Evolution of microstructure and mechanical properties of electroplated nanocrystalline Ni–Co coating during heating

Ni–Co alloy coating is applied to crystallizer surface to enhance its wear resistance for continuous steel casting. This study investigated the effects of heating temperature on the hardness, strength, and elongation of Ni–21 wt% Co coatings obtained by electroplating. The evolution of microstructure was analyzed by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results indicated that the as-deposited coating had a mixed structure of nanocrystals and microcrystals, containing numerous nanotwins and stacking faults. Its hardness, tensile strength, yield strength, and elongation were 371 HV, 1170 MPa, 1117 MPa, and 13.8 %, respectively. After low-temperature heating, the number of nanocrystals decreased, whereas the fraction of microcrystals increased. At heating temperatures of 100–200 °C, a reverse Hall–Petch phenomenon occurred owing to the formation of annealing twins and the transformation of dislocation-emission mechanisms, resulting in a stable hardness and tensile strength. As the heating temperature was further increased, the grains continuously grew, thereby decreasing the hardness and tensile strength. After annealed at 400 °C, the elongation reached 32.5 %, whereas the hardness, tensile strength, and yield strength decreased to 200 HV, 662 MPa, and 577 MPa, respectively.