Study On Microstructure And Properties Of Laser Cladding StelliteX-40 On Piercer Material

Steel tube is called industrial blood vessel and have an important position in the national economy.All countries in the world attach great importance to the development of production technology of steel pipe.The piercer is one of the most important and most vulnerable tools in the production of seamless steel tube,and is mainly used for piercing the solid tube blank.The piercer is subjected to the high axial pressure while receiving the friction with high temperature tube blank.The piercer will experience water cooling after piercing each seamless tube.This high temperature,high pressure and thermal cycle make the piercer service life extremely short,about tens to several steel tubes per piece.SMS MEER has developed a new technology Premium quality surface coating（PQSC）,which can increase the service life of traditional piercer by up to 30 times.The technology consists of three parts:cooling system optimization,mechanical structure optimization and piercer coating design.The first two parts have already been disclosed in related patents.In-depth research is needed on the design and preparation of the piercer coating to develop key technologies for the preparation of high life piercer.As a new material surface modification technology,laser cladding can significantly improve the strength,hardness,wear resistance,corrosion resistance and high temperature oxidation resistance of the surface of the parts.Compared with other surface modification technologies,laser cladding has many advantages:（1）Fast cooling rate,which can obtain the special microstructure that can not be obtained by equilibrium solidification;（2）Low heat input,parts are not easy to deform;（3）Wide application range,theoretically applicable to all metal materials;（4）Metallurgical bonding;（5）Easy to achieve automation.Therefore,it is expected to prepare a protective coating with high temperature wear resistance,high temperature oxidation resistance,good thermal shock resistance and tight adhesion to the substrate through laser cladding technology,in order to improve the service life of the piercer.In this paper,the laser process parameter are optimized by laser clad single channel and multi-channel StelliteX-40.The StelliteX-40 coating with different mass fractions of Y₂O₃ coatings were prepared by the optimal process parameters.The microstructure and properties of coatings were investigated by optical microscope,scanning electron microscope,X-ray diffractometer,micro Vickers hardness tester,tubular resistance furnace and high temperature friction and wear tester.The effect of the addition of Y₂O₃ on the microstructure and properties of laser cladding Stellite X-40 alloy.The influence of Y₂O₃ addition on the microstructure and properties of laser cladding StelliteX-40 alloy was studied.The results show that the optimum process parameters for laser cladding are laser power 2100 W,scanning speed 10 mm/s,overlap rate 50%and spot diameter 4mm.With optimized process parameters,a compact structure with no macro defects can be obtained.The micro structure of each coating is planar crystal at the junction with substrate,columnar dendrites and cellular dendrites are mixed in the middle of the coating,turning dendrites in near-surface.The coating micro structure of all components is based on y-Co dendrites,and the intergranular structure is（γ-Co+Cr23C6）eutectic structure.With the addition of Y₂O₃,the coating structure was first refined and then coarsened,and the StelliteX-40+0.2%Y₂O₃ coating microstructure refining effect was the most significant,followed by StelliteX-40+0.4%Y₂O₃ and StelliteX-40+0.6%Y₂O₃,and the StelliteX-40 coating structure was the coarsest.The hardness of each coating from high to low is StelliteX-40+0.2%Y₂O₃,StelliteX-40+0.6%Y₂O₃,StelliteX-40+0.4%Y₂O₃ and StelliteX-40,the results are consistent with the degree of refinement of coatings.It is believed that the addition of Y₂O₃ can improve the hardness through refining the coating structure.As a result of the high temperature oxidation and thermal shock tests of each coating,it can be known that as the amount of Y₂O₃ increases,the high temperature oxidation resistance and thermal shock resistance of the coating gradually increase.The analysis of samples after experiment suggested that the addition of Y₂O₃ can promote the preferential oxidation of Cr,which is beneficial to the rapid formation and densification of Cr₂O₃ oxide film which can improve high temperature oxidation resistance and thermal shock resistance of coatings.The Cr₂O₃ oxide film can function as a barrier between coating and pin,and a dense and highly adherent oxide film can be used to lubricate and reduce the wear,so the high temperature wear resistance of StelliteX-40+0.6%Y₂O₃ coating is optimal.The hardness,high temperature oxidation resistance,thermal shock resistance and high temperature wear resistance of laser cladding StelliteX-40/Y₂O₃ are superior to piercer steel,so it is feasible to use laser cladding to improve the service life of the piercer.