Quenching of Bearing Races - Influence of Rewetting Behaviour On Distortion

Immersion quenching is a widely used technique in heat treating shops, because these techniques provide higher heat transfer coefficients than the most alternative gas quenching techniques. The disadvantage of immersion quenching in evaporating fluids is the complex heat transfer mechanism which consists of the three phases film boiling, nucleate boiling and convection. Especially the transition from film to nucleate boiling – the rewetting of the sample surface – is a complex process which leads to an extreme local dependence of the heat transfer coefficient of the cooled work pieces. Therefore immersion quenching is very difficult to control and leads often to distortion. Especially oil quenching of thin walled bearing races can result in large changes of ovality and conicity with comparable high scattering. To understand these results two series of experiments were done. At first single rings from austenitic steel were quenched under different conditions in a special quenching tank and the rewetting behaviour was documented by use of a CCD-camera. Secondly bearing races from SAE 52100 were quenched in a standard tank. For reference a single peace quenching in a highly symmetric gas nozzle field was done. Before and after heat treatment these rings were measured by a coordinate measuring system and the changes in dimensions, ovality, triangularity, and conicity were determined. During the presentation some video films of the rewetting behaviour will be presented and the corresponding movement of the rewetting front will be compared with the results of size and shape changes.