A case microstructure may consist of martensite, retained austenite and bainite, and depends upon alloy and cooling rate. The impact of bainite on fatigue is not well known. Sometimes carburized parts are intentionally austempered to increase the amount of bainite and achieve higher toughness. How does the amount of bainite affect strength and fatigue? This article will attempt to answer this question.
The SAE 8620 test bars in this experiment were through-carburized at 927 C for 26 hours. This heat-treat cycle does not completely through-carburize the 0.200 inch gage diameter, but typically leaves the core hardness at or above 55 HRC. All test bars were finished machined and polished prior to heat treatment. After heat treatment and prior to testing, a final polish using 600 grit emery paper was done. This would remove some or all of the intergranular oxidation (IGO).
The percent of bainite was controlled by isothermally quenching the test pieces at an elevated temperature.
- Iteration #87 was direct quenched in cold oil after carburizing to achieve 100 percent martensite.
- Iteration #88, with 25 percent bainite and 75 percent martensite, was reheated to 857 °C for 30 minutes and quenched to 232 °C and held for 30 minutes.
- Iteration #89, with 50 percent bainite and 50 percent martensite, was reheated to 857 °C for 30 minutes and quenched to 232 °C and held for 50 minutes.
- Iteration #90, with 75 percent bainite and 25 percent martensite, was reheated to 857 °C for 30 minutes and quenched to 232 °C and held for 150 minutes.
- Iteration #95, with 100 percent bainite, was reheated to 857 °C and quenched to 316 °C and held for 240 minutes.
After heat treatment all iterations were metallurgically evaluated to confirm the amount of bainite predicted in the TTT diagram.
The mechanical properties, fatigue strength, and hardness are shown in Table 1. A graphical representation of strength versus the percentage of bainite is shown in Figure 1. From 0 to 50 percent bainite the ultimate strength is constant at around 1400 MPa. At 75 to 100 percent bainite the ultimate strength increases to around 1800 MPa. From 0 to 50 percent bainite the yield strength decreases slightly from 1,252 MPa to 1,114 MPa. Above 50 percent bainite the yield strength increases to a maximum 1,556 MPa at 100 percent bainite. The highest combination of yield strength and ultimate strength is at 100 percent bainite.
A graph of percent elongation versus the percent of bainite is shown in Figure 2. From 0 to 75 percent bainite the elongation gradually increases from 1 to 1.3 percent. From 75 to 100 percent bainite the elongation dramatically increases to 15 percent. The highest combination of yield, ultimate strength and elongation is at 100 percent bainite.
A graph of fatigue strength versus percentage bainite is shown in Figure 3. There is no clear correlation as the maximum fatigue strength of 758 MPa is at 25 percent bainite. The fatigue strength of all other iterations is relatively constant at just below 600 MPa. The difference in fatigue strength is likely due to normal variation.
A graph of surface hardness versus percentage bainite is shown in Figure 4. From 0 to 50 percent bainite the surface hardness is relatively constant at around 700 Brinell or 63 HRC. At 75 percent bainite the surface hardness decreases to 653 Brinell or 60 HRC, and at 100 percent bainite the surface hardness further decreases to 514 Brinell or 52 HRC.
A graph of ultimate strength and fatigue strength versus hardness is shown in Figure 5. From 514 to 653 Brinell the ultimate strength increases slightly from 1,785 MPa to 1,883 MPa. Above 653 Brinell the ultimate strength decreases rapidly to as low as 1,390 MPa. This is a documented relationship that was presented in the prior blog article, “Hardness versus Strength.” Ultimate strength increases linearly with hardness to about 550 Brinell and then decreases rapidly above that. From 514 to 682 Brinell the fatigue strength is relatively constant at around 600 MPa. At 710 Brinell fatigue strength increases to 758 MPa. It is tempting to conclude the increase in fatigue strength is related to the increase in hardness. However, in the previous blog article, “Hardness versus Fatigue Strength” it was shown that fatigue strength is largely a percent of the ultimate strength. It must be pointed out that there is a considerable amount of scatter or variation to the data in this relationship. It is believed the difference, and lack of difference, in fatigue strength observed in the current study is normal variation and not related to any change in hardness or microstructure.
The optimum condition for maximizing yield and ultimate strength, as well as ductility and fatigue strength is the 100 percent bainite condition. However, this condition also comes with a lower than normal hardness for most carburized components. This would likely reduce contact strength and fatigue, which must be considered in a component such as a gear.