Fatigue crack propagation in nine steels, type SAE 43XX, from 0.20 to 1.00 % C, for the simulation of the fatigue behavior in a carburized layer of the SAE 4320

The objective of this paper is to evaluate the crack propagation rate of the SAE 4320 steel carburized layer. For the simulation of a carburized layer, samples made of SAE 43XX were employed, varying the content of carbon from 0.20 to 1.00%. Specimens were copper layer electroplated, and then, they were heat treated in a cycle of carburizing, quenching, and tempering in five different temperatures to expose them to the thermal effects without diffusion of carbon. The results of the microhardness for the steels and for the analyzed conditions are presented in this work. The curve of microhardness has the same profile of a carburized layer for the SAE 4320 heat treated in the same conditions. The crack growth rates as a function of delta K for three tempering temperatures are plotted and the curves are shown. These results show that when the hardness is high (at 200 °C tempering temperature), there is a scattering of the curves and for the case of lower hardness (at 600 °C tempering temperature), the curves are closer. With increasing of tempering temperature there is a decreasing of the hardness and a significant effect of the metallurgical condition of the resistance of fatigue crack growth. Furthermore, with decreasing of the carbon content there is a very significant increase on the resistance of the fatigue crack growth. As a result of that, in the case of a carburized layer there is a raise of the fatigue crack growth resistance when the crack grows into the steel, from the surface (higher hardness) to the core (lower hardness).