Developments in martensitic and bainitic steels: role of the shape deformation

Steels can be designed to exploit the coordinated motion of atoms during the bainite and martensite reactions. In this way, it has been possible to make exceptionally strong and tough alloys in bulk form, and at an affordable price. Crystal sizes can be reduced to between 20 and 180 nm, by annealing heterogeneously deformed martensite, or by forming bainite under conditions where there is no atomic mobility. Engineering failures are dominated by fatigue, the effects of which are exacerbated by welding and the associated residual stresses. Alloys can now be designed such that the deformation caused by transformation eliminates residual stresses, with extraordinary improvements in the fatigue life of constrained assemblies of metal. These important achievements are described here, within a framework which relates the atomic mechanism to the engineering scale. (C) 2004 Elsevier B.V. All rights reserved.