Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718
Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.
Effects of micro-stresses from machining and shot-peening processes on fatigue life
An investigation of the effects of tensile machining and compressive shot-peening residual stresses on fatigue life is presented. The paper is focused on fatigue life modelling using the finite element method (FEM). A representative broached notched specimen under three-point bending is modelled as a 2D plane stress geometry using the FEM. Inconel 718 mechanical and fatigue material properties at 600°C are used. Machining and shot-peening residual stresses are mapped to the macro FE models using mathematical algorithms around the notched area of the specimen and their influence on the fatigue life experiencing high cycle fatigue (HCF) and low cycle fatigue (LCF) is investigated. The results show that the compressive shot-peening residual stresses significantly increase the life at HCF compared to the LCF.
Effect of Heat Treatment on High Temperature Crack Growth under Static Load in Alloy 718.
Abstract : Alloy 718 in its conventional heat treated form possesses very poor resistance to crack growth under static load in the temperature range of 425 to 760 C. Since this crack growth is found to be sensitive to the microstructure, a detailed investigation of the effect of heat treatment on the crack growth behavior has been made and various heat treatments have been evaluated with respect to their resistance to this crack growth. Among the heat treatments evaluated it is shown that solution at 926 C for 10 hours and age at 734 C for 48 hours would significantly improve the crack growth resistance without sacrificing its tensile properties. This improved resistance is attributed to the formation of Ni3Cb precipitates which effectively strengthen the grain boundaries. (Author)