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Alloy Steel Castings
Bimac Corporationwww.bodycote.com
Hot Isostatic Pressing
                               

HOT ISOSTATIC PRESSING (HIP) OFFERS THE POSSIBILITY OF ELIMINATING CLOSED POROSITY, VOIDS AND CREEP VOIDS IN CAST COMPONENTS.
HIP can also improve the homogeneity of the microstructure and the material properties.

This is especially important in the case of cast parts which are exposed to very high stresses and repeated thermal cycling, such as hot work tool steel glass moulds.

The Bodycote HIP division provides a HIP service which permits the designer to use the wrought properties of steel whilst retaining the advantage of casting to shape.

Principle
A variety of casting processes offer the attractive possibility of producing cost effective near net shape parts.

One disadvantage is that material properties may be reduced by internal porosity, when compared with forged material.

The application of a high temperature, high pressure gas environment provides the means by which internal porosity can be removed, utilizing plastic yielding, creep and diffusional effects.

Diffusion bonding of the void surfaces during the final stage of densification ensures that defects are completely removed.

The HIP treatment is conducted at a temperature at which excessive grain coarsening is avoided.

Defects of considerable dimension may be removed by the process, and the solubility of oxygen (at temperature) ensure that closed voids are virtually free from oxide inclusions.

Coach Cycles
Large HIP vessels offer the best economy of scale if they can be satisfactorily filled.

A regular, typically weekly, schedule of shared HIP cycles, at a variety of specified parameters, permits optimum vessel utilization and thus the best piece price for the customer.

Such shared cycles composed of similar alloys have been termed "coach cycles".

The requirement of a coach cycle is that parts should be made from the same or similar materials or that it is possible to HIP them using the same HIP parameters.

Coach cycles are offered for nickel and cobalt superalloys as well as steel and alloys of aluminum (the aluminum coach cycle being called DENSAL®).

Advantages
The removal of shrinkage defects and other pores and voids as potential initiators of failure, exerts a considerable positive effect upon component properties.

Low and high cycle fatigue and stress rupture properties are enhanced.

Use of HIP thus widens the range of titanium alloy compositions possible for castings in comparison with wrought systems, with the resultant property benefits.

In addition, significant cost savings may be achieved with an investment cast and HIP route in comparison to machining from solid.

HIP treatment may also provide improved homogeneity of the cast structure, with the result that chemical machining and corrosion susceptibilities are improved.

 

Casting Densification

  


Casting densification is still the most important commercial application of the HIP process, accounting for around 60% of the U.S. sales. It was first applied commercially in the early 1970s. Most of the early applications took advantage of the ability of HIP to improve fatigue life and ductility in critical materials through the closure of internal porosity in complex shaped investment castings. The earliest examples were jet engine blades, vanes, compressor wheels and structural castings as well as orthopedic implant materials like hip joints.

In recent years, commercial applications have flourished as HIP cost and delivery has improved through the use of larger and faster equipment in regional settings. Coach cycles (optimized HIP cycle for common materials) have made it possible for small customers to share space in larger HIP units at more cost-effective prices.

Typical HIP’d components now include automotive parts, pump bodies, valves, vacuum chambers, sterile enclosures, etc. where residual property levels causes high rejection rate, unacceptable property levels and surface finishing problems after machining. Cast alloys that are routinely HIP dandified include aluminum, nickel-, and cobalt- and iron-based superalloys, steels, stainless steels and titanium.