Effects of Sulfur and Overloads on the Fatigue Performance of Low-Alloy Steels

In earlier posts, the effect of sulfur on the fatigue performance of low-alloy steel was examined.  It was noted that sulfur is present as manganese sulfides in steels, and these sulfides are highly plastic at hot working temperatures.  During hot working the manganese sulfides elongate in the hot working direction giving rise to directionality effects for some mechanical properties.  When testing is carried out transverse to the elongated manganese sulfides, properties such as ductility and notch toughness are significantly reduced compared to results obtained parallel to the manganese sulfides.  The earlier posts also showed constant amplitude fatigue properties exhibited a directional behavior.  Results obtained from testing transverse to elongated sulfides showed fatigue properties were again significantly reduced compared to the fatigue properties observed from testing parallel to elongated sulfides.  The degree of reduction in fatigue properties increased with increasing sulfur level and hardness.

In a more recent study, the effects of overloads on the fatigue properties of a low-alloy steel were examined.  Heats of SAE 4140 steel with three different sulfur levels were secured.  Testing was carried out on specially prepared quenched and tempered steel sections which permitted machining fatigue specimens transverse to elongated sulfides.  Test identifications and associated sulfur levels are shown in Table 1.  The steel sections were all heat treated to HRC 40.

blog-post-28-table-1

Similar to what was noted in the most recent post, Effect of Overloads on the Fatigue Performance of a Forged Microalloyed Steel, a fatigue testing protocol was implemented in which high-amplitude cycles are inserted between groups of low-amplitude cycles.  The test protocol is shown schematically in Figure 1.  As can be seen, the load history consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  An effective strain-life curve is determined for the small cycles, and then compared to results obtained under fully reversed constant-amplitude conditions.  Fatigue testing was also performed under constant amplitude conditions for comparative purposes.

No. 28 Fig. 1 - New.jpgFigure 1

Figure 2 shows a composite plot of both the constant amplitude and overload results obtained for all three sulfur levels.  The data for Iteration No. 116 (0.004%S) is designated as “U Lo S”, the data for Iteration No. 117 (0.012%S) is designated as “Lo S”, and the data for Iteration No. 118 (0.077%S) is designated as “Hi S”.

The strain-life curves obtained at constant amplitude show results similar to what have been reported in previous posts.  The results at 0.004% sulfur are close to those at 0.012% sulfur, with the 0.012% sulfur results being slightly below those at 0.004% sulfur.  The fatigue results at 0.077% sulfur are significantly below those obtained at the other sulfur levels.

The overload fatigue results mirror the constant-amplitude data at somewhat higher-strain amplitudes.  The data are very close together at 0.004% sulfur and 0.012% sulfur, and the results at 0.077% sulfur are lower.   As strain amplitude is reduced in the long-life regime, the effect of sulfur level on overload results is minimized.

no-28-fig-2-newFigure 2

These results agree with data reported in earlier posts.  Fatigue performance is reduced if loading occurs transverse to elongated manganese sulfides formed during hot working.

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