Formation and Growth Kinetics of Reverted Austenite During Tempering of a High Co-Ni Steel
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THE combination of material properties such as good strength and hardness on one hand and excellent fracture toughness on the other hand is favorable in many applications, e.g., structural parts in airplanes. A steel class, which perfectly satisfies this property combination, are high Co-Ni secondary hardening steels.[1,2] The excellent toughness properties of these steels are assigned to thin austenite films, which are present preferentially along martensite lath boundaries. Despite different opinions about the toughening mechanism of the austenite films, it is principally asserted that a higher stability, i.e., resistance against phase transformation to martensite, of these films leads to a larger toughness of the material.[3–9] In Aermet 100, which is the most prominent representative of this steel class, two different types of austenite are present—retained austenite and the so-called reverted austenite.[6,7,10] Retained austenite is the product of an incomplete martensitic transformation after austenitization. Sato[7] found that this type of austenite has a negative MARINA GRUBER and GERALD RESSEL, Scientific Staff, STEFAN MARSONER, Manager Tooling, and REINHOLD EBNER, Company Director, are with the Materials Center Leoben Forschung GmbH, Roseggerstrasse 12, Leoben 8700, Austria. Contact e-mail: [email protected] FRANCISCA ME´NDEZ MARTI´N, Group Leader, is with the Department of Physical Metallurgy and Materials Testing, Montanuniversita¨t Leoben, Roseggerstrasse 12, Leoben 8700, Austria. SARAH PLOBERGER, Researcher/Product Development Special Materials, is with the Produkt- und Verfahrensentwicklung fu¨r das Segment Sonderwerkstoffe, Bo¨hler Edelstahl GmbH & Co KG, Mariazeller Strasse 25, Kapfenberg, 8605, Austria. Manuscript submitted May 2, 2016. Article published online September 21, 2016 5932—VOLUME 47A, DECEMBER 2016
influence on fracture toughness properties because of its low stability and its early transformation to brittle martensite during deformation. Reverted austenite is formed during a subsequent tempering treatment in high Co-Ni steels.[11,12] As it is described by Haidemenopoulos et al.,[8] the main difference to retained austenite is that reverted austenite can be altered in composition and size during tempering, which both affects the stability. In the literature it is reported that pronounced Ni or Mn enrichments in reverted austenite as well as the nm size of the films lead to an increased stability.[13–16] It was found by Ayer[6] and Haidemenopoulos[3] that reverted austenite in high Co-Ni steels exhibits such properties. However, the influence of carbon, one of the most powerful austenite stabilizers, on the formation as well as the growth mechanism of reverted austenite has not been precisely studied in the literature of high Co-Ni steels until now.[3–8] Thus, in order to obtain more information about the microstructural evolution of high Co-Ni steels, this work concentrates on the behavior of martensite, reverted, and retained austenite upon tempering with focus on the redistribution of
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