Surface Modification of a High-Speed Tool by Combined Tungsten and Nitrogen Saturation

Abstract—The importance of this work is caused by the tightening of the performance requirements for high-speed tools due to the widespread introduction of automatic lines and numerically controlled machines. Increasing the tool life is also necessary to reduce the consumption of expensive alloying elements, primarily tungsten. Solving these problems requires the use of technologies for hardening cutting surfaces. The TCT (thermochemical treatment) processes combining diffusion surface alloying with nitrogen saturation have shown their efficiency in surface hardening of various steels. The aim of this work is to study the combined surface saturation of high-speed steel with tungsten and nitrogen to increase the durability of small tools. Experimental studies are carried out on samples and small-diameter drills made of R6M5 steel. An installation for nitriding in multicomponent media is used for laboratory experiments on the combined TCT process. Tungsten metallization is carried out by a slip method with parallel nitriding of tool in a glow discharge. To determine the conditions that provides the necessary temperatures for saturation with tungsten and nitrogen, the temperatures of steel samples at the surface and in the core are measured at various current pulse durations in a heating phase. Metallographic analysis demonstrates TCT forms a modified surface layer 10–15 μm thick in R6M5 steel. The structure of the layer is an internal nitriding zone, which consists of a solid solution of tungsten and nitrogen in iron and fine tungsten nitride inclusions. Precipitation hardening and solid-solution hardening provide a twofold increase in the microhardness of the modified W–N layer compared to the alloy base. A transition diffusion zone of nitrogen martensite has been revealed under the hardened layer; it creates a smooth microhardness gradient from the layer to the core, which protects it from embrittlement, peeling, and spalling. Metallophysical modeling using an earlier developed technique is used to calculate of the hardening of the modified layer (yield strength increment). As the tungsten concentration in the layer increases, the fraction of the component of precipitation hardening by W2N particles is shown to increase. Full-scale tests under production conditions demonstrate that the tool with a hardened layer has increased resistance. The durability of drills, which is determined as the number of drilled holes before failure, increases by 2.2 times on drilling 30KhGSA steel and by more than 7 times on drilling a VT-23 titanium alloy.