Grain Boundary Walls Influence Yield Points and Brittle Fracture in Iron and Steel

Accepted theories explaining a principle cause for sharp yield points and the ductile-brittle transition in hypoeutectic plain carbon steels can be supplemented by the existence of hard grain boundary walls. Atomic Force Microscopy revealed that grain boundaries between adjoining grains in these steels consist of brittle, probably cementite (Fe 3 C), walls enclosing the grains. These walls inhibit the movement of dislocations between adjoining grains until the walls fracture transversely when the steel is ductile. These walls also have longitudinal cracks. At low temperatures, when the stress required for such fracture reaches the Griffith requirement for crack propagation, fracture propagates longitudinally along the walls until the crack length is sufficient for trans-granular fracture. Then the steel fractures in a brittle manner. Yielding in pure iron differs. It is explained by theories where the Cottrell atmosphere of interstitial carbon atoms pin dislocations, by disordered atoms at the grain boundaries, and by different dislocation slip directions between adjoining grains