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In this series, we will examine the concepts and features of forging grain flow. This article will cover the basics of grain flow that occur during metalworking processes. It will also discuss how grain flow can be observed in a forged part. In future articles, the effect of forging parameters on grain flow, the influence of machining after forging, grain flow in open-die and large forgings, and forging design considerations for grain flow will be discussed.
Grain flow is one of the major benefits cited for the use of forgings. Unfortunately, there are misconceptions on the topic, which include the underlying causes of grain flow, the benefits that can be accrued from grain flow and how to achieve an optimum grain flow. In the best case, grain flow results in a delighted customer and a forging that thrives in a critical service application.
To begin, let us provide a definition of grain flow in forgings. Grain flow is a directional orientation of metal grains and any inclusions that have been deformed by forging. Individual grains are elongated in the direction of the metal flow or plastic deformation. More importantly, nonmetallic inclusions, particles and other imperfections inherited from the casting process are elongated in the direction of grain flow. It should be noted that grain flow occurs to some degree in all metal-forming processes, not just forging.
Observations of Grain Flow
When examining the interior of a forging, the grain flow becomes obvious. Figure 1 shows the grain flow in a forged and machined component. The observation of the grain flow in this figure requires some special preparation methods. After the forging has been sectioned, it needs to be ground and polished similar to a metallographic sample. The major difficulty with this step in the process is that forgings are usually substantially larger in size than small samples for metallographic analysis. Care must be taken in the preparation to ensure that the surface is flat and not beveled. After polishing is completed, an etchant (a solution with acid) is applied to the polished surface. The standard method for preparation of the steel forging for examination of the grain flow is described in ASTM E-381 - Method of Macroetch Testing, Steel Bars Billets, Blooms and Forgings....