Differential resistivity measurement for monitoring annealing
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INTRODUCTION
THE annealing of cold worked metals has been followed and analyzed by numerous workers using hardness measurements, optical and electron metallography, density, X-ray diffraction, and more recently, positron annihilation, l-4 However, some of the methods require sophisticated apparatus and all are very difficult to use for monitoring during annealing, particularly for metals which do not recrystallize near room temperature. In reviewing metal properties which might be considered for monitoring annealing while it is under way, especially under industrial conditions, electrical resistivity stands out as one which is relatively easy to measure and amenable to instrumentation, recording, and standardization. Moreover, many advances have been made in recent years in refining measurement to the point where resistivity changes may be followed continuously and with a high degree of accuracy. Following resistivity changes at temperature is an attractive idea because the measurements are not affected by cooling, quenching, or possible age-hardening effects. As for industrial annealing, if a monitor is to be used at all, there is no reasonable alternative to one which can be used to follow structural and property changes as they occur. Strand and batch annealing are the main industrial heattreatment practices used for cold rolled sheet and strip. Strand annealing is a rapid, continuous process which results in a recrystallized product of very fine grain size. When a larger and more controlled grain size is required, e.g., for deep drawing operations in mild steel, batch or box annealing is used. Coils of metal are enclosed in a bell-type furnace and heated above the recrystallization temperature, soaked for a predetermined time, and then furnace cooled, the complete heating-holding-cooling cycle often taking several days. Monitoring is carried out by placing thermocouples within the coils, and past experience is used to decide the annealing temperature and soak-time. It has become increasingly clear in recent years that in order to optimize the annealing conditions and economize on energy consumption and 'turn-around' time, a sensor which R. A. L. DREW is Research Associate, Department of Metallurgical Engineering; W. B. MUIR is Associate Professor, Department of Physics; and W. M. WILLIAMS is Birks Professor of Metallurgy, Department of Metallurgical Engineering. All are with McGill University, Montreal H3A 2A7, Canada. Manuscript submitted June 1, 1982. METALLURGICALTRANSACTIONS A
depends on resistivity changes during annealing would be extremely useful. The present study was made in order to see whether an accurate, reasonably inexpensive method could be devised for following resistivity changes continuously during annealing. There have been many investigations of resistivity changes during the annealing of metals. Clarebrough et al,5'6 for example, demonstrated that electrical resistivity decreases by about 4 to 10 pet during the transition from the cold worked to the recrystallized state. They showed t
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