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High-pressure densification has been an accepted process for decades, proven by thousands of companies around the world to improve the performance and durability of critical parts. Advances in materials research, press and furnace designs, uniform rapid cooling capability, and digital control techniques make hot isostatic pressing (HIPping) more viable and affordable than ever for producing the strong, long-lasting products that are increasingly in demand from globally competitive manufacturers in all kinds of industries.
HIP BasicsHot isostatic pressing is a forming and densification process using heated gas (most commonly argon) under very high pressure. Unlike mechanical force, which compresses a workpiece from one or two sides, isostatic pressure is applied uniformly on all sides of an object, eliminating porosity without changing the object's net shape. The process can be used to treat preformed metal, ceramic or composite parts, and for the compaction of containerized powder shapes. Maximum standard operating pressures can be specified from 1500 to 30,000 psi, and higher pressures can be provided for special applications. Maximum temperatures are typically around 2000°C.
Direct or glass-encapsulated HIPping has long been employed to densify engineered ceramic products such as silicon nitride ball bearings and thread guides, boron nitride injection nozzles and control rod pins, insulators, electrodes, and pump components for the chemical industry. HIP often reduces or eliminates the need for surface finishing, machining or other secondary operations. Many new applications for HIPped ceramics have emerged, including:
- Ballistic protective glass, using spinel and aluminium oxynitride (ALON)
- Composite joint prosthetics, combining metal stems with sintered and HIPped powdered ceramic socket balls
- Body armor for military and law enforcement personnel
- Zirconia and alumina dental implants
Improved ProcessesTechnological advances in HIPping include increased cycling speeds, larger press sizes and precise control systems. Depending on the characteristics and sensitivity of the materials being processed, HIP cycles have been known to last as long as 20 hours. Great strides have been made to reduce this time to as little as five hours, which lowers per-unit costs. This cycle reduction is made possible by increasing the press' designed heating and cooling rates, while also reducing the process soak time whenever justified by previous experience and results.
Many HIP presses can be equipped with advanced rapid cooling furnaces that have, in some cases, cut the total cycle time in half or more. A programmable variable speed fan circulates cooler gas uniformly throughout the work zone, and the cooling rate can be precisely controlled to avoid cracking in thermally sensitive ceramic parts. This feature not only reduces cycle times, but also adds the possibility of combining the HIP process with solution heat treating.
An obvious way to lower unit costs of processed parts is to process a higher number of parts per cycle, and the trend toward larger HIPs is fueled to a great degree by the obvious advantage that larger units can process more parts. The bigger work zones also bring the benefits of HIPping to larger castings, pre-forms and containerized powder shapes.
In the last decade, significant improvements have been made in the computer control systems used to regulate the HIP cycle. These systems offer far more precise temperature control and more uniform heating throughout the multiple zones used in the newer HIP furnaces. This results in higher throughput of quality parts and greatly reduced scrap loss. The temperature also fluctuates less around set point, which avoids the wasteful entry and venting of process gas when the signal is "bouncing."
HIP's ability to produce near-net-shape parts with up to 100% of theoretical density and zero internal porosity has proven itself many times over. The steady growth in practical applications for engineered ceramics presents new opportunities to employ the latest advancements in HIP technology.
For additional information regarding hot isostatic pressing, contact Avure Technologies, Inc., 3721 Corporate Dr., Columbus, OH 43231; (614) 891-2732; fax (614) 891-4568; e-mail firstname.lastname@example.org; or visit www.avureae.com.
SIDEBAR: Giga-HIPsHIPs are getting bigger, and the largest of all has recently been ordered from Avure Technologies AB, Vasteras, Sweden, by Kinzoku Giken Co. Ltd., a Japan-based contract processor. Scheduled for installation in 2009 at the company's Himeji factory, this giga-HIP will stand over 41 ft and weigh more than 600 tons.
The giant work zone measures 79 in. in diameter and almost 14 ft in height. This unit is part of a two-press order that will support Kinzoku Giken's growing hot isostatic pressing business in Japan, China and Southeast Asia. Together, the presses will be used to densify large castings and create parts from powder metal for the electronics, aerospace and petroleum industries.
Several other large presses are currently being built and installed by Avure for contract processors. Bodycote of Camas, Wash., will soon be operating a unit with a work zone measuring 66 in. in diameter and 8.3 ft high, the latest of five presses of similar size purchased by the company. Howmet Castings, Whitehall, Mich., also has several large HIPs, including a recently ordered 42 x 65 in. model.