Recent trends in Hot Isostatic Pressing (HIP) technology: Part 2 – Powder based HIP products
The HIP systems described in Part 1 of this review have often been designed for specific applications such as converting powder into fully dense products (metal, carbide and ceramics) by pressure assisted sintering, post-HIP of sintered products to eliminate porosity, and the production of bimetallic products.
The HIP temperatures can range from 1100°C for powder consolidation of PM tool steels, to 1500°C for the elimination of porosity in hardmetals (cemented carbides), and exceed 2000°C for HIPing of ceramics.
Pressures of up to 200 MPa can be achieved. The length of the HIP cycle can take several hours so there are economic advantages in having as big a workload in the press as possible.
The development of ‘mega’ (1.6m diam.) and ‘giga’ HIP (2.05m diam) systems in recent years has come alongside the significant increase in powder production capacity, particularly high grade spherical shape powder particles suitable for HIP produced by gas atomisation.
This has resulted in a wide choice of alloy powders such as stainless steels, tool steels and high speed steels, as well as nickel-base and cobalt-based super alloys which are now being produced in large quantities in a variety of particle sizes at competitive prices. Fig. 1 shows a typical gas atomised metal powder used for HIP.
The conversion of gas atomised powders into 100% dense products having near-net shape components often weighing several tonnes, with a refined microstructure and mechanical properties superior to cast or wrought equivalents, has made hot isostatic pressing an essential PM processing route for a growing number of application areas.
PM high alloy steels
The use of HIP to produce PM tools steels was developed simultaneously, but independently, in Sweden by Stora Kopparberg and ASEA (now Erasteel) and by Crucible Materials Corp in the USA (Crucible Particle Metallurgy Process – CPM) in the 1970s.
The use of gas atomised tool steel powders and HIP led to the elimination of segregation found in cast materials, and the very fine distribution of primary carbides in the HIPed microstructure resulted in superior mechanical properties such as compressive strength, hot hardness, and toughness.
The high wear resistance of PM tool steels helped to secure increasing market share in the intervening decades and they are now extensively used in metal forming tools and other applications.
PM high speed steels are also produced by the HIPing of gas atomised powders, and one example is the HIPed gear cutting hob shown in Fig. 2. These PM HSS materials are said to bridge the gap between conventional high-speed steels and cemented carbides.
The early landscape of PM high alloy steel producers has changed significantly over the past decade. ASP® grades are now produced by Erasteel, a member of the ERAMET Group.
When Sandvik Powdermet, based in Surahammar, Sweden, acquired Metso Powdermet AB from the Finnish Metso Oy Group in 2006 it acquired a leading producer of near-net-shape HIPed PM components ranging in weight from 100g to 15t covering high alloyed steels, stainless steels, Ni and Co base alloys and metal matrix composites (MMC). The PM process route used by Sandvik Powdermet is shown in Fig. 3.
This has given the company a foothold in the offshore components market, utilising its near net shape expertise to produce complicated high alloy PM HIPed components including for subsea development projects. Areas of importance are HIPed components for multi-fluid analysis, wye-pieces for pipeline installations, manifolds for topside and subsea installations, valve bodies for choke and control valves, and swivels for FPSOs.
Manifolds are, for example, used for collecting oil/gas from well heads as well as for water injection. They are subjected to sour service and high pressure, and therefore require high mechanical strength and corrosion resistance (Fig. 4). The Duplex stainless FPSO Swivel component has internal passages for oil/gas, water injection and power connection.
Although Metso Materials Technology no longer produces HIPed components in-house, the company’s Engineered Materials & Components Division in Lokomo, Finland, continues to develop and use HIP technology for a growing range of applications.
One example given by the company  is a superaustenitic 254 SMO stainless steel ball valve used in desulphurisation plant in oil refineries. The HIPed components are required to operate in highly corrosive hydrogen sulphide environment containing oil/gas. The as-HIPed valve half is shown on the left in Fig. 5 after the removal of the HIP capsule and Fig. 5 right shows the as-machined part. The HIP route produced a material with superior properties and allowed easy and reliable ultrasonic inspection.
Undoubtedly one of the most impressive Metso applications for PM high alloy steels are the stainless steel end covers for the CERN particle accelerator located in a 27km long ring-shaped tunnel 100m underground near Geneva. The choice of end cover material was a fully dense HIP stainless steel 316LN grade of which Metso supplied some 2700 pieces, (Fig. 6). Metso has also supplied around 20 tonnes of HIPed PM stainless steel 316LN radial plates for the ITER fusion reactor project.
Bodycote in Surahammar, Sweden, has since 2010 also been operating a very large HIP unit and Erasteel doubled its powder production capacity in Söderfors, also in Sweden, opening a new large capacity gas atomiser in 2011.
Improvements in gas atomisation technology to produce finer, cleaner, high purity nickel-base powders has seen PM superalloy near net shape (NNS) components produced by HIP find increasing applications in the aero and land based gas turbines as well as other sectors such as energy.
Static and dynamic mechanical properties of HIPed superalloy products can be at the level, or very close of extruded + forged products . HIP parameters and final heat treatment conditions can be readily adapted to control the grain size and precipitate distribution in the microstructure of the PM superalloys.
This has led to the development of specific PM superalloy grades with higher alloying content than conventional cast and wrought grades, which allow aero engines to withstand higher combustion temperatures and pressure ratios resulting in increased fuel efficiency and performance. It would be difficult, if not impossible, to produce similar superalloy compositions by the cast and wrought routes.
The HIP route also allows the production of bi-metal parts with dual properties. One example is a turbine disk with high yield strength at the intermediate temperatures in the hub and with high creep resistance at the rim .
A further significant development has been the computer modeling of the HIP process which has led to the production of NNS PM superalloy components. A French consortium of SNECMA, Turbomeca and Tecphy (now Aubert & Duval) and separately VILS in Russia have successfully developed modeling technology to produce complex shaped as-HIPed components. As-HIPed PM superalloy turbine discs produced by VILS are shown in Fig. 7. An as-HIPed turbine casing made from SYP3 (Astroloy-type grade PM superalloy) for the SNECMA CFM 56-5 aero engine is shown in Fig.8.
Another significant application for hot isostatic pressing is the production of sputtering targets used for coatings, magnetic memory materials and microelectronic layers. High purity, homogenous materials with 100% density can be produced from chromium, refractory metals, ceramics or other materials which cannot be made by melting technology.
HIPed sputtering targets may be further HIP diffusion bonded to a suitable supporting substrate or backing plate. Bodycote HIP, a leading producer of HIPed components and provider of HIP services, has developed capsule technology which the company states opens up a range of sizes, designs and chemical composition of sputtering targets. Fig. 9 shows some HIPed chromium powder billets.
Promoting HIP technology
Although the first in a series of successful international conferences on isostatic pressing was organised by the author and his team at MPR Publishing in the UK as early as 1978, and a specific ‘International HIP Conference’ series was started in Lulea, Sweden in 1987, the awareness of PM HIP technology and its potential was still considered to be limited compared with some other PM technologies.
In 2009 the MPIF established a separate ‘Isostatic Pressing Association (IPA)’ in North America to provide more information and education on the area, and in 2010 the EPMA established a European counterpart in the form of the European PM HIP Group (EPHG). Adeline Riou (Erasteel, France), chair of the EPHG, stated that collective actions by the trade associations can be an effective way to improve the awareness of HIP. The group has published a free educational brochure on HIP that can be downloaded as a PDF (opens in a new window).
The EPHG will be organising a Special Interest Seminar on ‘Materials Properties & Materials Selection for Hot Isostatic Pressing’ (Wednesday, September 19 2012) to coincide with the EuroPM2012 Conference and Exhibition scheduled for Basle, Switzerland, September 16-19 (www.epma.com/pm2012).
There will also be technical sessions devoted to HIP at the PowderMet2012 Conference & Exhibition, Nashville, June 10-13 (www.mpif.org), and at the PM2012 Powder Metallurgy World Congress & Exhibition, Yokohama, Japan, October 14-18. (www.pm2012.jp)
Further information on the activities of the isostatic pressing groups can be obtained from the two trade associations www.mpif.org and www.epma.com.
 P. Siitonen (Metso Materials Technology, Finland), ‘PM HIP Components for O&G and Other Demanding Applications’. Presentation at Workshop on HIP Processing of Materials for Aggressive Environments’, Derby, November 16, 2011.
 G. Raisson (Aubert & Duval, France). Presentation at Workshop on HIP Processing of Materials for Aggressive Environments’, Derby, November 16, 2011.
 G. Raisson, etal (Aubert & Duval, France). ‘Production of Net-Shape Static Parts by Direct HIPing of Nickel-base Supealloy Prealloyed Powders’. Paper presented at Euro Superalloys 2010, and published in Advanced Materials Research, Vol 278, 2011, pp 277-292.
 S. Davies (Bodycote HIP Ltd, UK). ‘HIPing – The Process and what you can do with it’. Presentation at Workshop on HIP Processing of Materials for Aggressive Environments’, Derby, November 16, 2011.
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