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When discussing the strength of forged or milled steel parts, grain flow refers to how the molecules in the metal are aligned. Although not as visible, grains in a piece of steel are like the wood grain in a piece of lumber. The grain flow determines the strength of the metal piece. For engineered steel pieces, manufacturers and engineers identify the direction of the grain flow to evaluating the piece's design characteristics and strength.
The grain flow in a piece of engineered or forged steel is often not visible to the naked eye. Even under a magnifying glass, the rough steel surface makes it impossible to evaluate the grain flow without some kind of test. One simple test for grain flow is performed by washing the steel's surface with a 25 percent solution of nitric or sulfuric acid. Flood the face of the steel with acid, and as soon as the surface begins to etch, wash the acid off with pure water. The acid will etch the surface, revealing the grain flow pattern.
Another solution for quick grain flow examination is etching the surface with a 3 : 2 : 1 solution of water, hydrochloric acid and hydrogen peroxide. This solution will also oxidize the surface of the steel after just a few minutes exposure. Wash the oxidizing solution away, and when the piece is dried, the grain flow is revealed clearly.
Review the Forging Process
Metal working processes orient the grains in metals in predictable patterns. Metals which are hot or cold rolled have grain flow patterns that are elongated in the direction of the final tooling. Metals which are heat treated or machined also have grain patterns which are also predictable, and uniform. By gaining knowledge of the forging or machining process, the grain flow patterns of metals can be identified.
In some cases, polishing the casting will reveal the underlying grain flow. This doesn't work for all metals, such as rough iron beams, and structural steel. But in carefully engineered parts, when the surface is polished, and the surface imperfections removed, the grain flow is slightly visible in the mirror like surface with a magnifying glass, or under a low power microscope.