The
forging ratio is the ratio of the change in the cross-sectional area of a metal billet during
forging. In simple terms, it's the comparison between the metal's size before and after the forging process. This ratio is crucial for determining whether the
forging process is effective and how much the metal has deformed. As the forging ratio increases, the metal's internal structure changes in a big way, defects removed, the grains become finer and more uniform, and the metal's mechanical properties improve.
The forging ratio directly influences how much the metal deforms during the process. A larger forging ratio means more deformation, which leads to finer grains and better distribution of impurities and inclusions. When the forging ratio is low, the metal might still have many dendritic (tree-like) grains and non-metallic inclusions. But as the ratio increases, these impurities and irregular grains break up and rearrange, resulting in a more uniform structure.
When the forging ratio reaches a certain level, the metal's structure changes in a more noticeable way. The coarse, dendritic grains in a steel ingot get crushed during forging, stretching and deforming in the main direction of deformation. This is where grain recrystallization and the redistribution of excess phases (like carbides and non-metallic inclusions) come into play. With higher forging ratios, impurities align along the deformation direction, creating a "fibrous structure" that shows up as "flow lines" in the metal.
The creation of a fibrous structure is one of the most noticeable results of the forging process. This structure gives the metal directionality, meaning its properties differ depending on the direction of the fibers. It's similar to the fibers in wood, metal formed with a fibrous structure shows different mechanical properties depending on which direction you test it in. When you test along the fiber direction (longitudinal direction), the metal has better strength and toughness. But when tested perpendicular to the fiber direction (transverse direction), the properties are weaker.
The size of the forging ratio directly affects how well the fibrous structure forms. At a forging ratio of 2, you can start to see the fibrous structure. But when the ratio is between 4 and 5, the fibers are more pronounced. A larger forging ratio doesn't just create clearer fibers; it also improves the metal's density and strength. So, the forging ratio plays a big role in determining both the metal's mechanical properties and its overall quality.
The forging ratio also affects the metal's density and how well defects are eliminated. Studies show that when the forging ratio is 2, the density of the metal is close to its maximum. If you go beyond a 2.5 forging ratio, the density improvement starts to level off. The larger the forging ratio, the more likely it is that internal voids and loose areas in the metal will get compacted, resulting in a denser structure.
Additionally, porosity and looseness in the metal improve as the forging ratio increases. For most forged parts, when the ratio is over 2 (and with the right process and equipment), the porosity can be kept between 0.5 and 1.0, achieving a dense, "forged" structure. However, for larger alloy steel forgings, especially those that require complete breakdown of dendritic grains, a forging ratio of 5 or more is often needed for optimal results.
The choice of forging ratio directly affects the complexity of the forging process and the quality of the final product. If the ratio is too small, the deformation may not be enough to eliminate defects in the ingot. That's why you often need a higher forging ratio to ensure the metal is uniform and dense. In practice, to get high-quality forgings, it's important to choose the right forging ratio based on the intended use and material characteristics.
For instance, when forging steel ingots, a ratio of 2 to 3 can significantly improve the fibrous structure by elongating the metal. But if you use upset forging followed by elongation, the forging ratio needs to be between 4 and 5 to get clear fibrous patterns. The larger the ratio, the stronger the directionality of the fibers, which further enhances the metal's mechanical properties. For large-section alloy steel forgings, a ratio of over 5 is often necessary to ensure complete breakdown of dendritic grains and improve the metal's density.
Choosing the right forging ratio is crucial for quality control. In the forging process, it's not just about the ratio size—it's also important to consider how the process, equipment, and materials work together. If the forging ratio is too small, defects may remain in the metal. On the other hand, if the ratio is too large, it could overload the equipment and increase costs. So, it's essential to find a balance. Choosing the right forging ratio ensures that the metal flows and deforms properly, leading to a high-quality forging.
The forging ratio is a key factor affecting the structure and properties of the metal. It influences everything from the metal's density to the formation of its fibrous structure and mechanical properties. By adjusting the forging ratio, you can improve the metal's internal structure, refine the grain size, and create a directional fibrous structure that boosts the metal's strength. In the end, choosing the right forging ratio not only improves the quality of the metal but also helps reduce production costs and ensures that the forged part meets its requirements. Therefore, selecting the correct forging ratio should be done carefully, taking into account the specific requirements of the forging process and the material being used, to optimize results and enhance the metal's overall performance.
