Objective The forming quality of high- precision stainless steel sheets during cold rolling is directly affected by the uniformity of the surface hardness of Cr12MoV cold rolling work rolls. Therefore, enhancing the surface hardness uniformity of cold rolling work rolls has consistently been a focal point of research and development for rolling mill manufacturers worldwide. Laser cladding technology, which is a core element of green manufacturing, has been widely used for the surface strengthening and repair of rolling mill rolls. However, cladding layers are prone to developing defects, such as cracks, owing to the significant thermal stresses and element segregation that occur within them because of the rapid heating and cooling during laser cladding. Currently, many scholars often use substrate preheating methods to address this issue. However, studies investigating on whether substrate preheating can enhance the surface hardness uniformity of cladding layers are limited , particularly in the context of M2 steel. In this study, we introduce a substrate preheating process in laser cladding experiments to improve the surface hardness uniformity of cladding layers. Through numerical simulations of temperature and stress fields, coupled with microstructural analysis, we explore the impact of substrate preheating on the surface hardness uniformity of cladding layers. Methods Single-track M2 steel cladding layers are prepared on both preheated and unheated Cr12MoV roll substrates through laser cladding. First, the forming qualities of the cladding layers prepared using the two processing methods are compared. Second, the microstructure characteristics of the cross-sectional and longitudinal sections of the cladding layers are examined using metallographic microscope, with a comparative analysis of the structures at the same depth within the cladding layers. Third, X-ray diffraction is used to compare and analyze the phase composition of the cladding layers. Subsequently, the surface hardness of the cladding layers is measured at 3x10 locations using a microhardness tester. The surface hardness uniformity of the cladding layer is characterized based on the mathematical standard deviation method. Finally, numerical simulations of the temperature and stress fields of the cladding layers prepared using the two processing methods are conducted using Workbench software. Based on the aforementioned investigations, the impact of substrate preheating on the surface hardness uniformity of the cladding layers is analyzed. Results and Discussions Prior to substrate preheating, the surfaces of the cladding layers are not flat with noticeable cracks, and the wetting angle between the cladding layer and the substrate is 61.3 degrees (Figs. 5 and 6). The cross-sectional and longitudinal microstructures of the cladding layers consist of a mixture of columnar and equiaxed crystals (Figs. 7 and 8). The range between the highest and lowest values of the surface microhardness is 448.2 HV. The surface hardness fluctuation coefficient of the cladding layers is 83. After substrate preheating, the surfaces of the cladding layers become flatter with no noticeable cracks. The wetting angle decreases to 31.7 degrees (Figs. 5 and 6). The cross-sectional and longitudinal microstructures of the cladding layers are transformed into a single morphology dendritic structure (Figs. 7 and 8). The range between the highest and lowest values of the surface microhardness decreases to 176.1 HV, and the hardness fluctuation coefficient decreases to 46.3. Numerical simulations of the temperature field reveal that substrate preheating increases the peak temperature of the melt pool by approximately 130 degrees C . However, during the solidification process of the melt pool, the temperature gradient decreases from the inside to the surface (Fig. 13). Numerical simulations of residual stresses show that substrate preheating reduces the peak residual stress on path 1 from 475.4 MPa to 369.6 MPa, and the range between the highest and lowest residual stresses decreases from 101.9 MPa to 50.1 MPa (Fig. 16). On path 2, substrate preheating reduces the average residual stress from 580.9 MPa to 425.2 MPa, and the residual stress gradient decreases (Fig. 17). Conclusions In this study, M2 steel cladding layers are prepared by laser cladding on a Cr12MoV roll substrate at 180 degrees C. The preheating of the substrate makes the surfaces of the cladding layers flatter, enhancing uniform surface hardness distribution in the cladding layers. Furthermore, a comparative analysis of the cladding layer structural types before and after substrate preheating shows that the diversity of morphological structures of the cladding layer is restrained after preheating. This suggests that substrate preheating promotes consistency in the nucleation and growth conditions during the phase formation process in different regions of the cladding layer surface. In addition, substrate preheating reduces the temperature gradient and residual stress in the cladding layers, and the fluctuation range of the residual stress. The results of the simulations of the temperature and stress fields also indicate an improved consistency in the crystalline growth environment during phase formation. Overall, our study demonstrates that, during laser cladding, substrate preheating leads to M2 steel cladding layers without crack defects, with enhanced forming quality and more uniform hardness distribution.
