| Under the overall trend of advocating energy conservation and environmental protection in today’s world,the development of efficient and energy-saving coal-fired units has become an urgent task for the thermal power industry.The development of 700℃advanced ultra supercritical coal-fired power generation technology(steam temperature>700℃,pressure>35MPa)is the most direct way to improve the power generation efficiency of thermal power units in a short time,and also the fastest way to realize clean coal power generation in a short time.Welding joint is the weak link of high-temperature components in the whole power plant,which is directly related to the manufacturing and installation quality and safe operation of high-temperature components.Therefore,it is necessary to systematically study the service performance of welding joint,so as to protect the long-term and effective operation of ultra supercritical units.In this paper,solid phase welding of HT700 nickel iron base superalloy was realized by using inertia friction welding technology.The effects of post weld heat treatment and long-term heat exposure on the microstructure of welded joint were studied.The corrosion behavior and saturated steam oxidation behavior of welded joint in simulated coal ash/flue gas were compared between as welded and post weld heat treatment.The results are as follows:(1)The friction welding process produces a great deal of heat,resulting in the dissolution of a small amount of M23C6 strengthening phase at the grain boundary and the decline of mechanical properties.After post weld heat treatment(PWHT),more continuous M23C6 and uniformly distributed y’ phases are precipitated at grain boundaries and in grains,respectively.With the increase of solution temperature,the grain size becomes larger and more M23C6 precipitates at the grain boundary.When the heat treatment process is 1140℃×20min(water cooling)+1020℃×1h(water cooling)+650℃×16h(air cooling)+820℃×4h(air cooling),the average microhardness of the welded joint is the highest.Long time heat exposure at 750℃ will increase the grain boundaries spacing,coarsen M23C6 carbides,merge and grow γ’ phases,and reduce the microhardness of welded joint.No harmful phase was precipitated after thermal exposure,indicating that HT-1 has good thermal stability.The grain size of different welding areas of the same sample from small to large is in the order of weld center area,heat affected area and base metal area.(2)After steam oxidation of welded joints and PWHT welded joints,the oxide film composition from the outer layer to the inner layer is mainly a small amount of iron oxide,NiCr2O4 and a large amount of Cr2O3,and a small amount of internal oxidation product Al2O3.After coal ash/flue gas corrosion,the corrosion products of welded joints and PWHT welded joints are rich in Fe and Cr oxides,and a small amounts of internal sulfide corrosion products of Al2O3,TiO2 and Ti.The results show that the smaller the grain size and the thinner the oxide film are,and the weld fusion line has the thinnest oxide film thickness.(3)Because the grain size of the welded joint is small,the corrosion resistance elements can diffuse to the surface of the welded joint more timely along the grain boundary,and the dense inner and outer double-layer oxide film can grow rapidly at the same time in the early stage of oxidation and corrosion,so as to delay the corrosion;The results show that the grain size of the welded joint after PWHT is larger,and the formation time of continuous internal oxide layer is later than that of the welded joint,which leads to the larger oxide film thickness of the welded joint. |
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