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FIBROUS STRUCTURE OF FORGINGS : GRAIN-FLOW

In forging, metal flows similar to the flow of sand-cement mortar while being squeezed. Forging causes the grain flow and randomly dispersed small inclusions and segregations found in cast metal to become elongated in same direction as the metal is caused to flow. If a forged part is cut in a plane aligned with the direction and the surface is ground smooth and then immersed in an acid solution the exposed metal will appear to naked eye to have a fiber-like structure. These fibers are non-metallic inclusions or segregated phases that are elongated or "flowed" in the direction of  working. Grain-flow, fiber structure, flow lines and forging fibers are terms used to describe this effect. The grain-flow of forged part resembles in many ways the grain of wood. Like wood, the strength and toughness of  this metal is greatest in the direction of the fiber. In the directions at right angles to these fibers , the strength is normally no greater than that found in dense, sound casting.

If, on forging, the grain-flow is in a direction parallel to the principal grain-flow it is said to flow in its correct fiber direction, and if otherwise, it is said to flow in its incorrect fiber direction. 

The effect of these flow-lines is to produce marked directional properties in steel. For static load, the directions of these flow-lines are not so important, but these are important on parts where shock and fatigue are encountered, The ductility and resistance to impact of the metal as less in a direction at right angles to the flow-lines than parallel with them. Special care is, therefore, taken in the making of forgings for gears, crankshafts or other highly stressed parts to have the metal flow in the most favorable direction in all portions of  the forgings. It is common practice for purchasers of forgings to specify the desired directions for - the fiber-flow lines to meet the stresses expected under actual service conditions. 


Grain flow - fibre structure


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