In the previous article we examined the relationship between the core hardness of carburized steels to both strength and fatigue. In that study the hardness was controlled by the tempering temperature used during the quench and temper heat treatment. In actual practice the core hardness of carburized parts is a function of the steel hardenability, the cooling rate of the quenchant used, and the cross section size of the part. Hence, with a given steel and quenchant the hardness is controlled by the cooling rate.
In iterations 119-130 test bars were produced from 8620, 4320, 9310 and 20MnCr5 steel. These bars were intended to represent the core of carburized components at three different hardness levels. Rather than to control the hardness by quench and tempering the hardness was controlled by machining bars to various diameters and then oil quenching and tempering at 177 C. This more closely represents what occurs during actual carburizing.
Table 1 shows the fatigue strength, mechanical property and hardness data for iterations 119-130. Again, the fatigue strength is the stress level to achieve 1 million cycles.
Table 1: Core Hardness Properties Iterations 119-130
Figure 1 shows the ultimate strength versus hardness for the four grades of steel. Shown are the actual data points as well as the upper and lower bounds. A nearly linear relationship is shown between hardness and strength. Also shown are the upper and lower bounds from the previous article where the relationship did not appear to be linear. There is overlap in the two data sets, however the most recent data appears to be in the lower portion of the first. Once again, the steel grade or alloy content does not appear to be significant. The high alloy 9310 steel and medium alloy 4320 steel do not appear to be any stronger than the low alloy 8620 and 20MnCr5 steels at any given hardness level.
Figure 1: Ultimate Strength versus Core Hardness, Iterations 119-130
Figure 2 shows the fatigue strength versus hardness. Again, the actual data points are shown along with the upper and lower bounds. The relationship between fatigue strength and hardness is linear and the slope appears to be similar to the strength versus hardness plot above. Also shown are the upper and lower bounds from the prior article. In both cases the relationship is linear, however in the previous data set the slope is more shallow and there appears to be more scatter or variation.
Figure 2: Fatigue Strength at 1 Million Cycles versus Core Hardness, Iterations 119-130