Effect Of Welding Oxidation And Grinding Treatment On High Temperature Water Oxidation Of Nuclear Grade 304L

304L austenitic stainless steel is widely used in the design of nuclear safety grade mechanical components of nuclear power plant,which bears the load of operation conditions and radioactive containment function.Welding and grinding operations are often required in the manufacturing and installation of the above components,which results in the oxidation of the material surface or the change of the roughness,and damages the passivation film originally formed at room temperature.Therefore,the research on the effect of 304L welding oxidation and grinding on high temperature water oxidation not only help understand the phenomenon,process and mechanism of the effect of welding oxidation and grinding on the surface of matrix,but also benefit to evaluate the degree of the influence of the above process on the corrosion.These studies will lay a theoretical foundation for the safety evaluation of nuclear safety equipment,and provide a general theoretical support and engineering guidance for the development and optimization of welding and grinding process of nuclear safety equipment.Taking nuclear grade 304L and its weld as the research object,the oxide film on the surface of the weld metal and base metal in TIG welding without inert gas protection and the changes of the base metal surface under different grinding conditions with different abrasive size sandpapers were studied.The rules and mechanism of the influence of the welding oxide film and surface grinding on the oxidation in high temperature water were investigated.At the same time,the composition,structure,formation process of oxide film and surface properties,crystal defects of the surface after grinding were analyzed in details by means of optical microscopy,laser confocal scanning microscopy,scanning electron microscopy,energy dispersive spectrometry,Auger spectroscopy,X-ray photoelectron spectroscopy,laser Raman spectroscopy,focused ion beam,transmission electron microscopy and so on.The model of welding oxide film growth and the model of structure and performance evolution during surface grinding were established.The laws of influence of welding oxide film and grinding on the oxidation behavior of high temperature water were found and the relevant mechanisms were revealed.The main conclusions were as follows:(1)The whole weld metal showed the characteristics of "isothermal"oxidation,and the oxidation of Cr to form CrO3 led to the poor chromium oxide film.The typical structure of oxide film were composed of loose Fe2O3 particles in the outer layer,tight Fe3O4 and FeCr2O4 particles in the inner layer,NiO along the film-metal interface and cracks caused by expansion stress.Based on the typical structure,the second law of thermodynamics and the analysis of oxygen potential,the growth process of oxide film was reproduced and the growth mechanism was revealed.(2)The base metal near the fusion line showed characteristic of "gradient"oxidation,and the trend of oxidation degree and film thickness were consistent with the trend of welding peak temperature field.In the macroscopic morphology,the blue region was Cr-O film,and the two sides of the region extend outward successively were Cr-Fe-O region and Fe-Cr-O region.Combined with the typical oxidation process and thermodynamics analysis of chromium rich and chromium poor films,the "four regions" and its oxidation mechanism of temperature effected on the composition of oxide film were deduced,and The oxidation law of the whole welded joint was revealed in an integrated way.(3)After 90 days of high temperature water oxidation,the oxide particle size of the surface covering the chromium rich welding oxide film was significantly smaller than that of other areas(including areas with and without welding oxidation),and the oxide particle size of other areas were similar.Based on this phenomenon,the saturation effect of high temperature water oxidation determined by the structure and thickness of oxide film was found.The relationship between the welding oxidation film and oxidation rate under high temperature water was analyzed numerically.The inverse function relationships between chromium rich film,chromium poor film and oxidation rate were deduced.The theory and empirical formula of film thickness calculation under the condition of high temperature water oxidation accumulated on welding oxidation film were established.Based on the analysis of the welding oxidation model and the high temperature water oxidation model,the key mechanism of the high temperature water oxidation accumulated on the welding oxidation film was obtained,that was,the dissolution of iron oxide and the growth of NiFe2O4.(4)In the grinding process,with the increase of abrasive size,the surface roughness and hardness gradually increased,while the compressive stress gradually decreased,but it had no effect on the average strength.The difference of plastic deformation and elastic deformation caused by the difference of abrasive particle size determined the performance of the grinded surface.The relationship between the transformation depth and grain size of nano-crystals and the sub region(peak\slope\valley)of wear mark was found by the stereoscopic characterization of wear mark.The process of ?-austenite(FCC)??'-martensite(BCC)caused by coarse abrasive particles and the mechanism of K-S shear were revealed.At the same time,it was found that the dislocation density of the surface layer increased about 2.5×105 times due to the fine abrasive particles,which revealed dislocation slip mechanism of plastic deformation and the mechanism of dislocation increment.(5)After 90 days of high temperature water oxidation,the surface of the samples with different grinding treatments all formed a double-layer oxide film with iron oxide as the outer layer and chromium rich oxide as the inner layer.The coarser the abrasive particles,the more Fe2O3 and less Fe3O4 in the outer oxide film,the faster the high temperature water oxidation.Furthermore,according to the relationship between the surface metal crystal defects and the structure of high temperature water oxidation film,it was found that coarse abrasive particles were more likely to promote high temperature water oxidation.Based on the quantitative analysis of the linear and faceted crystal defects caused by different abrasive grains,a numerical model of crystal defects promoting high temperature water oxidation was established.The microscopic mechanism of the thin inner chromium rich oxide film and the thick total oxide film caused by coarse abrasive grains was that the increase of nano-martensite grain boundary caused by coarse abrasive had less promotion on the diffusion of Cr than the increase of dislocation density caused by fine abrasive.