Deep Rolling Response of Notched Medium Carbon Bar Steels

The effects of deep rolling were evaluated by reviewing the fatigue performance of three medium-carbon (0.4 C) bar steels representing microstructural classes characteristic of forging steels used for crankshaft and other automotive applications. Deep rolling is a surface deformation process whereby a radially symmetric work piece undergoes a surface deformation operation. The steel grades included a quenched and tempered alloy steel (4140) that demonstrated a high yield stress and low strain hardening rate, a non-traditional bainitic experimental grade (1.2 Mn, 0.72 Si) containing high amounts of retained austenite with low yield stress and high strain hardening rate, and a ferritic/pearlitic grade (1.3 Mn, 0.56 Si) with a low yield stress and medium strain rate hardening rate.

A reproducible test methodology to assess fatigue behavior was developed, based on flex-beam, fully reversed, S-N type laboratory fatigue testing. The as-received fatigue behavior of the three steels was characterized to provide a basis for comparison with the deep rolled condition. The deep rolling process was optimized in terms of rolling load based on peak fatigue life at an imposed nominal stress level for each steel. These conditions were used to process fatigue specimens for the final deep rolled condition. From the deep rolled specimens, the hardness profile and fatigue behavior were characterized. The fatigue data, fracture characteristics, and hardness profile data, are interpreted based on the consideration of the microstructure and corresponding strain hardening behavior as measured in compression and tensile tests.