Hot Isostatic Pressing has gone from the research lab to full-scale regular production in the course of the last 50 years. The future of this technology looks bright as the demand for improved material properties, the use of powder materials, and the desire to produce net or near net shape parts accelerates.

Fig. 1. Resistance-heated furnace temperature source inside the vessel

The HIP process, originally known as gas-pressure bonding, was developed at Battelle Memorial Institute's Columbus, Ohio, laboratory in 1955. The original application was the diffusion bond cladding of zirconium to zirconium-uranium alloys for nu-clear fuel elements. Around the same time, ASEA-Sweden was utilizing the isostatic application of pressure to compact the first synthetic diamonds.

Fig. 2. Near-net shape, large stainless steel part

HIP Process

The HIP process uses the combination of elevated temperatures and high pressure to form, densify, or bond raw materials or preformed components. The application of the pressure is carried out inside a pressure vessel, typically utilizing an inert gas as the pressure-transmitting media. A resistance-heated furnace located inside the vessel is the temperature source (Fig.1). Parts are cold loaded into the vessel, and pressurization occurs usually simultaneously with the heating. Parts are then cooled inside the vessel and removed.

Fig. 3. Fully dense silicon-nitride wear components

The primary purpose of the process is to obtain full density and improved mechanical properties within a given part. Since the beginning of the 1950s, a number of processes utilizing HIP have evolved. These include:

  • Powder Processing. This process, starting with metal or ceramic powders, utilizes HIP to obtain homogeneous microstructures with full density within a final part. The process can be used directly for compaction of parts or for billet compaction as a preform for further treatments. With full density being achieved at temperatures below those required for sintering without pressure, improvement of the material and fatigue properties is also achieved. One of the first commercial HIP'd powder processes was for the making of high-speed steels (HSS). Cemented carbides, for superior wear properties, and the making of net shape, large, stainless-steel parts followed (Fig. 2). Other materials, such as beryllium, and other applications, such as advanced sputtering targets, have utilized the process throughout the years. Fully dense silicon-nitride wear components (Fig. 3) that are processed utilizing a glass-encapsulation technique and the HIP process have expanded the powder applications beyond metals.
  • Defect Healing. Internal porosity in cast material can be removed during the HIP process, thus giving castings a more predictable, longer life, and reducing scrap. Commercialization of the use of HIP in the defect healing of castings was applied to nickel and titanium base superalloys used in the aerospace engine industry. In addition to the utilization for new componentry, the rejuvenation of turbine blades for the restoration of thermally reduced properties has also been demonstrated. Prosthetic devices for hips, knees (Fig. 4), as well as other medical and dental applications, utilize HIP to achieve the product's necessary fatigue properties. HIP of aluminum castings in automotive and other industrial applications proves that the processing economics of HIP are not limited to only the very expensive materials.
  • HIP Cladding. Here HIP's application of pressure and temperature can be used to metallurgically bond dissimilar materials in order to utilize the specific characteristics of each material within a given combination. For instance, a wear-resistant material can be bonded to a less expensive and softer substrate material. This use has proven successful in the lining of valve bodies in corrosive applications where the more expensive nickel-base alloy material is only placed where the valve sees the corrosive environment and less expensive forged steel material is used in the majority of the valve.

    Fig. 4. Prosthetic devices for hips and knees

    HIP Equipment

    The increased use of the HIP process has pushed the need for, and been accelerated by, the advances in the develop-ment of the HIP equipment itself. Hundreds of production-size HIP systems are in operation around the world, with di-ameters ranging from 250 mm to 1.7 meters. Many more research-size vessels exist as well. Typical pressures used in production are 100 to 200 MPa, and most processing temperatures range from under 1000°C (1832°F) to over 2000°C (3632°F). While the basic components of a HIP system remain the pressure vessel, furnace, compressors and controls (Fig. 5), all have undergone significant changes that have helped drive down the per-pound/per-piece price of the proc-ess, as well as having enabled the process to gain precision and reliability.

    Pressure vessel designs have followed and pushed regulatory change. Increased cycle life, higher pressure processing and safety have been the main drivers. New furnace designs and furnace materials have expanded the process' temperature parameters. Furnaces of molybdenum, steel and graphite can be selected based on the process requirements and process needs of maximum temperature, cleanliness or overall economy. Advanced computer controls now monitor and control one or a plant full of HIP systems.

    Fig. 5. Example of large HIP installation

    Processing economics, as well as some final material properties, have greatly benefited from the development of advanced cooling technology. By utilizing the highly convective nature of argon gas, combined with the ability to build multiple layer vessels with internal cooling and advanced furnace designs, large loads in HIP cycles are able to cool down faster. With rates possible up to 500°C per minute, this hundredfold increase in cooling rates has cut many process cycles by more than half and has also allowed for the possibility of combining the HIP cycle with a solution heat treatment (Fig. 6). This ability to gas quench rather than liquid quench can minimize distortion and part cracking, minimize surface reactions, and provide for more homogenous properties throughout the part.

    HIP's industrial base is ever expanding as the demand for improved material properties, the use of powder materials, and the desire to produce net or near net shape parts accelerates. Improvements in HIP equipment, the shortening of processing times, and the ever-improving economics make the HIP process a more viable choice for an ever-increasing array of materials.

    Fig. 6. HIP cycle curve


    Hot Isostatic Pressing began as a curiosity in the 1950s and has matured to successful production operation today for many materials applied in various applications. Its primary purpose is to achieve fully dense parts for the enhancements of mechanical and fatigue properties. PM-densification to near net shape parts has become an economic fabrication route applied in a number of industries. PM-billets compacted to full density are used as preforms for further treatments such as forging, rolling or extrusion. HIPing of investment castings eliminates subsurface porosity to improve properties and quality assurance. HIP cladding is an effective way to fully bond dissimilar materials together for better wear and corrosion resistance. The equipment has advanced steadily for increased throughput and attracts for more advanced processing such as HIP quenching. IH

    For more information: Franz Zimmerman can be reached at Avure Autoclave Systems Inc., 1603 Pershing Ave., Erie, PA 16509; ph. (814) 868-1408; e-mail: Jerry Toops can be contacted at Avure Autoclave Systems Inc., 3721 Corporate Drive, Columbus, OH 43231; ph. (614) 891-2732; e-mail