Strength and impact behaviour of age hardenable copper containing steels

Abstract

The strength and impact behaviour of Cu containing low C–Mn–Al steel plates has been examined in relation to their age hardening behaviour. The steels, containing 0, 1·22, and 2·0 wt-%Cu, were austenitised at 910°C and slowly cooled to ensure a ferrite–pearlite structure and then aged at temperatures in the range 525–650°C for 1 h. Copper refined the grain structure, and the coarse (40–60 nm) incoherent precipitation, present before aging, increased the strength by 60–80 MPa. In the fully aged condition, the very fine coherent precipitation increased the strength further by ∼80 MPa. Of the two types of precipitation, the coarser incoherent Cu precipitation was most effective in increasing the strength without materially influencing the impact behaviour (indeed, a small improvement was noted) and the greater the volume fraction, the better the properties obtained. Thus, the steel containing 2%Cu, with its greater volume fraction of incoherent precipitation, gave the best combination of strength and impact behaviour. In contrast, the coherent precipitation strengthened the steel, but at the expense of impact behaviour in a similar manner to normal precipitation hardening from microalloying additions. The excellent impact behaviour shown by the present Cu containing steels has been attributed to the ability of the soft Cu precipitates to deform, thus relieving the stress concentrations associated with dislocation pile ups.