Study On Reheat Cracking And Creep Performance In The Cghaz Of T23 Steel

T23 steel is a low-alloy ferritic heat resistant steel,which was developed on the basis of 2.25Cr-1Mo steel with absorbing the adventage of12Cr2MoWVTiB steel.And it's widely used in ultra-supercritical bolier water wall,reheater and preboiler.However,many leakage incidents had occurred in the tube of T23 steel water wall in high service temperture condition.And failure usurally occurred in the weldment or coarsed-grain heat-affected zone?CGHAZ?.The present thesis study on the influence of second welding thermal cycle on the microstructure and reheat cracking sensitivity,as well as the creep deformation of CGHAZ of T23 steel.Gleeble 3500 themo-mechanical simulator was used to prepare the microstucture of CGHAZ and reheat CGHAZ and evaluate reheat cracking sensitivity of reheat CGHAZ by the strain-to-fracture?STF?test.Results show that rational control the peak temperature range of the second welding thermal cycle to A_c1-A_c3 can reduce reheat cracking sensitivity because of partial austenitizing of CGHAZ.Precipitated phase,grain size,grain boundary character,micro-strain near the cracked grain boundary were observed by optical microscopy?OM?,scanning electronic miscroscopy?SEM?and electronic backscattered diffraction?EBSD?.The fractal dimension method was applied to quantitative analyzing grain boundary character.It was concluded that coarsed iron-rich M₃C on grain boundary at high temperature weaken grain boundary because of the incoherent boundary between grain and M₃C precipitate,leading to nucleation of crack and cavities.Refined grain can improve deformation resistent ability of CGHAZ.Zigzag grain boundary could restrain crack from propagation along the grain boundary.The strength difference between grain boundary and grain interor influences crack propagation.Big volume CGHAZ was prepared by heat treatment funaces and creep test was carry out at the temperature of 550?,under the stress of 300MPa-400MPa.Minimum creep strain rate increases with increasing of stress.The relational expression between minimum strain rate and stress could calculate the creep limit stress in a given minimum creep strain rate.The relasional expression between creep fracture time and creep stress could calculate the fracture time in a given creep stress.It was found that the creep damage failure mechanism of CGHAZ of T23 steel is vacancy diffution and cavities growth on the low stress through calculating the creep damage tolerance coefficient and analyzing crack morphology.For the high stress condition,the creep failure machanism could be explained by the coarsed precipitate.