In the previous posting, it was shown that, as a result of the presence of elongated manganese sulfide inclusions, the fatigue properties of low alloy steels can exhibit directionally dependent behavior. Fatigue properties obtained in a test direction transverse to the elongated manganese sulfides were found to be inferior to those obtained in a test direction parallel to the elongated manganese sulfides. In this posting, fatigue properties in the transverse direction are explored further as functions of hardness and sulfur level.
SAE 4140 steel was again selected for evaluation. Heats of steel with three different sulfur concentrations were examined, and specimens from each were evaluated at two hardness levels. As described in the previous posting, 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 and hardness for the steels tested.
The microstructure of all of the test sections was martensite.
Figure 1 shows the strain-life fatigue curves obtained for the three sulfur concentrations at the lower hardness level shown in the table above. The strain-life curve for 0.004 wt% sulfur is given by Iteration No. 99, the strain-life curve for 0.012 wt% sulfur is given by Iteration No. 80, and the strain-life curve for 0.077 wt. % sulfur is given by Iteration No. 81. As can be seen, fatigue performance deteriorates as sulfur level is increased, especially in the high strain amplitude, short life regime. The decrease in performance on increasing sulfur from 0.004 wt. % to 0.012 wt. % is relatively modest; however, the reduction in performance on increasing the sulfur concentration to 0.077 wt. % is much more significant.
Figure 2 gives the strain-life curves developed for the three sulfur concentrations at the higher hardness level. Iteration No. 98 gives the strain-life curve for 0.004 wt.% sulfur, Iteration No. 76 gives the strain-life curve for 0.012 wt.% sulfur, and Iteration No. 77 shows the strain-life curve for 0.077 wt.% sulfur. As in the case of the lower hardness level, fatigue performance is reduced as sulfur level is increased. In this case, however, the decrease in performance on increasing sulfur level from 0.004 wt. % to 0.012 wt. % is more pronounced in the high strain amplitude, short life regime.
These results reinforce the data exhibited in the last posting. Fatigue performance is dependent on sulfur level when the loading direction is transverse to the elongated manganese sulfide inclusions. Also there appears to be a somewhat higher sensitivity to sulfur level as hardness is increased. These results need to be considered in component design.