Air Cooling Analysis of Forged Bearing Steels

Abstract: This thesis work was done in cooperation with Ovako Sweden AB in Hofors to better understand and improve cracking-related problems in bearing ring production and cooling after forging in particular. This goal was achieved by doing a literature search and executing cooling experiments in live production. Cracks have appeared after forging when heating and forming rings or in storage before reheating larger models. Hard, brittle centers were the origin of the cracks. The literature search suggested that connected martensite structures and residual stresses resulting from cooling rate, cooling gradients, austenite grain size, and segregation were problematic. A slower cooling rate, especially in the pearlite transformation zone, was identi ed as a possible solution. This hypothesis was tested by cooling on plates and under a box to reduce the cooling rate. Plates were ine ective, while the box experiments led to reduced hardness and less martensite. Isolation, the temperature under the box, and the placement of the forgings were critical factors. Forgings spending longer time at higher temperatures showed improved properties, supporting the hypothesis made. However, more tests need to be performed before making any  finnal conclusions.