Effects of Sulfur Level and Anisotropy of Sulfide Inclusions on Tensile, Impact, and Fatigue Properties of SAE 4140 Steel

During metal forming processes such as rolling and forging, deformable manganese sulfide (MnS) inclusions become elongated. Such elongated MnS inclusions can have considerable adverse effects on mechanical properties, if the inclusions are not aligned with the loading direction. The objectives of this study were to evaluate and compare fatigue, monotonic tensile and CVN impact behavior of SAE 4140 steel with high (0.077% S), low (0.012% S) and ultra low (0.004% S) sulfur contents at two hardness levels (40 HRC and 50 HRC). The longitudinally oriented samples at 40 HRC, where MnS inclusions were oriented along the loading direction, did not exhibit any significant sensitivity of tensile or fatigue properties to the sulfur content. For the transversely oriented MnS inclusions, however, the monotonic tensile test results indicate very low ductility of the high sulfur material at both hardness levels, where specimens failed shortly after yielding. The yield strength of the materials, however, did not differ significantly with the sulfur content. With regards to fatigue strength, the high sulfur material had up to 25% lower fatigue strength than the ultra low sulfur material under transverse loading. The scanning electron microscopy (SEM) inspection of failed fatigue test specimens revealed that the fracture topography of the high sulfur material were very rough and jagged, indicating several cracks originating from MnS inclusions. The fracture surfaces of the other two materials with lower sulfur content exhibited typical fatigue fracture surfaces with clear initiation and final fracture regions.