Abstract
High-temperature tensile deformation of alpha iron was studied in the temperature range 500 to 900°C over a wide range of strain rates from 18 to 3.1×10−6 l/sec. The steady-state flow stress or maximum flow stress, σM, can be correlated with temperature, T, and strain rate, \dotε, approximately by the following deformation equation; \dotε=A·σMm·exp\left(−\dfracQRT\
ight), in which m=4.9 and Q=74 kcal/mol above the Curie temperature, Tc, and m=5.2 and Q=85 kcal/mol below Tc. These values are nearly equal to those obtained in creep experiments. The apparent activation energy for deformation determined under the stress normalized by elastic modulus (σ⁄E) is 61.0 kcal/mol above Tc and 62.5 kcal/mol below Tc, being almost the same as those for self-diffusion in alpha iron.
Metallographic investigation was done on specimens quenched by hydrogen gas instantaneously after deformation. Stable equiaxial subgrains were formed before reaching the steady-state in the high stress range, whereas in the low stress range subgrain formation was finished during the steady-state. The mean subgrain size, d, is unchanged in the region of high strain and is expressed solely in terms of σM, independent of the initial grain size and T or \dotε. The relation between σM and d is approximated by the equation, σM=K⁄d (K is constant) in the stress range below σM\simeq5.4 kg/mm2. This relation is altered above 5.4 kg/mm2 and K becomes several times larger than that in the low stress range. These results indicate that the deformation in the stress range below 5.4 kg/mm2 is controlled mainly by the dynamic recovery process assisted by the migration of vacancies, and the deformation in the high stress or hot working range is controlled by the dynamic recovery process and during the deformation the dislocation glide is considered to play an important role.
