Controlled drawing to produce desirable hardness and microstructural gradients in alloy 302 wire
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I.
INTRODUCTION
WIRE drawing is an important fabrication technique and is typically applied prior to the heading of various fasteners such as bolts, nails, screws, and rivets. Headability, which is most often expressed as a qualitative description of a wire’s ability to plastically deform consistently without fracture during heading, depends on process parameters such as temperature, strain rate, strain path, and lubrication and on the characteristics of the feed wire such as flow stress, strain rate sensitivity, strain hardening rate, ductility, cleanliness, soundness, chemical homogeneity, and surface condition.[1–10] Although the study of headability is fairly extensive, the particular role of microstructure has not been widely explored. Matsunaga and Shiwaku[11] investigated the headability of spherodized, annealed, and normalized carbon steel and found that the microstructures varied from better to worse, in that order. Stainless steels can be comparatively hard to head for various reasons such as microstructural instability, elevated tendency for galling, and poor work hardening rate.[12–16] Of the austenitic grades, alloys 305 and 302HQ (Carpenter Technology Corp., Reading, PA) are typically used in severe heading operations and their heading characteristics have been studied in detail.[13] At high strain rates, nickel stabilizes the austenitic structure, reducing the work hardening rate. Similarly, copper has also been
M.P. RIENDEAU, Principle Materials Scientist, is with Kaiser-Hill, LLC, Golden, CO 80402-0464. M.C. MATAYA, Associate Scientist, is with Safe Sites of Colorado, Golden, CO 80402-0464. D.K. MATLOCK, Armco Foundation Fogarty Professor and Director of the Advanced Steel Processing and Products Research Center, is with the Colorado School of Mines, Golden, CO 80401. Manuscript submitted March 28, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
shown to reduce the work hardening rate in alloy 302, increasing cold headability.[17] In a recent study, three heats of 302HQ wire were shown to exhibit distinctly different resistance to shear fracture during heading despite minimal differences in tensile properties, chemistry, inclusion content, and morphology.[18] Fracture was associated with the formation of microscopic shear bands. The heat with the best headability was found to have a distinctly finer grain size at the surface of the wire, which apparently led to increased toughness through the homogenization of strain on a fine scale. Such microstructural gradients have been beneficially employed to a limited extent in other applications.[19,20] The purpose of this investigation was to systematically study the development of a duplex grain size in alloy 302HQ wire. The effects of wire drawing process parameters such as total applied reduction, pass schedule, and die angle on the resultant strain gradients and the microstructural response of the nonuniformly strained wire to subsequently applied recrystallization heat treatments were investigated. II.
EXPERIMENTAL PROCEDURE
A. Material The che
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