 189.0x103sec | 20.5% | 20.6% | |
| Rene77 | Casting | 1,253K | 152MPa | 183.6x103sec | 19.4% | 37.0% |
| Casting+HIP | 1,253K | 152MPa | 244.8x103sec | 22.0% | 55.0% | |
| IN792 | Casting | 1,143K | 310MPa | 630.0x103sec | 9.2% | 6.5% |
| Casting+HIP | 1,143K | 310MPa | 1,018.8x103sec | 12.1% | 22.0% | |
| Rene80 | Casting | 1,143K | 310MPa | 149.4x103sec | 2.5% | 2.5% |
| Casting+HIP | 1,143K | 310MPa | 507.6x103sec | 11.5% | 17.0% | |
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