Mechanical fatigue behavior of chromium and nickel plain carbon steels

The influence of chemical composition on fatigue behavior and fatigue crack growth rates is variegated. Thus, chromium and nickel affect the fatigue limit but have no influence on the mean values of crack growth rates described by the Paris equations. The effect of carbon on the fatigue limit and the propagation of fatigue cracks is utterly different: It affects the exponentm on the right-hand side of the Paris equation, and the higher the content of carbon, the more pronounced its effect. Carbon also affects the dependencesm=f(Tr),m=f(HB),m=f(R), and logC=f(m) by changing the microstructure of the steel and, hence, the average fatigue crack growth rates. If the microstructure of a steel contains a ferrite phase or interlamellar ferrite in pearlite colonies, as observed in 0.2% and 0.4% carbon steels tempered at temperatures of 400°C and 600°C, then the mechanism of crack propagation is mainly connected with the formation of striations through the ferrite phase. In this case, the values ofm lie in the range of 2–4. This conclusion is made on the basis of data presented in [9, 10, 14, 15]. For a martensite or tempered martensite microstructure, as in 0.4% carbon steels as-quenched and tempered at 200°C, the predominant mode of fracture is intergranular separation and void coalescence, and the values ofm lie in the range of 4–6. For intermediate values of K, low-carbon steels (0.2%) are weakly sensitive to the mean stresses. This agrees with results obtained for other materials with a ductile mode of crack propagation. For 0.4% carbon steels tempered at 600°C, the exponentm increases from 3.5 to 5 asR increases from 0.1 to 0.85. Most likely, this is explained by an increased role for intergranular separation.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 62–67, January–February, 1995.