Fatigue Properties of Case-Carburized SAE 8620

In recent postings, the strain-controlled fatigue properties of the high-hardness case and the lower-hardness core of carburized low-alloy steels were compared.  The properties of the case were developed through simulation by diffusing carbon completely through fatigue-specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.  It was shown that at short lives, the softer core out-performed the high-hardness case.  At long life, the fatigue properties of the high-hardness case were found to be superior. 

This work was expanded to examine the fatigue properties of specimens carburized to a specific case depth, and to compare them with the properties developed through separate heat treatments simulating case and core. 

SAE 8620 was selected and two sets of specimens were heat treated to simulate case and core respectively, as described above.  In addition, a third set of specimens was carburized to develop a case depth of 0.03 inches.  The simulation heat treatments of the first two sets of specimens resulted in one set with a high-hardness martensite case, and a second set with a lower hardness, mixed-bainite/martensite core.  A composite microstructure was developed in the third set of specimens, which consisted of a high-hardness martensite case near the surface, and a lower hardness, mixed microstructure at the center.
The mechanical properties and hardness obtained for the three conditions are as follows:

      Location         Yield Str.        Tensile Str.         Red. in Area        HRC
                                  MPa                  MPa                          %                                                              

      Core                   1420.0            1683.0                      36.2              45

      Case                   1125.3            1868.7                        0.7              58

      Case/Core         1356.6            1764.1                      13.9        (See Fig. 1)

Figure 1 shows the hardness profile developed for the case/core composite specimens.  The hardness at the surface is about 58 HRC and the hardness of the core is about 45 HRC.

Figure 1

Figure 2 shows the strain-life curves for all three conditions.  The strain-life curve for Iteration No. 39 shows the fatigue properties of the simulated core, and the strain-life curve for Iteration No. 71 shows the properties for the high-hardness simulated case.  The fatigue properties for the case/core composite specimens are given by the strain-life curve for Iteration No. 62.

Figure 2

The data show that, as has been demonstrated in earlier postings, the high-hardness simulated case exhibits better long-life fatigue properties.  At short life however, the data shows that the simulated core exhibits superior properties to those of the case, which suggests that the case may be vulnerable to cracking due to overloads.  A comparison of this data with that of the case/core composite shows that at long life, the case/core composite exhibits fatigue properties very close to those shown by the high-hardness simulated case.  This suggests that the long-life properties of a carburized part are controlled largely by those of the case. 

At short life however, the fatigue properties of the case/core composite are between those of the simulated case and core.  This indicates that at short life, the properties of a carburized part will be a combined function of case and core.  The case core/composite data are superior to those of the high-hardness case, but do not approach those of the lower hardness core.

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