Fracture and fatigue crack growth analysis of rail steels

Abstract

Low carbon baintic steel shows promising potential, especially in critical components such as frogs and switches. Microstructural analysis of J6 bainitic rail steel was performed and compared with the microstructure of premium pearlitic rail steel. The bainitic microstructure revealed a mixture of tempered martensite and ferrite associated with intralath carbides. Typical pearlitic microstructure with a fine lamellar aggregate of very soft and ductile ferrite and very hard carbide cementite was observed. The mechanical properties, plane stress fracture toughness, K I c , and the fatigue crack growth behavior of the two steels were evaluated. Test specimens were machined from railheads of each material using electrical discharge machining (EDM). Rectangular unnotched and notched specimens were used for the mechanical properties and fatigue evaluation respectively. 1/2T compact tension specimens were used for the K I c evaluation according to ASTM E399. The J6 bainitic steel has ultimate strength, yield strength, and elongation to failure of about 1500 MPa, 1100 MPa, and 13% respectively. These values are higher than those for pearlitic steel. It was found that the average K I c for the bainitic rail steel is 52 MPa √ m, while that of the premium pearlitic steel is 41 MPa √ m. Fatigue studies showed that the crack speed for the bainitic steel is lower than that for the pearlitic steel over the entire range of the energy release rate. The bainitic steel exhibits a higher rate of crack deceleration in the second stage, as indicated by the lower slope of the fatigue crack propagation kinetics curve in comparison with the pearlitic steel. This attests to the superior fatigue damage tolerance of the bainitic rail steel and provides evidence to support the superior rolling fatigue damage tolerance of the bainitic rail steel reported in the literature.

Mechanical and fracture analysis of welded pearlitic rail steels

“…Both curves consist of linear elastic behavior followed by nonlinear plastic behavior until failure. However, the elastic modulus of the welded pearlitic rail steel is higher than that of the parent pearlitic rail steel, with that for the welded rail being 275 GPa and for the parent rail being 200 GPa [Aglan and Gan 2001]. The figure also shows that the nonlinear plastic portion of the welded rail's curve falls below that of the parent rail.…”

“…A comparative study was conducted on both the welded and parent pearlitic rail to study the effects of welding on the mechanical properties of the rail. The stress-strain relationships of the pearlitic rail steels used were previously studied [Aglan and Gan 2001;Aglan et al 2004;2007]. Figure 5, left, shows the average stress-strain curves for the welded and parent pearlitic rail steel.…”

“…The ratio of the weld-to-parent ultimate tensile strength is known as the weld efficiency [Orange 1966]. The tensile strength of pearlitic steels has been proven to be dependent on the ferrite-cementite lamellae spacing, with a smaller spacing giving a higher strength [Aglan and Fateh 2006;2007]. The thermal cycle associated with welding may cause the mechanical properties in the HAZ to be degraded by grain coarsening, precipitation, and by segregation of trace impurities [Kim et al 2001].…”

“…where σ is the residual strength for a notched specimen, a is the crack length, and f (a/W ) is a geometrical correctional factor, which is given by the following equation [Aglan and Fateh 2007]:…”

Caracterização Microestrutural E Estudo Cinético De Transformação De Fases Em Dois Aços Standard E Premium De Aplicação Ferroviária

“…O manganês, elemento -gêneo, expande o campo de austenita estável, deslocando a composição eutetóide para a esquerda e a temperatura eutetóide para baixo, reduzindo a formação de ferrita proeutetóide. A adição de Cr, Mo e V, bons formadores de carbonetos, restringem o crescimento dos filmes de cementita na perlita e, quando austenitizado, controlam o crescimento de grão austenítico do aço [7][8][9][10][11]. Alguns fabricantes de aços para trilhos buscam rotas alternativas para se atingir a classe Premium sem adição significativa dos elementos pré-citados, mas acelerando o resfriamento do trilho após laminação, de forma a obter uma microestrutura perlítica muito fina, sem formação de martensita.…”