Study On The Microstructure And Properties Of Thick-walled G115 Heat-resistant Steel Fusion Welded Joint

The existing state of China’s resources and energy structure indicates that thermal power will continue to play a significant role in the country’s energy structure for some time to come.While thermal power generating provides the electricity that people need on a daily basis,it also has negative effects on the environment and energy use.The development of ultra super-critical(USC)thermal power technology,which increases the steam temperature and steam pressure of heat-resistant boilers in thermal power generation units to improve the thermal efficiency and decrease coal consumption,has gradually grown in importance as a means of lowering CO2 emissions,improving the environment,and saving energy.The higher steam parameters put forward higher requirements for the performance of heat-resistant steel used in steam boilers,from both the performance and cost considerations,to enhance the upper limit of the use of heatresistant steel is the key to solving this problem,so the development of high-temperature section heat-resistant steel is the most critical factor in the development of ultra-supercritical thermal power units.G115 steel is a new type of martensitic heat-resistant steel developed independently by China’s General Research Institute of Iron and Steel over a period of more than 10 years on the basis of P92.G115 steel has good organizational stability in the range of 630-650℃,and is also the best choice for thick-walled parts of large diameter pipes and reheat inlet pipes in the temperature range of 630-650℃.In this study,two welding processes,tungsten inert gas shielded welding(TIG)+ manual arc welding(SMAW)and TIG + dynamic vibration wire feeding tungsten arc welding(TIP TIG),are used to investigate welding processability.The test material is large-diameter thick-walled08Cr9W3Co3 VNb Cu BN(G115)heat-resistant steel pipe.The welded joint is well formed and no obvious defects exist.The microstructure and properties of the various regions of the joint were examined using metallographic(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),electron backscatter diffraction(EBSD),and other microscopic test methods,as well as impact properties,hardness,electrochemical corrosion,and other performance test methods.The microstructural transformation of the joint and its impact on the mechanical properties and corrosion performance were also examined.These are the results:Both welding process methods can obtain high-quality G115 thick wall pipeline joints,and the welded joints have good metallurgical bonding.The base metal(BM),heat affected zone(HAZ),and weld seam(WZ)are clearly visible,and no obvious macroscopic defects such as pores,cracks,slag inclusions,and lack of fusion are found in the joint.The microstructure analysis shows that the microstructure of the joint obtained by the two welding processes is similar,both of which are tempered Martensite,accompanied by a large number of precipitation phases.WZ is mainly composed of ferrite and Martensite,and the microstructure morphology is equiaxed crystal and columnar crystal.The middle and upper parts of the weld are mainly columnar crystal structures with different growth directions,and the bottom of the weld is mainly fine equiaxed crystal.The microstructure of FGHAZ and CGHAZ consists of original austenite grain boundaries(PAGBs)and flat noodles martensite.A large amount of precipitates were observed at the grain boundaries and inside the grains.The grain size of TIP TIG welds is smaller than that of SMAW welds.Mechanical properties analysis reveals that the hardness distribution trend in the welded joint cross-section of the different areas(cover,filler,and root)is essentially the same,gradually increasing from the base material to the weld zone,and the heat-affected zone between the base material and the weld,the two processes under the joint hardness difference is not significant.After high-temperature tempering treatment,the impact absorption of the welded joint can meet the requirements of >31 J(GB/T 229-2020 standard).SMAW weld zone average impact absorption for 45.4 J,heat-affected zone average impact absorption for 74.5 J,while TIP TIG weld zone average impact absorption for 59.3 J,heat-affected zone average impact absorption for 103 J.The results show that TIP TIG is filled with the cover to obtain a better toughness of the weld.The fracture microscopic morphology analysis shows that the microscopic fracture in the initiation zone mainly consists of tough nests of different sizes accompanied by deep holes,and the fracture morphology in the extension zone shows the river-like characteristics of quasidissociation fracture,and the quasi-dissociation fracture surface is flat,and its deconstruction surface consists of a large number of tearing ribs characterized by tough nests.The fracture analysis shows that the joint fracture form is a typical mixed tough and brittle fracture mode with bias toward brittle fracture.Electrochemical corrosion performance analysis shows that under the same corrosion conditions,the corrosion current density of BM is the smallest and that of WZ is slightly higher than that of HAZ.This is due to the large amount of uniformly distributed precipitation phase in the joint after post-weld heat treatment,and the content of the precipitation phase gradually increases with the extension of tempering time.These precipitated phases contain a large amount of Fe and Cr elements,and the increase of precipitated phases greatly reduces the proportion of Cr elements,resulting in a weakening of the protective effect of the passivation layer on the surface of the joint,which leads to an increase in corrosion current and a decrease in corrosion resistance in the WZ and HAZ zones.Overall,the BM of the joint has the best corrosion resistance,the WZ has the worst corrosion resistance,and the HAZ is between the two.