Fatigue mechanisms in FV520B, a turbine blade steel
An investigation has been undertaken to examine the effect of microstructure on the mechanical properties of FV520B, a precipitation hardenable martensitic stainless steel. This high performance grade of stainless steel was heat treated to three commercially available material specifications, namely the peak hardened, standard and softened overaged conditions. These three precipitation hardened conditions were found to exhibit a range of tensile properties. In order to determine the role of the microstructure, a full materials characterisation programme was performed. The investigative techniques used to characterize the microstructures, were Transmission Electron Microscopy (TEM); Analytical Scanning Electron Microscopy (ASEM); optical microscopy; dilatometry and X-Ray Diffraction (XRD). The microstructural phases and features identified were measured and quantified wherever possible. The effect of the material microstructure and environment on the fatigue properties of FV520B have been investigated. Fatigue tests were performed under uniaxial loading conditions at a stress ratio R (omin/omax) of -1. The tests were undertaken using highly polished specimens to determine the fatigue strength of the three precipitation hardened conditions. The test conditions employed were air and a corrosive 3.5% sodium chloride environment, at pH2 and ambient temperature. The role of the microstructure and the effectiveness of the tensile strengthening mechanisms on the fatigue and corrosion fatigue strength have been discussed. Using SEM, the fatigue crack nucleation mechanisms prevalent within the three microstructures in air and the chloride environment have also been identified. For the peak hardened material, nonmetallic inclusions dominated the fatigue crack nucleation process in air and chloride environments. For the softened overaged condition, multiple site nucleation due to slip band cracking was the prevalent mechanism especially at higher nominal stress amplitudes. The tolerance of this high strength material to small defects at higher stress levels and the actual size of the critical microstructural defects initiating failure have also been highlighted. The microstructure has been shown to strongly influence the processes of fatigue crack nucleation, Stages I and II crack propagation and the concept of the microstructure acting as barriers and providing resistance to crack growth have been discussed. The effectiveness and the size of these microstructural barriers to crack growth have been considered. This discussion has led to the proposal of a model that facilitates flow stress and fatigue lifetime predictions as a function of the quantity of a key microstructural phase. The key microstructural phase, namely reverted austenite affected both the tensile and fatigue properties of FV520B as a function of the heat treatment. The standard overaged material was found to exhibit the greatest resistance to fatigue crack propagation.