In this article we will examine the strength and fatigue life found in the core of a part when using typical low carbon alloy carburizing steels. Table 1 shows the data found with this type of material and heat treatment through Iteration No. 110. The data shown are the iteration number, steel grade, fatigue strength at one million cycles, yield strength, ultimate strength, reduction of area, elongation and reported hardness. These samples were all quench and tempered using simulated carburizing cycles. However, in many cases the tempering temperature was elevated in order to control the level of hardness to a more typical value found in a part. Because of the small test-bar diameter, the core hardness of all grades would be quite high if the normal 177 C tempering temperature was used after oil quenching.
Figure 1 shows ultimate tensile strength versus hardness. It is evident there is a reasonably good relationship between hardness and strength for these samples. Higher hardness provides increased strength within the range shown. Most of the grades in this study were the more common low-alloy variety such as 8620, 5120, 8822, 41B17M and 86B20. However, a few iterations used medium and high-alloy grades such as 4620, 4320 and 9310, and these are pointed out in the figure. It is interesting to note these grades follow the same curve of strength versus hardness and do not offer any benefit in this regard.
Figure 1: Ultimate strength versus hardness for low carbon alloy
carburizing steels through iteration 110
Figure 2 shows the fatigue strength at one million cycles versus hardness. There appears to be a relationship between high-cycle fatigue strength and hardness, but it is not as well defined as the previous example. Fatigue strength appears to increase with increasing hardness, but there is a significant amount of scatter. This indicates there may be other factors affecting the life other than hardness, such as the heat of steel, the heat treatment, and the machining process. Once again, the higher alloy steels do not appear to offer any benefit as all data fall within the same scatter band.
Figure 2: Fatigue strength versus hardness for low carbon alloy
carburizing steels found through iteration 110.
It must be remembered we are only looking at the core properties in this comparison. An actual carburized component is a composite of the case and core. The performance of only the case or core by itself may not be indicative of the performance of the final part.