Effect of cold work and aging on mechanical properties of a copper bearing microalloyed HSLA-100 (GPT) steel

Investigations have been carried out on the effect of cold work and subsequent aging on mechanical properties of a Cu-bearing HSLA-100 steel microalloyed with Nb and Ti. Aging at 400°C after various degrees of cold work (25–70 pct) exhibits multiple hardness peaks. The treatments cause significant improvement in hardness and tensile strength, but at the cost of impact strength. Cold work also causes deterioration in ductility, which again improves on subsequent aging. The C70A treatment involving 70 pct deformation exhibits maximum response to age hardening giving a hardness of 465 VHN and a UTS of 1344 MPa, but with low values of ductility (5 pct) and impact energy (24 J). C50A treatment involving 50 pct cold work and aging results in an optimum combination of mechanical properties. This treatment in the second hardness peak stage yields a hardness of 373 VHN, UTS of 1186 MPa together with a ductility value of 11 pct and impact energy of 109 J. Scanning electron microscopic studies of fracture surfaces reveal that the impact fracture occurs by formation of dimples and nucleation and growth of voids and cracks. Fracture in tensile specimens is caused by formation of voids and cracks at high density striations. Formation of voids and cracks is also assisted by the presence of precipitated carbide particles.