Microstructural sources of toughness in QLT-Treated 5.5Ni cryogenic steel
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I.
INTRODUCTION
FERRITIC steels which are intended for structural use at cryogenic temperatures are thermally processed to lower the ductile-brittle transition temperature to below the intended temperature of service. In conventional 9 pct Ni steel this treatment involves a straightforward intercritical temper at a relatively low temperature within the two-phase (c~ + 3,) region. During the intercritical temper a fine, dense distribution of austenite phase is precipitated along the boundaries of the dislocated laths of the martensite structure. This austenite is thermally stable on cooling to at least 77 K. It lowers the ductile-brittle transition temperature by breaking up the crystallographic alignment of the martensite laths within packets through a toughening mechanism described in Reference 1. Ferritic steels of lower nickel content can also be toughened for cryogenic service2"3as can ferritic iron-manganese steels containing no nickel at all. 4'5 But in these cases more elaborate thermal treatments are required. The most commercially important of these alternative heat treatments is the "QLT" treatment, which is a 3-step heat treatment employed for toughening the 5-6Ni steels developed some years ago by the Nippon Steel Corporation in Japan and by Armco Steel in the United States. The QLT treatment is the fourth of those diagrammed in Figure 1. It involves two sequential intercritical heat treatments after initial quenching. The first of these (L) is an intercritical anneal in the upper range of the two-phase (c~ + 3') field. The second (T) is an intercritical temper at lower temperature within the two-phase field. While the QLT treatment has been used with commercial success for some years, the fundamental reasons for its J.I. KIM is Research Scientist with Thomas J. Watson Research Laboratory, IBM, Yorktown Heights, NY 10598; C. K. SYN is Research Scientist, Phillip McKenna Laboratory, Kennametal, Inc., P.O. Box 639, Greensburg, PA 15601; and J. W. MORRIS, Jr. is Professor of Metallurgy, University of California, Berkeley, CA 94720. Manuscript submitted September 3, 1981. METALLURGICALTRANSACTIONS A
effectiveness remains somewhat unclear. The present research was undertaken to clarify the microstructural sources of toughening through this heat treatment. In previous papers we discussed the dominant mechanism by which a distribution of precipitated austenite lowers the ductilebrittle transition of ferritic steel, 1 and determined the approximate composition of the precipitated austenite in Nippon 5.5Ni steel as a function of heat treatment, including the QLT treatment. 6 In the work reported below we studied the interplay of composition and microstructure in determining the ductile-brittle transition temperature of 5.5Ni steel, with particular emphasis on the benefit achieved from the QLT treatment. II.
EXPERIMENTAL PROCEDURE
The alloy used for this investigation was a commercial Fe-5.5Ni steel which was provided by the Nippon Steel Corporation. Its composition was determined to be: Fe-5.86Ni- 1.21Mn-0.
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