Absence of yield points in iron on strain reversal after aging, and the Bauschinger overshoot

Abstract

The yield phenomenon is absent in low carbon steel if the direction of straining after full aging is opposite to that just before aging. That this is not the consequence of macroscopic residual stresses is demonstrated by torsion experiments with thin-walled tubular specimens provided with a shallow circumferential groove, in which yield takes place by widening of a single circumferential Lüders band with the elimination of significant internal stresses. If the straining is interrupted after partial yielding of the annealed specimen and then stress is re-applied immediately without aging, the consequences are opposite. Yielding continues at the normal level if the direction of straining is not changed; if it is reversed, there is a normal Bauschinger effect in the previously yielded volume, but the Bauschinger stress–strain curve rises without discontinuity considerably above the former yield level. When the Lüders band begins to widen, the stress drops abruptly from the overshooting Bauschinger curve to the normal yield level and remains constant until the end of the yield. Repeated strain aging with straining in the same direction gives yield points rising with the total strain roughly as the strain hardening rises with the strain along the uninterrupted stress–strain curve. After each yielding the rate of strain hardening is roughly the same as it is in uninterrupted straining at the same strain, but the curve is displaced to higher stress levels which rise with the number of aging treatments. The “permanent strain-age-hardening” may reach 30% or more of the normal yield stress at the same strain.