Surface treatments of tool steels

The main aim of surface treatments for tool steels is to create a structure with gradient mechanical properties that improve resistance to friction and wear without compromising resistance to oxidation and corrosion, and resistance to mechanical, thermal and surface fatigue without compromising resistance to crack propagation and spalling. The processes commonly used to achieve these property gradients are mechanical (surface nanostructuring by mechanical attrition), thermal (surface quenching treatment) or thermochemical. The tribological characteristics of the surface can also be enhanced by thin coatings a few micrometers thick, providing protection against oxidation, corrosion or overheating in service. These coatings are produced by conventional liquid deposition techniques (notably electroplating), as well as by dry deposition techniques using chemical (CVD) and physical (PVD) methods. The combination of the property gradient obtained by thermal or thermochemical surface treatment and the hard coating obtained by one of the aforementioned techniques enables the surface properties of the tool steel to be optimized for its conditions of use. For certain applications, notably in the field of hot or cold forming tooling and certain wear parts, it is possible to use hot isostatic hardfacing or compaction, to obtain thick coatings, which can even reach several millimeters in thickness in the latter case. The last family of processes substantially enhances resistance to thermal fatigue and deformation, thanks to the use of nickel- or cobalt-based alloys with high hot mechanical properties.

The first part of this article covers mechanical, thermal and thermochemical structural transformation treatments. The second part covers thin coatings obtained by the liquid or dry process, and the last part covers thick coatings obtained by hot isostatic reloading or compaction.

At the end of the article, readers will find a glossary and a table of the notations used.