It is well known that sulfur has a major impact on some of the properties of low alloy steels. Sulfur is present in the form of manganese sulfides, which are highly plastic at hot working temperatures. As a result, during the hot rolling of various steel mill products, these sulfides become elongated in the hot rolling direction. This in turn imparts directionality to some of the properties. For example, the values of ductility and notch toughness, when measured transverse to the elongated manganese sulfides, are often significantly less than those obtained from measurements longitudinal or parallel to the elongated sulfides. It is of interest therefore to determine if strain controlled fatigue properties exhibit the same directionality.
SAE 4140 steel, quenched and tempered to a constant hardness, was selected for evaluation. Heats of steel with two different sulfur concentrations were examined. Testing in the longitudinal direction was carried out on through-induction hardened steel bars. Transverse testing was conducted on specially prepared, quenched and tempered, steel sections which permitted machining fatigue specimens transverse to the elongated manganese sulfides.
The table below summarizes the pertinent details of composition, test orientation and properties for the steels tested.
The microstructure of the through-induction hardened bars used for longitudinal testing was a mixture of martensite and a small amount of bainite. The microstructure of the specially prepared, quenched and tempered, sections used for transverse testing was martensite.
As can be seen, the ductility in transverse direction was significantly less than that observed in the longitudinal direction for both sulfur levels. Furthermore, there was no effect of sulfur level on ductility in the longitudinal direction; however, there was a significant reduction in ductility in the transverse direction when sulfur level was increased.
Figure 1 shows the strain-life fatigue curves obtained in both the longitudinal and transverse directions for the two low sulfur levels shown in the table above. The strain-life curve for the longitudinal direction is given by Iteration No. 96, and the strain-live curve for the transverse direction by Iteration No. 80. The fatigue performance in the longitudinal direction is slightly better than in the transverse direction especially in the high strain amplitude, short life regime.
Figure 2 gives the strain-life curves developed in both orientations for the high sulfur levels given in the table above. Iteration No. 97 gives the strain-life curve in the longitudinal direction, and Iteration No. 81 shows the strain-life curve in the transverse direction. The fatigue properties in the longitudinal direction are clearly superior to those in the transverse direction at all levels of strain amplitude.
The strain-life curves in the longitudinal direction appear to show little dependence on sulfur level, whereas there is a stronger dependence of fatigue performance on sulfur level in the transverse direction. This finding parallels the effects of sulfur on ductility as shown in the table above. Thus, to the extent that manganese sulfides are present in a component subject of fatigue loading, the direction of loading needs to be considered in design.
In future postings, the effects of sulfur will be investigated further, not only as a function of sulfur level, but also as a function of hardness.