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.