Investigations of the effect of ultra-rapid heating on the softening of a cold rolled low-carbon steel

Ultra-rapid annealing deals with softening of a metal or alloy by using heating rates greater than about 500°C/second. Electrical resistance heating was used in the present investigation to heat specimens at rates in the range of 1000 to 5000°C/second, and it has been confirmed that very rapid softening ( ultra-rapid annealing ) can be obtained in cold reduced low carbon steels in heat treatment times of a fraction of a second. The resistance heating apparatus was designed originally by A. Pearce and developed over a number of years in the Department of Materials Engineering at the University of Wollongong. It basically consists of a controller, an electrical system and a water quench device. Tensile shaped specimens can be heated to temperatures in the range of 500 to 1100°C in a fraction of a second. A three-wire thermocouple arrangement was applied in the present investigation to measure the specimen temperature accurately and reproducibly. A computer model has been established using an implicit method to predict the temperature profile and to design the optimum shape of the specimen for ultra-rapid annealing. Less than 1% discrepancy was found between thermocouple measurements and computer predictions. A decarburized low carbon steel was cold rolled with reductions in the range 40 to 80%. The deformed material was ultra-rapid heated to study the effect of heating rates on softening temperature range. It was found that for heating rates between ~1000 to 5000°C/second, the softening process is enhanced or accelerated as the heating rate increases. This result confirms, for resistance heating and for higher heating rates, results reported previously for salt bath and induction heating. Batch annealing in the softening temperature ranges of ultra-rapid annealing was applied to the same material to draw comparison with ultra-rapid annealing in a study of the recrystallization kinetics and mechanisms. The results suggest that recrystallization during ultra-rapid annealing is controlled by nucleation, while recrystallization during batch annealing depends on both nucleation and grain growth. Compared with the batch annealed case, partially restored ultra-rapid annealed samples showed less tendency to form a well developed subgrain structure and greater structural heterogeneity. Despite these difference and a remarkable acceleration in the rates of recovery and recrystallization, there was little evidence of any novel mechanisms for restoration during ultra-rapid annealing. Both recovery and recrystallization were accelerated, with recovery being promoted relative to recrystallization. These findings are inconsistent with a proposal that enhanced softening occurs by very rapid recrystallization due to suppression of recovery. The enhancement of softening by ultra-rapid heating could be restricted by a prior recovery treatment at 100°C, but the rate of softening was still high relative to that characteristic of batch annealing and the rate increased sharply as the heating rate increased. The observations are consistent with a suppressed solution model in which dissolution of carbon and/or nitrogen is restricted by very rapid heating, thus allowing restoration to occur freely without the retarding effects of significant concentrations of interstitial solutes. Tensile tests were conducted on the annealed materials. The ultra-rapid heating rates resulted in an increase in ductility and toughness, compared with the batch annealing treatment. Ultra-rapid annealing also resulted in a decrease in yield stress and tensile strength and increase in ductility, toughness and plastic strain ratio (R) as the temperature and the amount of recrystallization increased. The major original contributions of the present investigation are (1) establishing a reproducible resistance heating method for ultra-rapid annealing; (2) developing a computer model to accurately predict the specimen temperature profile during ultra-rapid heating; (3) demonstrating that ultra-rapid annealing by resistance heating results in a substantial acceleration of the rate of softening of a cold rolled low carbon steel; and (4) showing that the phenomenon of enhanced softening is not due to suppressed recovery, but is consistent with suppressed solution of carbon and/or nitrogen atoms