Issue 37

B. Jo et alii, Frattura ed Integrità Strutturale, 37 (2016) 28-37; DOI: 10.3221/IGF-ESIS.37.05 28 Focussed on Multiaxial Fatigue and Fracture Deformation and fatigue behaviors of carburized automotive gear steel and predictions Bonglae Jo, Yongbo Shim Iron and Steel Development Team, R & D Division of Hyundai Motor Company, 150, HyundaiYeonguso-ro, Namyang-eup, Hwaseong-si, Gyunggi-do, Korea., Shahriar Sharifimehr, Ali Fatemi Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, 2801 W. Bancroft Street, Toledo, OH 43606, USA, A BSTRACT . The fatigue behavior of carburized components such as automotive transmission gears is very complex due to hardness and microstructure difference, residual stresses and multi-axial stress states developed between the case and the core. In addition, automotive gears in service, commonly used in helical type, are actually subjected to complex stress conditions such as bending, torsion, and contact stress states. This study presents experimental and analytical results on deformation behavior of carburized steels, widely used in automotive gears, under cyclic stress conditions including axial and torsion loadings. Axial fatigue tests and rotating bending fatigue tests are also included. Predictions of cyclic deformation and fatigue behaviors of the carburized steel with two-layer model are compared with experimental results. The carburized steel investigated in this study exhibited cyclic softening under both axial loading and torsional loading. Predicted results with simple two-layer model for the cyclic deformation and fatigue behaviors were comparatively similar to the experimental data. K EYWORDS . Carburizing; Case hardening; Gear steel; Cyclic softening; Fatigue; Prediction. INTRODUCTION arburizing, one of the case hardening heat treatments, has been widely used in automotive parts due to significant improvements in component durability at a comparatively economic cost. The objective of carburizing of steels is to obtain a high-carbon plate type martensitic case with excellent wear, and good fatigue and pitting resistance in combination with a tough low-carbon core. In addition, the compressive residual stresses developed in the surface region result in a significant enhancement of the long life fatigue resistance of carburized steels. Carburized steels consist usually of three regions with different microstructures, different hardnesses, and different material properties such as surface, and transition zone and core. Moreover, residual stresses developed by case hardening heat treatment and Poisson’s ratio mismatch between the hard elastic case and the soft core usually cause multi-axial stress C