表面激光熔覆H13/NiCr-Cr_(3)C_(2)复合粉末熔覆层性能研究

Properties of Surface Laser Cladding H13/NiCr-Cr_(3)C_(2) Composite Powder Cladding Layer

采用激光熔覆技术在H13基体表面制备H13与Cr3C2-NiCr复合粉末的熔覆层,首先借助扫描电子显微镜(SEM)、能谱仪(EDS)和X射线衍射(XRD)观察分析粉末和涂层的微观组织与相结构以及两者的结合特征,然后通过热震试验测试熔覆层的热疲劳性能,接着测试涂层表面和截面的显微硬度,最后使用高温摩擦磨损试验机测试各种因素对基体与熔覆层耐磨性的影响。结果表明,85%H13+15%NiCr-Cr3C2复合粉末的熔覆层质量最好,复合粉末主要由Fe-Ni相和Fe-Cr相组成,熔覆层主要物相为马氏体、Cr3C2、Cr7C3和(Cr·Fe)7C3。经过激光熔覆处理后,熔覆层的显微硬度随着Cr3C2-NiCr含量的增多而增大,熔覆层表面的显微硬度值接近1100 HV,熔覆层的平均显微硬度为920 HV。在相同条件下,基体的磨损深度显著大于熔覆层,表明熔覆层的耐磨性明显优于基体。

Objective With the development of science and technology and its needs in practical engineering,metal parts are often subjected to extreme conditions,such as high alternating stress,high temperature,high speed,and high corrosion.Therefore,solving the problem of repairing failed parts under extreme conditions is urgent and complicated.It is necessary to analyze and evaluate the failure mode and service life of parts and seek suitable repair materials and process methods.In this study,the hot working die of H13steel commonly used in engineering is taken as the background,and the strengthening and repair under extreme conditions are taken as the starting point,and investigates the laser cladding strengthening and remanufacturing technology to strengthen various parts suitable for operation under extreme conditions.Repair provides a certain reference significance.Recently,there have been successful study results based on laser cladding;however,the study on Cr3C2-NiCr powder as a laser cladding material is relatively rare.Therefore,Cr3C2-NiCr is selected herein as the cladding material where Cr3C2,as a reinforcing phase,can improve the wear resistance,heat resistance,and hardness of the mold surface.Its physical properties resemble those of H13steel,thereby reducing the melting cracking caused by material mismatch during the coating process.As an adhesive,NiCr can play a transitional role between the substrate and cladding material and improve the heat and corrosion resistance of the bonded part.Methods Laser cladding technology is used to prepare a cladding layer of H13and Cr3C2-NiCr composite powder on the surface of the H13substrate.The microstructure and phase structure of the cladding powder and coating and the bonding characteristics of the coating and substrate are observed and analyzed using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD).The thermal fatigue property of the cladding layer is tested using a thermal shock test.The microhardness of the coating surface and section are measured using a microhardness tester.The influences of various factors on the wear resistance of the substrate and cladding layer are tested using a high-temperature friction and wear tester.Results and Discussions The quality of the cladding layer of the Cr3C2-NiCr composite powder prepared via laser cladding is related to its volume fraction ratio.Compared with other proportioning schemes in the experiment,the cladding layer quality achieves the best performance using the ratio of 85%H13and 15%NiCr3C2(Fig.2).The grain distribution of the cladding layer is uniform;the direction of the dendrite is generally along the substrate and points to the surface of the cladding layer at a certain angle(Fig.3).The structure in the cladding layer is dominated by dendrite,and the main phases are martensite,Cr3C2,Cr7C3,and carbide(Cr·Fe)7C3,among which the martensite diffraction peak is the most obvious(Fig.5).The surface scanning atlas show that the binding mode between the cladding layer and matrix is metallurgical bonding(Fig.4).Thermal shock tests show that sample No.3with this ratio scheme achieves the best thermal fatigue performance(Fig.8).The microhardness of the cladding layer surface of the six samples are tested,and the microhardness of the cladding sample surface significantly increased compared with the substrate(Fig.12(a)).The surface microhardness of the cladding layer of sample No.3is measured.The results show that the surface hardness of the cladding layer is the highest,with a microhardness value of approximately 1100 HV.With an increase in the distance from the surface of the cladding layer,the microhardness value of the cladding layer decreases gradually.At 0.8mm from the surface,owing to many carbides and aciculate martensite dispersed in the tissue,the hardness is approximately 790.65HV(Fig.12(b)).Due to the occurrence of self-quenching in the surface of the matrix,the hardness increases compared with other regions of the matrix.The microhardness of the laser cladding layer increases by approximately 350HV compared with that of the substrate,which can strengthen the surface of H13steel.The wear resistance test of the cladding layer showes that under the same conditions,the wear depth of the cladding layer is lower than that of the matrix,indicating that the wear resistance of the cladding layer is higher than that of the matrix(Fig.13).Conclusions Under the determined process parameters,the cladding layer quality of 85%H13+15%NiCr-Cr3C2composite powder prepared via laser cladding achieves the best performance and the composite powder exhibits the Fe-Ni and Fe-Cr phases in the XRD pattern.The main phases of the cladding layer are martensite,Cr3C2,Cr7C3,and carbide(Cr·Fe)7C3,among which the martensite diffraction peak is the most obvious.This suggests that the martensite transformation is relatively complete in the structure obtained after cladding.After laser cladding treatment,the microhardness of the cladding layer is significantly increased and it increases with increase in the Cr3C2-NiCr content.The surface microhardness of the cladding layer is close to 1100HV,which is approximately twice that of the substrate(570 HV).The average microhardness of the cladding layer(920 HV)is increased by approximately 350HV compared with that of the substrate,achieving the purpose of strengthening the surface of the H13steel.Under the same conditions,the wear depth of the substrate is significantly greater than that of the cladding layer,indicating that the wear resistance of the cladding layer is better than that of the substrate.