Silicon nitride materials for hot working of high strength metal wires

For hot rolling, tools are nowadays made of cemented carbides. In service, these rolls suffer from wear and thermal fatigue. Due to the properties of ceramics, their use could cause improvements in tool behaviour. In field tests – when rolling materials with high deformation resistance – cracks developed in the silicon nitride rolls, which grew for a long time period before large parts of the rolls broke apart. In more moderate rolling conditions the rolls operated safely. A FE model is used to analyse the in-service behaviour of cracks in the silicon nitride rolls. For the observed crack path the stress intensity factor of the cracks is determined using the weight function method. It increases up to a crack depth of around 0.35 mm and then decreases again with increasing crack depth. This explains the observed pop-in-type growth of cracks after an overload. Depending on the rolled materials, the popped in cracks have a length of up to 1.2 mm. The further growth of the cracks to a length of several millimetres, which is caused by a fatigue growth mechanism, needs several thousand additional revolutions.