Study On Microstructure And Properties Of Thermal Spray Stellite X-40 Coating On Piercing Plug Steel

Steel pipes occupy an important position in modern industrial production.With China's efforts to increase the development of oil,natural gas and other energy sources and the implementation of China's "West-to-East Gas Transmission" strategy,domestic demand for steel pipe products is increasing.The piercing plug is one of the most important parts for the production of seamless steel tubes,and the consumption of piercing plug in the process of piercing is huge.The working condition of piercing plug is very bad,withstanding the highest temperature of 1200? and complex alternating stress,and the plug will be subjected to a water cooling to reduce surface temperature after each pipe-passing operation.The piercing plug is prone to thermal fatigue in the cold and hot working environment,which is very easy to cause damage in the work.Therefore,improving the service life of piercing plug is an urgent problem to be solved in scientific research.Stellite X-40 is a Co-based high temperature wear resistant alloy that can work stably at 800?.The alloy structure is mainly composed of solid solution matrix phase and carbide strengthening phase.The existence of alloying elements such as chromium and tungsten at high temperature can improve the solid solution strengthening ability and carbide dispersion strengthening ability of the matrix of y-Co solid solution,which makes the alloy have good impact and wear resistance.A layer of Stellite X-40 alloy protective coaling is prepared on the piercing plug surface by thermal spraying and laser remelting method to improve the high temperature strength and wear resistance of' the piercing plug surface.Stellite X-40 alloy coating are prepared on the surface of perforated steel by plasma spraying,high-speed flame spraying and laser remelting.The microstructure and phase of the coating are analyzed by means of optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffractometer.The hardness.thermal shock resistance and high temperature are resistance of the coating are tested by micro vickers hardness tester,high temperature tube furnace and high temperature friction and wear tester.It is determined that the piercing plug steel grade is 20CrNi3MoV.The microstructure is granular bainite,the average micro Vickers hardness is 322 HV0.2.the yield strength is 804.25 MPa,and the tensile strength is 985.05 MPa.The breaking elongation rate is 7.69%.Plasma spray coating and high velocity flame spray coating are typical layered structures.The plasma spray coating has a pore size of 5.92%,an average microhardness of 335 HV0.2.The high velocity flame spray coating has a porosity of 2.45%,a microhardness of 525 HV0.2.The plasma spray coating is ineffective after 20 times of thermal shock experiments,and high speed flame spray coating failed after 10 times of thermal shock experiments.The wear resistance of the base material is the worst,the sample weightlessness of wear is 42.3 mg,and the friction coefficient is 0.52.The high velocity flame spray coating has excellent high temperature wear resistance,but the coating has the highest friction coefficient,the reason is that the coating occurs a slight peeling during friction.The sample weightlessness of wear is 24.5 mg,the friction coefficient is 0.56.The weight loss is 57.9%of the matrix.The plasma spray coating is the second,but the lowest friction coefficient,and the sample weightlessness of wear is 28.8 mg,the friction coefficient is 0.33,the weight loss is 68.1%of the matrix.A layer of alloy oxide film generated on the surface of thermal spraying coating which plays a certain role in reducing wear,which makes the thermal spraying coating have higher resistance to high temperature wear than the matrix material.The coating is remelted by the following process parameters that the laser power is 440 W,the scanning speed is 9 mm/s,the defocusing amount is+4 mm,and the protective gas flow rate is 3.5 L/min.The remelted coating and the substrate are metallurgically bonded,the coating layer structure disappears,and defects such as pores and cracks are eliminated.The remelted structure is composed of a Cr-rich phase and a carbide-rich phase precipitated between the matrix phase of the ?-Co solid solution and the dendrites.After remelting,the thermal shock resistance of the coating is improved,The coating failed to undergo 50 thermal shock tests.The hardness of remelted coating is reduced,and the average micro Vickers hardness is 335.9 HV0.2.Compared with the high-speed flame spray coating.The high temperature wear resistant performance of laser remelted coating is reduced,the weightlessness of wear is 29.7 mg,the weight loss is 70.2%of the matrix.The friction coefficient is 0.43,which is lower than the friction coefficient of high velocity flame spray coating.The experimental results show that Stellite X-40 alloy thermal spray coating and laser remelted coating have good high temperature wear resistance,flame spray coating has the best wear resistance,remelted coating has the best thermal shock resistance.the preparation of Stellite X-40 alloy coating on the perforated top surface can further improve the service life of the piercing plug.