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Why can forging make a metal tougher in one direction than another?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

Forging improves the mechanical properties of metal by shaping it through hammering or pressing, a process that significantly alters its internal structure.12

During the forging of a bar, the metal's grains become greatly elongated in the direction of the material's flow. This structural change substantially increases the toughness of the metal along the axis of this elongation. Conversely, this process can result in the metal being somewhat weakened in directions transverse to that flow.3

Because of this directional property, a key aspect of effective forging design is ensuring that the resulting flow lines are oriented to align with the direction of maximum stress the part will encounter when placed in service. By controlling this grain orientation, engineers can optimize the performance and durability of the finished component.3