Combined Influence Of Residual Stress And Impurity Phosphorus On The Brittlement Of Heat-affected Zones In SA508Gr.4N RPV Steel

As a new generation of reactor pressure vessel steel,SA508 Gr.4N steel has preferable toughness,strength,hardenability and welding performance compared to the previous generation of steel.In terms of chemical composition,it increases the content of chromium and nickel and reduces the content of manganese.Although the change of its composition can achieve a better microstructure,it also increases the sensitivity of the material to neutron radiation.As the core component of the pressurized water reactor(PWR)nuclear power plant,the reactor pressure vessel needs to withstand the erosion of high temperature,high pressure,radiation,and corrosive fluids during long-term service.Among them,the coarse-grained area of the welding heat-affected zone is the weakest part of the weld,which is prone to sudden brittle fracture.It is known that neutron irradiation will cause non-equilibrium segregation of phosphorus in steel,thereby reducing the intergranular bonding strength of the iron lattice.In this project,the thermal aging method is used to simulate the effect of phosphorus element segregation on the brittleness of the coarsegrained region of SA508 Gr.4N steel welding heat-affected zone.This article takes SA508 Gr.4N steel as the research object and uses Gleeble thermal simulation method to obtain a uniform and enlarged welding heat-affected zone.The welded samples were subjected at 620 ? for 6 hours stress relief annealing and aging treatments for 500 hours at 560 ?,800 hours at 530 ? and 1200 hours at 500 ?respectively,to obtain different amounts of phosphorus grain boundary segregation.Through the analysis of the internal microstructure of the sample under different conditions,it is found that the microstructure of the CGHAZ after welding is primarily consisted of lower bainite and a little bit granular bainite structure,while the microstructure after annealing still exhibits lath shape,which proves that the steel has enough tempering stability.Meanwhile,after annealing,the carbides inside the material are redistributed,and a large number of chain carbides precipitate along the lath boundary of bainitic ferrite.Charpy impact experiments were carried out on the samples as welded and after stress relief annealing+ aging at different temperatures.The impact fractures at different temperatures were analyzed,and it was found that for the samples after thermal simulation,as the temperature decreased,the fractures showed dimples,intergranular and transgranular cleavage fracture characteristics in turn.For the impact fracture of the sample after thermal aging,even if the temperature is lowered to-190?,the cleavage fracture characteristic still cannot not be observed.This is due to the grain boundary segregation of phosphorus induced by thermal aging,leading to grain boundary bond strength is reduced.Comparing the ductile to brittle transition temperature of the specimens under different conditions,it is found that the transition temperature of the samples after stress relief annealing + 560 ? aging is lower than that of as welded.This is mainly because the stress relief annealing eliminates the residue stress inside the specimens during thermal cycling,which brings down the yield strength of the steel.The effect of hardening embrittlement is more significant than the effect of nonhardening embrittlement caused by the phosphorus grain boundary segregation,and the final performance is that the sample is tougher.At the same time,due to the amount of phosphorus equilibrium grain boundary segregation increases with the decrease of aging temperature,the ductile-brittle transition temperature of the sample after aging at 500 ?and 530 ? is significantly lower than the other two cases.Combining the influence of welding residual stress and phosphorus grain boundary segregation on the fracture characteristics of the sample,a schematic diagram of the relationship between fracture stress and temperature is obtained.Considering that the mechanical properties test of nuclear materials usually requires using of small-sized specimens,Abaqus finite element analysis software is adopted to compare the distribution of the internal plastic zone inside the miniaturized specimen and the standard Charpy impact specimen at the moment of impact load.The size effect will cause the central area of the small sample to lose a part of the constraint,which makes it easier to enter the plastic state,that is,ductile fracture is more likely to occur.Therefore,a certain degree of compensation is often required when evaluating the fracture performance of materials with small-sized specimens.