Strength and impact behaviour of age hardenable copper containing steels

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 degrees C an d slowly cooled to ensure a ferrite-pearlite structure and then aged at tem peratures in the range 525-650 degrees C for 1 h. Copper refined the grain structure, and the coarse (40-60 nm) incoherent precipitation, present befo re aging, increased the strength by 60-80 MPa. In the fully aged condition, the very fine coherent precipitation increased the strength further by sim ilar to 80 MPa. Of the two types of precipitation, the coarser incoherent C u precipitation was most effective in increasing the strength without mater ially influencing the impact behaviour (indeed a small improvement was note d) and the greater the volume fraction, the better the properties obtained. Thus, the steel containing 2%Cu, with its greater volume fraction of incoh erent precipitation, gave the best combination of strength and impact behav iour. In contrast, the coherent precipitation strengthened the steel, but a t the expense of impact behaviour in a similar manner to normal precipitati on hardening from microalloying additions. The excellent impact behaviour s hown by the present Cu containing steels has been attributed to the ability of the soft Cu precipitates to deform, thus relieving the stress concentra tions associated with dislocation pile ups. MST/4225.