Specific grinding energy causing thermal damage in helicopter gear steels

Abstract

Machining operations are an intrinsic part of the manufacturing industry and, as a result, they are under constant scrutiny for development and technological enhancements. Thermal damage that occurs during machining processes is undesirable due to its detrimental effects. Grinding temperatures generated during grinding are a direct consequence of the energy input to the process. Grinding burn is one such type of thermal damage. Excessive grinding temperatures cause thermal damage to the workpiece. From microhardness distributions in the subsurface of hardened steels, visible burn is found to be accompanied by reaustenitization of the workpiece. With burning, rehardening of the steel also occurs. Rehardening is a consequence of reaustenitization followed by the formation of untempered martensite, which can be identified after etching. The objective of this research is to detect the onset of burn in the grinding operation and to detect the transition stage. In this research, thermal damage (burn) in carburized and hardened helicopter gear steel caused by grinding was investigated. AISI 9310 and X53 gear steels, used in helicopters and tilt-rotor aircraft, respectively, were prepared and heat-treated by a production partner, Bell Helicopter. Vasco X2M gear steel, used in helicopters, was prepared and heat-treated by a research partner, U.S. Army. Grinding tests were conducted on these steels. Nital etching was used to detect grinding burn. Models were established to predict onset of thermal damage for AISI 9310, X53, and Vasco X2M steel based on specific grinding energy determined from grinding force measurements. The model was compared to results for other steels reported by Malkin and was found to be in agreement. The models can be employed in a control strategy to avoid grinding burn.