The contribution of precipitation to strain ageing in low carbon steels

Abstract

At first strain ageing in a low carbon steel is concerned with atmosphere locking of dislocations, but mechanical tests have shown that solute segregation beyond this first stage increases strength by a form of precipitation hardening. A comparison of the change in lower yield stress ΔY and in electrical resistivity during strain ageing shows that there is a sudden decrease in the effectiveness of solute segregation, in further increasing ΔY, at the end of the first stage. Estimates based on the Cottrell-Bilby equation (8) suggest that this occurs at dislocation atmosphere densities not greatly in excess of one atom per atom plane. Segregates formed just beyond the completion of atmosphere locking disperse rapidly on re-straining and ageing, and are probably solute clusters. Continued ageing gives more stable precipitates, probably along the dislocations, but an electron microscopical examination has shown that, at ageing temperatures of 60°C or less, these remain extremely small. The increase in ΔY during the early stages of precipitation is approximately proportional to the number of solute atoms segregating to unit length of dislocation, and is insensitive to the dislocation density L. But at higher segregate concentrations (~3–10 times that required for atmosphere locking) continued segregation is less effective in increasing ΔY, also ΔY tends to increase with L. The increment in strain hardening capacity due to precipitation is sensitive to the amount of prestrain, and appears to be approximately proportional to L 3 2 . Assuming precipitation is on dislocations, this agrees with the relationship suggested by Fisher et al. (16)