Prediction Of Creep Crack Initiation In Weld Joint Of P92 Pipes With Strip Defects

With the continuous development of society,people focus on efficient use of energy and energy saving and emission reduction.Because of its high power generating efficiency,ultra supercritical unit has been widely used in large power plant.However,welded joint under higher temperature and pressure is weak part,especially when the joint contained defect.Therefore,the integrity of pipes containing defects must be evaluated accurately to maintain safety and save money.Welded joint,which directly affects the operation safety of the power plant,is an irreplaceable connection method for generating units.Since small weld defects cannot be avoided completely during welding,they will accelerate the creep failure speed of welded joint after the long period of operation in high temperature and high pressure environment.Therefore,it is very important to evaluate the safety of welded joints.Some researchers have conducted many studies on creep crack initiation and growth and the current integrity assessment methods are relatively mature for creep crack assessment.However,there are no specialized assessment methods for volume defects under creep conditions.In this paper,the creep crack initiation of welded joint of P92 steel pipe with long strip defects is studied.Based on damage mechanics theory,numerical simulation technique is applied to study the creep damage mechanism of pipes containing embedded spherical defects,and then life prediction model is established.The main research work and conclusions are summarized as follows:(1)The creep crack initiation position is independent with tube size,loading conditions,defect geometric dimension and so on.The initial position of creep damage always occurs in the tip of defects;the rupture stress and stress triaxiality in the defects tip are benefit to the creep cavity nucleation and growth.(2)Creep crack initiation time is related to defect geometric dimension.The longer of the strip defects in the circumferential direction,the less the effective bearing area of the load,resulting in the decrease of creep crack initiation time.The thinner defect leads to more serious stress concentration around crack tip,and consequently reduces creep crack initiation time.The strip defects near the weld root and the defects with large width to thickness ratio are constrained by groove,and has less creep crack initiation time.(3)Finally,through the concept of average equivalent stress,the relationship between the geometric dimension of the defect and the creep crack initiation time is established,and the prediction model of the creep crack initiation time is proposed.