Microstructure And Properties Of Low Temperature Transformation Metal By Laser Cladding

Laser cladding is an advanced material surface treatment technology.The rapid cooling and rapid heating in the cladding process can cause residual tensile stress in the cladding layer,thereby increasing the stress corrosion cracking and reducing the use performance.The low temperature phase transformation(LTT)alloy powder reduces the martensitic transformation point(Ms)by controlling the carbon content and adding alloying elements to form a high-alloy martensitic phase.The volume expansion of the martensitic transformation at low temperature is used to counteract the thermal contraction in the cooling process,so as to improve the residual stress.After the cladding forming,there is no need to carry out the subsequent treatment of stress relief,thereby improving the production efficiency.Laser cladding is highly process-friendly.Choosing the right combination of process parameters is the basis for obtaining a good cladding layer.This article first analyzes the role of alloying elements in steel,and on the premise of guaranteeing the comprehensive mechanical properties of the cladding layer,Cr-Ni are the two main added elements that reduce the martensitic transformation temperature,and Mn,Mo,Cu,V and other trace elements are additionally added.Finally two alloy powders LTT1 and LTT2 were determine with the same content and different elements in the alloy system.The phase transition temperature of the alloy cladding layer was measured by a thermal dilatometer.The volume expansion rate of the martensitic phase transition in the cladding layer was calculated.Through the control of single factor variable method and orthogonal experimental method,the laser cladding process parameters are studied.By analyzing the influence of selected factors on the macroscopic quality and dilution rate of the cladding layer,a set of cladding process parameters with good forming is finally determined.The selected process parameters were used for laser cladding experiments and the tempering treatment was used in the cladding layer to compare the microstructure and properties of the cladding layers before and after tempering.The microstructure of the cladding layer was observed and analyzed by 3D confocal laser scanning microscopy,scanning electron microscopy and XRD.It was found that the microstructure of the cladding layer of LTT was formed by lath martensitic and retained austenite distributed between martensitic plates.After tempering,the crystal grains of the cladding layer are refined and the martensitic lath boundaries become blurred.There is an element transition from the cladding layer to the substrate and the cladding layer forms a strong metallurgical bond with the substrate.After tempering,the elemental transition in the fusion zone is almost unchanged.The point scanning of different areas of the cladding layer was performed.It was found that the content of elements in different areas did not change much and the distribution was even.The retained austenite structure was found in the cladding layer by the XRD detection technology.The content of the retained austenite of the LTT2 cladding layer was slightly higher.After tempering the retained austenite was decreased.It was found that the microhardness and wear resistance of the cladding layer were improved compared with the substrate by testing.The maximum microhardness of the LTT1 cladding layer is 448.7HV,which is 2.3 times higher than that of the substrate.After tempering,the microhardness of LTT1 cladding layer decreased slightly.More contains of V element in LTT2 cladding layer produces secondary hardening during tempering,which made the microhardness and wear resistance of LTT2 cladding layer increased after tempering.The X-ray diffraction technique was used to measure the residual stress of single and multi-lap cladding layers.It was found that the surface of the LTT cladding layer produced different degrees of compressive stress,indicating that the low-temperature phase change material significantly improved the residual stress of the cladding layer.The residual compressive stress value of the LTT1 cladding layer is greater than that of the LTT2,and the lapped cladding layer stress value is greater than that of the single cladding layer.The maximum residual compressive stress value of the cladding layer reached 496.69 MPa,which appeared on the surface of LTT1 lapped cladding layer.