Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungste
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UCTION
NAVAL applications require the use of steels with high strength and high toughness over a wide range of welding and service conditions. A 10 wt pct Ni steel has been developed with ballistic resistance, strength, and toughness values exceeding those of steels currently used, and is now being considered as a candidate material to replace existing high strength, low alloy steels.[1] The yield strength in the optimally heat-treated condition is 908 MPa and the Charpy impact toughness at 189 K ( 84 C) is 147 J,[1] thus demonstrating that this steel meets the low temperature toughness requirements for the intended application. The steel obtains high strength from the formation of martensite and good toughness from the addition of nickel and the concomitant, associated mechanically induced transformation of austenite to ERIN J. BARRICK and JOHN N. DUPONT are with the Lehigh University, 5 East Packer Ave, Bethlehem, PA, 18015. Contact e-mail: [email protected] DIVYA JAIN is with the Department of Materials Science & Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL, 60208. DAVID N. SEIDMAN is with the Department of Materials Science & Engineering, Northwestern University, and also with the Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, IL, 60208. Manuscript submitted June 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite, known as the transformation-induced-plasticity (TRIP) phenomenon.[1,2] For some time, the achievement of simultaneous high strength and toughness in TRIP steels has been understood.[3] The progressive formation of martensite in the presence of stress or strain causes a higher rate of work hardening and relieves stress concentrations; the resulting outcomes are increases in strength, ductility, and toughness.[4,5] To accomplish this, the alloy content and thermomechanical treatments are designed to stabilize austenite at room temperature. The success of the TRIP principle hinges on the stability and volume fraction of austenite present, which is determined by the composition and heat treatment. This 10 wt pct Ni steel obtains the necessary austenite for the TRIP mechanism through a three-step quenching, lamellarization, and tempering (QLT) heat treatment. QLT heat treatments were originally developed for lower-Ni steel such as 5.5 wt pct Ni steel, but have been adapted for higher Ni content steels in the last 20 years.[6–14] The quenching step (Q) involves heating in the single phase c region, and both the lamellarization step (L) and tempering treatment (T) are conducted between the Ac1 and Ac3 temperatures in the two-phase (a + c) field.[12] The Q treatment is responsible for generating a lath martensite microstructure, which provides high strength of the alloy.[6] During the L treatment, austenite forms on the
prior austenite grain boundaries and/or the martensite lath boundaries and becomes enriched in austenite-stabilizing elements. However, some of the austenite formed at this stage is not thermally stable,
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