Research On Ratcheting Behaviour And Fatigue Performance Of Small Bore Piping Socket Weld In Nuclear Power Pipeline

The socket weld of stainless steel pipe is easy to be assembled,so it is widely used in the nuclear power piping system.However,during the operation of the nuclear power unit,the small bore pipe socket welding joint causes fatigue cracking and seriously affect the safe operation of nuclear power.In this thesis,the cyclic behaviour and ratcheting behaviour of 304 L stainless steel and ER308 L stainless steel welding wire were studied from the aspects of material and structure.The uniaxial cyclic experiments are carried out for the base metal and the weld metal.The results shows that the cyclic behaviour of the base metal is divided into three stages: initial hardening,softening and obvious secondary hardening.The ratcheting behaviour includes the initial decline stage of the high ratcheting strain rate,the constant ratcheting strain rate stage and the accelerated ratcheting strain rate stage before fracture.The weld metal is characterized by cyclic softening except the first several cycles,where a slight cyclic hardening occurs,and it exhibits a continuous cyclic softening in the remaining life.The ratcheting behaviour is divided into three stages: the initial low ratcheting strain rate stage,ratcheting strain rate stable growth stage and accelerated ratcheting strain rate stage.Based on the macroscopic cyclic loading experiments,the dislocation structures and their evolutions in the base metal and the weld metal during different stages of strain cycling and ratcheting deformation are observed by transmission electron microscopy (TEM).The micro-mechanisms of strain cycling and ratcheting deformation for two materials are qualitatively explained.For the base metal,the dislocation patterns evolve from simple dislocation structures into complex and high density dislocation structures.The relationship between deformation twins,shear bands and dislocation wall/channel structures,dislocation cells controls the cyclic hardening behaviour and ratcheting behaviour.However,for the weld metal,the dislocation patterns evolve from high and complex dislocation structures into low density dislocation structures gradually,which leads to cyclic softening behaviour and accelerated ratcheting deformation.The pre-groove and the toe dressing are used to improve the original socket weld.The X-ray and finite element simulation techniques are used to study the welding residual stress distributions in socket weld.The results show that the pre-groove and the toe dressing can effectively improve the residual stress state at the root and weld toe.The tensile residual stresses are reduced significantly,even the compressive residual stresses are generated at weld toe,and the stress concentration at the weld toe is reduced.Thus,the fatigue performance is improved.According to the actual loading condition,the cyclic loading tests are carried out for three types of socket weld.The experimental results show that the fatigue performance of the socket weld is improved by 70% at maximum through the two ways of the pre-groove and the toe dressing.A new fatigue life assessment approach,which is called Modified W?hler Curve Method (MWCM),is used for estimating fatigue life of the socket weld.The prediction accuracy is improved by improving parameter.The assessment approach by MWCM is convenient and has accurate prediction of the low fatigue life of the socket weld,which have an important significance for engineering practice.