EFFECT OF SOME MICROSTRUCTURAL CHARACTERISTICS ON THE FRACTURE TOUGHNESS AND FATIGUE CRACK GROWTH RESISTANCE OF FIVE DIFFERENT PEARLITIC STEELS FOR RAILROAD APPLICATION

Luiza Pessoa Moreira¹; Thiago Gomes Viana¹; Leonardo Barbosa Godefroid¹; Geraldo Lúcio de Faria¹; Luiz Cláudio Cândido¹

¹ UFOP

doi:10.20906/CPS/COB-2015-0895

Resumo

Two different types of eutectoid steels are used nowadays in the Brazilian construction of railroads (not manufactured here). Premium and Standard types of rails are classified depending on the presence of certain alloying elements and the heat treatment done on the rails head. This work characterize the microstructure and determines the fracture toughness and fatigue behavior of five of those steels (four Premium and one Standard), coming from different countries. The microstructure was evaluated by light optical and scanning electron microscopy (SEM). Grain size and pearlite interlamelar spacing were measured. In addition, tensile tests, fracture toughness tests (linear elastic fracture mechanics - KIC), and fatigue crack propagation tests (da/dN x deltaK) were performed. The fracture surfaces of the specimens of all tests were analyzed by SEM to confirm the material fracture mechanism. Metallographic examination showed the presence of fine pearlite in all steels, with a normal distribution of inclusions, but with significant differences between the microstructural characteristics. Mechanical tests showed that exists a significant difference among the steels. The microstructural aspects were correlated with the results of fracture mechanics and fatigue crack propagation in order to show the effect of the microstructure on the mechanical behavior of the steels and provide relevant information for an adequate selection of steel for railroad applications. It was confirmed that a more refined microstructure steels has greater tensile resistance, lower value of fracture toughness KIC and worse behavior in fatigue, with smallest value of fatigue propagation resistance in crack growth region I (deltaKTH). The fractography confirmed the brittle behavior of those materials.

Palavras-chave: Microstructure; Fracture toughness; Fatigue crack growth resistance; Railroad steel