Causes And Mechanism Of Reheat Crack Formation In Membrane Wall Tube Of Supercritical Circulating Fluidized Bed Boiler

With its high efficiency and low pollution,supercritical circulating fluidized bed boiler has become the main development direction of thermal power generation.Recently,however,in the electric field at home and abroad,the leakage and tube explosion caused by the reheat crack of TP347 H membrane wall tube plate in the thermal affected zone of superheater frequently occur,which seriously affect the safe and stable operation of the thermal power unit.Therefore,the formation reason and mechanism of reheat crack of membrane wall screen of supercritical circulating fluidized bed boiler TP347 H were studied to provide theoretical support for the design,manufacture and operation of membrane wall screen of supercritical circulating fluidized bed unit.The blind hole method and Ansys finite element software were used to study the welding residual stress of TP347 H membrane wall screen.Then,Ansys was used to calculate the stress state of the membrane wall screen under the actual working condition,and the results showed that the temperature distribution had a great influence on the stress distribution of the in-service membrane wall screen,and the temperature stress and residual stress were the most important parts of the axial stress of the membrane wall screen.The membrane wall screen cracks under the joint action of these two stresses.The thermal crack sensitivity of TP347 H thermal affected zone and the base material was evaluated by thermorestor-w thermal restraint simulation device.Then the mechanical properties,fracture,grain size and precipitated phase were analyzed.The results show that the reheat crack sensitivity in the heat-affected area presents a C curve with temperature,and the reheat crack sensitivity is the highest at 750℃.The mechanical properties,fracture,fracture characteristics,grain size and precipitates also change with the temperature,which affect the reheat crack sensitivity.To clarify reheat crack formation mechanism,heat affected zone of 750 ℃ heat affected zone and heat crack using EBSD and TEM on crack initiation and extension process are studied,results show that the TP347 H heat affected zone and heat crack nucleation is not coherent,due to the deposition and matrix craze,larger separation material grain boundary on both sides of the large difference of grain orientation,etc.The propagation of reheat crack in TP347 H heat-affected zone is due to the weakening of grain boundary at high temperature,so the crack expands along grain boundary.The weakening of grain boundary at high temperature is caused by the increase ofgrain boundary at large Angle and the formation of precipitation-free precipitation zone at grain boundary during precipitation.Four models in the process of cracking and expansion are summarized,which provide the basis for the manufacture and installation of TP347 H membrane wall tube plate of superheater of supercritical circulating fluidized bed boiler.