Avaliação das propriedades mecânicas de ligas modelo de aços com alto teor de manganês para aplicações criogênicas

Abstract

High manganese steels have being studied as a cheaper alternative to replace austenitic stainless steels and Fe-Ni alloys in storage tanks and liquefied natural gas pipes (LNG) applications operating at cryogenic temperatures. This type of application requires that materials have a good toughness to cryogenic temperature. Stacking fault energy (SFE) defines the mechanisms of deformation in high-Mn steels, influencing the behavior of these mechanisms under deformation. Four high-Mn steels with Mn (20-30%) and C (0.2-0.6%) contents were studied by analyzing the microstructure, tensile mechanical properties, Charpy impact toughness, and CTOD and J-Integral fracture toughness in tests performed at room and cryogenic temperatures. We sought to relate the results of tests performed with the mechanisms of deformation resulting from the steelmaking process or from severe service utilization with stacking fault energy calculated from the chemical composition, temperature and microstructure. The microstructure of alloys is formed by austenitic matrix, and when deformation is present the appearance of mechanical twins at studied temperatures, also confirmed by SFE calculation, classifying the alloys as TWIP steels. The tensile mechanical properties were higher at cryogenic temperature than at room temperature, with consequent reduction in elongation. The 30Mn26C (30% Mn; 0.26% C) alloy showed surprising ductility and impact toughness at cryogenic temperature, presenting values of 28% in elongation and 73.2 J in Charpy impact energy when compared to the other alloys at this temperature. In correlation with yield stress, ultimate tensile strength and strain hardening rate with SFE, an inverse proportionality was verified in relation to temperature tested. While, in the relationship of SFE with tensile ductility and Charpy impact toughness, the proportion is direct. The fracture toughness of alloys at room temperature was excellent, the alloy with lower Mn content and higher C content was the most tenacious. The fracture toughness at the cryogenic temperature of alloys was not satisfactory due to the presence of heterogeneities in alloys manufacturing process. Higher Mn contents alloys and with Al addition obtained better fracture toughness at cryogenic temperature. In terms of toughness at cryogenic temperature, the 30Mn26C alloy is the most indicated, because it obtained the best results of fracture toughness (m = 0.152 mm; Jm = 178.1 kJ / m2), together with the 22Mn53C alloy, and the highest impact energy value (73.2 J) of all.