| With the continuous development of the natural gas industry,the high acid gas fields that contain H2S/CO2 are entering the stage of comprehensive development.In the producing of these gas fields,the corrosive medium will cause serious corrosion damage if the selection of material is not reasonable or the means of anticorrosion are improper.Inconel 625 alloy possesses excellent comprehensive properties.The corrosion resistance of this alloy is satisfied with the requirements of material serviced at severe corrosion condition,which is stipulated by API 6A and NACE MR0175/ISO 15156 standard.However,the price of Inconel 625 is about 10-15 times as expensive as the ordinary sulfur resistant steel.The high cost has seriously restricted its bulk application.Therefore,it is of great theoretical value and wide prospect of engineering application to carry out the research on surface modification of Inconel 625 alloy using the cladding method.In this study,the key issues researched are controlling of the welds formation quality,regulation of microstructure and residual stress,relationship between microstructure and corrosion performance,and heat treatment process of the Inconel 625 overlaying deposited using hot-wire pulsed tungsten inert gas(HPTIG).Based on systematic research,reasonable cladding process and heat treatment process are put forward.According to the proper process proposed,the weldment possesses favorable mechanical performance,and the cladding can meet the requirement of corrosion resistance in the environment,where contains H2S/CO2.In view of the controlling of the welds formation quality using HPTIG,the regression models between the process parameters and formation quality were established using the response surface method(RSM),on the basis of cladding experiments designed by central composite design(CCD)method.And,the models were verified by significant testing,lack of fit testing,residual analysis,and proof experiments.All results indicate that the models can well reflect the relationship between the process parameters and formation quality of the welds,and that the models can be applied to predict the formation quality of the welds.Meanwhile,the effect of process parameters on the formation quality of the welds was revealed.The main effect of process parameters on the weld height is Vs>?>f>Ip>Ib.The main effect of process parameters on the width and dilution of welds is ?>Vs>f>Ip>Ib.The interaction influence on weld height is Ip?>Ib?>IpIB.For the dilution,the interaction effect of Ib? is more important than that of Ip?.And,the process parameters were optimized with the objective of the height,width and dilution of the welds using the models established.The dilution of the optimized weld is less than the welds without optimized.Using the optimized parameters,multi-track welding experiments were carried out to establish the geometry model of the weldment.On the basis of the geometry model,the 3-D finite element model of temperature filed and mechanical filed were established,combining Fortran,Python with Abaqus software.The progressive filling of the welds is realized by element birth and death method.And,the validity of the model was verified.The results show that the model is reliable and can accurately reflect the distribution of welding temperature and stress.To improve the microstructure of the cladding,and to reduce the stress of the weldment,the influence of interval time and welding paths on cooling rate,temperature gradient and stress were analyzed.The results show that reasonable control of the interval time and optimal welding path are beneficial to improve the uniformity of the microstructure,and to reduce the amplitude of the stress in the weldment.Based on the optimized process parameters and welding strategy obtained from simulation,multi-track single layer and two layer cladding were carried out.The evolution of microstructure and distribution of precipitates of the cladding were studied using scanning electron microscope(SEM),X-ray diffraction(XRD),energy dispersive spectrometer(EDS)and other methods.The microstructure evaluation from the fusion interface to the top of cladding is planar,cellular,columnar and equiaxed grains.The cladding layer is mainly composed of ?-Ni,Laves phase and MC phase,which are distributed in intergranular zoon.Moreover,the different crystal morphology corresponds to different distribution of precipitations.The pitting corrosion and intergranular corrosion performance of the cladding was measured using electrochemical impedance spectroscopy(EIS),potentiodynamic scanning technique and immersion testing,combining with corrosion morphology.The experiment results indicate that the corrosion resistance of the single cladding layer is mainly affected by the dilution of base metal.Therefore,its corrosion resistance is obviously worse than that of the two layers.The pitting corrosion resistance and intergranular corrosion resistance of the samples,which cut from the different thickness of the second cladding layer,are different.Meanwhile,the results pointed that the large grain size and less precipitations are benefit to the pitting corrosion resistance and intergranular corrosion resistance of the cladding.In order to improve the corrosion resistance of the cladding and the mechanical performance of the weldment,the influence of heat treatment temperature on the microstructure and performance was explored undering the same holding time.And,the process of heat treatment was designed with the consideration of the material difference of cladding and substrate.The research indicate that Laves phase is partly dissolved after heat treated at 750?,and that there is ? phase precipitated at the grain boundary.With the increase of heat treatment temperature,the ? phase increases and grows.Additionally,the influence of heat treatment temperature on the corrosion behavior of the cladding was revealed using electrochemical testing,immersion testing and coupons testing,combining with corrosion morphology and product.The results show that the intergranular corrosion resistance and H2S/CO2 corrosion resistance are significantly deteriorated with the increase of the ? phase.Combining with the tensile testing and fracture morphology,the influence of the heat treatment on the mechanical properties of the cladding was also researched.It is indicate that a small amount of ? phase is benefit to the tensile strength and yield strength of the cladding.But,the mechanical properties of the cladding decrease dramatically with the increase of ?phase.Based on the above results,a reasonable heat treatment process was obtained with the consideration of corrosion resistance and mechanical properties of the weldment. |
PDF Full Text Request