Study On Failure Mechanism Of Main Steam Pipe Elbow In Supercritical Units

The higher steam parameters makes the supercritical units higher generator efficiency,but brings new challenges to plant operator.Transmitting steam between the boiler and the steam turbine,main steam pipeline integrity plays a decisive role in safe operation.Cracking accident occurs frequently at elbow of main staem pipe.The poor quality control of the metallurgy of the elbow steel,hot bending,the post-bending heat treatment,and the post-weld heat treatment may cause the deterioration of the material and the failure of the equipment.The study on the failure mechanism of the main steam pipe and elbow,it is great significance for guiding the pipe production and the operation of the power plant equipment.In this work,the cracking accident of a super critical P91 steel main steam pipe is studied.Macroscopic inspection and chemical composition testing,micro structure analysis,routine mechanical performance testing,intergranular corrosion resistance test,Tafel polarization curve and other experimental methods were used.And analyzed the failure mechanism of elbow with reference to manufacturing process of elbow.According to the testing results,the Si and Al element content of the elbow pipe section is higher than the standard,and the Mo5 N,Nb contents are lower than the lowest limit of the standard.The great difference of grain size between the tempered zone and the normalizing zone in the heat affected zone caused metallographic continuity bad.In addition,the hardness of the elbow is lower than the standard and the tensile test is not qualified.The presence of inclusions makes the corrosion resistance of the heat-affected zone of the elbow pipe and elbow greatly reduced.The results of the intergranular corrosion test and Tafel polarization curve all show that corrosion resistance of the elbow and its heat affected zone is worse than the straight and the its heat affected zone.According to the results of the experiments,the phase transition of the coarse grain structure was observed because of the welding heat cycle,which resulted in a great difference in the grain size between the normalizing zone and the tempering zone of elbow.The scouring of high temperature steam exfoliated the inclusions in the heat affected zone of the elbow.Then a pitting pit was formed.Under the action of pipeline system stress and residual tensile stress of elbow heat affected zone,the corrosion activity of this area increased,and the corrosion crack extended outward along the inclusions until the crack expands to through-wall cracks.In the study of P91 heat treatment,simulating elbow material degradation mechanism,it was found that straight pipe normalizing at 1200? for 2 h would result in the same material deterioration as the elbow.The standard tempering treatment(heat preservation at least 60min at 730?760?)cannot eliminate the deterioration of the material,and they must be normalized and tempered qualifiedly to recover.Thus,the material deterioration of the elbow occurred when the hot bending process or post-bending heat treatment process in manufacturing process of elbow.The disqualification of the chemical composition of the elbow has a great influence on the heat treatment performance.Reaching the same material deterioration,The heat treatment temperature(1100? for 2 h)of the elbow is lower than straight pipe(1200? for 2 h).The presence of inclusions makes the pitting resistance of P91 steel lower.The Tafel polarization curve shows that the coarse grains and the presence of inclusions make the P91 steel's corrosion potential decrease and corrosion current density increases.