| Three primary components(pressure vessel,regulator and steam generator)of nuclear power plant are made of forged low alloy steel with its inner surface deposited by austenite stainless steel weld metal(one layer of 309L and two layers of 308L).The properties of the weld overlay cladding exposed to service environment(280～320℃ solution)play an important role in safe operation of nuclear power plant.Besides,for the stainless steel weld overlay cladding materials including austenite and ferrite phases,thermal aging embrittlement is likely to occur after a long term service,which acts as the key factor for the service safety of nuclear power plant.In this paper,the investigation on the microstructure,thermal aging behavior and their effects on the oxidation behavior of domestic 308L stainless steel cladding material were conducted.By means of optical microscope(OM),scanning electron microscope(SEM),transmission electron microscopy(TEM)methods,the microstructure evolution of stainless steel weld overlay cladding was investigated;combined with SEM and TEM analysis effect of thermal aging on mechanical property and deformation behavior was studied by small punch and in-situ tensile tests respectively,;combined with weight gain(loss),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),focused ion beam(FIB)and TEM,the oxide film formed on the cladding material in high temperature and high pressure water(350℃ and 20MPa)was characterized by static autoclave.All the results were used to illuminate the microstructure evolution,mechanical and oxidation behavior degradation mechanism of domestic stainless steel cladding material and simultaneously reveal the correlation between the microstructure evolution and properties change of the cladding material.The main results were listed as follows:(1)Austenite and worm(or island)-like ferrite phases formed in the E308L stainless steel weld overlay cladding.Higher Cr content led to formation of more ferrite content,and carbides were found along austenite/ferrite phase interface after PWHT.Higher Cr content enhanced the pitting resistance and compactness of the oxide film to reduce metal amount oxidized and dissolved,which mitigated the weight changes and the formation of Fe-rich oxides.PWHT promoted more and deeper pitting holes along the austenite/ferrite phase interface due to formation of carbides,which resulted in an increase in metal amounts oxidized and dissolved,and were also responsible for more Fe-rich oxides and higher weight changes.(2)EQ308L stainless steel weld overlay cladding showed a duplex-structure including austenite and 10 vol.%polygonal ferrite phase.Thermal aging had no obvious effect on the volume fraction of ferrite,but could cause microstructural evolution by spinodal decomposition and G-phase precipitation in the ferrite phase.The precipitates along the dislocation and within ferrite matrix contained the face-centered cubic structural G-phase and silicide phase.Silicide phase,supposed to be(Fe,Mn)3Si phase,preferred to form with G-phase along the dislocations and within ferrite matrix.Silicide phase showed a cube-on-cube relationship with ferrite phase and G-phase.Heterogeneous distribution of elements within Ni-Mn-Si clusters was responsible for silicide phase formation during thermal aging process.(3)Through small punch test,small punch energy decreased with the increase of thermal aging time,wherein,the rapidly degradation part in the range of 0～1000 h was controlled by the spinodal decomposition,while the slow decrease part in the case of aging time longer than 1000 h was related to the formation of G-phase and silicide phase and spinodal decomposition.Thermal aging contributed to hardening of ferrite phase and then,promoted the embrittlement susceptibility of aged ferrite phase.Curvilinear slip bands formed in the aged ferrite phase,and nucleation of microcracks occurred at the austenite/ferrite phase interface along these slip bands.The hardening of the ferrite and high stress concentration on austenite/ferrite phase interface resulted in brittle fracture and phase boundary separation after thermal aging.(4)For the in-situ tensile test,intensive straight slip bands formed in unaged austenite and ferrite phases.The interaction between the slip bands and phase interface contributed to the initiation of microcracks.Deformation behavior of austenite phase was not affected by thermal aging,while the plastic deformation of the aged ferrite phase proceeded via formation of curvilinear slip bands.Existence of spinodal decomposition and precipitaties(G-phase and silicide phase)resulted in preferential initiation of microcracks along the curvilinear slip bands.Microcracks nucleated along phase boundary with carbides,and hardening effect of ferrite phase lowered the microcracks formation along carbide/ferrite phase boundary.(5)Thermal aging showed no obvious effect on the oxidation behavior of austenite phase but resulted in thickness and composition change of oxide film formed on the aged ferrite phase.The oxide film formed on the unaged cladding material consisted of 100-nm-thick inner oxide layer(FeCr2O4 and Cr2O3)and Fe-rich outer oxide particles(Fe3O4)with a size of less than 500 nm.By comparison,300-nm-thick Fe-Cr inner oxide layer(FeCr2O4,Cr2O3 and Fe3O4)and outer oxide particles(Fe3O4 and FeCr2O4)with a size of less than 700 nm formed on the aged ferrite phase.Spinodal decomposition was considered as the main factor to promote the oxidation of the aged ferrite phase,in the meanwhile,G-phase and silicide phase was believed to enlarge the level of Cr-depleted,which further facilitated ions dissolution and oxidization of the aged ferrite phase during corrosion test.In addition,existence of carbides along phase boundary facilitated oxidation of the cladding material in high temperature water. |
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