Effect of second phase on the fatigue crack growth in AlSl 4340 steel

Abstract

Increasing applications of AISI 4340 steel necessitate more precise characterization of its mechanical properties so that designers can better ensure failsafe behaviour in service. The fatigue fracture process of a component includes crack initiation and crack propagation. C r a c k initiation behaviour can be evaluated by studying low cycle fatigue and high cycle fatigue. The crack propagation life depends on fatigue crack growth rate and fracture toughness. Many researchers have conducted fracture toughness measurement [1-3], dynamic fracture propagation [4-6] and bending and torsion fatigue tests [7] in 4340 steel. It has been shown that the fracture toughness can be improved by using higher austenitizing temperature (1100-1200 °C) or through high temperature thermomechanical treatment [3]. However, little work has been done on the fatigue crack growth rate of 4340 steel [8]. The second phase plays an important role in determining the mechanical properties. The second phase hard carbide particles in a tool steel will develop stresses at the boundary of the matrix and the inclusion during external cycling loading [9] and reduce the low cycle fatigue properties. Zaczyk et al. [10] indicate that the absorbed energy during stable crack growth depends upon the amount the morphology of second phase particles in ductile steels. The resistance to shear instability and fracture toughness also relies on second phase particle dispersions [11]. Tomita and Okabayashi have proposed two heat treatments that can produce a dutile second phase in the martensitic matrix of 4340 steel [12-15]. The first process is isothermal heat treatment (IHT), which produces a lower bainite second phase, and the second is interrupt quench treatment (IQT), which produces a highly tempered martensite second phase. The existence of the ductile second phase can significantly improve the mechanical behaviour such as tensile properties, impact energy [12-14] and plane strain fracture toughness [15]. However, Tomita and Okabayashi did not address the fatigue properties. In the study reported here we investigated the effect of types and volume fraction of the second phase on the fatigue crack growth rate in AISI 4340 steel. The results are compared to those of conventional quenched and tempered 4340 steel. A commercial AISI 4340 steel was used in this study. The chemical composition was (wt%: C 0.39, Si 0.24, Mn 0.61, P 0.02, S 0.01, Ni 1.46, Cr 0.67, Mo 0.17). The martensitic transformation temperature Ms determined by a dilatometer was 330 °C. Based on the Ms temperature, three different heat treatments were carried out to produce ductile second phase in the matrix: isothermal heat treatment, interrupt quench treatment and direct quench treatment (DQT). Table I lists these heat treatment processes and specimen identification. The microstructures of specimens etched with a 3% nital solution were observed by optical microscopy. Fatigue crack growth tests in accordance with the ASTM E647 standard [16] were performed in a computerized MTS servo-controlled testing system. Compact tension specimens of thickness 5.4 mm and width 43.2 mm were used. A sinusoidal load with frequency 40 Hz and load ratio R = 0.1 was applied throughout the test. A crack opening displacement gauge was attached close to the notch of the specimen, and the growi~ag crack length was measured by the compliance method. Fatigue cycles and cyclic stress intensity factor at every moment of stressing could be recorded automatically. Two duplicate fatigue tests were performed for each specimen. The broken surfaces were studied by scanning electron microscopy (SEM) to understand