Surface Finish Effects on Fatigue Behavior of Forgings

Fatigue fractures are the most common type of mechanical failures of components and structures. It is widely recognized that surface finish has a significant effect on fatigue behavior. Forgings can be accompanied by significant surface roughness and decarburization. The correction factors used in many mechanical design textbooks to correct for the asforged surface condition are typically based on data published in the 1940's. It has been found by several investigators that the existing data for as-forged surface condition is too conservative. Such conservative values often result in over-engineered designs of many forged parts, leading not only to increased cost, but also inefficiencies associated with increased weight, such as increased fuel consumption in the automotive industry. In addition, this can reduce forging competitiveness as a manufacturing process in terms of cost and performance prediction in the early design stage, compared to alternative manufacturing processes. In this paper two surface conditions are evaluated, a smooth-polished surface finish to be used as the reference surface and a hot-forged surface finish, in order to quantify forged surface finish effect at several hardness levels (19 HRC, 25 HRC, 35 HRC and 45 HRC) on bending fatigue of a commonly used forged steel (10B40).