Study On Surface Strengthening And Stress Corrosion Behavior Of Welded Joints Of Pressure Vessels

Pressure vessels are generally operated in various corrosive media and subjected to varying degrees of corrosion.Stress corrosion cracking is one of the most important damages in the use of pressure vessels.Pure titanium is often used to manufacture of pressure vessels due to its excellent corrosion resistance.Welded joints become the weak link of the pressure vessels because of the uneven microstructure and welding residual stress.Ultrasonic shot peening(USSP)treatments were used to improve the microstructure and properties of pure titanium welded joints.And OM,TEM and XRD were used to characterize the surface structure and residual stress of the materials treated by different processes.The surface hardness and roughness of the materials were measured.The stress corrosion behaviors of materials in 10%HCL solution were tested by slow strain rate tension(SSRT)method.The corrosion behaviors of the materials in80?10%HCL solution were tested by methods of polarization curve,AC impedance and weightlessness.The corrosion morphologies were observed by SEM and the corrosion mechanisms were analyzed.In this study,TA2 pure titanium welded joints were respectively treated by USSP of0.1 A,0.15 A and 0.2 A.The strengthened layers were formed on the surface of materials.The grain refinement of the surface layer and the homogenization of the microstructure of the welded joints were realized,the surface microhardness was improved,and the residual compressive stress was introduced.The stress corrosion sensitivity index(I_SCC)of original sample was 25.61%in the corrosion system,which indicated that there was a tendency of stress corrosion.The index(I_SCC)reached the minimum value when pure titanium welded joint is treated by 0.15 A USSP,which was 28.78%lower than the original sample,there was no obvious tendency of stress corrosion.0.15 A was the optimal USSP processing parameter to improve the stress corrosion resistance of pure titanium welded joints in 10%HCL solution.Polishing and heat treatment were performed on 0.15 A USSP treated pure titanium welded joints.The stress corrosion behaviors of materials in corrosion solution were studied by SSRT experiments with different roughness of shot peening surface and different surface residual compressive stress.The results showed that the reduction of roughness contributes more to stress corrosion cracking resistance than the weakening of USSP strengthening effect,which can further improve the plasticity of materials.The stress corrosion cracking resistance of USSP specimens after heat treatment was lower than that of untreated specimens.USSP treatment enhanced the corrosion resistance of pure titanium welded joints in10%HCL solution at 80?.With the roughness decreased,the electrochemical corrosion resistance first decreased and then increased.Weightlessness corrosion test showed that with the decrease of roughness,the corrosion rate of materials decreased,the annual corrosion rate decreased by 18%,and the corrosion resistance increased.When polished to 2/5 of the strengthening layer,a large reduction in roughness could promote a more stable and compact protective passivation film on the surface,indicating that the best corrosion resistance is obtained.