Austenite transformation and age hardening of HSLA-80 and ULCB steels

Alternative materials to HY-80, quenched and tempered structural alloy steel, like the microalloyed steels HSLA-80 and ULCB, allow the suppression of the quenching and tempering heat treatment and offer better weldability due to their extra-low C content. In the HSLA-80 steel copper precipitation is one of the main hardening mechanisms available, while in the ULCB steel the contribution of the bainitic transformation plus solid solution hardening is vital. The aim of this work was to determine the continuous cooling austenite transformation (CCT) diagrams of both steels and its performance during age hardening. It was verified that both alloys developed a bainitic microstructure with low C content, commonly designed by the literature as “granular” bainite. The hardenability of the ULCB steel was greater than the HSLA-80 due to the presence of Nb, B and Mo in the first alloy. The age hardening behavior of these alloys was slightly different between each other. The HSLA-80 steel developed maximum hardness during a 600 °C age hardening, while for the ULCB steel this occurred at a 500 or 600 °C age hardening and took less time. Both steels showed a significant hardness decrease during the 700 °C age hardening, that was probably due to overaging and tempering effects.