Failure Analysis of Si₃N₄ Rolls for Wire Hot Rolling by Numerical Simulation of Thermal and Mechanical Stresses

Silicon nitride rolls for wire hot rolling have been tested in the rolling mill. After short employment for processing of ultra-high strength materials, cracks appeared in the roll calibre, which limited further application of the rolls. In order to find out possible causes for the damage, relevant mechanical properties of the roll have been determined and thermally and mechanically induced stresses were assessed by numerical simulation. It has been found that thermal stresses are of little relevance, whereas mechanically induced stresses are high enough to enhance subcritical crack growth resulting in the development of the macroscopic cracks, so that the observed damage of the rolls could have been adequately explained. One further result of the analysis was that flaws induced by grinding are decisive for the service time of ceramic rolls, so that special attention has to be paid to the machining of the roll calibre.