DEVELOPMENT OF THE R5 VOLUME 2/3 PROCEDURE TO ENABLE THE CREEP-FATIGUE CRACK INITIATION ASSESSMENT OF CARBURISED STAINLESS STEEL COMPONENTS

摘要

In a high temperature (>480°C) carbon dioxide rich gas environment, 300 series stainless steels typically develop a duplex oxide of magnetite and a nickel-chromium rich spinel. Whilst the oxidation behaviour is well understood, it has only recently been considered that this duplex oxide growth also results in carbon injection into the metal, resulting in a surface layer of carburised material up to approximately 500um deep. This carburisation results in the lamellar formation of carbides along grain boundaries. These brittle phases covering the grain boundaries are expected to result in a reduction in the affected material's creep ductility and fatigue endurance. At the same time, due to changes in local chemistry and microstructure, other material properties are affected in the carburised layer, such as creep deformation behaviour, tensile properties and cyclic stress-strain properties. Existing R5 methodology used to predict creep-fatigue crack initiation does not account for the presence of carburisation. However as the R5 Volume 2/3 assessment methodology aims to predict initiation of cracks of sizes similar to the depth of the carburised layer, there is likely to be a significant impact on the time to initiate creep-fatigue cracking. This paper briefly discusses the nature of carburisation and then focuses on the materials data and methodology changes required to obtain a more accurate and conservative estimate of creep-fatigue initiation times for carburised components.