Cr12MoV cold work die steel is widely used in the roll industry owing to its advantages such as low deformation, high abrasion resistance, and large bearing capacity. However, Cr12MoV rolls exhibit high hardness and brittleness, making them prone to spalling, pitting, cracking, and other microarea damage under service conditions. The harsh operational environment of rolls often involves substantial impact, extrusion, and external friction, leading to localized damage such as large-area spalling and cracking, ultimately resulting in roll scrap. Laser repair is a key method for restoring damaged rolls. This technique enables the formation of dense microstructures and metallurgical bonds at the interface, making it an important technology for repairing damaged Cr12MoV rolls. However, the surface hardness of the repaired layer exhibits fluctuations under the conventional laser mode, particularly in the interfacial transition zone. These variations negatively affect the rolling accuracy of high-precision plates and shorten the service life of rolls. This study explores the application of swing laser beams in welding to promote the transformation of columnar crystals to equiaxed crystals, addressing issues related to non-uniform surface hardness in repaired Cr12MoV rolls. This method aims to minimize hardness fluctuations in the interfacial transition zone, reduce metallurgical defects, and establish a framework for high-quality surface repair of Cr12MoV rolls.

The surface of a Cr12MoV roll was machined into a trapezoidal damage repair groove using fine engraving technology. The damaged area was then repaired at conventional laser and figure-8 swing laser modes. The following steps were undertaken: 1) comparing the repair layer molding quality at two laser modes; 2) analyzing and comparing the microstructure of the cross-section at the same depth of the repair layer using metallographic microscopy; 3) analyzing and comparing the phase composition of the repair layer via X-ray diffraction; 4) examining the distribution of main elements (Fe and Cr) in the repair layer via energy-dispersive spectroscopy line scanning; 5) testing the microhardness and wear resistance of the repair layer at different laser modes. By combining microstructural analysis, elemental distribution, and other findings, this study explored the mechanism through which the figure-8 swing laser influences the surface microhardness uniformity of the repair layer.

At conventional laser mode, the surface color of the repair layer is dull, and cracks are observed in the interface bonding zone (Fig. 7). The microstructure comprises a mixture of coarse columnar dendrites and equiaxed dendrites (Fig. 8, Fig. 9, and Fig. 10). The difference between the maximum and minimum surface microhardness values is 185.0 HV, with a microhardness fluctuation coefficient of 35.8 (Fig. 13). Large spalling pits are found in the repair layer after friction and wear tests (Fig. 16 and Fig. 17). Furthermore, the figure-8 swing laser considerably improves the appearance of the repair layer, imparting a metallic luster to the surface and eliminating cracks in the interface bonding zone (Fig. 7). The figure-8 swing increases convection in the molten pool, interrupts the growth of columnar dendrites, and transforms the microstructure into a uniform pattern of fine equiaxed dendrites (Fig. 8, Fig. 9, and Fig. 10). The difference between the maximum and minimum surface microhardness values is 61.1 HV, with the microhardness fluctuation coefficient reducing to 13.2 (Fig. 13). Surface microhardness uniformity is improved considerably, while the friction coefficient decreases from 0.66 to 0.56. In addition, the width and depth of the wear marks decrease considerably (Fig. 16 and Fig. 17).

The study compared the effects of conventional laser and figure-8 swing laser modes on the repair layer and found that the figure-8 swing laser substantially can enhance the morphology and quality of the repair layer. The surface displays a metallic luster, and cracks in the interface bonding zone are eliminated, resulting in a smooth and defect-free repair layer. Additionally, Fe and Cr are more uniformly distributed in the repair layer. Furthermore, the figure-8 swing laser reduces the temperature gradient and improves the solidification rate. These factors provide a stable grain growth environment, collectively promoting the transformation of columnar dendrites into equiaxed dendrites and reducing structural heterogeneity in the repair layer. The microhardness uniformity of the repair layer is improved, and as a result, the width and depth of the abrasion marks decrease considerably. These changes lead to an increase in wear resistance.