Spheroidization of medium-carbon steels
- PDF / 1,131,345 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 75 Downloads / 244 Views
9
International
Spheroidization of Medium-Carbon Steels J.M. O'Brien and W.F. Hosford Spheroidization experiments were made on a medium-carbon AISI 4037 steel, using both intercritical and subcritical annealing cycles. The results indicate that in the subcritical cycle the spheroidization occurred much more quickly than expected, so that shorter times were sufficient to achieve high formability. On the other hand, the hardness dropped faster in the intercritical cycle. Although more work needs to be done, these results suggest that using a subcritical spheroidization process instead of an intercritical process could achieve considerable savings in time, energy, and cost.
Keywords carbon steels, formability, heat treating, spheroidization
1. Introduction STEEL BOLTS are typically formed from rod by cold heading. In this process, rod is fed into the heading machine from a coil and is held in a set of dies while the protruding end is upset forged with a female die to form the bolt head. The deformation during cold heading is quite severe, and unless the steel has a high degree of formability, cracks (splits) may form at the outer edge. Bolts that are to be hardened by subsequent heat treatment are made from medium-carbon steel (0.35 to 0.50% C), alloyed for hardenability with elements such as chromium, manganese, and molybdenum. Rods of these alloy steels are hot drawn to final diameter, coiled, and cooled for delivery. The structure at this point is ferrite and pearlite, with the coarseness of the pearlite depending on the rate of cooling after coiling. J.M. O'Brien and W.F. Hosford, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
This microstructure is not ductile enough for cold heading. Therefore, a spheroidization anneal is performed to convert the carbide in pearlite to a spheroidal shape. These spheroidization treatments are long (12 to 20 h) and therefore energy intensive. Approximately 160,000 tons per year of medium-carbon steel is spheroidized for fastener applications alone. Any reduction of the temperature or time of the spheroidization could result in a major energy savings. A typical spheroidization treatment consists of heating the steel into the intercritical temperature region (740 to 760 ~ for 2 h and then slowly cooling below the lower critical temperature (700 to 715 ~ and holding at this temperature for 8 to 20 h before cooling to room temperature (Fig. la). Alternatively, the coil could be spheroidized by a subcritical annealing operation, simply by heating it to just below the lower critical temperature (700 to 715 ~ rather than first heating into the intercritical region (Fig. lb). Quicker spheroidization would be expected, because the pearlite would be finer than that developed on slow cooling through the lower critical temperature. Why the intercritical anneal is commonly used is not clear. Samueis (Ref 1) has given an excellent review of spheroidization. There is little in the literature that would justify the higher temperatures and l
Data Loading...
