Study And Application Of Temperature/stress Field Characteristics In New 9%Cr Heat-Resistant Steel Heavy Thickness Pipes During Welding And Post Weld Heat Treatment

New 9%Cr heat resistant steels have been widely used in some key components,including main steam pipes,headers and superheaters,in supercritical(SC)and ultra-supercritical(USC)thermal power plants.The weldability of 9%Cr steels has aroused widespread concern.Welding joints are generally considered as one of the weakest parts in the piping system of boilers due to the degradation of creep strength which will be aggravated by unwanted welding residual stress developed in welding zone and nearby base metal.Post weld heat treatment(PWHT)is an efficient way to improve the toughness of 9%Cr steel joints and relieve welding residual stress.Therefore,it is of great theoretical significance and engineering application value in the research and application of temperature/stress field characteristics in welding and PWHT for new 9%Cr heat resisrant steels.Martensitic transformation has great influence on the formation of residual stress in 9%Cr steel weld joints due to its high high hardenability and low transformation temperatures(about 100-400℃).A thermal-metallurgical-mechanical finite element(FE)computational procedure was developed based on ABAQUS code to simulate welding temperature filed and residual stress distribution in 9%Cr steel joints.The effect of martensitic transformation on stress evolution in multi-pass welding and the influence of martensitic transformation on residual stress in butt-welded pipes and butt-welded pipes were future investigated by using the computational procedure.The simulated result shows that the final weld pass has the major effect on the formation of residual stress in multi-pass butt-weld 9%Cr steel pipes.The compressive stress generated by martensitic transformation in former weld pass was relieved by latter weld pass.As a result,the effect of martensitic transformation appears to be confined to the final weld pass.Clear different effect of martensitic transformation on welding residual stress is found between plates and pipes.Tensile longitudinal stress in weld zone is efficiently reduced due to martensitic transformation for both joints,whereas tensile longitudinal stress is formed in the base metal nearby the weld zone for the pipe.On this basis,the article further explores ways to reducing residual stress by using martensitic transformation for 9%Cr steel weled pipes.The results showed that reduction of martensitic transformation temperature has little effect on welding residual stress.However,the tensile residual stress is significantly reduced when interpass temperature exceed 300℃.Local PWHT is usually used in field installation of 9%Cr steel pipes owing to the limited field condition.The parameters of local PWHT should be carefuly choosed for purpose of achieving desired effects of PWHT.In the present work,the usage of 9%Cr steels in SC and USC boilers was firstly investigated and the standards of local PWHT in China and abroad was compared.Typical specifications of 9%Cr steel pipe were choosed on this basis for on-site simulation of local PWHT.The influence of pipe dimensions(diameter,wall thickness)and local PWHT parameters(width of heated band and gradient control band)on the temperature field of local PWHT was future investigated.The temperature field of local PWHT for 9%Cr steels was simulated using FE method.The simulated results are generally in good agreement with the experimental measurements.A parametric study of temperature filed in local PWHT for 9%Cr pipes was conducted.The simulated results show that the increase of heated band witdh can efficiently decrease through-thickness temperature gradient of local PWHT,while the increase of gradient control band witdh can efficiently decreas the axial temperature gradient of local PWHT.The increase of pipe diameter and wall thickness would lead to a clear increasement of through-thickness temperature gradient.The influence of internal air flow of 9%Cr steel pipes on local PWHT temperature field was also studied.The results show that through-thickness temperature gradient was significantly increased by internal air flow.A clear circumferential temperature gradient was formed due to internal air flow.In addition,the effect of local PWHT on the stress relieving and the stress induced by non-uniform heat treatment were studied.The results showed that the residual stress of 9%Cr steel welded pipes was effectively relieved by local PWHT and no excessive stress was induced by local PWHT.The main problem of local PWHT for 9%Cr welded pipes is insufficient heat treatment temperature expeiclly in the inner wall.A local PWHT expert system based on radial basis function(RBF)neural network for 9%Cr steel pipes was developed by using JAVA.Appropriate local PWHT parameters for 9%Cr steel pipes can be obtained by the developed expert system quickly and accurately,which provides a great convenience for field construction.