基体预热对激光熔覆制备M2钢熔覆层表面硬度均匀性的影响

Effect of Substrate Preheating on Surface Hardness Uniformity of M2 Steel Cladding Layer Prepared Through Laser Cladding

研究了基体预热对M2钢熔覆层表面硬度均匀性的影响,利用数学标准差方法表征了熔覆层表面硬度均匀性。结果表明:基体预热前,熔覆层表面凸起明显,组织结构为柱状晶和等轴晶混合组织,表面显微硬度极差为448.2 HV,硬度波动系数达83;基体预热后,熔覆层表面平整度变好,组织结构转变为单一形态树枝晶,表面显微硬度极差为176.1 HV,硬度波动系数降低到46.3。通过温度场数值模拟,发现基体预热使熔池峰值温度提高,但在中心到边缘区间,温度梯度在熔池凝固过程中降低;通过残余应力数值模拟,发现基体预热不仅降低熔覆层内残余应力,还降低熔覆层内残余应力的波动幅度。最后,结合温度场、应力场模拟结果及组织结构,探讨了基体预热对熔覆层表面硬度均匀性的影响机理。

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 3×10 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°(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°(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℃.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℃.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.