Rolled-ring forging is a specialized forging process used to create seamless rings of different sizes and profiles. These forgings are widely used in aerospace, automotive, wind, bearing, and other industries, where a strong seamless ring is needed.
Rolled rings can vary tremendously in size, from small rings of a few inches in diameter, to those with diameters more than 25 feet in diameter. Weights can vary from a pound to many tons (Figure 1).
The Process of Ring Rolling
Ring rolling is applicable to a large variety of materials, depending upon application. Commonly used materials for ring rolling include:
- Carbon and alloy steel: General industrial applications, including automotive and wind turbine applications.
- Stainless steel: Offers corrosion resistance, making it ideal for harsh environments. This includes the precipitation hardening grades.
- Aluminum, titanium, and superalloys: Used in aerospace and high-temperature applications.
Preform
The initial material billet is heated in a furnace between 1,000-1,200°C depending upon the material. Aluminum alloys are forged at much lower temperatures (375-500°C). Superalloys are heated to 1,100-1,350°C, depending upon alloy grade, generally above the carbide and γ’ solvi. Proper temperature control is important to achieve the desired grain size and minimize grain growth.
Before rolling can begin, the heated billet undergoes an initial shaping process. The preform is positioned between the radial roll and the mandrel. A controlled force is applied to the preform, reducing thickness and initiating radial expansion. The billet is upset (compressed) to the desired thickness and achieves a uniform shape.
Piercing
In this process, a hole is created in the center of the billet using a punch press or a mandrel press. This transforms the billet into a doughnut-shaped preform, which serves as the starting material for the rolling process. Preform shaping is critical as it determines the initial distribution of material and ensures optimal deformation during the rolling phase.
The pierced preform is often placed back into the furnace to again heat the preform to the proper forging temperature for rolling.
Initial Rolling
The heart of the rolled ring forging process is the ring rolling operation, where the preformed doughnut is expanded into a seamless ring. This process is carried out using a ring rolling mill, which consists of:
- Drive roll: Applies compressive force to reduce wall thickness and spins the ring during rolling.
- Axial roll: Controls the height of the ring, ensuring dimensional accuracy.
- Mandrel or idler roll: This roll supports the ring from the inside and helps maintain circularity.
As the preform rotates, the idler roll continuously applies force to expand the ring. The mandrel prevents inward collapse, ensuring uniform thickness distribution. The material undergoes grain refinement, improving mechanical properties such as tensile strength and fatigue resistance.
The axial rolls apply controlled force along the height of the ring to maintain specified dimensions. This stage ensures the final ring does not exhibit excessive width variations or defects.
Grain flow is carefully directed to align circumferentially, which enhances resistance to stress and fatigue. The forging process eliminates porosity and refines the microstructure, significantly improving durability.
Roll to Size
As the ring grows in diameter, the idler roll increases pressure on the inside of the ring. In many ways, this is like a potter working in clay, to produce a round vase on a potter’s wheel. Instead of the potter’s finger, the idler roll moves the material outward while the ring is spinning. Like the potter’s hand on the outside of the vase, the drive roll spins the ring and provides back pressure to ensure the proper outer diameter.
This entire process is very quick, taking only a few seconds or minutes. Generally, once the pierced preform is removed from the furnace, the ring rolling process can be accomplished quickly, with internal friction maintaining the temperature throughout the ring rolling process.
A schematic of the ring rolling process is shown in Figure 2. Depending on the specific ring, the rolled ring is heat treated and machined to the final dimensions.
Advantages and Benefits of Ring Rolling
Versatility in Available Materials
Rolled rings can be made from any material that can be forged. This includes carbon steels, alloy steels, titanium, aluminum, and superalloys. Another advantage is the flexibility in size. Rolled rings can be made with a diameter of one inch to diameters of many feet. Wall thickness can also be widely varied depending on material and ring diameter.
Enhanced Metallurgical Properties
During the ring rolling process, the deformation of the preform is radial compression and axial extension. The grain is arranged continuously around the circumference, with no seam. This contributes to enhanced fatigue and creep strength, uniform grain size, and excellent internal quality.
Energy and Material Efficiency
With ring rolling, there is complete use of the material. There are no flash or draft angles to contend with. The outer and inner diameters are close to the desired dimensions, so minimal machining is required. Raw material is conserved.
Ring rolling is cost-effective. It can be highly automated and is a rapid process. The energy consumption is low compared to die forging.
The contact areas between the tool and workpiece are small, but the amount of deformation is large. This means that the required energy or force required to achieve the desired shape is much less than a die press of similar capability. Much lower pressures are required to achieve the final product.
Conclusions
Rolled-ring forging is an interesting forging process that combines precision, strength, and material efficiency.
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