Feature Article Hot Isostatic Pressing for the Casting Industry
by Magnus Ahlfors MSc., Quintus Technologies Inc.; James Shipley BSc., Quintus Technologies AB; Dr.-Ing. Inmaculada López Galilea, Ruhr-Universität Bochum
H
ot Isostatic Pressing (HIP) is the application of an extremely high isostatic pressure at approximately 85% of the melting point on a solid object. The pressure and temperature are sustained for a predefined time with the purpose to eliminate
internal voids and defects
in components to achieve 100% theoretical density. The pressure and heat are applied to the components via an inert gas, normally argon. By eliminating internal voids and defects in the material, stress concentrations and crack initiation points are eliminated which will improve the material properties. The main properties that are improved by HIP are fatigue, creep, ductility and fracture toughness. The data scatter in the properties is also reduced by eliminating the internal defects using HIP, which gives the material more predictive properties allowing increased design values and the use of thinner sections.
(a) HIP for Cast Components HIPing is widely used within the casting industries especially for components in
high demand applications and
critical components. Most of the cast components within a turbine engine are HIPed due to the high requirements on the material properties of such components. Examples are rotating parts like turbine blades and vanes but also housings and casings. HIPing is also commonly used for cast orthopaedic implants for joint replacement to improve fatigue resistance and high performance automotive parts like crank shafts and even engine blocks. Increasingly replacement of forgings is being considered by HIPed castings to reduce cost and lead time, especially for large components.
28 ❘ September 2019 ®
Figure 1: Micrographs of cast material showing typical (a) shrinkage porosity and (b) gas porosity [2]
Figure 2: Fatigue data for HIPed and as-cast material [3]
Defects in Castings For cast material there are two main types of internal porosity caused by two different mechanisms. The first one is shrinkage porosity which is
caused by the shrinkage that the molten metal undergoes upon cooling and solidification during the casting process. When the melt starts to solidify in the mould, the material shrinks and
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