In an earlier post, the fatigue properties of a microalloyed steel, in the form of hot-rolled bar stock, were compared with the properties of the same microalloyed steel in the form of forged crankshafts. In the most recent post, the effects of overloads on the fatigue properties of the microalloyed steel hot-rolled bars were examined. In this post, the results of further studies, which were conducted to evaluate the effect of overloads on the fatigue properties of the microalloyed steel forged crankshafts, are presented. These results are also compared with those obtained from the hot-rolled bars.
The chemical compositions and mechanical properties of both the hot-rolled bars and the forged crankshafts were reported in the earlier posts. However for convenience, they are shown again here in Tables 1 and 2 below.
The same steel grade was used for both product forms, and the mechanical properties are similar.
Similar to what was noted in the most recent post, to simulate the effects of overloads on the fatigue performance of the crankshafts, 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.
The comparative strain life fatigue properties for the forged crankshafts for both constant amplitude and overload testing are shown in Figure 2.
As can be seen, the effects of inserting overloads into the loading history has the same effect of reducing fatigue life of the forged crankshafts as was observed earlier for the hot-rolled bars. The data from the hot-rolled bars was combined with the data from the forged crankshafts shown in Figure 2 to produce the comparative graph shown below in Figure 3.
This fatigue life as a result of the overloads is quite similar for both the hot-rolled bars and the forged crankshafts, with a possible slight advantage to the hot-rolled bars. As was noted earlier for the constant-amplitude testing, the hot-rolled bars showed a slight improvement over the forged crankshafts in the long-life region. This was attributed to the slightly higher hardness and tensile strength for the hot-rolled bar stock. In future posts, the effects of overloads will be examined for other combinations of steel grade and processing.