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This study concerns that the high-silicon alloy steel treated by the laser hardening with an in-situ thermal treatment (LHITT) between M s and M d temperature. The shear band, such as nano-twin and ε-martensite is expected to be obtained by the aid of thermal stress during laser hardening process to strengthen the steel. The in-situ thermal treatment temperature of the steel was confirmed by thermal mechanical simulator and thermodynamic model. The residual stress of the hardening layer was measured by X-ray stress analyzer. The phase structures were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Moreover, the nano-hardness was evaluated by the nano-mechanical tester. The experimental results demonstrate that the stacking faults were generated by the laser thermal stress which drives high density dislocations motion on { 111 } γ planes. The shear band is formed by the overlapping of the stacking faults in austenite during the LHITT process. Besides, the high surface hardness (7.9 GPa) can be achieved by the LHITT. The hard points (9.5 GPa) in LHITT sample are related with the shear band as the analysis of the elastic modulus. [ABSTRACT FROM AUTHOR]

Abstract: The pulsed laser hardening with low-pulse frequencies enables a higher hardening depth compared with continuous wave laser hardening. To avoid melting a special pulse-to-pulse temperature control was developed. The temperature during hardening was calculated. Residual stresses after hardening were measured and simulated. Studies of the fatigue strength showed that pulsed laser hardening enables an increase of the fatigue strength of component-like specimens. The endurance limit was simulated with the weakest-link model on the base of hardness, roughness, and residual stresses. The weakest-link model also predicts crack initiation sites which were compared with the observed ones. The described calculations of the residual stresses and the endurance limit are applicable on variable geometries of components. So, in the field of development and construction it is possible to reduce the development cycles. [Copyright &y& Elsevier]

Abstract: Laser hardening provides benefits over the conventional hardening processes, including minimum distortion in the parts and the absence of a quenchant. This process is also faster than conventional hardening processes and can be used for selective hardening of specific areas of components. One known problem with laser hardening in steels, however, is back tempering when a large area is hardened by multiple, overlapping passes. This study focused on the development of a numerical model to predict the back tempering in multi-track laser hardening. A tempering model was combined with existing models of thermal behavior and phase change kinetics, which were developed earlier in the authors’ group, to predict three-dimensional hardness profiles after multiple track laser hardening. The combined model was first validated through multi-track laser hardening tests and then used to predict and optimize the laser hardened case depth in multi-track laser hardening of AISI 4140 steel. The predictions and parameters optimized to obtain maximum case depth with the least variation along width of the hardened zone were experimentally verified. Case depths up to 2mm were obtained with 5mm overlapping of laser tracks. [Copyright &y& Elsevier]

Three different steels with C contents of ∼0.2 wt.% and different contents of chromium, molybdenum and vanadium were similarly laser hardened after different preceding heat treatments. The laser-affected zones beneath the surface were characterised by microhardness, dislocation density, carbide type, size and distribution as well as grain size of the austenite. By a systematic study of the changes in microstructure of these laser-affected zones, the effective hardening mechanisms were separated and their influence on the material strength or the microhardness was classified. [Copyright &y& Elsevier]