In earlier posts, the fatigue properties of various carburized low alloy steels were examined. Both case and core properties were evaluated. Since carburized components are manufactured in various section sizes, it can be expected that cooling rates in the cores following carburizing will also vary, and have a subsequent effect on core microstructure and properties.
A study was initiated which 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 size. Microstructures, properties and fatigue performance were then determined.
Initial 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 15.2 mm diameter bar developed a microstructure which was estimated at 90% martensite, 8% bainite and 2% ferrite. The microstructure of the 30.5 mm diameter bar was estimated at 30% martensite, 60% bainite and 10% ferrite.
The strain controlled fatigue curves 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 percentage of martensite and higher hardness, exhibits improved fatigue performance compared to the larger diameter bar.
This data indicates that, in applications involving heavier section components, where the fatigue properties of the cores of carburized components may be critical, attention needs to be given to the cooling conditions following carburizing.