Microstructure And Properties Of Welded Joint Of C-HRA-5 Austenitic Heat Resistant Steel

With the increasing use of electricity in China's society,but the technical level of thermal power units is backward,power generation efficiency is low and emission pollution is serious,so the development of large-capacity,high-efficiency ultra-supercritical units is an important goal for the future development of thermal power,and is an important guarantee for the sustainable development of thermal power.The increase in steam pressure and temperature parameters in thermal power stations has put forward higher requirements for the materials of key parts.The superheater and reheater,as the parts of the power station boiler with the harshest service environment,have severely restricted the development of power station units.Therefore,the development of advanced boiler tube materials is the key to power generation technology.At present,C-HRA-5 as a new austenitic heat-resistant stainless steel,due to its high temperature oxidation resistance and excellent mechanical properties of high temperature is widely used.In the actual application process there will be a large number of C-HRA-5steel welding,therefore,the study of the microstructure of welded joints,the prediction of high temperature mechanical properties,lasting strength and the impact of intergranular corrosion susceptibility,not only can provide a theoretical basis for ensuring the quality of welding,but also to ensure the safe and effective operation of the unit has important practical significance.In this paper,for C-HRA-5 steel welded joints,the comprehensive use of metallographic optical microscopy,transmission electron microscopy,scanning electron microscopy and energy spectrometry,as well as room temperature mechanical properties test,high temperature tensile test and double-ring electrochemical kinetic potential reactivation method,etc.,to study the microstructure of different areas of C-HRA-5 steel welded joints,as well as its impact on the room temperature tensile properties,high temperature tensile properties Based on the Larson-Miller parameters,a mathematical model for predicting high-temperature enduring strength was established;the microstructure of the welded joint at different sensitisation temperatures was studied,as well as its effect on the susceptibility to intergranular corrosion.The microstructure of welded joints of C-HRA-5 steel in the supplied condition is an austenitic matrix and a precipitated phase.The weld zone is a coarse dendritic austenite organisation with solidifying grain boundaries(SGB),solidifying subgrain boundaries(SSGB)and migrating grain boundaries(MGB)in the weld zone near the fusion line.The heat-affected zone and the base material zone are both typically austenitic,with fine and agglomerated precipitation phases distributed diffusely in the matrix.The large heat input during the welding process makes the grain in the heat affected zone have a tendency to grow significantly.The precipitated phases of the weld metal are mainly?(Gamma)phase and the strengthening phase?'(Gamma prime).In the austenitic matrix in the heat affected zone,there is mainly MX phase distributed in the grain and at the grain boundaries.In the parent material zone,massive MX phase and short rod-like Z phase precipitate in the grain,and spherical MX phase and M23C6carbide precipitate at the grain boundaries.C-HRA-5 steel welded joints at room temperature under different rates(0.01 mm/s,0.05mm/s,0.1 mm/s)of tensile testing,the study found that when the tensile rate increases,dislocation movement is hindered,deformation impedance increases,making the tensile strength and yield strength of the welded joint increases with the increase in tensile rate,and the change in yield strength is more sensitive.The hardness of the welded joint in the supplied state is not uniformly distributed,the hardness of the fusion zone is the highest,the hardness of the base material zone is the lowest,the hardness of the heat affected zone is higher than that of the base material,and the hardness of the weld zone is higher than that of the heat affected zone.The hardness of the root of the weld is greater because the multi-layer multi-pass weld can act as a post-weld heat treatment.The content of Co elements in the weld zone is much higher than in the base material zone,and the Co elements strengthen the?'phase,making the hardness of the weld zone higher.Under high temperature conditions the welded joints of C-HRA-5 steel were subjected to tensile tests at different temperatures(650°C,700°C,750°C)and,as with the room temperature tensile tests,the welded joints were fractured at the weld,resulting in a lower strength in the weld zone probably due to coarse grain size or non-uniform chemical composition and segregation of elements in the weld zone.As the test temperature increases,the precipitates and inclusions in the tough nest increase and the dislocation plugging effect is enhanced,resulting in faster crack expansion and eventual fracture,making the strength and elongation of the welded joint continuously reduced.Based on the L-M parameters,the use of state functions and full differential characteristics,the establishment of a mathematical model for predicting the enduring strength of C-HRA-5 steel welded joints,700°C extrapolation of the fitted formula for lg?=2.76018-0.13151 lg?,extrapolation of 100,000 hours of enduring strength for ?105973=116.0203 MPa.higher than the ASME SA-213 standard requirements.After sensitisation at 650°C to 850°C,C-HRA-5 steel welded joints are organised as austenite in all areas.With the increase in sensitisation temperature,the grain size in the heat affected zone shows a trend of increasing and then decreasing,which is due to the increase in temperature making the second phase precipitation increase,and the precipitation phase has a nailing effect on the grain boundaries,inhibiting the growth of the grains.With the increase in sensitisation temperature,a large number of M23C6carbide precipitation along the grain boundaries,resulting in grain boundaries poor in chromium,thus causing intergranular corrosion,the welded joint susceptibility to intergranular corrosion increases.Through the microstructure analysis of C-HRA-5 steel welded joints,mechanical properties and intergranular corrosion sensitivity analysis,indicating that C-HRA-5 steel has good weldability,its application in ultra-supercritical boilers superheater and reheat apparatus has extremely far-reaching practical significance.