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. This etchant is called a macroetchant since it will reveal features of the forging on a scale that can be observed with the human eye rather than requiring a microscope.
ASTM E-340 provides a standard test method for the macroetching of metals and alloys. This standard states: “Forge shops … use macroetching to reveal flow lines in setting up the best forging practice, die design and metal flow. For an example of the use of macroetching in the steel forging industry, see ASTM E-381. Forging shops and foundries also use macroetching to determine the presence of internal faults and surface defects.”
This standard also provides the chemical composition for the etchants that can be used on a variety of metal forging alloys, including steels, aluminum alloys, stainless steels, high-temperature alloys (superalloys), nickel alloys, titanium alloys and magnesium alloys. Macroetching methods are also well described in Volume 9 of ASM Handbook Metallography and Microstructures.
It should be understood that the macroetching can be very aggressive and needs to be done in a safe manner. Also, because of the acid content in the etchant, the attack on the forging is substantial. The observed grain flow is due to the presence of particles and inclusions. The etchant will attack the interface region between these inclusions as well as the base metal. The etched surface appears to show inclusions that are very large, but that is not the case because of the acid attack. The actual area that is eaten away in the forging is much larger than the inclusions themselves. Do not be deceived into thinking that the steel or other metal is extremely dirty because of the methods that are used to observe grain flow. Even with relatively clean material, the grain-flow lines can be seen from an aggressive acid etch even though the number and size of the inherent inclusions are relatively small.
Effect on Mechanical Properties
The important implication about grain flow is that some mechanical properties vary with respect to orientation relative to grain flow. This fact is one of the major benefits ascribed to forgings. This variation in mechanical properties can be exploited so that the actual product has superior properties in a critical direction relative to those expected from the alloy composition itself.
However, we should be clear that not all of the mechanical properties will vary significantly with the grain flow. For example, strength and hardness are primarily controlled by the alloy chemistry and the heat treatment that is given to the forging. Grain flow will not have a major effect on the strength or the hardness of the alloy. In contrast, desirable properties associated with retarding crack propagation can see significant differences depending on the grain flow and the direction of the moving crack. So, properties like fatigue strength, impact toughness and ductility, which are measures of a material’s resistance to cracking (measured after fracture), can be significantly improved if the crack propagation direction and the grain flow are properly aligned. The optimum alignment occurs when the maximum principal stress (perpendicular to a potential crack or fracture) is aligned with the grain-flow lines.
When the properties of a metal are independent of direction, the material is described as being isotropic. Plastically deformed metals with grain flow have anisotropic properties. Figure 2 illustrates this principle of anisotropy with respect to grain flow. In this example, the grain flow is indicated in the block of metal.
Test samples are machined with three different orientations. The longitudinal sample has the grain flow along the direction of the long axis. The transverse and short transverse are oriented so that the grain flow is perpendicular to their long axis. When the longitudinal sample is tested, the crack or final fracture will be perpendicular to the long axis of the sample. So, for the longitudinal sample the crack propagation is perpendicular to the grain flow, whereas for the transverse and short transverse the crack or fracture that forms is somewhat parallel to the grain flow. Note that there is not a significant variation in the yield strength of this material with test-sample orientation. Going from the short-transverse sample to the longitudinal sample, the increase in yield strength is less than 3%. Since yield strength is a measure of when plastic deformation starts in the metal, it does not have a crack or fracture involved in the property measurement.
In contrast, the reduction in area and the elongation are measured in the sample after it has broken or fractured. Likewise, the impact energy is a measure of the material’s resistance to rapid crack propagation through it. The reduction in area increases by a factor of more than 5 and the elongation increases by a factor of 3 when comparing the short transverse test sample to the longitudinal test sample. The increase in the impact energy is almost a factor of 2.5. These changes are remarkable increases in these mechanical properties. The increases in fatigue and impact properties vary by material, processing conditions and microstructure. With proper design and understanding of the application, a forging offers an opportunity for significant improvements in critical mechanical properties.
The fundamental reason for the enhancement of properties when the test sample and the grain flow are aligned is due to the manner in which a crack or fracture will propagate through the material. Like the fractures that are observed in wood, a crack preferentially propagates in the direction of the grain flow. When the crack forms perpendicular to the grain flow, it will undergo numerous deflections as it moves across the sample. Each of these small deflections requires more energy and makes the material more resistant to this cracking or fracture. Hence, certain mechanical properties are increased when a sample is tested in the longitudinal direction.
When tested in the transverse or short transverse directions, the crack can propagate very easily along some of the inclusions, requiring less energy for the fracture process. This reduction in energy requirement causes the mechanical property to be lower. It is this change in the ease or difficulty of crack propagation that is the root cause for the change in the mechanical properties due to grain flow.
Summary
We have provided a definition of grain flow in this introductory article. We have also examined how grain flow is observed in forgings and some of the implications of grain flow in general on the mechanical properties of a forging – especially those that are a measure of crack or fracture resistance. Depending on the orientation of the grain flow and the direction in which a crack propagates, these mechanical properties can be enhanced or diminished. In the next article, we will examine the relationship more specifically between grain flow and the forging process.
Acknowledgments
Content for this paper was primarily developed by Scientific Forming Technologies Corporation in partnership with SCRA Applied R&D and FIA. The material was initially developed as a Forging Design Seminar under the FAST program, a multi-year, industry cost-shared program sponsored by the Defense Supply Center Philadelphia and Defense Logistics Agency – Research and Development.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or cvantyne@mines.edu. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or jwalters@deform.com.
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