The Mechanism of brittle fracture in CN3MN grade superaustenitic stainless steel /

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2015
Başkan, Mertcan
Steel is one of the most widely used materials in almost every major engineering application. Their uses extend from the large constructions in the capitals to the small kitchenware products in houses. Steel has been known since the ancient times, and the properties of steel have been enhanced for hundreds of years by the help of the improvements in metallurgy and material science. Steel has been almost at the center to the discovery-based development of new alloys with notable properties. The steel family has been still growing with the innovations in compositions, heat-treatments and production techniques. Attributed primarily to its remarkable deteriorative properties, CN3MN, is one of the significant Super Austenitic Stainless Steel (SSS), which is a subclass of austenitic stainless steels. Main application areas of CN3MN are marine constructions and oil wells due to their excellent corrosion resistance and superior toughness. However, according to the results of recent time temperature transformation (TTT) diagrams, incorrect heat-treatments for as short as 15 minutes causes fracture toughness of the alloy to decrease by 50% as embrittlement occurs. Wrought steel pieces are typically smaller in size and can be heat-treated and cooled rapidly in order to avoid embrittlement. However, in the case of cast superaustenitic stainless steel, cooling rate is much slower so the regions suffered from such embrittlement is extended. The focus of this study is to understand the mechanisms of the embrittlement problem after annealing heat-treatments for very short times i.e. 30 seconds to 16 minutes at 927°C. The fracture surfaces were investigated by using scanning electron microscope (SEM). Failure analysis results showed a ductile to brittle failure transition for relatively short annealing times. The first reason for embrittlement was defined as the precipitates formed at the grain boundaries. Since the intermetallic precipitates are very brittle, they deteriorate the mechanical properties when the brittle network is formed on the grain boundaries. Average precipitate size in 30-second annealed specimen was found to be about 400 nm with a high number density. The crystal structure of the precipitates formed in short-term annealing could not be matched with previously determined secondary phases in austenitic stainless steel. The kinetics of the nucleation and growth was determined to be very sluggish in the previous studies. In this study, it is shown that the newly determined precipitates have relatively high nucleation rates. This is related to inhomogenous distribution of Mo after the homogenization. These Mo-rich regions are thought to trigger a rapid precipitation. Second mechanism observed in CN3MN is transgranular embrittlement. The heat-treated specimens contained some cracks with zigzag pattern. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in micron-length scales. The reason of having such faulted regions was connected to having low stacking fault energy. Stacking fault energy of austenite phase in CN3MN was calculated to be between 12-30 mJ/m2 which is quite low for a metallic alloy. Another implication for low SFE was the dislocation distribution. Low SFE promotes entangled dislocation structure instead of subgrain formation, which limits the dislocation motion. The low SFE suppress the crosslinking and climb processes, which are the most active phenomena to maintain dislocation motion. The stacking-fault observed in austenitic matrix changes the FCC by HCP sequence within the defected nano-metric regions. Therefore, the vast majority of transgranular cracks were formed within the brittle HCP-sites resulting into catastrophic transgranular fracture.

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Citation Formats
M. Başkan, “The Mechanism of brittle fracture in CN3MN grade superaustenitic stainless steel /,” M.S. - Master of Science, Middle East Technical University, 2015.