Two-stage linear ln[sinh(ασ)]
vs. 1/
T relations indicative of double
QHW behavior, are obtained from anisothermal multipass flow curves generated by testing Nb/Ti microalloyed steel in torsion (in the range 1250-800ºC), except for the case of the high cooling rate-short interpass time tests, which gave rise to single-stage plots, indicative of single
QHW behavior. Above the
Tnr, the
QHWU (apparent activation energy for hot working corresponding to the upper temperature range) is little affected by test variables (interpass time and cooling rate) and type of deformation (isothermal continuous and anisothermal multipass deformation give virtually equivalent
QHWU, values above the
Tnr). However, below the
Tnr, the
QHWL, corresponding to the lower temperature range, becomes sensitive to test variables, and may show considerable deviation from
QHWL obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the
Tnr, relevant to a multipass hot working operation, can be well described only by the
QHWL obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the
QHWL and the
Tnr can be divided into three regions. Within the short interpass time region (1.8 to 10 sec), the two parameters show opposite trends, while beyond 10 sec they correlate well. Within the 1.8 to 10 sec region the
Tnr is assumed to be controlled by Nb in solid solution, while the
QHWL is controlled by both solute and precipitation effect. Beyond 10 sec precipitation is the only controlling mechanism. In addition to the
Tnr derived from Mean Flow Stress (MFS) and ln[sinh(ασ)]
vs. 1/
T plots, the recrystallization limit,
Trl, which correlates well with both
Tnr's, and the recrystallization stop,
Trs, temperatures are derived from % Fractional Softening (FS)
vs. 1/
T plots.
View full abstract