Austenite decomposition during continuous cooling of an HSLA-80 plate steel
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I.
INTRODUCTION
DEMAND for steels possessing good combinations of strength, toughness, and weldability has led to the development of steels with very low carbon levels, over 1 wt pct of copper for precipitation hardening, and additions of other substitutional alloying elements, such as manganese, nickel, chromium, and molybdenum, for hardenability. At least 0.02 wt pct of niobium is added for grain-size control. These alloys are described by various ASTM specifications, tl,2,3] and the United States Navy has certified a similar alloy for ship construction which is referred to as HSLA80 [4] because of the nature of the microstructure and a minimum yield strength of 550 MPa (80 ksi). The HSLA-80 designation will be used to refer to the alloy studied in the present work, whereas the A710 designation will be used to refer to steels with a range of compositions similar to, but notably wider than, that of HSLA-80 steel. In addition to these alloy designations, other terms and trade names have been used to identify similar alloys.tS-~2] The effects of austenitizing temperature, cooling rate after austenitizing, aging temperature, aging time, cooling rate after aging, and alloy chemistry have been studied in
S.W. THOMPSON, Associate Professor/ISS Professor, and G. KRAUSS, John Henry Moore Professor, are with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. D.J. COLV1N, formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines, is Captain, United States Army, and Assistant Professor, Department of Civil and Mechanical Engineering, United States Military Academy, West Point, NY 109965000. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICAL AND MATERIALSTRANSACTIONS A
A710 steels and similar alloys.[ 12-17] Additionally, welding issues have been addressed in several publications (e.g., References 4, 6, and 18 through 23). Despite the low carbon content of these steels, moderate hardness and strength are produced as a result of a fine-grained base microstructure and the formation of fine-scale copper precipitates during aging. Relatively high alloy content (nominally, in wt pct, 0.55Mn, 1.15Cu, 0.85Ni, 0.75Cr, and 0.2Mo) produces low austenite transformation temperatures which result in finegrained ferritic microstructures. Additionally, some elements, e.g., chromium and molybdenum,t'] significantly retard copper precipitation during cooling so that only fine copper precipitates are present after the subsequent aging heat treatment. However, the phase transformation phenomena associated with this class of steels have not been documented in as much detail as compared with medium-carbon, alloy steels. Typically, the microstructures of HSLA-80 s
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