Study On Stress Corrosion Cracking Of Austenitic Stainless Steels In Chloride Solution

Austenitic stainless steel is widely used in the manufacturing of pressure equipment such as boiler, waste heat boiler, reactor, heat exchanger and pressure pipeline etc. During operation, especially in the coastal areas, pressure equipment are vulnerable to erosion of Cl- and susceptible to stress corrosion cracking problems. According to the pressure equipment operated in the Cl- environment,4 kinds of common austenitic stainless steel materials (S31008 S34778 S30403 and S31603) are selected as the research object. In this paper, the study on the stress corrosion cracking behavior of austenitic stainless steel exposed to acid solution containing provided the theory basis for both the use and maintenance of existing austenitic stainless steel pressure equipment and the material selection in the design and manufacture of pressure equipment in the future.The flow field inside the elbow is analyzed by the finite element method. The distribution of pressure generated by the fluid is acquired which is shown that:the minimum pressure appeared on the inner wall surface and the maximum pressure appeared on the lateral wall surface. The amplitude of pressure variation is so small that can be ignored. So it is reasonable that setting the internal pressure as 1.OMPa equally in the fracture mechanics calculation of the crack on the elbow model inner surface. The static analysis module of ANSYS software is used to the fracture mechanics calculation to obtain the value of the stress intensity factor at the crack. The results show that:The stress intensity factor get to be the largest in the middle and decreased gradually to the side of the crack.The 4 kinds of stainless steel materials are tested with the method of slow strain rate test (SSRT) in different environments and electrochemical test in the same environment. The specimen fracture of SSRT is observed by the Scanning Electron Microscope (SEM). The three-dimension model of the specimen with pit and crack is carried out. The distribution of the stress at the pit and crack that can predict the direction of the crack during constant strain stretching is analyzed numerically. The results show that:Stress concentration appears in the middle of the pit and perpendicular to the direction of the force. This indicates that the crack most likely appears here and extends along the direction perpendicular to the force. When crack exist, the stress concentration appears at the bottom of the crack tip which will accelerates the crack growing.The results from the SSRT and the electrochemical test of austenitic stainless steel materials in different environments were show that:In corrosive environments, the fracture surface of the austenitic stainless steel specimen appears brittle fracture characteristics such as cleave stairs, tearing ridge and herringbone pattern. (1) In chloride solution, with the temperature increasing, the resistance against stress corrosion of stainless steel S31008 increased first and then decreased. When the temperature get to 120 ℃, it get to be the best. Improving pressure is helpful to strengthen resistance against stress corrosion of stainless steel S31008. (2) In the test environment, as the temperature increases, the resistance against stress corrosion of S34778 stainless steel decreased first and then increased, the worst is at 260 ℃. However, the change of pressure has little influence on the resistance against stress corrosion. (3) The resistance against stress corrosion of S30403 stainless steel in chloride solution is increased with the temperature increasing and decreased with the pressure increasing. (4) The resistance against stress corrosion of S31603 stainless steel in chloride solution is decreased with the temperature increasing. Increasing the pressure significantly improves its resistance against stress corrosion in chloride solution. (5) The electrochemical test result show that:At normal temperature and pressure in NACE solution, the order of the resistance against corrosion of the 4 types of stainless steel is S31008> S34778> S30403> S31603.The grey system theory is adopted to analysis the significance effect of temperature and pressure on the resistance against stress corrosion of austenitic stainless steel material. The results show that:The temperature has a greater effect than the pressure on the resistance against stress corrosion of austenitic stainless steel material especially for the S31008 and S30403.