| Duplex stainless steel is widely used in marine equipment due to its good corrosion resistance.However,it will encounter unavoidable corrosion problems in harsh and complex marine environment,and the most common one is stress corrosion cracking(SCC).It is of great significance to fully study the stress corrosion cracking behavior of duplex stainless steel in marine environment to ensure the structural integrity of marine equipment.Taking 2205 duplex stainless steel as the research object,a small punch stress corrosion test(SPT-SCC)device was developed and designed.The influence of marine corrosion environment on the stress corrosion cracking of 2205 duplex stainless steel and its welded joints was systematically studied from the aspects of conventional slow strain rate tensile(SSRT)test and small punch stress corrosion test.The main research work and conclusions are as follows:(1)Effect of environmental media on SCC behavior of 2205 duplex stainless steel was investigated based on SSRTThe slow strain rate tensile test of 2205 duplex stainless steel was carried out in different environmental media and strain rates.The results show that there is no SCC phenomenon when the strain rate is 1.5×10-5s-1 in distilled water and seawater solution,but the SCC phenomenon is obvious when the strain rate is 0.6 mol/L Na Cl+0.1 mol/L HCl seawater mixed solution.It is found that when the strain rate is greater than 2×10-5s-1,2205 The results show that the stress corrosion sensitivity coefficient of duplex stainless steel is less than 25%,which is in the safe zone;the strain rate is reduced to1×10-5s-1,and the sensitivity coefficient is greater than 25%or even 35%,which is in the range of dangerous zone and brittle fracture zone,and SCC will occur in 2205 duplex stainless steel;the range of strain rate between1×10-5s-1 and 2×10-5s-1 is in the dangerous zone,which may occur.The analysis of fracture morphology shows that there are some brittle fracture characteristics of stress corrosion cracking,such as corrosion crack initiation,corrosion crack propagation and river pattern around the corrosion crack in1×10-5s-1 0.6 mol/L Na Cl+0.1 mol/L HCl seawater mixture solution,which further demonstrates the rationality of stress corrosion cracking sensitivity coefficient region diagram.(2)Mechanical properties of 2205 duplex stainless steel welded joints were investigated based on digital image correlation technique(DIC)The DIC based uniaxial tensile test of 2205 double-phase stainless steel welded joint shows that:in the cloud image of the full-field strain field of the welded joint,the strain in the weld area is large,the neck contraction is located in the weld area,the strain in the base metal area is small,and the heat affected zone appears uniform transition.Local stress-strain curves were constructed through strain field data to obtain the yield strength of different areas of the welded joint,and the yield strength increased from the base metal to the weld.Through the analysis of the microstructure of each area of the welded joint,it is found that there are differences in the metallographic microstructure of the base metal,the heat-affected zone and the weld.The image processing and analysis software(IPP)calculated that the two-phase ratio of austenite and ferrite in the base metal area is close to 1:1,and the austenite content in the heat-affected zone and the weld area is much greater than that of ferrite.The uneven distribution of two-phase microstructure in dual-phase stainless steel will lead to the decrease of the comprehensive properties of the materials,so there are differences in the local mechanical properties of the welded joints.(3)Evaluation of stress corrosion susceptibility of 2205 duplex stainless steel welded joint based on small punch testBecause SSRT can't distinguish the difference of stress corrosion sensitivity in different areas of welded joint,a small punch stress corrosion test device was designed and built on the existing AGS-D universal material tensile testing machine.The repeatability test of the built small punch stress corrosion test device was carried out,and the repeatability of the test results was good,meeting the requirements of small punch test.Based on the advantage of micro sampling of small punch sample,the stress corrosion cracking sensitivity of 2205 duplex stainless steel welded joint base metal,heat affected zone and weld in different H+concentration mixed seawater solution was studied,and the difference of stress corrosion cracking sensitivity in different areas was compared.The stress corrosion sensitivity coefficient of SPT-SCC was established based on the load displacement curve.The results show that the fracture energy sensitivity coefficients of base metal and HAZ are 55.7%and 59.0%respectively when the concentration of H+is 1.5 mol/L,while that of welding material is 1.0 mol/L The results show that the stress corrosion resistance of welding material is weaker than that of base metal and heat affected zone.Combined with the small punch fracture morphology,it is found that the radial crack perpendicular to the main fracture can be observed on the fracture surface of the sample in the H+concentration of 1.0 mol/L solution,which shows the typical stress corrosion brittle cracking characteristics,while the base metal and heat affected zone show the typical stress corrosion brittle cracking characteristics in the H+concentration of 1.0 mol/L solution The fracture surface of the sample in mol/L solution is neat and smooth without radial cracks,which belongs to the ductile cracking without SCC.It also proves that the stress corrosion resistance sensitivity of the welding material is weaker than that of the base metal and heat affected zone.Through the research of this paper,a small punch stress corrosion test device was developed.From the conventional slow strain rate tensile test and small punch stress corrosion test,the influence rule and mechanism of seawater environment medium on the stress corrosion cracking behavior of 2205 duplex stainless steel and its welded joint were explored.Based on DIC,the distribution rule of local mechanical properties of 2205 duplex stainless steel welded joint was systematically studied.These researches are of great significance to ensure the structural integrity of duplex stainless steel offshore equipment. |
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