In earlier posts, the effect of cooling rates on the core fatigue properties of carburized steels were examined. The study involved subjecting low alloy steel bars of various diameters to a carburizing thermal cycle without the presence of carbon in the furnace atmosphere. The goal was to simulate carburized cores with varying cooling rates by varying section sizes.
An initial series of data was developed on SAE 4320 steel, a nickel-chromium-molybdenum low alloy steel. Two bar diameters were employed: 15.2 mm (0.6 in.) and 30.5 mm (1.2 in.). The mechanical properties and hardness values of the two bar diameters following the carburizing thermal cycle are shown below.The constant amplitude, strain controlled fatigue properties for both bar diameters are shown in Figure 1. Iteration 122 shows the fatigue properties for the 30.5 mm diameter bar, and Iteration 124 shows the fatigue properties for the 15.2 mm diameter bar.
It can be seen that the smaller diameter bar, with the higher hardness, exhibits improved fatigue performance compared to the larger diameter bar.
In follow-up testing, the two bar diameters of SAE 4320 were subjected to overload fatigue testing to determine if hardness influenced overload fatigue performance. As was described in other posts, to simulate the effects of overloads on fatigue performance, 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 2. 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. Effective strain-life data is determined for the small cycles, which can then be compared to results obtained under fully reversed constant amplitude conditions.
Figure 3 shows a simple plot of the overload fatigue data obtained for the two SAE 4320 bar diameters. Iteration 146 gives the results obtained for the 30.5 mm diameter bar, and Iteration 148 shows the results obtained for the 15.2 mm diameter bar. The data exhibits some scatter, but the band for the 15.2 mm diameter bar lies at the upper edge and above the band for the 30.5 mm diameter bar. This indicates that the higher hardness of the 15.2 mm diameter bar resulted in improved overload fatigue performance.
It should be noted however that, due to the scatter, additional results are needed to more completely define the role of hardness on overload fatigue.