Hot Isostatic Pressing

Hot Isostatic Pressing

Topics CoveredBackground

The HIP’ing Process

Advantages of HIP’ed Materials

HIP and Maching

What Type of Things can be Produced by HIP’ing?

What HIP’ing can be used for

Cladding

Areas where HIP’ing is Utilised

HIP in the Gas Turbine Industry

HIP vs Conventional Foundry Technology

Summary

BackgroundIn 1976, Howmet Corporation became the first company to offer hot isostatic pressing (HIP) services to the aerospace industry. The HIP process, which subjects a component to elevated temperatures and pressures to eliminate internal microshrinkage, helped engineers respond to the aerospace industry’s increasingly stringent regulations. HIP enabled engineers to design components so they could meet specifications for use in critical, highly stressed applications.

The HIP’ing Process

The HIP process provides a method for producing components from diverse powdered materials, including metals and ceramics. During the manufacturing process, a powder mixture of several elements is placed in a container, typically a steel can. The container is subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder. The container is then sealed and HIP’ed The application of high inert gas pressures and elevated temperatures results in the removal of internal voids and creates a strong metallurgical bond throughout the material. The result is a clean homogeneous material with a uniformly fine grain size and a near 100% density.

Advantages of HIP’ed Materials

The reduced porosity of HIP’ed materials enables improved mechanical properties and increased workability. The HIP process eliminates internal voids and creates clean, firm bonds and fine, uniform microstructures. These characteristics are not possible with welding or casting. The virtual elimination of internal voids enhances part performance and improves fatigue strength. The process also results in significantly improved non-destructive examination ratings.

HIP and Maching

One of the primary advantages of the HIP process is its ability to create near-net shapes that require little machining. Conventional manufacturing methods use only 10-30% of the material purchased in the final product the rest is removed during machining. A HIP’ed near-net shape part typically uses 80-90% of the purchased material. As a result, machining time and costs are significantly reduced. The strong combination of improved raw material use and greater machining efficiency that results from the HIP process has driven the growth of HIP’ed powder metal parts manufactured from nickel-based and titanium alloys. In fact, HIP has become the standard ‘bill of material’ on virtually all powder metal components produced by Howmet’s HIP operation.

What Type of Things can be Produced by HIP’ing?

The HIP process enables engineers to produce materials of all shapes and sizes, including cylindrical billets, flat rectangular bar billets, solid shapes with complex external geometry, and complex shapes with internal cavities. Because powder metals do not have the directional property characteristics of forgings, the HIP process can produce materials from metallic compositions that are difficult or impossible to forge or cast. Howmet’s expertise in HIP powder compaction is displayed in the manufacture of abrasive tips, figure 1. Abrasive tips are uniquely layered compacts of ceramic and metallic powders which are used for turbine blade wear protection.

Figure 1. Ceramic and metal sides of finish machined abrasive tips

What HIP’ing can be used for

The HIP process is now not only used for densifying castings, but in many other

applications such as diffusion bonding of dissimilar materials, component repair and powder metal consolidation. In the powder metal market, Howmet applies HIP technology in four separate areas:

         Consolidation of powder metals (PM)

         Creation of PM shapes

         Production of near-net shapes

         Cladding.

Cladding

Another fundamental application of the HIP process is cladding. Cladding is the selective bonding of hardfacing materials onto various substrate surfaces. A less expensive material is coated with a thin layer of powdered metal, creating a buffer on its wear surface. This reduces costs by placing expensive, wear resistant materials only where they are needed. As a result, wear resistant properties are improved without incurring unnecessary cost penalties. An additional benefit of cladding is that it can create bonds between otherwise incompatible materials such as metal, intermetallic, and ceramic powders.

Most notably, cladding is used in the production of diesel engine valve lifters, figure 2. Here, the hardfacing material (tungsten carbide) is bonded to a lower cost material such as an alloy steel. Howmet has also used this capability in a number of other industries, such as the offshore and plastic extrusion equipment industries, where cladding is used to fortify various components including valve bodies, thick wall casings, and compound tubes, figure 2.

Figure 2. Left, diesel engine valve lifters with HIP clad carbide hardfacing and right, cross section of 4140 steel extrusion barrel with HIP clad inside diameter of

nickel alloy hardfacing.

Areas where HIP’ing is Utilised

Today, HIP has expanded well beyond aerospace products and is finding new applications in a range of industries, including automotive (turbocharger wheels and diesel engine valve lifters), medical (prosthetic devices), petroleum (valve bodies) and chemical processing. HIP offers engineers in these industries greater design freedom than was previously possible with conventional processes such as forging and casting. Parts which cannot be made by the more conventional processes are now possible using HIP, figure 3. An example of this is the dual alloy wheel. In this the hub, which is made of a HIP consolidated powder metal, is bonded to a cast outer ring through the use of the HIP process. The resulting part has excellent tensile properties in the hub and high stress rupture properties on the outer ring.

Figure 3. A Selection of parts made by HIP’ing

HIP in the Gas Turbine Industry

A rapid rise in the use of the HIP process followed the intensification of standards within the gas turbine industry. These standards required the elimination of shrinkage porosity in investment cast components, such as increasingly complex airfoils.

HIP vs Conventional Foundry Technology

Conventional foundry technology was not up to the task. By developing HIP, Howmet met mechanical property requirements and eliminated shrinkage porosity defects. HIP provided the means to produce the desired high density, fine grain material.

Summary

Since it’s inception in 1955, the use of the HIP has grown steadily in the powder metal and casting densification fields. During the last 25 years, HIP has become a proven process in the production of aerospace and industrial gas turbine parts, and the future looks bright. New markets have developed for rocket engines, satellites and aerospace airframe castings. HIP continues to be used more frequently in the production of powder metal parts and shapes. Cladding and near net shape technology are on the rise with significant growth expected in the production of sputtering targets. As with any technology, awareness by industry is the key to growth. With this in mind, a HIP council was recently formed consisting of equipment manufacturers and HIP suppliers. The inaugural meeting of the Hot Isostatic Pressing Council of the Advanced Particulate Materials Association took place on 30 August, 1999 at Bodycote IMT,

Andover, Massachusettes. This council will now address the areas of safety, marketing and technology, to further enhance the capabilities of the HIP process. 

Primary author: Steven Mussman

Source: Materials World Vol. 7 no. 11, pp. 677-78, November 1999.

For more information on Materials World please visit The Institute of Materials.

Hot Isostatic Pressing (HIP) is a method by which a workpiece is processed under the simultaneous application of high isostatic pressure gas like argon, generally over 98MPa(1000kgf/cm2), and high-temperature, generally over 1000 ﾁ C.HIP is an indispensable process of manufacturing for industries such as sintered hard alloys, ceramics and super alloys, and a prospective technology which fill the requests of the coming generations in which high-value added products will be more and more in demand.

Production technology

 Daido steel offers the diversity of manufacturing equipment and high-caliber  production technology supplying high-quality target materials and evaporation materials which only a general specialty steel manufacturer can offer. 

 E B F ( Electron-beam Melting Furnace ) 

Features

 Water-cooled copper mold remelting lamination solidification under highly vacuum state

Melting atmospheric pressure

 10-10 Torr

Melting materials

 High melting-point metalic(Nb,Mo,Ta,W)

V I F ( Vacuum Induction Melting ) 

Features

 Refractory crucible melting under vacuum state

Melting atmospheric pressure

 -10 Torr

Melting materials

 High-grade specialty steel, super alloy

V A F ( Vacuum Arc Melting Furnace ) 

Features

 Water-cooled copper mold remelting lamination solidification under vacuum state melting atmospheric pressure 

Melting atmospheric pressure

 -10 Torr

Melting materials

 High-grade specialty steel, super alloy (Ti, Zr) 

P S C F ( Plasma Skull Casting Furnace ) 

Features

 Uncontaminated melting of granulated material in skull(Remelting of rod materials is also possible)

Melting atmospheric pressure

 760 Torr (Ar, regulated atmosphere) 

Melting materials

 Special metals, Alloy materials,  Reactive metals (Ti, Zr, Cr)

P P C F ( Plasma Progressive Casting Furnace ) 

Features

 Melting of granulated material in water-cooled copper mold 

Melting atmospheric pressure

 760 Torr (Ar)

Melting materials

 Reactive metals (Ti, Zr)

H P ( Hot  Press ) 

Features

 Sintering of powder in high temperature

Temperature

 2,000 °C max.

Atmosphere

 Vacuum, Ar 1 atmosphere

Pressure 

 250 kg/cm max.

Processed materials 

 W-Si, Mo-Si, Tb-Fe-Co

H I P ( Hot Isostatic Press ) 

Features

 Isostatic pressing in high temperature 

Temperature

 2,000 °C max.

Atmosphere

 Vacuum, Ar 1 atmosphere

Pressure 

 2,000 kg/cm max.

Processed materials 

 Cr, Cr-Alloy

Hot Isostatic Pressing

It may sound like some new, exotic dry cleaning process and though many have heard of "HIP", Hot Isostatic Pressing, few of us understand the many benefits of this materials process. Since it's largely misunderstood, many conservative engineers are reluctant to adopt HIPping as an element in their manufacturing designs, thus missing a valuable process tool.

HIP is a process that subjects a material simultaneously to both high temperature and high gas pressure, usually Argon, in vessels equipped with sophisticated control systems and telemetry.

Typically, the temperature is selected to permit limited plastic deformation of the material being processed in the solid state at an argon gas pressure of 15,000, 30,000, or at times, 45,000 psi (1,000 to 3,000 atmospheres) is isostatically exerted on the heated parts for a period of time. The chamber is then slowly cooled, depressurized and the parts removed.

(Figure 1. Typical HIP schematic.)

Since modern HIP chambers tend to be very large, huge multiples of small parts can be accommodated in a single HIPping run, thus rendering the unit cost of the process to a small number.

HIP can close internal porosity in a material without distorting external geometry, consolidate powder materials to 100% of theoretical density or form perfect diffusion bonds between similar or dissimilar materials.

All raw materials contain microscopic voids and "bubbles" of gas, from that standpoint, all raw materials can be considered porous. The advantages of HIP for such porous materials include the elimination of all internal porosity in simple or complex shapes with resulting improvement of mechanical properties such as ductility and fatigue life.

The HIP process falls into three categories: Densification, Powder Metallurgy and Composites, diffusing two like, or unlike metals together.

The benefits for powder materials include the ability to produce 100% dense billets and powdered metal near-net shapes at relatively low cost because of the reduction in machining costs. Near net shapes can be pressed to 100% density, Shape control is obtained by cans and mandrels designed using both CAD and FEA programs. Small variations in packing density lead to small distortions in the final shape. Mating surfaces, for instance, will almost always have to be machined. HIP can also achieve 100% diffusion bonds in clad composite materials.

Advantages

The decision to employ hard chromium plating would be dictated by the following needs and requirements:

1. Control of microstructure2. Higher content of alloying elements3. High material purity4. Near net shape components up to 12,000 kg5. Complex capsule and material design

Typical HIP products now include automotive parts, pump bodies, valves, vacuum chambers, bearings, sterile enclosures, etc. anywhere residual porosity causes high rejection rates, unacceptable property levels and surface finishing problems after machining. Commercial alloy applications include steel, stainless steel and aluminum castings. Not limited to metals, the process is very versatile, having been used to densify ceramics, plastics, glasses and many other materials.

Many design engineers will avoid the use of low-cost castings in applications that require the freedom from porosity, for example for high-vacuum surfaces or containers, for surfaces that need to be antiseptically cleanable, or for fatigue-critical applications. They need to know that a HIP densified casting is a very cost-effective alternative to machining parts from a wrought blank.

Material densification via the HIP process, densifies to the material's theoretical limit. This has several advantages including, improvement of mechanical properties, enhancement of physical assets and ease of manufacturing. Typical applications have been for fatigue-critical applications like nickel and titanium alloy structural castings for jet engines, turbine blades and vanes, and cast cobalt-chromium or titanium alloy orthopedic implants, such as hip joints.

The HIP process allows for the manufacture of composite or dissimilar materials, thus avoiding welds or costly, gasketed assemblies. The same high temperature and pressures may be applied to achieve the diffusion bonding of encapsulated metal powder to a solid material or the diffusion bonding of different solid materials. Thereby providing for the combination of properties such as a stainless steel body diffusion bonded to a titanium nozzle that is nearly free from residual stresses.

(Figure 1. Diesel engine valve lifter.)

For some material combinations, an interlayer is required to prevent brittle phase transformation or to minimize thermal expansion differences between materials for bonding.

The diesel engine valve lifter shown in figure two is a good example of a clad product. This product replaced a troublesome design that used furnace brazing to apply a tungsten carbide wafer to a steel lifter body. The HIP bonded valve lifter proved to be significantly more reliable and saved a substantial amount of money in scrap and repairs. In total, in the past five years Bodycote-IMT of Andover, MA reports that they have produced well over 3 million lifters without a single field failure.

Hot isostatic pressing can provide many benefits by stabilizing a material, removing residual stresses, densifying and eliminating voids and occlusions. The process "homogenizes" an alloy and in most cases, the properties of the material are enhanced, providing greater stability and wear characteristics.

With cast materials, parts can be cast to near net shape and HIP'd, thus eliminating costly machining and additional machining stresses. A good example would be aluminum castings, notoriously thermally unstable to the point where producing a cast aluminum mirror was once considered virtually impossible.

However, a large 12" flat stabilization mirror is being used in the fire control system of the FVS Bradley tracked vehicle. Originally, a thick, 6061-aluminum alloy blank was heavily machined and ribbed in order to provide light weighting of the backside of the mirror. More material ended up in chips than remained in the mirror structure. In an

effort to reduce costs, a cast aluminum mirror of A-201 alloy was produced. The mirror blank was HIP'd and the only post machining required was for the two mirror trunnions. The mirror face was subsequently diamond fly-cut to an optical surface. Being theoretically dense, there were no voids or occlusions to mar the optical surface.

The final cast and HIP'd mirror proved to be far more thermally stable than it's machined counterpart. In addition, this material processing reduced the final production mirror cost by more than 30%.

Beryllium and titanium are both sintered materials and both are extremely difficult to machine.

(Table 1. These tensile properties show the difference between treated and untreated and HIP tensile bars from 7 x 7 3/4 in permanent mold plates of several different aluminum alloys.)

Beryllium is not only a costly material but it is also prone to micro-cracking or "twinning" during the machining process. Final dimensions require extremely careful machining with very small cuts of .0005". This is followed by an acid etch, hopefully, to final dimensions. By HIP'ing beryllium powders to near-net shape, high machining costs, are thus eliminated and the process radically enhances the properties of the material.

The highly and oft misstated toxic effects of beryllium aside, HIP'ing Be to near net shape greatly reduces the final cost of a part made from this remarkably strong and lightweight, material.

Cast Titanium aircraft engine parts are routinely HIP'd for greater reliability and performance. Today's jet engines could not function without the HIP process to densify and improve this difficult cast material.

The HIP process is recognized as a means of providing enhanced soundness or integrity, increased density and improved properties to w wide rane of materials. The process significantly improves the mechanical properties an fatigue strength aluminum alloy, sand and permanent mold castings. It has proved capable of eliminating microporosity resulting from the precipitation of hydrogen and the formation of internal shrinkage during solidification. The HIP advantage is of importance in the manufacture of castings subject to radiographic inspection when required levels of soundness are not achieved in the casting process.

At elevated temperatures and pressures voids formed by hydrogen precipitation in the castings are collapsed and healed as are shrinkage voids uncontaminated by hydrogen.

Mechanical densification such as forging did not provide the same results, even at elevated temperatures. Typical tensile improvements in Aluminum castings are illustrated in table one.

October 1, 1999 by Fred Hochgraf

Published October 1, 1999, in our Nuts & Bolts, Volume 11 newsletter. Click here to view the entire newsletter.

http://nhml.com/resources/1999/10/1/hot-isostatic-pressing

http://nhml.com/index.cfm new hampashire materials laboratory ltd.

A hot isostatic pressing apparatus (HIP apparatus) comprises a vertically cylindrical high-pressure vessel comprising a high-pressure cylinder 1 and upper and lower lids 2 and 3; a bottomed cylindrical casing 6 capable of housing workpieces 9 and a resistance-wire heater 11, a heat insulating structure 16 equipped with a gas flow regulating valve 15 and

formed in a bottomed cylindrical shape on the outside of the casing 6 so as to cover the casing 6, and a heat sink 17 having a water cooling mechanism provided in the space defined by the heat insulating structure 16 and the inner surface of the high-pressure cylinder 1, which are provided within the high-pressure vessel; and a pressure medium gas stirring fan 12 for promoting the temperature uniformity of the space of the treatment chamber 7 for housing the workpieces 9, the stirring fan being arranged on the lower lid 3 side within the casing 6, whereby the cooling to a temperature range of 100° C. or lower which allows a quenching treatment and the safe manual handling of workpieces can be efficiently performed.

New HIP-ping method HIP–ping by super-high pressure liquid

It is known that microscopic intergranular cavities decrease substantially the strength of materials. Fractures in parts usually start from these cavities.

The current technology for reducing this problem is by treating many critical parts by a process of Gas pressure (up to 1500ATM) integrated with high temperature (up to 1300 C}, after production.

“ Hot Isostatic Pressing” or “HIP-ping “ closes and seals the inter granular cavities in materials (picture #1) and therefore increases their reliability.

At present, many critical parts for aircraft engines, some rocket units and etc. are treated by HIP-ping process.

As some heat resistant steel alloys are very fragile, they are not made by plastic deformation processes and they are produced by casting. Parts from such alloys, for example, blades for gas turbines, suffer from micro and macro-cavities after casting. These cavities decrease considerably strength of parts and they must by eliminate.

At present, HIP-ping process is sole technological process, that can do it.

“MLC Extrusion Systems” has developed an innovative method for treatment of materials, the “ Hot Isostatic Pressing by Liquid”, which has match technical and economical advantages.

(see picture #2)

The treated parts are loaded in an electric oven. The hot parts are later loaded unto the high pressure working chamber , which is located between stamps of hydraulic press .

It is filled with working liquid, the press stamp falls and presses the plunger creating a super high pressure (14,000-18,000 ATM) into the chamber.

The loaded parts are soaked under this super high pressure for 20-60 seconds and they are unloaded after cleaning the chamber by inert gas from receiver .After the unloading the system is ready for another cycle. The treatment by super high pressure allows to “close” both micro-cavities and even macro-cavities. Therefore the quality of such “healing” process is very elevated. Moreover, energy consumption is very low due to the incompressibility of the liquid, and due to heating the parts outside the working chamber. Investment in the equipment is very low, especially on availability of a hydraulic press; the technological process is simple, reliable, and safe and may be easily automated.