Tensile failure behavior of plain carbon steels at elevated temperatures
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I.
INTRODUCTION
MOST
II.
EXPERIMENTAL P R O C E D U R E
hot forming and shaping of steels is done at temperatures where the stable phase is austenite. Dynamic recrystallization occurs during the plastic deformation of this fcc phase, rendering it highly ductile at temperatures above 1050 ~ almost regardless of the degree of alloying. But below that temperature, fracture initiation at grain boundaries can precede the onset of dynamic recrystallization. ~ This intermediate temperature embrittlement is troublesome in casting and welding as well as in hot forming, especially when the fine precipitation of carbides and nitrides is possible. Although the mechanism of the embrittlement is not well understood, its occurrence has been studied extensively. 2-~0Furthermore, the conditions controlling the precipitation processes are receiving some attention ~ because these carbide and nitride precipitates are used in microalloyed steels to impede austenite grain growth. One purpose of this paper is to provide some information on the embrittlement of austenite when manganese sulfide is the embrittling species. Another is to examine the failure behavior of the phase mixtures adjacent to the austenite region, as shown in Figure 1. For these mixtures, the fracture of the ferrite-plus-austenite mixture has already been studied in some detail, 9'~~ but the high-temperature failure behavior of the ferrite-plus-pearlite and pearliteplus-cementite mixtures has received very little attention, although their plastic flow behavior is documented in a companion paper. |4 Finally, throughout the paper it will be emphasized that the failure behavior can be resolved into two major components. The first is plastic tensile instability, which is important for hot forming operations where moderate strains are encountered. The second is fracture which, if it occurs at small strains, can cause severe embrittlement in casting and welding processes.
Tension testing was done in a vacuum furnace mounted on a constant-strain-rate machine. The chemical compositions of the materials are given in Table I. The 22.7 kg ingots were hot rolled at 1260 ~ to 13 mm-thick plate, from which cylindrical button-head type tension specimens with a diameter of 3.2 mm, a gage length to diameter of 11.5, and a fillet radius of 0.25 mm were machined with their principal axis parallel to the rolling direction. Before heating, the furnace was evacuated to 0.3 mPa and then back filled with flask argon to a pressure of about 13 Pa. The specimens were generally annealed for l hour at a temperature TA above or at the deformation temperature To. The range of the phase regions was determined metallographically and by dilatometry. The results agreed with the phase transition temperatures inferred from the plastic deformation behavior. 14
P.J. WRAY is with Inland Steel Company, East Chicago, IN 46312. Manuscript submitted April 16, 1984.
Fig. 1 - - S c h e m a t i c illustration of the phase fields in the Fe-C system at intermediate temperatures and less than 2.0C.
Austenite
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