Sintering/Powder Metallurgy: Hot Isostatic Pressing (HIP) Technology
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.
HIP ProcessThe 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.
HIP EquipmentThe 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.
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.
SummaryHot 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
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