Residual stress and heat treatment – process design for bending fatigue strength improvement of carburized aerospace gears∗

Abstract It is well established that carburization of low alloy steels promotes compressive residual surface stress upon quenching, and that compressive surface stresses enhance fatigue life. In an effort to build on these established facts, a project is in-progress to improve helicopter gear fatigue life the application of intensive quenching to achieve deeper compressive surface stress. Under US Army Sponsorship, DCT has demonstrated the feasibility of improving the bending fatigue life of Pyrowear 53 steel gears by achieving deeper compressive residual stress in carburized and quench hardened parts. Computer simulations of the conventional heat treatment practice and an intensive quenching process were conducted to analyze these heat treating processes in terms of metallurgical response and residual stress development. The timing and location of phase transformations during the quenching process was found to be critical to achieving the optimum residual stress state for service life. Beginning with simple notched bar coupons and progressing to full test gears, the physical bending fatigue results for these heat treated components are discussed in relation to the combined heat treatment residual and gear loading stresses from the computer simulations.