The contribution of atmosphere locking to the strain-ageing of low carbon steels

Abstract Cottrell locking of dislocations and precipitation hardening can both contribute to the increase in the lower yield stress which develops during the strain-ageing of low carbon steels. An analysis of stress-strain relationships based on the Petch equation, σ y = σ i + k y d − 1 2 ,(8,11) allows a separation of the contributions of these two sources of hardening. Observations of the development of changes in the tensile stressstrain relationship during ageing have shown that atmosphere locking develops first, causing an increase in k y d − 1 2 with little change in σi. An estimation based on the Cottrell-Bilby equation(1) suggests that this first stage is completed when the dislocation atmosphere density reaches a value between one and two atoms per atom plane. After completion of the first stage, further segregation of solute has little or no effect on dislocation pinning: the increases in yield stress are then due to increases in σi and must be caused by solute cluster formation or precipitation. In a strain-aged steel, the contribution of atmosphere locking to the lower yield stress is rather small compared with that in an annealed steel of the same grain size. This may be a consequence of the finer scale of the dislocation network in the cold worked material.