The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating setup to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 µm thick TiCN and α-Al2O3 films, respectively. Diffraction data from the WC-Co phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 seconds, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm 2. The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750°C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300°C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction setup revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.
The results of high spatially resolved X-ray diffraction (XRD) analyses of residual stresses in laser-line hardened 42CrMo4 tempering steel samples are comparedwith the results of numerical process simulations and carefully discussed. Samples were locally line hardened at different maximum temperatures (950 °C, 1150 °C) and with different laser-beam feeds (200 mm/min, 800 mm/min) for the investigations. In addition to X-ray diffraction analyses, the effect of the process parameters on the formation of local microstructures was also examined. The results show that experimentally determined compressive residual stresses in process zones transverse and parallel to the laser track increase as temperatures decrease and feed increases. The dimensions of hardened zones (width, depth) affected by laser hardening at lower maximum temperatures are clearly smaller than those affected by laser hardening at higher temperatures, whilst the impact of laser-beam feed is less pronounced. A new model was developed for numerical simulation of laser-line hardening processes, showing good agreement between numerically calculated and experimentally determined microstructures in the process zones. The results of residual stresses calculated by simulation also exhibit good qualitative and largely also quantitative agreement with experimentally determined residual stresses. Partly, the simulation predicts some local deviations in the distribution of residual stresses.
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