Introduction
Pipe bending is a key process used in manufacturing to create customized angles and radii in piping systems across many industries. There are two main methods – cold bending and hot bending – each with advantages and limitations depending on the alloy, diameter, thickness and degree of bend required. This report provides an overview of the pipe bending process, details the key differences between cold and hot bending techniques, and explores their applications and suitability for various bending scenarios.
Before Bending Pipes and Tubes
Regardless of the bending method, proper preparation of the pipe or tube is crucial. This involves cleaning and deburring the ends, ensuring straightness within specification, and accurately measuring and marking the bend line. Pipe ends may need to be beveled to promote full penetration welds after bending. Non-destructive testing using methods such as ultrasonics or x-rays verifies there are no unacceptable defects prior to bending.
What is Cold Bending?
Cold bending involves bending pipe at or near room temperature, without any external heating of the material. As the pipe is bent, the outer surface undergoes tension while the inner surface is compressed, causing the material to work harden and reduces ductility. This makes it more difficult to form tight bends without cracking or wrinkling of the pipe wall. Cold bending is best suited for softer materials like copper, plastic, and aluminum, as well as small diameter steel pipes up to around 2-3 inches depending on wall thickness. Thinner walls and larger diameters increase the risk of cracking during cold bending.
Cold Bending of Pipes
The pipe is bent using one of several methods – roll bending using grooved rolls to progressively form the bend, yawl and mandrel bending using internal support along with mechanical advantages like gears or hydraulics to apply pressure, or three-roll bending which sandwiches the pipe between C-shaped rollers. Multiple passes with pipe rotation in between may be needed to incrementally form tight bend radii without exceeding the material’s bending limits. Heat treatment after bending can help relieve internal stresses and cracking susceptibility.
Hot Bending
Hot bending involves heating the pipe to soften it prior to bending, reducing the work hardening effect and allowing for sharper bend radii to be achieved. Common heating methods are induction heating, which heats the pipe surface and internal volume indirectly using coils, or direct open flame heating in a controlled manner. Temperature is a critical factor – too low and bend forms will be difficult, too high and the material properties may deteriorate. Proper hold times are required to ensure an even temperature profile before bending. At temperature, bending is performed on the still malleable pipe using bending machines capable of the tighter radii and forces involved. Cooling methods like water sprays are used after shaping to harden the bend form.
Cold Bending vs. Hot Bending
Key differences are outlined in Table 1. Cold bending is generally favored for smaller diameter pipes up to 2-3″ using techniques like roll bending. Above this size, alloy strength increases the risk of cracking, necessitating hot bending using mandrel or three-roll bending machines capable of inducing the necessary curvature. Hot bending also allows bending larger diameter pipes with thinner walls that would be unbendable using cold techniques due to reduced ductility. However hot bending demands more specialized equipment and heating controls and precautions to avoid overheating or uneven cooling after bending.
Table 1. Cold vs. Hot Bending Comparison
Attribute Cold Bending Hot Bending
Suitable diameters Up to 2-3″ typically Above 2-3″
Temperature At/near ambient Raised to 800-1200°F
Effects on material Increases work hardening Softens to bend easier
Tightest bend radii Larger 3-5x diameter Smaller 2-3x diameter
Cracking risk Higher above 2″ diameter Lower with pre-heating
Equipment Roll/yawl machines Mandrel/three-roll machines
Controls complexity Less equipment/controls Demand heating/cooling controls
Application Examples
Cold bending is commonly applied to softer alloys like copper tubing for plumbing applications, thin-walled steel conduit, and plastic pipes. Larger steel pipes for oil/gas pipelines or structural tubing require hot bending to achieve the tightly curved elbows and crosses needed. Stainless steel and high-strength alloys require hot bending regardless of diameter due to work hardening susceptibility. Heat input also reduces residual stresses imposed during bending that could promote future cracking.
Conclusion
In conclusion, both cold and hot bending techniques provide effective methods to form bends in piping systems, with suitability depending on material type, size and bend severity. While cold bending entails simpler equipment, hot bending allows tighter radii and processing of larger high-strength pipes. With proper preparation, controls and techniques, either method can produce robust, structurally sound pipe bends suited for their various applications transporting media. Overall, understanding limitations and optimizing bending parameter selection ensures dimensional accuracy and long service lifetimes of the finished pipe configurations.
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