Hard facings are used in a number of different types of components generally to provide improved wear, erosion, and corrosion resistance to the substrate. A typical application is to a valve seat which may be subjected to high wear loads and high flow rates such that a hard wearing surface is required.
Traditionally, a common application method has been to weld deposit the hard facing onto a softer substrate, e.g. the main body material of the component. This can result in poor quality micro-structures, e.g. cast/inhomogeneous structures being created that may not provide the required wear/erosion/corrosion resistance. This can be a particular issue for Nuclear applications where the hard facing is cobalt based. If the facing deteriorates and releases cobalt based debris, the debris (crud), if it becomes activated, can contribute significantly to the overall plant radiation activation level. Also, depending upon the geometry, deposition process, the expansion coefficient difference between the facing and substrate, and the cooling rate, a detrimental tensile residual stress can be generated. This can cause cracking of the facing which, depending upon the in-service loading conditions, could lead to crack propagation into the substrate, potentially threatening the structural integrity of the component; a leak or structural failure could result.
One application method to address the potential poor micro-structure that may be created from weld deposition methods, is to use a Hot Isostatically Pressed (HIPed) hard facing. This is where a wear surface is created from the solid state, rather than using a localised melting route, by the HIP consolidation of powder produced by a gas atomisation process. Homogenous, isotropic, finer grained, and defect free microstructures can be created via this method which generally exhibit improved resistance to wear/erosion/corrosion. Also, where the HIP consolidated facing is HIP bonded to the substrate, a beneficial compressive residual stress is created that can arrest any crack propagation into the substrate if cracking of the facing was to occur.
For Nuclear applications it is imperative the material quality of a hard facing is assured. It is essential that care is taken particularly in the production of the powder to ensure it does not contain undesirable defects/contamination.
This paper presents: the benefits that can be achieved with creating component hard facings using the HIP process, the key defects/contamination that may be present, and the quality control measures recommended to assure the material quality.