Multistage strain aging of low-carbon steels

In the present study, a new multistage torsion test was developed to study the effect of temperature, solute level and interpass time (IPT) on static strain aging behaviour. The objective of the present work was to study strain aging under conditions closer to actual wire drawing (i.e. large strains, high strain rates and multistage strain aging) using torsional deformation. The torsion test allowed for a rapid, accurate and controlled method to simulate a range of microstructures and the subsequent testing of the metallurgical properties of the material. The multistage torsion test was found to be suitable for the investigation of multistage strain aging in low-carbon steels under conditions of large strain and high strain rate. Major characteristics of the multistage torsion flow stress-strain curves reported for the first time include: (i) pronounced upper yield points and/or strength increments (ΔY) after each aging step indicative of static strain aging; (ii) high initial work hardening rates, followed by very low work hardening in the presence of ΔY; and (iii) serrated flow at temperatures of 200°C and higher indicative of dynamic strain aging. It was found that in general multistage strain aging was detrimental to the ductility of the material. It was also found that strain aging was in itself dependent on the strain aging history of the material, decreasing with increasing prior strain aging.