Yield behavior of a mild steel after prestraining and aging under reversed stress
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I.
INTRODUCTION
WHEN cold-worked metals are deformed plastically in the reverse direction to prestraining, the Bauschinger effect is almost invariably observed. This effect should be eliminated before the metals are used as engineering components, because it causes a reduction in fatigue strength as well as in static flow strength during reverse straining.[1,2] For cold-worked steels, stress-relief annealing at about 600 7C is the most commonly applied heat treatment for removing the Bauschinger effect. But it is accompanied by a reduction in work hardening induced by prestraining.[3] That is, the yield stress in the same direction as the prestrain is significantly reduced by recrystallization. Warm working has been found to be very effective in preventing a loss of work hardening.[4] Steels deformed in the temperature range from 250 7C to 300 7C exhibit a pronounced increase in work hardening due to dynamic strain aging, and then the Bauschinger effect at room temperature can be reduced significantly.[5] However, undesirable ‘‘blue brittleness’’ effects, such as a decrease in ductility and the formation of bluish oxides, also occur. Static strain aging is a type of behavior, usually associated with the yield-point phenomenon, in which the strength of a steel is increased by heating to around 100 7C to 300 7C after cold working. This treatment is not so effective in removing the Bauschinger effect.[6,7,8] Nevertheless, the effectiveness of such strain aging may be improved by stress aging (strain aging under stress).[9,10] This article describes an attempt to give the same yield stress in subsequent tension and compression to the tensileprestrained mild steel by means of stress aging. The main aim was to increase the compressive yield stress without altering the tensile yield stress significantly. Thus, aging was done under compressive stresses at temperatures below the blue brittleness temperature. From tension and com-
pression tests at room temperature, the critical aging stress at which the yield stresses in the two reloading directions become equal was determined as a function of aging temperature. Furthermore, forward and reverse torsion tests were carried out on specimens which had been stress-aged after a torsional prestrain, and these results were compared to those of the tension and compression tests. II.
EXPERIMENTAL PROCEDURE
A. Specimens and Equipment The material used was a mild steel supplied in the form of a round bar of 30 mm in diameter. The chemical compositions are given in Table I. Thin-walled cylindrical specimens of 20-mm o.d., 1-mm wall thickness, and 40-mm parallel length were machined. All specimens were fully annealed at 910 7C for 1 hour in a vacuum furnace before testing. The mechanical properties are given in Table II. A hydraulic-type multiaxial testing machine equipped with a heating furnace was used for this investigation. It enabled a cylindrical specimen to be loaded axially and twisted. The maximum capacities of this machine are a 49kN axial load and a 490-Nm torque. In orde
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