Room-temperature aging of manganese-alloyed high nitrogen duplex stainless steels
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I. INTRODUCTION
THE room-temperature aging of both ferritic[1,2,3] and austenitic[4–7] alloys containing interstitial carbon has been well documented in the literature. According to Thilo et al.,[1] ferritic flat carbon steels prestrained by 2 pct show only minor room-temperature age hardening, with an increase on the yield strength of the order of 5 MPa after 10 days of room-temperature aging, whereas advanced ferritic grades such as dual-phase steels show a higher aging response of 20 MPa increase in yield strength when aged in the same conditions. Room-temperature aging studies on dual-phase steels have shown that after the formation of martensite, an increase of 40 MPa in the 0.2 pct offset yield strength is possible after 1 week of aging, and an increase of 20 MPa is observed within 15 minutes.[8] Carboncontaining austenitic alloys also show a small increase in flow strength when aged for short times at room temperature. Rose and Glover have shown that for an aging time of 15 minutes, austenitic alloys containing carbon show an increase in flow strength of about 15 MPa, and in the presence of martensite, an increase in flow strength of 20 MPa is shown after 5 minutes at 80 °C.[5] However, there is still little information available regarding room-temperature aging of carbon- and nitrogen-bearing manganese alloyed duplex stainless steel alloys containing ferrite and austenite as the starting microstructure (with transformation of austenite to martensite possible during straining). This paper reports on the unusual room-temperature aging behavior of such alloys.
MARCO MILITITSKY, Graduate Research Assistant, and BRUNO C. DECOOMAN, Professor, are with the Laboratory for Iron and Steel Making, University of Ghent, Ghent, Belgium. Contact e-mail: decooman@ postech.ac.kr JOHN G. SPEER, Professor, is with the Advanced Steel Products and Processing Research Center, Colorado School of Mines, Golden, CO. NICO DE WISPELAERE and NURI AKDUT, Research Managers, are with NV OCAS, Arcelor Group, Zelzate, Belgium. Manuscript submitted August 17, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
A. Casting and Processing Two laboratory cast and processed duplex stainless steels were used for this study. Both alloys contain carbon and nitrogen, and the main difference between the alloys is the manganese content, where the highly alloyed grade has 16.3 wt pct manganese and the lower alloyed grade has 9.5. For the remainder of this paper, the alloy with 16.3 wt pct manganese will be referred to as ‘‘high manganese’’ (HM) and the alloy with 9.5 wt pct manganese ‘‘low manganese’’ (LM). Both alloys were cast as 20-kg rectangular ingots in an open-air induction oven. The cast blocks were air cooled to room temperature, then heated to 1250 °C for 30 minutes before hot rolling to 80 pct thickness reduction, with finishing temperature above 950 °C, and immediately quenched in water after hot rolling. Before cold rolling, all sheets were subjected to an annealing cycle of 10 minutes at 1050 °C, followed by water qu
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