Wear performance of surface hardened PM steel from pre-alloyed powder

Powder metallurgy (PM) processing of steels typically results in a material characterized by residual porosity reducing the mechanical strength of the material. That is why mechanical components produced from PM steels for high demanding applications often require a surface hardening in order to improve both the fatigue and the wear resistance. Among the surface treatments, gas nitriding assures economic advantages but the presence of interconnected porosity also makes the core structure involved by the diffusional phenomena. In this paper gas nitriding is proposed in combination with shot peening, as a surface densification technique, to improve the wear performances of PM components from prealloyed powder. 40mm diameter, 9.85mm thickness disks were pressed at 7.0g/cm3 density from a pre-alloyed powder (1.5% Cr, 0.5% Mo), with 0.8% graphite. The disks were sintered at 1120°C, 30min, in N2-H2 (90-10) atmosphere in an industrial equipment. Four conditions of the disks were examined: as-sintered, sintered and gas nitrided, sintered and shot-peened, sintered-shot peened-gas nitrided. The disks were characterized by experimental tests: dimensional variations, macro and microhardness profiles, LOM observations, surface texture analysis, residual stress analysis by X-Ray technique and pin on disk wear tests. The results of the wear tests were fulfilled with observations by SEM and new interesting interpretations of the involved mechanisms were proposed by the application of the energetic approach and non-standard wear tests

Friction coefficient measurements to assess tooling behaviour in hot rolling processes

In hot rolling, tool wear is an important issue, as well as friction and lubrication phenomena. An adhoc designed tribometer, able to reproduce industrial contact conditions, was applied to investigate a variety of tool-workpiece surface interactions during hot rolling. In this work, the experimental methods were tailored to assess friction coefficient for the severe case of piercing of hot steel billets by means of plugs in seamless pipe production. Tool samples were machined from commercial steel plugs. Before testing against the counterbody representing the steel billet, they were subjected to the same industrial treatment performed on the plugs to develop a proper oxide acting as lubricant and thermal barrier. Through ring-on-ring tribological experiments, factors affecting friction such as surface oxide, temperature, billet material (0.40%C steel vs. high-alloy P91 steel), sliding speed and contact pressure were investigated, even when surface plastic deformation occurred. The set-up methodology was also demonstrated to be suitable to investigate wear resistance of hard chromium coatings for mandrels