Hot Isostatic Pressing (HIP)

The production of a PM HIP component is leaner and shorter than usual conventional metallurgy processes. The cost of HIP relative to energy and materials costs has decreased by 65% over the last two decades.

 

HIP - A high quality cost effective solution.

 

  • Introduction

    Hot Isostatic Pressing (HIP) is a process to densify powders or cast and sintered parts in a furnace at high pressure (100-200 MPa) and at temperatures from 900 to 1250°C for example for steels and superalloys. The gas pressure acts uniformly in all directions to provide isostropic properties and 100% densification. It provides many benefits and has become a viable and high performance alternative to conventional processes such as forging, casting and machining in many applications.

    Its positioning is very complementary to other powder metallurgy (PM) processes such as Metal Injection Moulding (MIM), pressing and sintering, or the new additive manufacturing technologies. It is even used in combination with these PM processes for part densification and the production of semi finished bars or slabs.

    A wide range of component types can be manufactured thanks to HIP. Its capabilities include large and massive near net shape metal components such as oil & gas parts weighing up to 30 tonnes, or net shape impellers up to one metre in diameter. Equally it can be used to make small PM HSS cutting tools, such as taps or drills made from PM HIP
    semi-finished products, which can weigh less than 100 grams, or even very tiny parts such as dental brackets.

    As a result, HIP has developed over the years to become a high-performance, high-quality and cost-effective process for the production of many metal (or ceramic) components.

    Source: Olle Grinder, Euro PM2009

  • HIP Design Guidelines

    Introduction

    These design guidelines provide some hints regarding PM HIP component design and manufacturing, so that potential users of the technology understand better the possibilities and issues specific to the PM HIP technology.

    Component designers can choose between four different options when considering the PM HIP technology:

    • Simple shapes such as round, tubular or flat bar that will either be further machined or forged and rolled
    • Near-net-shapes (NNS) which will reduce the need for machining or welding
    • Complex net-shapes (NS) which eliminate the need for machining in the functional parts of the component. This provides more freedom in designing components and geometries impossible to machine
    • Bimetal or composite construction, where a metal powder layer will be HIPped for instance on a conventional metal substrate, as an alternative to PTA or spraying technologies. In this case, powders are only used in the functional area of the component.

    Computer Modelling

    Computer modelling is used, in combination with the experience of HIP design engineers, to simulate accurately the powder densification and shrinkage behavior and to achieve optimum container geometry and dimensions.

    Computer modelling allows optimization of the HIP process in particular for complex geometries. It also allows designers to get as close as possible to the finished shape, thereby eliminating expensive machining operations or avoiding any risk of undersize part.

    Computer modelling is useful in particular in the case of sharp corners, when there are different container thicknesses or for complex net shape parts.

    Container Materials

    Container materials and thickness are very important parameters when designing a PM HIP part.

    The container must satisfy the following considerations:

    • It must be strong enough to maintain shape and dimensional control prior to and during HIP.
    • It must be soft and malleable at HIP temperature.
    • It must be compatible with the powder being processed and not penetrate nor react with the powder mass.
    • It must be leak proof both at low and high pressures.
    • It must be weldable for secure sealing and be removable after HIP.

    The most common container materials are low carbon steels or stainless steels. However in specific cases, containers can be made of high temperature material such as titanium or glass for the compaction of refractory materials. The normal container thickness is between 2 and 3mm.

    Near Net Shape container construction (courtesy of Kennametal HTM)

    Container Shrinkage

    During Hot Isostatic Pressing, the container shrinkage is not isotropic and depends on many parameters such as:

    • Container material
    • Container overall geometry
    • Container thickness
    • Positioning of container welds
    • Variations in powder fill density within the container

    For instance the end plates of a straight cylindrical container will not shrink radially to the same extent as will the cylindrical section. This results in an end effect called « elephant's foot » or « hourglass effect ».

    Effect of sheet thickness in HIP container design
    (F. Thümmler, Introduction to Powder Metallurgy, published by Institute of Materials, London, 1993)

    Positioning of Container Welding Seams

    The positioning of welded seams on the container has a major impact on its deformation behavior during hot isostatic pressing. This must be taken into account when designing PM HIP part as shown in the table below which shows the advantages and disadvantages of each method.

    Regular deformation and less welding work are the main benefits of method 3. In method 4, the construction reduces the stress on welded seams because the gas pressure is applied on both sides. Therefore, design methods 3 and 4 are usually preferred to methods 1 and 2.

    Container Deformation

    Container materials and thickness are very important parameters when designing a PM HIP part.

    Tolerances of PM HIP parts

    Container materials and thickness are very important parameters when designing a PM HIP part.

    Summary

    Container materials and thickness are very important parameters when designing a PM HIP part.

     

  • Microstructure of PM HIP Parts

    Introduction

    Thanks to the rapid solidification process, fine and regular microstructures can be obtained thanks to the PM HIP technology, with strength values similar to those of forged parts.

    Stainless Steels

    When using HIP with stainless steels companies can obtain components with:

    • An excellent combination of toughness and strength
    • Isotropic mechanical properties
    • Same or better mechanical properties than forged products
    • Same or better corrosion resistance than forged products

    High Speed Tool Steels

    The use of HIP with tool steels enables:

    • Longer tool life
    • More reliable tool life
    • Better fatigue strength
    • Better wear resistance due to higher carbide content
  • Benefits of HIP Technology

    The main benefits of the PM HIP technology

    PM HIP technology offers many benefits in the following key areas:

    • Component quality and performance
    • Due to the fine and isotropic microstructures produced by HIP
    • Reduction of the number of welds on complex parts
    • Dense, without segregation

    Design Flexibility

    • Near-Net shapes, Net shapes or Bimetal construction
    • Use of composite materials
    • Freedom of part sizes and production series
    • Freedom of alloys

    Cost Reduction

    • A lean manufacturing route, leading to shorter production leadtimes
    • Reduction of machining needs
    • Producing single parts where previously several were required
    • Less NDT needed and easier NDT

    Reduced Environmental Impact

    In the case of near-net-shape and net-shape parts due to the excellent material yield compared with conventional metallurgy

    PM HIP Uses and Applications

    This document focuses on HIP technology for the compaction of metal powders in a metal container. In this case, the powder is compacted through pressure while the temperature will ensure diffusion on the contact surface between powder grains, until all hollow spaces are closed so that a 100% density is achieved.

    However Hot Isostatic Pressing is also widely used for:

    • The densification of cast parts
    • The densification of sintered powder parts such as cemented carbides or ceramics
    • The densification of MIM parts
    • Diffusion bonding between metal parts

    Thanks to this wide range of uses HIP is currently employed for the manufacture of parts used in many industry sectors often in business critical and aggressive environments. Some examples of these include:

    • Energy
    • Process Industry and Tooling
    • Transportation and Aerospace
    • Nuclear and Scientific
    • Oil and Gas

    Comparison with other manufacturing technologies

    PM HIP technology is often chosen as an alternative to conventional technologies such as forging and casting. In particular it can offer the following features:

    • Improved material properties, provided by the fine and homogenous isotropic microstructure
    • Improved wear and corrosion resistance, through extended alloying possibilities
    • Reduction of the number of welds and associated cost and inspection issues
    • With the near net shape option, two separate welded parts can be produced in one single step
    • The bimetal option, using expensive materials only in functional areas
    • Reduction of machining costs, thanks to near net shape or net shape options
    • New solutions to produce complex internal cavities, which are difficult or impossible to machine

    The benefit of PM HIP technology increases compared to cast or forged parts, especially when:

    • Using high value materials such as alloyed steels or nickel- and cobalt-base alloys, because of the near net shape or net shape possibilities
    • Producing small series of large and complex shapes
    • Where processing costs are high, due to a combination of multiple operations such as machining, welding and inspection.

    In summary HIP provides innovative solutions to shorten manufacturing cycle times and to produce small series of parts.

 

HIP Main Process Steps

  • Powder Manufacturing

    The most suitable metal powders for Hot Isostatic Pressing are produced by gas atomisation because of :

    • The perfectly spherical powder shape
    • The high fill density, thanks to the spherical shape and particle size distribution
    • The excellent reproducibility of particle size distribution, ensuring consistent and predictable deformation behavior
    • The wide range of possible alloys, due to the rapid solidification rate.

    Note: gas atomisation is a physical method to obtain metal powders, like water atomization or centrifugal atomisation, as opposed to chemical or mechanical methods.

    SEM picture of gas atomized powders, courtesy of Erasteel

    The gas atomisation process starts with molten metal pouring from a tundish through a nozzle. The stream of molten metal is then hit by jets of inert gas such as nitrogen or argon and atomized into very small droplets, which cool down and solidify when falling inside the atomisation tower. Powders are then collected in a can.

    A wide range of metal powders can be hot isostatically pressed. In addition to standard or customized compositions of steels, nickel-base and cobalt-base alloys, many powders are compacted by hot isostatic pressing such as Titanium, Copper, Lead, Tin, Magnesium and Aluminium alloys. Another benefit of the PM HIP technology process is that new alloy compositions which are impossible to cast or forge can be considered thanks to the rapid solidification process. Indeed, during hot isostatic pressing, the elements do not have time to segregate like in cast parts, because the temperature is below the melting point
    (~0.8 x T solidus).
    This possibility has been very valuable for metallurgists to invent new alloy compositions for instance in the field of :

    • Tool steels for higher wear or temperature resistance
    • Stainless steels for high corrosion resistance in difficult environments
    • Composite materials e.g. wear resistant metal and ceramic composites
  • Container Manufacturing

    Container manufacturing involves the following steps:
    • Container sheet cutting and forming/shaping
    • Assembly of steel sheets and optionally pipes and metallic inserts by TIG welding
    • Leak testing, by evacuating the container and introducing helium or argon under pressure. If a leak is detected and located,repair is undertaken.

    The integrity of welds is critical, otherwise when the vessel is pressurized, argon will enter the container and become entrapped in the powder mass. The argon will remain in the material and argon-filled pores will strongly deteriorate the mechanical properties.

  • Container Filling and Outgassing

    Once assured that the container is leak-free, the powder is introduced via a fill-tube. In order to achieve maximum and uniform packing of the powder, which is necessary to ensure a predictable and consistent shrinkage, a vibration table is used. Vibration will allow the powder to better fill narrow spaces and remote areas. In special cases such as critical aerospace applications, the filling operation is done under inert gas or vacuum to minimize contamination of the powder.

    The next step is outgassing to remove adsorbed gases and water vapor. After outgassing, the fill tube is welded to seal the container. The absence of leaks is critical. Otherwise, when the HIP vessel is pressurized, argon will enter the container and become entrapped in the powder mass, creating argon-filled pores with damaging effects on the mechanical properties.

  • The HIP Process

    During the hot isostatic pressing process, the temperature, argon-gas pressure and holding time will vary depending on the material types.

    After filling and closing, the HIP vessel is evacuated to eliminate the air. Then, while heating up, Argon gas pressure is increased in the vessel. After reaching the calculated pressure, the increase in pressure is done through gas thermal expansion. In the holding time, gas pressure and temperature are constant. After this, a rapid cooling takes place, with decreasing pressure and temperature.

    • Chosen temperatures are below approx. 0.8 x T solidus, to avoid having a liquid phase.
    • The gas used is generally Argon but in special applications, other gases or gas mixtures are used.
    • The rise in pressure is built up with a compressor.
    • The gas pressure is equal inside and outside the insulation. But the gas density is higher outside the insulation than inside because of the lower temperature.

    Modern HIP systems can feature uniform rapid cooling (URC) which circulates lower temperature gas to cool the part at a controlled rate of up to 100°C/min. The HIP quenching technique cuts cycle time dramatically by shortening the cooling stage by as much as 80%. It also provides the benefit of combining heat treatment with HIP in a single step. The uniform rapid cooling restricts grain growth and thermal distortion of the parts and avoids surface contamination by using high purity argon gas.

    A HIP unit consists mainly of a pressure vessel, a heating system and an Argon gas system. Various HIP constructions are available:

    • with or without a frame (For pressures above 100 MPa and HIP diameters above 900mm, frame construction is chosen for safety reasons
    • with or without top screw thread locking systems
    • with different heating systems

    Molybdenum furnaces are used for temperatures up to 1350°C and carbon graphite/tungsten furnaces up to 2200°C. Inside the pressure vessel, insulation (ceramic fibers and Molybdenum sheets) is used to protect the steel pressure vessel against the heat and to hold the high temperature inside the insulation. The bottom, cover and pressure vessel are water cooled to protect the sealing ring and the vessel against the heat.

    In large HIP units, diameters can reach 2200mm and height of more than 4000mm, with a capacity of up to 30 tonnes.

  • Container Removal

    After HIP, the container can be removed (when the container is not to be re-used) by :

    • Machining
    • Acid pickling
    • Slipping off
  • Post Processing Operations

    After container removal, various additional operations can take place, including:

    • Heat treatment
    • Machining
    • Finish grinding
    • Surface treatment
    • Assembly
  • Quality and Testing

    Depending on the size and value of the parts being made various types of quality testing will be undertaken. Two of the most common are ultrasonic testing and dye penetrant inspection. CAT scanning is also used in critical high value applications.
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