Effect of chemical composition and microstructure on the fatigue crack growth resistance of pearlitic steels for railroad application.

Abstract

This work characterized the microstructure and evaluated the mechanical behavior of two pearlitic steels used in Brazilian railroads, a C-Mn-Si steel and a V-microalloyed steel. The microstructures were observed by light optical, scanning electron and atomic force microscopy. Prior austenite grain size, pearlite colony size and pearlite interlamelar spacing were measured. Continuous cooling transformation diagrams for both steels were obtained. The mechanical behavior was evaluated by tensile tests, hardness tests, fracture toughness tests (linear elastic fracture mechanics - KIC), and fatigue crack propagation tests (da/dN×ΔK). The fatigue tests were performed with different R-ratios (R=0.1 and 0.7) applying two recent empirical models proposed by Rubaie et al. (2008) and Jones et al. (2012) to predict the material behavior. Different loading histories including overloads in function of the crack size were also applied. The fracture surfaces of all tested specimens were analyzed by scanning electron and atomic force microscopy. The presence of vanadium in steel can certainly provide a more refined microstructure, changing the mechanical properties. However, an appropriate range of simple chemical composition and an adequate thermomechanical processing can provide a steel in accordance to standard specifications for railroad application without the need of microalloying element additions that could make the final product more expensive. In the specific case of fatigue, both steels presented a dependence to fatigue R-ratio. The two empirical models were satisfactory aiming to fit the experimental data, although the equation of Jones et al. is simpler to be manipulated. The studied steels also had a dependence to fatigue overloads that can represent a positive effect related to crack growth retardation with increasing fatigue overloads, but also a negative effect, because increasing the crack size for which the overload is applied and increasing the number of overloads, the intensity of retardation effect decreases. These fatigue results are important to predict the actual behavior of steels used in the railway sector and to perform a proper maintenance control, avoiding a premature failure and a consequent catastrophic accident.