Effect of carbon content on the plastic flow of plain carbon steels at elevated temperatures
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INTRODUCTION
S I N C E 1953 the principal account of the effect of carbon on the plastic flow of austenite has been that of Feltham. 1He documented the carbon dependence of the steady-state flow behavior, assuming that for the constant-stress mode of loading, such behavior was characterized by the minimum strain rate. A more complete view of the plastic-deformation behavior is often needed for the high-temperature processing of steels. For example, the flow-stress behavior at small strains is important in casting 2 and welding processes, whereas the whole work-hardening region is of interest for the design of schedules for controlled-rolling of plates. In the present work the constant-strain-rate mode of loading has been used for an examination of the transient deformation of a series of plain carbon steels with different carbon contents. This provides for the process designer a documentation of the effect of temperature and strain rate on the stress-strain behavior of the steels in the austenite range. It also reveals for the metallurgist the effect of an interstitial solute on the plastic flow behavior of austenite. Thus, with increasing carbon content the flow stress in the transient region decreases primarily because of a decrease in the rate of work hardening. The flow stress reduction is only partially accounted for by the increase in grain growth with carbon content.
II.
E X P E R I M E N T A L PROCEDURE
A detailed description of the tension-testing procedure was given previously? Cylindrical buttonhead-type specimens having a gage-section diameter of 3.17 mm (0.125 in.) were deformed in an argon atmosphere. The range of strain rates was 5.5 x 10 .6 to 2.3 x l 0 z sec 1 and the deformation temperature ranged from 850 to 1400 ~ Specimens were annealed for one hour and, except for a few special cases,
P. J. WRAY is Associate Research Consultant, Basic Research Division, Mechanical Sciences Section, Research Laboratory, U.S. Steel Corporation, Monroeville, PA 15146. Manuscript submitted November 7, 1980. METALLURGICAL TRANSACTIONS A
were immediately deformed at the same temperature. With the constant-strain-rate test, strain is proportional to time, and true stresses can be calculated as F(exp e)/Ao where F is the force, e the true plastic strain, and Ao the initial cross-sectional area. The chemical compositions of the 0.80 Mn, 0.25 Si plain carbon steels are given in Table 1.
III.
EXPERIMENTAL RESULTS
To illustrate the kind of data that were obtained in this study, the stress-strain curves for the 0.051 C steel deformed at different temperatures at 2.3 x 10 -2 sec 1 are presented in Figure 1. The range of strain over which the flow stress can be measured sensibly is restricted by the onset of dynamic recrystallization during deformation. 4 Thus, the stress-strain curves in Figure 1 are terminated just after dynamic recrystallization has begun, and the observed maxima are not related to plastic instability. Carbon-Content Dependence. The variation of the flow stress with carbon content at different st
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