Effect of Microstructure and Mechanical Properties on the Stress Corrosion Cracking Assessment of Type 304 Stainless Ste
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Effect of Microstructure and Mechanical Properties on the Stress Corrosion Cracking Assessment of Type 304 Stainless Steel Using Slow Strain Rate Tests A. Contreras1*, S. L. Hernández1, E. Terres1, R. Galvan2 1
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Instituto Mexicano del Petróleo, Eje central Lázaro Cárdenas Norte 152, San Bartolo Atepehuacan, C. P. 07730, México. Tel: +52 5591758194. * E-mail: [email protected]
Unidad Anticorrosión, Instituto de Ingeniería, Universidad Veracruzana, Av. S.S. Juan Pablo II s/n, Fracc. Costa Verde, C.P. 94294, Veracruz, México.
ABSTRACT The stress corrosion cracking (SCC) of the commercial austenitic stainless steel type 304 was investigated as function of test temperature, microstructure and mechanical properties in acidic chloride solution (25 wt.%-MgCl2) using slow strain rate tests (SSRT). Susceptibility and mechanism of SCC was investigated using SSRT performed at strain rate of 1 x 10-6 in/s in a glass autoclave containing a magnesium chloride solution at 20, 50 and 80°C. The SCC assessment was carried out in function of the results of time to failure ratio (TFR), elongation ratio (ELR), ultimate tensile strength ratio (UTS-R), strain ratio(eR), yielding strength ratio (YSR) and stress rupture ratio (SR-R). This assessment was complemented by some scanning electron microscopy (SEM) observations, in order to determine the type of fracture and its features. SSRT results indicate that 304 stainless steel was susceptible to SCC at 50 and 80°C. SCC susceptibility increases as the temperature increase. By the contrary, the mechanical properties decreases with temperature increase. SEM observations showed a ductile type of fracture, indicating that cracks appear to be originated from the pits, increasing the number of cracks as the temperature increases. Corrosion pits are one of the main potential sites for surface crack initiation. The stress concentration in the pits will be the nucleation site for cracks. Keywords: Fracture, Corrosion, Stress-strain relationship, Scanning Electron Microscopy, Steel. INTRODUCTION Austenitic stainless steel AISI 304 is extensively used in the petrochemical, automotive, aviation, and cuisine industry among others. Type 304 Stainless steel (SS) is used where both the mechanical properties and corrosion are required. This chromium-nickel steel commonly used commercially contains 18 wt% chromium and 8 wt% nickel. From all the stainless steel is one of the most economical and used. The SCC susceptibility of stainless steel depends on the alloy composition, structure, heat treatment applied and of course from the environment and temperature. The austenitic (300 series) stainless steels can be susceptible to SCC in certain chloride environments, whereas the ferritic and martensitic grades (400 series) can be susceptible to hydrogen embrittlement. The terms “Chloride-SCC”, “Caustic-SCC” and “polythionic acid-SCC” are often used to describe SCC of stainless steels in those types of environments [1] When considering the SCC of stainless steels, it should be good distinguish between cracking in
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