Technological watch: Materials used in the tooling field : High Speed Steels (High Speed Steel HSS) - Coatings and additional treatments

To improve their performance and extend their service life, HSS tools can benefit from a number of additional treatments, such as conventional heat treatments, water, air and oil quenching, but also using new processes such as laser quenching or cryogenic treatments, as well as the coating of the active areas - more specifically the tips and cutting edges - by depositing titanium carbide, tungsten carbide, titanium nitride or other combinations of physical reinforcement.

Optimisation of conventional heat treatments

Like all advanced technologies, the development of high-performance tool steels has undergone numerous developments, both in terms of technology and knowledge of materials. In addition to traditional annealing, quenching and tempering processes, the heat treatment of tool steels has benefited from a number of improvements: control of treatment atmospheres, control of annealing conditions according to the chemical composition of the alloys, treatment in vacuum furnaces with pressurised gas quenching.

Cryogenic treatment

When quenching steels with a carbon content of more than 0.5%, cooling to room temperature does not result in the complete disappearance of residual austenite, and thus the complete transformation of the structure obtained by quenching (martensite). A low temperature treatment (liquid nitrogen) makes it possible to control the complete transformation of the structure by reaching the temperature at the end of the martensitic transformation.

In other words, cryogenic treatment, which is still the subject of much research and experimentation today, consists of heating and cooling the tool part under extreme conditions in order to release all the stress according to the principle of zero entropy at absolute zero temperature. This makes it possible to avoid internal stresses as well as dimensional variations caused by uncontrolled material transformation.

Laser hardening

Laser hardening is a selective surface hardening technique by transformation of the solid state (martensitic hardening, tempering and severe quenching) or fusion of the surface to be treated (refusals, alloying, cladding and dispersion hardening), excluding coupling fluid. The treatment time, on the order of one second, is much shorter than that of other more conventional processes. The most commonly used process is the martensitic hardening process which is applied to carbon steels and cast irons. The application of the laser beam very rapidly increases the surface temperature (up to 1000°C/s), which transforms the surface layer into austenite. Rapid cooling caused by thermal conduction in the mass of the workpiece then leads to the martensitic transformation of the steel, generating a very fine microstructure with high hardness.

Compared to conventional processes, laser hardening offers many advantages, mainly the improvement of wear resistance, by concentrating on localised heating zones with a rapid hardening speed that limits deformation. From an economic point of view, laser hardening combines productivity, reproducibility and overall quality of the delivered product.

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