Previously, 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.
In the most recent posting, fatigue properties of specimens of SAE 8620 steel, carburized to a specific case depth, were compared with the properties developed through separate heat treatments simulating case and core. The data showed that at long life, the case/core composite exhibited fatigue properties very close to those shown by the high hardness simulated case. At short life however, the case/core composite fatigue properties of the case/core composite were between those of the simulated case and core.
An additional set of tests were conducted on a higher alloy steel grade, SAE 4320. Again, 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 0.04 inches.
The mechanical properties and hardness values obtained for the three conditions were as follows:
Location Yield Str. Tensile Str. Red. in Area HRC
MPa MPa %
Core 920.0 994.0 63.0 30
Case 1250.0 ——- 2.1 60
Case/Core 1344.5 1705.3 55.5 (See Fig. 1)
Figure 1 shows the hardness profile developed for the case/core composite specimens. The hardness at the immediate surface is about 56 HRC, which increases to about 58 HRC at a depth of about 0.005 inches. This effect is probably due to the presence of some retained austenite in the martensite case. The hardness of the core is about 45 HRC.
Figure 2 shows the strain-life curves for all three conditions. The strain-life curve for Iteration No. 49 shows the fatigue properties of the simulated core, and the strain-life curve for Iteration No. 50 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. 63.
The data show that, as was demonstrated for SAE 8620 steel in the previous posting, the high hardness simulated case exhibits better long life fatigue properties. At short life however, the data shows that the simulated core exhibits superior fatigue properties to those of the case, again suggesting that the case may be vulnerable to cracking due to overloads. Data for 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. At short life however, the case/core composite fatigue properties of the case/core composite are between those of the simulated case and core. This overall behavior was also exhibited by SAE 8620 steel, which was noted in the previous posting. At long life the fatigue properties of a carburized part appear to be controlled by the high hardness case, whereas at short life the fatigue properties are an average of both the case and core properties. In the short life regime, fatigue life of the case/core composite exceeds that of the core but does not approach that of the case.