In the previous post (Dec. 30, 2016), the effects of cooling rate on both constant amplitude and overload core fatigue properties of a carburized Ni-Cr-Mo alloy steel (4320) were compared. As was noted, the study involved subjecting 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. Test results showed constant-amplitude fatigue performance improved with the development of higher hardness at higher cooling rates. Results also indicated fatigue performance under overload conditions improved.
A similar comparison has been completed on a Mn-Cr low alloy carburized steel, 20MnCr5. Two bar diameters were evaluated as before: 15.2 mm (0.6 in.) and 60.9 mm (2.4 in). Both bar diameters were heat treated using a carburizing thermal cycle, without the presence of carbon in the furnace atmosphere. As in the previous post, the aim was to simulate carburized cores at different cooling rates. The mechanical properties and hardness values obtained for the two bar diameters are shown in the table below.
The constant amplitude strain-controlled fatigue results for the two bar diameters, described in an earlier post, are shown in Figure 1. The 60.9 mm diameter bar is represented by Iteration No. 128, and the 15.2 mm diameter bar is represented by Iteration No. 130. The higher hardness of the smaller diameter bar, resulting from a higher cooling rate, shows better fatigue performance particularly in the runout regime.
To examine the effects of overloads during fatigue testing, a test protocol was used as in previous posts (Dec. 30, 2016) and illustrated in Figure 2. The load history in the protocol consists, which has been described in other posts, was undertaken, and which is shown in Figure 2. The load history in the protocol 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.
In Figure 3, Iteration No. 152 gives the overload fatigue results for the 60.9 mm diameter bar, and Iteration No. 154 shows the results for the 15.2 mm diameter bar. As in the case of the 4320 overload data described in the previous post, the data scatter band for the 15.2 mm diameter bar lies slightly above that of the 60.9 mm diameter bar. This indicates improved overload fatigue performance results from higher hardness developed through higher cooling rates.