Mechanism And Application Of Phase Transformation To Reduce Residual Stress Of Deposited Metal

Wear is one of the main reasons for the failure of parts,which not only increases the cost of production,but also causes huge waste of resources.The repair of damaged parts or wear resistant composite coatings prepared on the surface of ordinary materials by using advanced surface strengthening technology can significantly prolong the service life of parts and improve economic efficiency.During the deposition process,the severe thermal shrinkage will form a large and concentrated residual tensile stress in the material,increasing the tendency of brittle fracture and stress corrosion cracking and reducing the wear resistance and fatigue strength of the deposited layer,even causes the cladding layer cracking.To eliminate the adverse residual tensile stress,heat treatment after welding or mechanical methods often used in the production.The methods not only increase the workload and the difficulty of processing,but also can not be applied to some special complex parts.The low temperature transformation alloy cladding layers use the volume expansion of the martensitic transformation to compensate for the tensile stress caused by the thermal shrinkage,and even to obtain the residual compressive stress,so as to avoid the formation of the deposited crack.In addition,this method can greatly improve the quality and efficiency of the deposited layer,and has broad application prospects.Theoretical and experimental methods are used in this paper to analyze the effects of different alloying elements on the properties of steel and martensitic transformation temperature.Combined with the Schaeffler phase diagram,three groups of low temperature martensitic transformation alloy powders with different ratios were designed(LTT1,LTT2,LTT3).The LTT alloy powders were deposited on the Q235 by argon tungsten arc(TIG),and no cracks or other defects were found in the cladding layer.The microstructure and alloying element content of the deposited layer,diffusion of alloying elements in fusion zone and phase composition were analyzed by optical microscope(OM),scanning electron microscopy(SEM),X ray diffraction(XRD).The results show that the LTT cladding layers are composed of martensite and a small amount of retained austenite(8%~12%).The obvious diffusion of alloying elements occurs near the fusion zone and a strong metallurgical bond is formed between the cladding layer and the substrate.The alloying element content of the deposited layer is slightly reduced due to the diffusion of elements and the burning of alloying elements.The microhardness and wear resistance of LTT cladding layers were tested.The results show that the hardness and wearresistance of the low temperature transformation cladding layers have been greatly improved compared with the substrate material.The hardness the LTT2 cladding layer is the highest,the maximum can reach 557.2HV,and wear resistance increased 60.85 times;In order to study the applicability,microstructure and properties of LTT alloy powder under different conditions.The LTT2 alloy powder was sprayed on the Q235 substrate by plasma spray welding,which was denoted as LTT4 spray welding layer.The spray welding layer and the substrate form a metallurgical bonding with no defect.The microstructure of LTT4 spray welding layer is martensite and a small amount of retained austenite,the residual austenite content is 10.18%.Compared with the LTT2 argon arc cladding layer,the density and dispersion of the LTT4 plasma sprayed layer are reduced,the grain size also grows slightly,so the hardness value decreases,the maximum is 531.3HV,and the wear resistance is 47.22 times higher than that of the substrate.Gleeble 3500 was used to test the strain curves of LTT coating and spray welding layer.The phase transformation of LTT1,LTT2 and LTT4 start at the temperature of 110 ?,210?and190?,and ends at the room temperature with a phase expansion of 0.35%,0.51% and 0.42%respectively.The phase transformation of LTT3 is between 300? and 160?,of which with a phase expansion of 0.3%.The X ray method was used to measure the residual stress of the cladding layers.The results showed that,the residual compressive stress with different degrees is obtained in the LTT cladding layer and spray welding layer.The peak residual compressive stress of LTT1,LTT2,LTT3 and LTT4 can reach to-276 MPa,-361 MPa,-229 MPa and-351.2MPa respectively.The volume expansion of the martensitic transformation can effectively compensate the tensile stress caused by thermal shrinkage.