Effect Of Microstructure And Thermal Aging On The Stress Corrosion Cracking Of The Nuclear Grade 316LN Stainless Steel Welded Joint In High Temperature Water

The stainless steel of the primary coolant pipe and its welded joints are susceptible to stress corrosion cracking under the harsh operating environment with high temperature and high pressure water of the nuclear power plant,which has serious influence on the reliability of the nuclear power plant.Meanwhile,due to the long service life of nuclear power plant,the microstructural characteristics of stainless steels will be changed by the long-term service at the elevated temperature,resulting in degradations in mechanical integrity and corrosion resistance of the components.As the changes in the mechanical properties along with degradation of the corrosion resistance of the thermally aged material have profound repercussions on the reliability of the components,the thermal aging of the stainless steel and its welded joints under the harsh operating environment is a key issue in the nuclear power plants.In order to establish the performance degradation mechanisms of the stainless steel and its welded joints during the long-term service under the harsh operating environment of the nuclear power plant,attempts have been made to evaluate the microstructural evolutions and properties of the thermally aged material.In this study,the inhomogeneous microstructure and its thermal aging evolutions of the nuclear grade 316LN stainless steel multi-pass weld joint were investigated by Xray diffraction,scanning electron microscopy,electron backscatter diffraction,transmission electron microscopy and high resolution transmission electron microscopy.The effects of inhomogeneous microstructure and thermal aging microstructure evolution on the high temperature mechanical properties,deformation and cracking behavior of different regions of the weld joint were studied by high temperature small punch test.A novel and simple high-temperature water small punch testing methodology was developed to investigate the stress corrosion cracking behavior of the stainless steel under the high-temperature water environment.The influence mechanism of inhomogeneous microstructure and thermal aging on stress corrosion cracking performance of the as-received and thermally aged welded joint in high temperature and high pressure water were studied.The main results were listed as follows:(1)The mechanical properties and deformation behavior in different zones of 316LN stainless steel multi-pass weld joint were strongly influenced by the local inhomogeneous microstructure.In the heat affected zone,the dislocation density and residual strain were high,and the hardness and strength were improved.In the weld metal,there were numbers of needle-like ferrites,and the cracks tended to initiate at the ferrite-austenite interface and preferentially propagate along the needle-like ferrite,the strength and plastic deformation of the weld metal were reduced.In the fusion zone,cracks tended to initiate and propagate on the weld metal side due to the low strength and poor plastic deformation in the weld zone.(2)The microstructural evolutions mechanism of the 316LN stainless steel weld joint were affected by the local inhomogeneous microstructure and stress state in different regions.Long-term thermal aging at 400℃ had obvious influences on the density and structure of dislocations in 316LN base metal,leading to a lowdensity dislocation,in addition,multiples,coplanar dislocation arrays,stacking faults and extended dislocations were formed due to the microstructure recovery during the long-term thermal aging,and the hardness of the base metal increased slightly.By contrast,in that weld joint,the residual strain was high,the microstructure of austenite phase had no obvious change,but spinodal decomposition and precipitation of G phase occurred in ferrite phase,which increased the lattice mismatch in ferrite and the strain of weld metal,hence significantly increased the hardness of the weld metal.(3)The elevated temperature deformation mechanism of the austenite phase in the base metal,heat affected zone and weld metal were strongly influenced by the long-term thermal aging.Both dislocations slip and deformation twinning played an important role in plastic deformation in the as-received austenite phase,while multi slip dominated the plastic deformation process of the thermal aged austenite phase under the elevated temperature.On the other hand,the hardening of ferrite due to long-term thermal aging increased the deformation incongruity between ferrite and austenite phases,which made dislocations more likely to plug up at the phase interface and leaded to crack along the phase interface.(4)A novel and simple high-temperature water small punch testing methodology was developed,which can investigate the stress corrosion cracking behavior of the stainless steel under the high temperature and high pressure water environment.The stress corrosion susceptibility of the material in high temperature and high pressure water obtained by using of high-temperature water small punch test was basically consistent with that obtained by using the standard slow strain rate tensile test,and the deformation and cracking mechanism of the samples were also consistent,which indicates that the high temperature water small punch test can accurately evaluate the stress corrosion cracking susceptibility of materials in high temperature and high pressure water environment.(5)The stress corrosion crack susceptibility and the crack initiation and growth behavior of 316LN stainless steel weld joint in high temperature and high pressure water environment were strongly influenced by the different deformation and hardening mechanisms and grain boundary characteristics of the inhomogeneous microstructure in different zones of the welded joint.In the base metal,the cracks tended to initiate and propagate along the slip bands and grain boundaries,which leading to a mixed fracture mode of intergranular and transgranular in base metal,and the stress corrosion cracking susceptibility of base metal was low.In the heat affected zone,the stress corrosion cracks preferentially propagate along grain boundaries,although it can initiate along both the grain boundaries and slip bands.The fracture mode was intergranular fracture.The hardening of heat affected zone and the reduction of ∑3 special angular boundaries were believed to be responsible for the increased susceptibility to stress corrosion cracking.In the weld metal,the cracks can initiate and propagates along the ferrite-austenite phase boundary,slip band and austenite grain boundary.In the fusion zone,in contrast to the fracture feature in a room temperature air environment,stress corrosion cracks tended to initiate and propagate in the heat affected zone side in high temperature and high pressure water environment.(6)The change of stress corrosion cracking susceptibility in different regions of the welded joints after thermal aging was related to the evolution of the local inhomogeneous microstructure.Long-term thermal aging resulted in the increase of stress corrosion cracking susceptibility of austenitic stainless steel base metal in high temperature and high pressure water,which may be related to the increase of intergranular corrosion susceptibility of the thermally aged base metal.The stress corrosion cracking properties of heat affected zone had no obvious change after long-term thermal aging,which may be related to the unobvious change of microstructure.In weld metal,spinodal decomposition of ferrite and the precipitation of G phase had little effect on tress corrosion cracking susceptibility of weld metal after long-term thermal aging.The stress corrosion susceptibility decreased in the order of heat affected zone,base metal and weld metal.