Surface finish effect on fatigue behavior of forged steel

The overall objective of this study was to conduct a systematic and comprehensive experimental investigation to evaluate and quantify forged surface finish effect at several hardness levels (19 HRC, 25 HRC, 35 HRC, and 45 HRC) on fatigue behavior of a commonly used forged steel. Specimens were subjected to reversed cantilever bending and rotating bending fatigue. Two surface conditions were evaluated, a smooth-polished surface finish to be used as the reference surface, and a hot-forged surface finish. The heating methods used for forging were gas furnace heating as well as induction heating, to allow comparison of the two heating methods, as decarburization depth differs between the two methods. Since shot blasting is commonly used as a forged surface cleaning process with the additional benefit of inducing compressive residual stress, the hot-forged surface finish was evaluated with and without shot blasting. Limited amount of testing was also conducted to investigate the effect of the flash left by the forging process. Fatigue test results indicate that the historical data commonly used for the as-forged surface condition are overly conservative. New forged surface finish factors and curves as a function of hardness or tensile strength and fatigue life were developed based on experimental data.