The effects of heat treatment and cold working on the room-temperature and cryogenic mechanical properties of Fe-SOMn-QA
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JAMES E. KRZANOWSK[ is Assistant Professor, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824. Manuscript submitted October 14, 1987.
METALLURGICALTRANSACTIONS A
carbon-rich zones form a somewhat disordered version of the L' 12 perovskite structure (prototype CaTiO3). Spinodal decomposition has not been observed in alloys with 0.5 pct or less carbon. The room-temperature and cryogenic mechanical properties of the Fe-30Mn-9AI-0.gc alloy are reported in this note. The carbon content of this alloy is higher than those previously examined for cryogenic mechanical properties. The previous studies indicate that a carbon content above 0.5 pct C will reduce the cryogenic ductility, but this effect can be offset by the higher aluminum content of the alloy. In addition, the spinodal decomposition and ordering reactions can be used to age harden the alloy. The alloy used in this study was vacuum induction melted by Carpenter Technology Corporation. The ingot was homogenized at 1423 K for 24 hours, and then press forged from a cross-sectional area of 50 cm 2 to 3.6 cm 2 from a furnace temperature of 1423 K. To make tensile test specimens, 0.3 cm • 2 cm • 10 cm rectangles were cut from the bar stock, and then hot rolled to a thickness of 0.16 cm. Flat tensile specimens with a 2.54 cm gage length were machined from these pieces. The mechanical testing program was limited to standard tensile testing at room temperature, 197 K, and 77 K. The yield strength was measured at 0.2 pct offset strain, and the percent elongation reported is the total elongation measured after fracture. The effects of heat treatment and cold working were examined in addition to test temperature. All samples were annealed at 1223 K for 20 minutes, and then quenched into iced brine to suppress to the extent possible the spinodal decomposition and ordering reactions. Several of these samples were aged for 4 hours at 823 K; another set was cold-rolled to approximately 30 pct reduction in thickness and then similarly aged. The microstructures of the specimens prior to mechanical testing were examined by optical and transmission electron microscopy. Optical microscopy of the alloy in annealed/ quenched condition revealed a single-phase microstructure with a grain diameter of 30 micrometers. A TEM micrograph of an annealed/quenched and aged specimen is shown in Figure 1. The spinodal structure is clearly shown in this image. Sidebands about the (200) reflections were observed, as well as superlattice reflections. The results of the mechanical testing program are shown in Figure 2. The annealed and quenched specimens exhibited increasing yield and tensile strengths with decreasing temperature. The percent elongation in these specimens was nearly constant with temperature, with values ranging from 46.5 to 49.5 pct. The aged specimens similarly exhibited increasing yield and tensile strengths with decreasing temperature. However, while good ductility was maintained at 293 K and 197 K, little ductility was observed at 77 K. The per
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