1992 Volume 33 Issue 6 Pages 577-584
The effect of cold working on the recovery and recrystallization behavior of lath martensite has been investigated in a 0.2% C low alloy steel by means of optical and transmission electron microscopy. Cold working in the lath martensitic steel causes an intrusion of slip bands into the matrix and this results in the destruction of lath martensitic structure and the formation of dislocation cell structure around slip bands. The volume fraction of such a damaged martensite increases with increasing deformation. In the specimens subjected to heavy cold rolling above 80% reduction, the undamaged lath martensitic structure could rarely be observed and dislocation density becomes one order of magnitude higher.
On the annealing at 973 K (below AC1 temperature), there is a large difference in the recovery and recrystallization behavior between the damaged and undamaged areas. Recrystallized ferrite grains preferentially nucleate within the damaged area and grow to encroach into the undamaged area where the recovery rate is slower than that in the damaged area. With increasing prior deformation, the recovery and recrystallization of the matrix are markedly promoted, corresponding to an increase in the volume fraction of the damaged martensite and in the density of dislocations.
Thus, it is concluded that cold working before annealing plays a role to destroy the lath martensitic structure and to increase the dislocation density, leading to an increase in both nucleation and growth rate of the recrystallized ferrite grains.
