| The stainless steel is widely used in industrial production due to its good mechanical properties and machinability.However,stainless steel equipments are prone to stress corrosion crack(SCC)in chloride solution.Moreover,owing to the lack of obvious symptoms,the SCC will cause serious accidents which would be detrimental to the safety of life and property.Laser peening(LP),a state-of-the-art technology of surface treatment,is universally applied because of its unique advantages.It is extensively reported the improvement of LP on the mechanical strength,wear resistance and fatigue property.It is noted that the further and systematic research on the effect of LP on the resistance of SCC in 304 stainless steel weld is rarely revealed.The C-ring specimens of 304 stainless steel weld were treated by laser peening with 4 different energy densities in current work.The effect of LP on the surface properties were studied by the surface morphology observation,microhardness measurement,grain size analysis,residual stress test and XRD phase-analysis.The influence of LP on the SCC was investigated by the experiments of the C-ring specimens in 42%boiled magnesium chloride solution.Results are summarized as follows:(1)The residual stress of C-ring weld surface was measured by X-ray method for stress measurement to compare the residual stress distribution before and after LP.Results indicated that the residual stress was changed from tensile stress to compressive stress after LP on the surface of C-ring weld.The residual compressive stress became larger with the increasing energy density.When the energy density equaled to 5.659GW/cm2,the residual stress in the area of LP is about 357.5 MPa,which was higher than the yielding strength of the material.It was beneficial to enhance the resistance of SCC in 304 stainless steel weld.(2)The microhardness tester was applied to measure the microhardness of C-ring weld.The microhardness of weld specimens with LP was significantly higher than that without LP.As the distance increased along the thickness direction,the microhardness decreased gradually.The results of grain size analysis showed that high energy shock produced by LP developed severe plastic deformation on C-ring surface.The grain refinement was observed in the area adjacent to the C-ring surface.(3)The SCC resistance of 304 stainless steel weld in chloride solution was conducted by the experiment of C-ring weld specimens in boiled magnesium chloride solution.The initiation time of crack in the C-ring specimen with LP treatment was significantly longer than that without LP treatment.The specimen with higher energy density resulted in a longer initiation time of crack,meaning the better SCC resistance.The former results revealed that the compressive residual stress and grain refinement induced by LP effectively improved the SCC resistance of 304 stainless steel weld in chloride solution.(4)The fracture morphology and corrosion products were analyzed by scanning electron microscope(SEM).The SCC type of 304 stainless steel weld was mixed with transgranular and intergranular type in chloride solution.The main cracks contained bifurcation and secondary cracks.It was brittle mode with river and fan shapes on fracture surfaces,which was in accord with typical characteristics of SCC.Corrosion products were aggregated of Cl element.It can be inferred that Cl-destroyed the passivation membrane on the surface of metal and formed the pitting holes on the metal substrate.Consequently,the SCC initiated and propagated in the sites of pitting holes. |
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