In the field of metal processing, steel forging is a critically important manufacturing process. By applying pressure, the metal undergoes plastic deformation, thereby producing forgings with specific shapes and properties. During the forging process, the forging ratio is an extremely important parameter, as it is directly related to the quality, performance, and production cost of forgings. This article provides an in-depth discussion of the definition, function, influencing factors, and the importance of reasonably selecting the forging ratio.
The forging ratio refers to the ratio of the cross-sectional area of the metal before deformation to that after deformation during the forging process. It is an important indicator used to measure the degree of metal deformation. The calculation method of the forging ratio varies depending on different forging operations, but in general, it is based on the change in cross-sectional area before and after deformation.
The drawing forging ratio refers to the ratio of the cross-sectional area before drawing to that after drawing. The calculation formulas are as follows:
The forging ratio y is equal to F1 divided by F0, or y is equal to L0 divided by L1.
Here, F0 and L0 are the cross-sectional area and length of the ingot or billet before drawing, while F1 and L1 are the cross-sectional area and length after drawing. Through this ratio, the degree of metal deformation during the drawing process can be clearly understood.
The upsetting forging ratio is also referred to as the upsetting ratio or compression ratio. It is expressed as follows:
The forging ratio y is equal to F0 divided by F1, or y is equal to H1 divided by H0.
Here, F0 and H0 are the cross-sectional area and height of the ingot or billet before upsetting, while F1 and H1 are the cross-sectional area and height after upsetting. The upsetting forging ratio reflects the degree of compression during the upsetting process and is one of the important indicators for evaluating forging quality.
The forging ratio has a significant influence on the mechanical properties and structural characteristics of forgings. As the forging ratio increases, internal pores in the metal are compacted, cast dendritic structures are broken up, and both longitudinal and transverse mechanical properties of the forging are significantly improved.
However, when the drawing forging cross-sectional ratio exceeds 3–4, with further increases in the forging ratio, an obvious fibrous structure forms. This causes a sharp decline in transverse plasticity indices, leading to anisotropy of the forging. Therefore, when selecting the forging ratio, it is necessary to fully consider the performance requirements and process characteristics of the forging in order to achieve optimal forging quality.
An increase in the forging ratio helps improve the metal’s microstructure. During forging, as the forging ratio increases, internal pores are gradually compacted and cast dendrites are broken up, resulting in a denser microstructure. This improvement in microstructure directly leads to enhanced mechanical properties of the forging, including strength, hardness, toughness, and plasticity.
For example, when forging high-quality carbon structural steel and alloy structural steel, a reasonable forging ratio can increase the strength of the forging by 20%–30%, while toughness is also significantly improved.
The forging ratio also has an important influence on anisotropy. When the forging cross-sectional ratio exceeds 3–4, an obvious fibrous structure forms inside the forging. This fibrous structure causes differences in mechanical properties between the longitudinal and transverse directions, resulting in anisotropy.
In practical applications, such anisotropy may have adverse effects on service performance. For example, in components subjected to multi-directional stresses, anisotropy may lead to stress concentration and thus reduce service life. Therefore, when selecting the forging ratio, it is necessary to reasonably control the ratio according to the specific service requirements of the forging in order to reduce the impact of anisotropy.
The selection of the forging ratio is influenced by multiple factors, including metal material properties, forging process, forging temperature, and the shape and size of the forging.
Different metal materials have different flowability and deformation capabilities during forging, and therefore require different forging ratios. Materials with good flowability, such as low-carbon steel, can use smaller forging ratios, while materials with poor flowability, such as high-alloy steel, require larger forging ratios. This is because materials with poor flowability require greater deformation during forging to achieve the desired structure and performance.
Different forging processes also affect the forging ratio. For example, in cold forging processes, due to poor material flowability, larger forging ratios are required to ensure sufficient deformation. In hot forging processes, better flowability allows for smaller forging ratios.
In addition, different forging equipment and die designs also influence forging ratio selection. For example, in free forging on hammers, greater flexibility allows the use of larger forging ratios, whereas in die forging, forging ratio selection must be more cautious due to die constraints.
Forging temperature is one of the important factors influencing the forging ratio. At higher forging temperatures, metals exhibit better flowability and deformation capacity, allowing for smaller forging ratios. At lower forging temperatures, metals have poorer flowability and deformation capacity, requiring larger forging ratios to achieve the required deformation.
Therefore, in actual production, appropriate forging ratios must be selected based on the specific forging temperature.
The shape and size of the forging also influence forging ratio selection. Forgings with complex shapes or large sizes require larger forging ratios to ensure sufficient deformation and the required structure and properties. Forgings with simple shapes and small sizes can use smaller forging ratios.
In addition, service requirements also affect forging ratio selection. Components subjected to high stress require larger forging ratios to improve strength and toughness, while general-purpose components may use smaller forging ratios to reduce cost.
When selecting the forging ratio, the following principle should be followed: on the premise of meeting all forging requirements, the smallest possible forging ratio should be selected. This reduces forging difficulty and cost while helping to improve forging quality and performance.
For free forging on hammers, forging ratios should be selected according to forging type and service requirements.
- Shaft forgings produced directly from ingots:
- Forging ratio based on the main section ≥ 3
- Forging ratio based on flanges or other protrusions ≥ 1.75
- Shaft forgings produced from billets or rolled materials:
- Forging ratio based on the main section ≥ 1.5
- Forging ratio based on flanges or other protrusions ≥ 1.3
- Ring forgings: forging ratio generally ≥ 3
- Disc forgings:
- Produced directly from ingots: upsetting forging ratio ≥ 3
- Other cases: upsetting forging ratio generally > 3, but the final operation should be > 2
For high-alloy steel billets, forging must not only eliminate structural defects but also ensure relatively uniform carbide distribution. Therefore, larger forging ratios must be used.
For example:
- Stainless steel forging ratio: 4–6
- High-speed steel forging ratio: 5–12
This is because greater deformation is required during forging to break up carbides and distribute them uniformly, thereby improving forging performance.
For large forgings, forging ratio selection is particularly important. The raw materials for large forgings are mainly steel ingots, which inevitably contain defects such as looseness, inclusions, and gas pores.
The purpose of forging is to break up the cast structure, refine grains, homogenize chemical composition, and improve mechanical properties. To ensure internal quality, the forging ratio must be controlled between 3 and 7. Ratios below 3 result in insufficient deformation, while ratios above 7 lead to shape distortion.
In actual production, concave upsetting plates are recommended for ingot upsetting, as they provide better load distribution and help improve forging quality.
At present, three categories of process schemes are used to improve forging ratio uniformity: changing billet–die boundary conditions, changing billet shape, and changing upsetting deformation modes. These methods effectively improve forging ratio and geometric uniformity, thereby enhancing forging quality.
By changing billet–die boundary conditions, metal flow behavior during forging can be improved, thereby enhancing forging ratio uniformity. For example, in die forging, optimizing die design to create smoother die surfaces reduces friction and promotes more uniform metal flow.
Additionally, adjusting die temperature and lubrication conditions further improves metal flow and forging ratio uniformity.
Changing billet shape is another effective method to improve forging ratio uniformity. For example, in large forgings, billets can be designed with pre-deformed shapes to promote more uniform deformation.
Optimizing billet dimensions and shapes allows better adaptation to die geometry, thereby improving forging ratio uniformity.
Changing the upsetting deformation mode is also an important method to improve forging ratio uniformity. For example, modifying flat-anvil upsetting deformation can effectively improve forging ratio and geometric uniformity.
In actual production, advanced upsetting processes such as multi-directional upsetting and stepped upsetting can be adopted to achieve more uniform deformation and improved forging ratio consistency.
The forging ratio has important practical significance in forging plant production, especially for large forgings such as wheel forgings, ring forgings, shafts, and cylinders.
Wheel forgings are typical large forgings, and forging ratio selection must comprehensively consider material properties, shape, size, and service requirements. Generally, the forging ratio for wheel forgings should be controlled between 3 and 5.
A reasonable forging ratio ensures internal quality and improves strength and toughness, meeting the service requirements of transportation equipment.
Ring forgings are widely used in aerospace and machinery manufacturing. Their forging ratio should be selected according to specific service requirements and is generally ≥ 3.
A reasonable forging ratio ensures internal quality and improves strength and toughness under complex service conditions.
Shaft forgings are common products in forging plants. Their forging ratio selection depends on material properties, shape, size, and service requirements and is generally ≥ 3.
A reasonable forging ratio ensures internal quality and improves strength and toughness to meet mechanical equipment requirements.
The forging ratio is an important parameter in metal forging processes and is directly related to forging quality, performance, and production cost. Reasonable selection of the forging ratio effectively improves internal quality and mechanical properties while reducing cost and improving productivity.
In actual production, forging ratio selection must comprehensively consider metal material properties, forging process, forging temperature, and forging shape and size. Optimizing forging ratio selection provides important technical support for forging production and promotes technological progress and sustainable development in the forging industry.
