Age hardening in a Cu-bearing high strength low alloy steel

The copper age hardening response in a low alloy variant of the ASTM A710 type steel has been investigated at temperatures of 450, 500, and 550 degrees C for three pre-treated conditions (as rolled ferrite, bainitic ferrite, and martensite). Transmission electron microscopy has been used to follow microstructural changes and their relation to the variations in the hardness curves. The results indicate that the age hardening responses of both the martensitic and bainitic ferrite structures are much higher than that of the as rolled condition, and this observation is rationalised in terms of higher solute Cu content, higher density of dislocations and greater uniformity of solute copper atoms in the pre-treated condition providing a multitude of nucleating sites for copper precipitation. Moreover, it has been found that the peak hardness in the martensitic and bainitic structures was attained when fine epsilon-copper precipitates are predominantly observed on dislocations. Compared to the martensitic and bainitic structures, the presence of pre-existing interphase epsilon-copper precipitates, as well as the formation of additional copper-rich clusters and precipitates from supersaturated ferrite contribute to the aging response in the hot rolled condition.