Study of residual thermal stresses in hard-alloy coatings obtained by electropulse technology

Qualitative and quantitative estimates of the temperature fields and level of the residual thermal stresses (RTS) in a steel substrate and hard-alloy coating obtained by electropulse technology (EPT) are presented. The estimation was carried out using the finite element method (FEM) and universal COMSOL Multiphysics® software to simulate applied problems. The results of the simulation showed that the higher the rate of mechanical loading, the smaller the depth of heat penetration into the substrate, the residual thermal stresses being localized in the zone of thermal influence near the interaction surface. At the same time, a thin layer of steel cannot cause considerable stresses in the bulk of the hard alloy. The stresses in the steel layer reach the yield point and the layer deforms without formation of large tensile stresses in the hard alloy. A criterion has been obtained that makes it possible to reveal the range of coating application parameters in which the impact of the steel substrate on the formation of residual thermal macro-stresses in the bulk of the hard-alloy coating is minimum. Electropulse equipment has been developed for application of the hard alloy coatings. Standard x-ray sin²Ψ-method (rotation method) is used for experimental evaluation of the macrostresses in the zone of the steel contact with the coating. Studies have shown that both radial and axial stresses are compressive; the maximum absolute values of the stress are observed in the radial direction in the outer layers of the coating, while the axial stresses relax near the free surface. Radial stresses in the outer layers of the coating reach a value of -210 MPa, and axial -110 MPa. The stresses in the coating layers contacting with steel are also characterized by rather high values, of the order of -160 ... -170 MPa. A high level of stress is attributed to the fact that the coating is formed under external pressure. At the same time the compressive stresses are favorable for a hard alloy, since it exhibits high values of the ultimate compression strength.

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