Laser-ultrasonic monitoring of ferrite recovery in ultra low carbon steel

The static recovery kinetics of warm deformed ferrite have been characterized by a combination of in situ laser ultrasonics and stress relaxation measurements. During recovery the ultrasonic velocity change decreased whilst the ultrasonic attenuation generally remained constant. The velocity change was explained in terms of dislocation damping, whilst for the attenuation the results were due to a combination of grain scattering and dislocation damping. From the ultrasonic velocity and attenuation measurements, the dislocation density and pinning point separation have been determined using a model in the literature. These values have been compared to those obtained from stress relaxation data for the same experimental conditions. The results showed that the difference in calculated dislocation densities differed by one or two orders of magnitude. The difference in the values of pinning point separation was about one order of magnitude. Finally possible reasons for these differences have been discussed with reference to the dislocation structure present during recovery.