Abstract
Ductile cast irons are often used for automobile parts that have casting surfaces with shot blast. The influences of surface roughness, transition of microstructures from surface to interior, varied defects and residual stress on the fatigue strength are all very complicated. The microstructure in the transition layer contains a work hardened layer, due to shot blasting, and a pearlitic or ferritic layer produced by casting. In this study, tension compression and plane bending fatigue tests were conducted by using shot blasted ferrite-pearlitic ductile cast irons, with casting surfaces, to quantitatively evaluate the influences of the above factors on the fatigue strength. The fatigue strength was evaluated based on the area parameter model. The fatigue cracks of the specimens at N f <1x10^7 started from casting defects, which were mostly located at sites under dents on the casting surfaces, or the dents themselves. However, the fracture origins of the specimens at
N f >1x10^7 were found at inner defects, due to the compressive residual stress existing at the casting surface and introduced by shot blasting. In order to evaluate the influence of these defects and dents on the fatigue strength, the defect size, area eff , with interaction between surface roughness and defects, and the defect size, area R , for the surface roughness were defined. The influence of the transitional layer on the fatigue strength was considered by applying an equivalent hardness, HVeq, taking the
transitional layer and the ferrite-pearlitic structure into consideration. Moreover, the relief of residual stress during fatigue tests was detected. The relieved residual stress was regarded as being mechanically equivalent to mean stress. Consequently, the fatigue strength of shot blasted ductile cast irons with casting surfaces was successfully evaluated for practical use.
Furthermore, fatigue tests using an actual part, a shock absorber bracket, were performed, and the usefulness of the quantitative evaluation method was verified. With respect to ultralong fractures exceeding N =107, ODAs (Optical Dark Areas), which were considered to be generated by the influence of hydrogen, were not observed at the sites around the defects. Although the data of the ultralong fractures in this study are insufficient, the fatigue limits of the shot blasted ductile cast irons, of which the fatigue origins are at the inner defects, are considered to exist at N 10^8
Keywords - Fatigue strength, Ductile cast iron, Surface roughness, Casting defect,
Residual stress, the area parameter model