Research On The Stress Corrosion Cracking Of FV520B Precipitation-hardening Martensitic Stainless Steel

FV520B is the new precipitation-hardening martensitic stainless steel developed from Crl3 martensitic stainless steel, which is the general material of impeller in centrifugal compressor in natural gas pipeline due to its high strength, excellent weldability and corrosion resistance. At present, the research about stress corrosion cracking(SCC) of FV520B are few at home and abroad, and country and companies urgently need the corrosion data of this material. This paper studies on the stress corrosion cracking mechanism, the influence of dielectric environment factors to SCC sensitivity, the critical potential and sensitivity rang of SCC, the nucleation condition of cracks in pit and the initiation and propagation process of stress corrosion cracks of FV520B precipitation-hardening martensitic stainless steel by slow strain rate tensile(SSRT) test, electrochemical corrosion test and finite element simulation, comprehensively evaluating the SCC behavior of FV520B stainless steel in service environment. The research fills the gap about the SCC of FV520B and also can meet the country and companies’needs about the corrosion performance of this material.The SCC behavior of FV520B general tensile samples and welded joint samples in the simulated real service environment were analyzed with SSRT test and electrochemical corrosion test combining the analysis of potential, fracture appearance and energy spectrum. The test results show that test temperature and H2S density have a greater influence on the SCC sensitivity of general samples. The corrosion current density and SCC sensitivity decrease first, then increase with temperature up. When the H2S density increases, the corrosion current density and SCC sensitivity all have a marked increase and the SCC aggravates. The CO2 density has a greater influence on the SCC sensitivity of weld joint samples than H2S density. The corrosion current density and SCC sensitivity of weld joint samples increase significantly when the CO2 density increases. The SCC sensitivity reaches the maximum value 64.82% in the test environment of 130℃,8.4 MPa,6% H2S and 39%CO2.The fracture surface shows obvious brittle fracture characteristics and there is a wide mud pattern at the edge of fracture, which is the typical fracture morphology of SCC. Weld joint samples all have pronounced tend of SCC in the 6 group test environments and are more susceptible to SCC than general samples in the same environments. The analysis of cracks and fracture morphology shows the corrosion mechanism of FV520B general samples in test environment is hydrogen induced stress corrosion cracking.The influence of pit shape and size on local stress concentration in the tensile samples and the position and condition of stress corrosion cracks nucleation was studied using the element remove simulation method. The maximum stress region locates in the bottom of pit on FV520B tensile samples. Instead, the location of maximum strain is near the pit mouth or shoulder and plastic strain exists in this region. The stress concentration factor of dish pit is biggest and the stress concentration phenomenon is most obvious. The plastic strain rate during pit growth gotten form simulation is in the sensitivity range of stress corrosion cracks nucleation, indicating that stress corrosion cracks are more likely to nucleate near the pit mouth or shoulder.Tensile failure process of plate samples and V-shaped single edge notched samples was simulated basing on cohesive zone model(CZM). The internal stress in corrosion product film and film-induced stress in matrix increase with the increase of film thickness. The thicker of corrosion product film, the smaller the crack initiation strength and the easier to initiate and propagate for stress corrosion cracks. Meanwhile, with corrosion product film increase, the SCC sensitivity of plate samples increases and KISCC of V-shaped single edge notched samples reduces, making the stress corrosion cracks more likely to propagate. Simulation results show that the stress corrosion cracks nucleate on the surface of corrosion product film and propagate gradually until film breaks with continuation of tensile load. Finally, stress corrosion cracks propagate into the matrix.