Segregation-Induced Enhancement of Low-Temperature Tensile Ductility in a Cast High-Nitrogen Austenitic Stainless Steel
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ile elongation near room temperature of low stacking fault energy (SFE) austenitic steels including stainless,[1–9] high Mn,[10–13] and high Ni[14] steels varies in three temperature regimes as follows. At the highest temperature range (regime I), tensile elongation remains more or less constant or exhibits a weak temperature dependence. At intermediate temperatures (regime II), elongation increases as the temperature decreases. The enhancement of ductility at reduced temperatures in regime II is commonly attributed to such deformationinduced microstructural changes as e martensite formation, the e-TRIP effect,[15] and deformation twinning, the TWIP effect.[2] These are common microstructural features of deformed austenitic stainless steels[16,17] and high Mn[12,18,19] steels and are thought to be consequences of high glide planarity. Enhanced glide planarity caused by JAVAD MOLA, MARCO WENDLER, Scientific Assistants, and ANDREAS WEIß, Professor, are with the Institute of Iron and Steel Technology, Technische Universita¨t Bergakademie Freiberg, Leipziger St. 34, 09599 Freiberg, Germany. Contact e-mail: [email protected] BENEDIKT REICHEL, Scientific Assistant, is with the Institute of Materials Science, Technische Universita¨t Bergakademie Freiberg, 09599 Freiberg, Germany. GOTTHARD WOLF, Professor and Director, is with the Foundry Institute, Technische Universita¨t Bergakademie Freiberg. BRUNO C. DE COOMAN, Professor and Director, is with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Manuscript submitted November 12, 2014. Article published online February 10, 2015 1450—VOLUME 46A, APRIL 2015
reduced cross slip of screw dislocations at lower temperatures has been correlated with an underlying temperature dependence of SFE.[20] The latter dependence is available for many fcc metals and alloys including austenitic steels.[20–24] The enhancement of tensile elongation at lower temperatures in regime II of austenitic stainless steels is interrupted at the temperature below which the deformation-induced formation of a¢ martensite is enabled, namely below the Mdcfia¢ temperature. The loss of ductility caused by the deformation-induced formation of a¢ martensite initiates the regime III of elongation, characterized by reduced elongations at lower temperatures. The large number of investigations in support of the detrimental effect of a¢ martensite formation on the tensile elongation of metastable austenitic stainless steels[1–7, 10–12] indicates that any possible contribution to the ductility of a TRIP effect would not be large enough to prevent the loss of ductility below the Mdcfia¢ temperature. This has been also suggested by the modeling of the a¢ TRIP effect contribution to the ductility.[25] The present study demonstrates how compositional inhomogeneities in a cast high-nitrogen austenitic stainless steel can cause deviation from the behavior described above. The chemical composition of the experimental steel was
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