Effect of Si on temperature dependence of non-propagation limit of small fatigue crack in a Fe-C alloy

Dynamic strain aging improves the non-propagation limit of a fatigue crack in ferritic iron alloys containing supersaturated carbon. However, upon increasing the test temperature, the non-propagation limit of the fatigue crack decreases owing to carbide precipitation. In this study, we present a guideline to improve high-temperature fatigue resistance in ferritic steels containing supersaturated carbon via Si addition that suppresses carbide formation. Compared with an Fe-0.017C binary alloy, an Fe- 0.016C-1.0Si alloy shows higher tensile strength at 293 and 433 K. The Si addition increased the fatigue limit at both temperatures as compared with that of the Fe-C binary alloy. The higher fatigue limit than that of the binary alloy at 293 K originated from the solid solution strengthening of Si, whereas the improved fatigue limit in Fe-0.016C-1.0Si at 433 K was attributed not only to the solution hardening, but also to the suppression of carbide formation at 433 K. With an increase in the temperature from 293 to 433 K, the reduction in the fatigue limit of the Fe-0.017C alloy was 65 MPa, while that for the Fe-0.016- 1Si alloy was only 40 MPa. These results indicated that the robustness against temperature can be improved by the addition of Si.