| Despite of the efforts made by different countries to develop sustainable renewable energy,it is undeniable that fossil energy is still irreplaceable in the future.Using fossil energy to generate electricity not only consumes coal,which is a kind of non-renewable energy source,but also produces SO2,CO,CO2,NO2 and other harmful gases and waste residues.In order to protect the environment and save non-renewable energy,improving the coal burning efficiency of thermal power station boilers has become an important research direction.Studies showed that improving steam parameters of a thermal power station generator set is an effective means to directly improve its coal burning efficiency,but at the same time improving boiler steam parameters,it also puts forward higher requirements for the materials used in the boiler.In the ultra-supercritical boiler,superheater and reheater are key components of thermal power units,to which the service conditions are more stringent,and they have to withstand high temperature,high pressure and smoke corrosion.The new Ni-Cr-Fe-W alloy(GH4985 alloy)is a deformed nickel-based super alloy specially developed by the Institute of metal research,Chinese Academy of Sciences for superheater and reheater.The alloy,which performs perfectly in high temperature tensile,durability and corrosion resistance in high temperature flue gas,is one of the alternative materials of the key part of 700? ultra-supercritical thermal power unit in the future.At present,the main connection mode of alloy tubes in superheater and reheater is welding,and the research on the welding performance of GH4985 alloy is still in the blank stage at the present stage,so it is particularly necessary to carry out researches on the welding performance of GH4985 alloy.In this thesis,4750HS alloy,HAYNES282 alloy and 4985HS alloy were selected as welding wire materials to weld GH4985 alloy.The influences of different heat treatment processes on the tensile and durable properties of the welding plate after welding were studied,and the microstructure changes of the welded joints were analyzed so as to provide support for GH4985 alloy welding wire selection and welding heat treatment process formulation.The main research results are as follows:4750HS alloy welding wire was adopted to weld with GH4985 alloy,and then the welding plate was conducted through the 780? × 15 h/AC aging treatment.The results showed that the tensile strength of welded joints at room temperature and at high temperature are basically consistent with the base material,but the high temperature endurance strength is obviously lower than the base material.Further microstructure analysis showed that after aging treatment,the joint microstructure was a typical welding microstructure composed of base material and columnar crystals,and the grain boundary of the base material would extend into the welding microstructure.HAYNES282 alloy welding wire was adopted to weld with GH4985alloy,and then the welding plate was conducted through two kinds of heat treatment system as 780? × 15 h/AC aging treatment,and 1010?× 2 h/AC+788?× 8 h/AC solid solution and aging heat treatment.The tensile strength at room temperature and at high temperature under the two heat treatments were compared and analyzed.The results showed that the durability of the joint after aging treatment is lower than that of the base material.The welding joint with solid solution and aging treatment has a longer lasting strength than the base material.The microstructure analysis showed that the welded joints of the two heat treatment systems are all typical welded microstructure composed of base material and columnar crystal.Among them,the grain boundary carbides of welded joints only after aging treatment were relatively thick;the grain boundary carbides at welding joints treated by solid solution and aging were relatively few and discontinuous.4985HS alloy welding wire was adopted to weld to weld GH498 alloy,and then the welding plate was conducted through two kinds of heat treatment system as 780?× 15 h/AC aging treatment,and 1150 ?×1 h/WQ+780 ?× 15 h/AC solid solution and aging heat treatment.The tensile strength at room temperature,tensile strength at high temperature and tensile strength at high temperature under the two heat treatment systems were compared and analyzed.The results showed that the properties of welded joints can reach the standard under both treatment systems,and the high temperature endurance of the joint is better than the base material.The microstructure analysis of the joint showed that the direct aging microstructure still retained the welding columnar crystal.However,the columnar crystals disappeared completely in the welded microstructure after solid solution and aging treatment,replaced by the equiaxed crystal structure.Compared with 4750HS and HAYNES282 alloy welding wire,4985HS alloy welding joint has more stable and excellent mechanical properties,making it an alternative candidate material for GH4985 alloy welding. |
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