What is grain flow in forge component?

Grain flow is a typical grain pattern generated during forging of metal. When a piece of metal is cold or hot forged by applying external force or pressure, then metal starts deforming normal to the direction of application of pressure. During the process of permanent deformation, metal grains get deformed & generate what is known as" flow lines ". This is called Grain Flow. It can be seen by chemically etching cross sectional piece of forge metal shown fig below.