AGE-HARDENING IN A CU-BEARING HIGH-STRENGTH LOW-ALLOY STEEL

The copper age hardening response in a low alloy variant of the ASTM A 710 type steel has been investigated at temperatures of 450, 500, and 550 degrees C for three pre-treated conditions (as rolled ferrite, bai nitic ferrite, and martensite). Transmission electron microscopy has b een used to follow microstructural changes and their relation to the v ariations in the hardness curves. The results indicate that the age ha rdening responses of both the martensitic and bainitic ferrite structu res are much higher than that of the as rolled condition, and this obs ervation 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 h ardness in the martensitic and bainitic structures was attained when f ine epsilon-copper precipitates are predominantly observed on dislocat ions. Compared to the martensitic and bainitic structures, the presenc e of pre-existing interphase epsilon-copper precipitates, as well as t he formation of additional copper-rich clusters and precipitates from supersaturated ferrite contribute to the aging response in the hot rol led condition.