Go With The Flow
by Royce Lowe
In processes such as open die forging and the production of seamless rolled rings, grain flow is important in that it beneficially affects the mechanical properties and the accompanying service life of the finished forged parts.
Grain flow may be defined as the creation of a directional grain structure formed from the heating and cooling during the forging process. Grain flow in forging is the directional orientation of metal grains during metalworking. It’s an important part of the forging process because it determines how the grains will push or pull when the metal is under stress or fatigue. During the metalworking process, the grains of the forged metal are heated and cooled to create an optimal grain structure that is directional, referred to as grain flow. The grain form is altered so they are aligned throughout the metal component instead of their random form in the cast ingot or billet.
In general, metals with aligned grains (along the direction of force application) exhibit improved performance under stress, as the grains can more effectively resist fracture and plastic deformation. The forging process refines the grain structure of a cast product (the ingot), which is random and likely showing structural weakness at grain boundaries, into an orderly granular arrangement. Any non-metallic inclusions in the cast product will also become similarly elongated. Any internal cracks or cavities will be consolidated. The amount and direction of open die forging, or ring rolling, will effectively improve the strength, fatigue strength, impact resistance, ductility, and overall durability of the finished part. This is particularly important in demanding applications such as aerospace, automotive, and heavy machinery.
In open die forging, an ingot or a billet is deformed between dies that are flat or relatively simple-shaped, using hammers or presses.
The metal undergoes significant deformation, which results in the elongation and alignment of the grains along the direction of the applied force. Because open die forging involves free deformation without confinement in a die cavity, the grain flow pattern will be highly influenced by the specific forging sequence and the geometry of the part being produced. Similarly, increasing amounts of deformation will result in improved mechanical properties. Forging temperature, die design, and forging sequence must all be controlled to ensure optimum grain flow, hence optimum mechanical properties.
Seamless rolled rings are produced through a specific forging technique designed to create rings or cylindrical shapes without the need for welding or seam formation. The process involves heating a thick metal billet, then piercing the center to create a hollow section, which is progressively reduced and elongated by rolling between rotating dies. As the metal is deformed, the grain structure becomes aligned along the direction of flow, typically circumferentially around the ring. The grain structure is oriented in two primary directions: radially and circumferentially. Radial grain flow is oriented perpendicular to the ring’s surface, while circumferential grain flow aligns around the circumference of the ring. The circumferential grain flow is especially important for components subjected to high rotational forces, such as turbine wheels and structural rings.
In open die forging, the elongation of metal grains along the direction of force improves the material’s ability to withstand high-stress environments, while in seamless rolled rings, both radial and circumferential grain flow patterns contribute to the strength and reliability of the final product. Again, as with open die forging, control of temperature and pass sequences during ring rolling are primordial and will serve to produce optimum mechanical properties.
Grain flow plays an essential role in determining the mechanical properties of components made through open die forging and seamless rolled ring processes. Strict control of the direction of grain flow and of the deformation process allows manufacturers to produce high-performance parts that exhibit superior strength, toughness, and resistance to fatigue.
All Metals and Forge Group has extensive experience in the manufacture of open die forgings and seamless rolled rings for a wide range of applications. The company is fully aware of the importance of producing the optimum grain flow, and the beneficial effects this has on the properties and performance of its customers’ products. AMFG’s Forging Capabilities Chart shows the various shapes, sizes, and weights that can be produced in the alloys of each metal family. You can find the Forging Capabilities Chart here.
