Study On Laser Cladding Technology And Performance Of Welded Joints Of Bimetallic Mechanical Composite Pipe

Bimetallic mechanical composite pipe have a great differences in chemical composition and the weak interface bonding,between the base material and the cladding material,it often have shedding,peeling and other phenomena in the welding process,which easily lead to joint welding defects and Follow-up welding inoperability.Therefore,it is one of the effective ways to solve the welding problem of mechanical composite pipe by using the welding method to transform the mechanical composite pipe end from mechanical combination to metallurgical combination.At present,the most common pipe end welding method is mainly MIG or TIG surfacing,but in the experimental study found that it still has the disadvantage of large dilution rate,low welding efficiency and large tube end deformation after welding.However,laser cladding technology has the characteristics of small welding heat input,fast cooling rate,low coating dilution rate and small thermal distortion.Based on this,This paper presents the use of laser cladding instead of pipe end welding to achieve L415 / 316 L mechanical composite pipe end of the metallurgical bond,and researching its welding process and the performance of the welded joint.In view of the immature research of high toughness corrosion resistant laser cladding powder,this paper selects or arranges one kind of low hardness,high corrosion resistant self-fluxing alloy powder and non-self-fluxing powder from the existing market powder,respectively,according to the material composition of the stainless steel mechanical composite pipe.And through the microstructure analysis and mechanical properties test,Contrast research the characteristics of microstructures and properties of laser cladding specimens,self-fluxing alloy powder and non-self-fluxing powder,in order to determine laser cladding powder type suitable for the mechanical composite pipe end,and achieving mechanical composite pipe welding.At the same time,according to the standard of composite pipe,the microstructure observation,mechanical property test and corrosion property test ofcomposite pipe welded joints were carried out,and the suitability of the welding process was comprehensively evaluated.The results show that both the self-fluxing alloy powder A and the non-self-fluxing alloy powder B cladding layer have the properties of low hardness and high corrosion resistance.Both of the two cladding layers consist of plane crystal,cellular crystal,columnar crystal and equiaxed crystal,the phase mainly composed of ?-Ni solid solution.Among them,the content of C and Si in the self-fluxing powder cladding layer is higher,and it exists in the ?-Ni solid solution,which makes the cladding coating more brittle.So,its cladding sample shows stronger tensile properties,impact resistance Performance and poor bending resistance.The cladded sample shows stronger tensile property and stronger impact resistance Performance,poor bending resistance.The non-self-fluxing powder cladding coating contains a small amount of C,Si elements and many alloying elements so that it make a good plasticity and toughness,which it also results that the cladding samples have excellent performances in tensile properties,impact resistance and bending performance.Tensile fracture SEM shows that the self-fluxing powder cladding coating is quasi-cleavage fracture and belong to brittle fracture.Whereas,the non-self-fluxing powder cladding coating fracture consists of a large number of dimples,which belongs to ductile fracture.When the organizational structure,chemical composition and mechanical properties are taken into account,the non-self-fluxing powder is more suitable for laser cladding for mechanical composite pipe.The end laser cladding process of L415 /316 L mechanical composite board was carried out by using non-self-fluxing powder B and ENiCrMo-3 welding consumables.the interfaces of welded joints were well combined,and there is no brittle and hard tissues were produced in all the interface regions.Only Heat Affected Zone of 316 L stainless steel,was irradiated by laser,produce a small amount of ferrite during rapid cooling process.The mechanical test results show that the welded joint has good tensile properties and bending resistance,the tensile strength can reach 530 MPa,which is higher than the 514 MPa required by the standard specification,which can meet the actual engineering requirements for the strength of composite joints.The intergranular corrosion test shows that the joint weld still has good bending performance after soaking in the boiling intergranular corrosionsolution for 24 hours,and the welded joint has excellent intergranular corrosion resistance.In summary,the laser cladding technology can achieve high quality welding of mechanical composite pipe and can be applied to the welding of mechanical composite pipe.