The Improvement of Stress-Relaxation Properties of PC Wire by “Warm-Straightening”

Abstract

Various properties of highcarbon PC wire treated by cold-straightening and bluing (“CS+B”) and those by warm-straightening (“WS”) were compared and the following results obtained.
It was found that the greater the bending deviation during straightening, the straighter was the processed wire and the more stable was the torsion value. There was an increase in the diameter of the wire in the “CS” process and in the “WS” process at intermediate temperatures, but it was found that at higher temperatures the diameter reached a maximum and then gradually decreased, becoming less than the original diameter above 400°C.
The maximum tensile stength and sectional-hardness were obtained for “CS+B” wire when treated at 200°C and 250°C respectively, while for “WS” wire the maximum values were obtained at 300°C. The “WS” wire had a lower stress-relaxation value than “CS+B” wire, this being paricularity evident at high autoclave temperatrue. One reason may be that the “WS” wire has a higher elastic limit than “CS+B” wire.
Minimum relaxation values for both “CS+B” wire and “WS” wire were obtained at a temperature approximately 50°C above that at which σ_0.2 and σ_0.02 showed a maximum and this corresponded with the results of creep tests performed at intermediate temperatures.
It is considered that in the “WS” process the dislocations increase and there are repeated lockings of these dislocations by solute atoms and that the composite effect of aging and process hardening results in a fixed structure of dislocations.