Corrosion Fatigue of a CrMnN Stainless Steel

Abstract

Abstract Austenitic CrMnN stainless steels have been developed as alternative with improved mechanical properties to replace austenitic CrNi stainless steels in certain applications. Stainless steels are often used in highly corrosive environments such as hot chloride solutions. In contradiction to CrNi stainless steels there are only very limited data available on corrosion fatigue of CrMnN steels in such environments. The investigated stainless steel contains 18% Chromium, 21% Manganese and 0.66% Nitrogen. Manganese is used to stabilize the austenitic lattice instead of Nickel and to increase the solubility of nitrogen, which also stabilizes the austenitic structure. S-N-curves were recorded on electrochemical polished samples in inert glycerine as a reference medium and in 62 vol-% CaCl2 solution, both at a temperature of 120°C. The highly chloride containing solution can come into contact with the investigated material in certain application. R value (ratio between upper and lower stress level) was slightly above Zero to avoid destroying of fracture surfaces. Fracture surfaces of representative specimens of the S-N-curves have been extensively investigated in a scanning electron microscope and are evaluated as a function of medium and applied stress. Furthermore, constant load tests for stress corrosion cracking, electrochemical tests and exposure tests were done to determine behaviour of the investigated steel in the high temperature solution. All methods of investigation are compared to each other and results are critically discussed.