Brief hot isostatic process (HIP)
Hot isostatic pressing (HIP) is a manufacturing process used to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and workability.
The HIP process subjects a component to both elevated temperature and isostatic gas pressure in a high pressure containment vessel. The pressurizing gas most widely used is argon. An inert gas is used, so that the material does not chemically react. The chamber is heated, causing the pressure inside the vessel to increase. Many systems use associated gas pumping to achieve the necessary pressure level. Pressure is applied to the material from all directions (hence the term "isostatic").

HIPing is one of the answers to solving your precision and complex casting and manufacturing needs.

HIP is a production process of unique benefit in the precision casting, powder metal, metal bonding and ceramics industries. HIP improves the performance and yield of precision castings. HIP consolidates powders of metals, ceramics or carbides into fully dense, complex parts wit net or near net shapes. HIP is also used for diffusion bonding similar and dissimilar materials and for applying wear or corrosion resistant coatings to parts exposed to stringent operating conditions.

Hot Isostatic Pressing is a process in which components are subjected to the simultaneous application of heat and high pressure in an inter gas medium. The pressure is uniform in all directions or isostatic. Using hot isostatic pressure, the material is changed, in simpler terms, to a ‘plastic state’ which collapses the voids. The clean surfaces of the voids bond together making the components or parts stronger. In most cases, voids that were collapsed do not change or alter the shape of the parts orcomponents.

Hot Isostatic Pressing results in startling improvements in materials’ mechanical properties, as well as significantly reduced scrap losses and decreased rework and weld repair. Of high significance is the marked reduction in the statistical spread or scatter usually associated with the cast material properties. The net result is improved reliability and efficiency of material utilization in case components.

Discuss the mechanics of the process

Credit: Sandvik

Production process for hot isostatic pressing
Powder metallurgically based hot isostatic pressing (PM HIP) technology for its near-net shape products.

The production process step by step
The production process from the melt to the finished product takes place in three stages. Powder is produced by inert gas atomization. The powder is canned in sheet metal capsules, giving the product the desired shape. The capsules are consolidated into full density under high pressure and temperature by hot isostatic pressing (HIP).

Melting and atomization
The melt produced in an induction furnace is tapped into an induction-heated ladle where further alloying elements can be added in a protective atmosphere. The ladle also permits stirring and temperature control of the melt throughout the process.
When the melt is tapped from the bottom of the ladle it is discharged directly into the atomization chamber. The molten steel is broken up by jets of inert gas and the atomized melt solidifies into small spherical particles of high purity and low oxygen content and with a diameter less than 500 micron. The powder is stored under inert gas hermetically-sealed vessels.

Canning
The powder is canned in capsules of mild steel, which are produced by sheet metal forming and welding. The capsule is designed to give the fully dense end product the desired shape. Compound products can be produced by designing capsules with separate compartments for different powders or enclosing parts of solid material together with the powder.

Hot isostatic pressing (HIP)
The capsules are placed in a hot isostatic press where they are subjected to high pressure and temperatures. The hot isostatic pressing parameters of pressure, temperature and time are predertermined to give the material full density.

Post treatment
Depending on the type of material and the application, the PM HIP products will be heat treated, machined and subjected to various types of quality control, such as ultrasonic inspection, dye penetrant testing, testing of mechanical properties.
The mild steel sheet used in the can remains on the product after the hot isostatic pressing and heat treatment and is removed by machining or by acid pickling.

Notes
For processing castings, metal powders can also be turned to compact solids by this method, the inert gas is applied between 7,350 psi (50.7 MPa) and 45,000 psi (310 MPa), with 15,000 psi (100 MPa) being most common. Process soak temperatures range from 900 °F (482 °C) for aluminium castings to 2,400 °F (1,320 °C) for nickel-based superalloys. When castings are treated with HIP, the simultaneous application of heat and pressure eliminates internal voids and microporosity through a combination of plastic deformation, creep, and diffusion bonding; this process improves fatigue resistance of the component. Primary applications are the reduction of microshrinkage, the consolidation of powder metals, ceramic composites and metal cladding. Hot isostatic pressing is also used as part of a sintering (powder metallurgy) process and for fabrication of metal matrix composites.

Advantages and disadvantages

Advantages of HIP products
The ability to manufacture tailor-made powder metallurgically-based (PM) HIP products with irregular shapes and complex geometry offers several advantages over castings, forgings and fabricated materials, both in terms of design flexibility and material properties.

Four main advantages with HIP products

1. Increased design flexibility
Hot isostatic pressing allows for considerable design flexibility in shaping components, making it especially suitable for products such as flanges, valve bodies, manifolds, hubs and pump parts. Often involving irregular shapes and small runs of a type and size, each HIP product/component can be individually designed so that it is guaranteed to meet its operating conditions.

2. Reduction of costly operations
The ability to manufacture products with irregular shapes and complex geometry means less need for costly operations like machining and welding.

3. Improved process safety
The elimination of critical welds makes HIP products contribute to increased process safety.

4. Enhanced material properties
The fine microstructure and isostatic pressure with which the HIP products are processed result in isotropic mechanical properties, in other words, properties that are equal in all directions. The isotropic properties, with no segregations, can contribute to, for example, lighter constructions. And resistance to corrosion and hydrogen-induced stress cracking (HISC).

Credit: Sandvik


Possible HIP Limitations
  •  Volumetric Shrinkage
  • Surface-Connected Porosity
  •  Pressurizing gas will enter pores and hold them open (need to HIP prior to machining).
  •  Incipient Melting
  •  If a compositional gradient (segregation) exists within a part, the local melting temperature may be lower than the HIP temperature.
  • Eutectic Melting
  • Solid state reactions between parts and support tooling must be considered.
  • Creep Deformation
  • Care must be taken with thin wall section parts.
  • Material Cleanliness.
  • HIP can only eliminate internal porosity; other defects such as inclusions will remain.


How HIP can improve

Application areas
Many HIP products are used for a wide range of applications within, for example, offshore and power generation. Here are some examples of hot isostatic pressed (HIP) products:
  • Hubs
  • Manifolds
  • Pump housings
  • Steam chests
  • Stressometers
  • Swivels
  • Turbine rotor shafts
  • Valve bodies
  • Wye pieces
Enhanced product properties
The ability to manufacture HIP products with irregular shapes and complex geometry offers several advantages over castings, forgings and fabricated materials, both in terms of design flexibility and material properties.
The fine microstructure and isostatic pressure with which the HIP products are processed result in isotropic mechanical properties, in other words, properties that are equal in all directions. The isotropic properties can contribute to, for example, lighter constructions.

How HIP can impact the design