Grain Flow in Forgings II – How Forging Changes Grain Flow
The mechanical properties of a metal with grain flow are anisotropic. The properties that involve a fracture or a crack (such as fatigue strength, impact resistance, tensile elongation) will be higher if the grain flow is along the longitudinal axis of the specimen being tested. This increase in mechanical properties is due to the greater resistance to crack propagation that occurs when the crack or fracture is occurring perpendicular to the grain flow. Grain flow can also affect the crystallographic orientation of the grains, leading to some anisotropy during plastic deformation (forging). This local anisotropy can cause localized flow and adiabatic shear bands in certain alloys.
Grain Flow in the Starting Billet
In the starting billet, or mult, grain flow is commonly in the longitudinal direction. It has this starting orientation because the billet was plastically deformed in a rolling mill forge by the supplier. To a lesser extent, ingot conversion to bar can include open-die, extrusion or radial forging operations. Most often the supplier will use a bar or rod mill to reduce the cross section of the ingot or continuously cast billet to a smaller cross-sectional size. This reduction helps to convert the cast structure into a more desirable wrought structure. Prior rolling, like forging, will help to close any porosity that may be present from the ingot casting.
As in forging, prior rolling will result in longitudinal grain flow with a more homogeneous composition. The grain flow for the bar/rod rolling is due to the same elongation of the gains and elongation of the particles and impurities in the metal. For example, steels that are more machinable often have some extra sulfur. The sulfur reacts with the manganese in the steel to yield a deformable compound, MnS. Often described as stringers in the steel, these MnS particles plastically deform during the rolling process and become elongated in the rolling direction.
Effect of Forging on Grain Flow
Although the billet or bar arrives at the forge shop with a grain-flow direction, the final grain flow observed in the product evolves during the forging process. For example, the flash region of a forging can have a very dramatic effect on the grain-flow orientation. Figure 1 illustrates this change. The original billet will have longitudinal grain flow that came from the prior rolling at the supplier’s mill, during which the billet is compressed in the longitudinal direction and fills the cavity. Near the end of the stroke, the excess metal will deform through the opening that is left for the flash and go into the gutter. The flash region is a critical part of the die design because it increases cavity pressure, which minimizes under-fills in the die cavity. The flash opening also causes the metal to deform in a very specific direction. The flowing metal will create a new grain-flow direction in the flash region during this deformation.
Figure 2 schematically shows the changes that the grain flow will have on the property of ductility as a function of the amount of deformation. Ductility is normally measured by the tensile elongation or the reduction in area from a tensile test. For both types of measurements the sample will have been broken (i.e., fractured) to obtain values for ductility.
When studying metal components without plastic deformation (i.e., as-cast), the ductility is typically relatively low, regardless of the direction (longitudinal or radial). The material has isotropic ductility but at a very small value. Low ductility is common in cast metal. This can be observed and measured as the metal is deformed in the supplier’s bar or rod mill.
During this rolling process, where plastic deformation is imparted to the metal, the metal transforms into a wrought product. Furthermore, ductility (especially in the longitudinal direction) is improved significantly. The longitudinal grain flow that is created in the bar during the rolling also manifests itself in the ductility to becoming anisotropic (i.e., the ductility varies with the direction in which the sample is tested). In this case, a sample in the longitudinal direction is more ductile than in the radial direction. Note the plastic deformation has improved the ductility in the radial direction relative to a casting, but the improvement is more dramatic in the longitudinal direction.
The next step in the production sequence is for the bar to be forged in a process similar to that shown in Figure 1. Because the forging is compressing in the longitudinal (axial) direction and the metal is expanding in the radial direction, the grain flow is undergoing a re-orientation, especially for the material close to the flash opening. If we were to extract test samples during the forging at different points in time during the die closure, we would find that the ductility of these samples would be changing. The ductility of the longitudinal/axial samples would start to decrease some, since the material is being pushed downward in that direction and the ductility in the radial direction would increase correspondingly.
As the forging is finished, the ductility in the radial direction will become higher than that in the axial direction near the flash (Figure 2). This change in the ductility is due to the change in grain flow that has been imparted into the metal during the forging. The initial axial grain flow in the starting billet has changed as a result of the forging process. This initial grain flow has been converted into radial grain flow, with the resultant change in properties as a function of testing direction. The mechanical properties that will be most affected by grain flow are those that involve a fracture or cracking (e.g., fatigue, impact, ductility).
Aspect of Forging and Grain Flow
Metals (and their grains and the particles) will deform in the path of least resistance during forging processes. This path is not always the one that we plan or desire. Hence, the die designer needs to understand the effects of various features on not only producing the part that the customer wants but the effect on the grain flow that occurs. The grain flow in a forging is a result of the forging shape, preform design, billet size, material behavior and processing conditions. Process simulation is frequently used to help the forging designer optimize the material flow during forging. Our next article will examine these specific effects in more detail.
In this second part of our grain-flow series, we discussed how the bars that arrive at the forge shop initially possess a grain flow due to the prior deformation imparted by the metal supplier. The prior deformation is normally done in a hot bar or rod mill and creates grain flow in the longitudinal direction.
We have also examined how forging can change the grain flow that is initially present in the bars that are received from a supplier. The change in grain flow will cause the properties that are dependent on a fracture (e.g., fatigue, impact, ductility) to undergo changes during the process, depending on the direction used to produce the test sample for measuring these properties. The forging will be anisotropic with respect to a number of mechanical properties.
In the next article of this series, we will be begin to examine details about the control of grain flow during forging in order to take advantage of the improved directional properties that can be obtained by grain flow.
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 firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.