Definition: Grain flow is a directional orientation of metal grains and any non-metallic inclusions that have been deformed due to forging. Individual grains are elongated in the direction of the metal flow or plastic deformation. More importantly, non-metallic inclusions, particles and other impurities inherited during the casting process are elongated in the direction of grain flow. Grain flow occurs to some extent in all metal-forming processes, and not just forging.
How to observe flow-lines: The forging needs to be ground and polished, after sectioning, similar to a metallographic sample. Then, macro-etching is carried out
Why flow lines are visible: One can observe grain flow-lines due to the presence of particles and inclusions. The etchant attacks the interface region between these inclusions as well as the base metal. The metal is corroded due to acid and the unaffected inclusions are visible as flow lines.
Significance: Mechanical properties vary with respect to orientation relative to grain flow. Properties like impact toughness, ductility and fatigue strength, which are measures of a material’s resistance to cracking, can be improved significantly by proper alignment of the direction of crack propagation and the grain flow. The optimum alignment occurs when the maximum principal stress (perpendicular to a potential crack or fracture) is aligned with the grain-flow lines.