Spheroidization cycles for medium carbon steels
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I. INTRODUCTION
HIGH strength bolts are made from alloy steels containing from 0.35 to 0.50 pct carbon and enough alloy to achieve the required hardenability. The alloys studied in this work are two of the most common alloys: AISI 1541 and AISI 4037. Wire rod was produced by hot rolling to the diameter of the bolt shank, cooling by fans, and coiling. After pickling and coating with lime, the coils are spheroidized to achieve the necessary formability. They are then sheared and cold headed, and threads are rolled onto the shank. Finally, the finished bolts are austenitized, quenched, and tempered. In the cold heading, the shank is held in a set of grips, while the end is struck with a female die with a cavity having the desired shape. The upsetting causes large tensile hoop strains that may cause splitting if the material has insufficient ductility. Figure 1 shows such a fracture. Spheroidizing provides the needed ductility for cold heading.[1] The spheroidization treatment is by far the most time consuming phase of bolt manufacture. Commercial spheroidization of coils usually takes from 10 to 24 hours depending on the alloy and the size of the load. A number of articles have been written about the mechanisms and kinetics of spheroidization.[2–10] However, very little has been written about the details of the heat treatments. Several articles[11–15] have recommended that the steels be heated into the temperature region between the upper and lower critical temperatures (between A1 and A3) prior to JAMES M. O’BRIEN, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan, is President, O’Brien & Associates, Blissfield, MI 49228. Contact e-mail: [email protected] WILLIAM F. HOSFORD, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted July 31, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
spheroidizing the carbides below the lower critical, but none of these compare the results of such an intercritical process with simply heating to below the lower critical (A1) (subcritical process), and none give data relating spheroidization and formability. A survey was made of commercial spheroidization practice by interviewing eight companies. None of the companies would allow their names to be identified. Of these, four were steel producers that spheroidize their own product before shipping to bolt manufacturers. One company simply buys steel wire, cold reduces, and spheroidizes it before selling to bolt manufacturers. The other three companies are bolt manufacturers who spheroidize the wire themselves before cold heading. Of the eight companies, seven use the intercritical process. One company was using a subcritical process, simply heating to below the lower critical and holding. However, that company was considering changing to the intercritical process. All of the producers agreed that the AISI 1541 steel was the more difficult to spheroidize. For that reason, most of the work in this article wa
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