Overview

Cold rolling is one of the important production processes for most common nickel alloy seamless pipes (such as: Monel 400, Inconel 600, Incoloy 800, Hastelloy C-276, etc.). It determines the size and performance of the nickel alloy seamless pipes & tubes. This article will explain the principle and function of cold rolling in easy-to-understand words and illustrations.

Process Principle and Important Parts of Cold Rolling

The main purpose of cold rolling is to reduce the outside diameter and wall thickness of the pipe tube billet to produce nickel alloy seamless pipes & tubes of different sizes. Its principle is to continuously squeeze and deform the pipe tube through the roll outside and the core rod inside it to achieve the purpose of reducing the outer diameter and wall thickness.

Roll

The roll of the nickel alloy tube is a disc-shaped part with a semicircular depression on the side (as shown in Figure 1). Its main function is to reduce the outer diameter of the nickel alloy seamless tube. Because it needs to be subjected to very large pressure during the rolling process without deformation, the material of the roll is generally high-carbon steel with high hardness. After quenching, the hardness of this material can ensure that it will not be deformed during the rolling process. In the current nickel alloy seamless pipe production process, two-high rolling mills and three-high rolling mills are mainly used.

The characteristic of the three-high rolling mill is that the rolling mill is composed of three rolls placed at 120° of each other(as shown in Figure 2). During rolling, the tube billet is deformed between the cylindrical core rod and three rolls with constant cross-section holes. Because the size of the roll groove is the same as the outer diameter of the finished pipe, the diameter deformation from the billet to the finished pipe is limited, so the output is low, but this rolling method has higher precision, so it is mainly used for finishing rolling process.

The feature of the two-high rolling mill is that the rolling mill is composed of two rolls placed 180° with each other(as shown in Figure 3). The tube billet is rolled and deformed between the tapered core rod and two rolls with annular groove blocks. The circumference of the annular groove block has a constantly changing section. The size of the groove is larger than the outside diameter of the tube billet and the size of the groove at the exit are equivalent to the outer diameter of the finished tube. Although it is difficult to design and manufacture deformation tools and it needs to be equipped with special roll pass processing machine tools, because of its remarkable advantages such as large diameter reduction, large wall reduction and high output, it is still widely used.

Whether it is a three-roll cold-rolling tube mill or a two-roll cold-rolling tube mill, the diameter and wall thickness of the rolled tube is deformed by passsing through the roll (outer mold) and the core rod (inner mold). Because the metal to be rolled will also generate lateral flow while extending longitudinally, that strip-shaped protrusions along the length will be formed between the gap of the two rolls. If this kind of protrusion cannot be effectively eliminated, the outer diameter size and shape tolerance (cylindricity) of the tube after rolling cannot be guaranteed.

Core Rod

In the cold rolling of nickel alloy pipes, the core rod is an important tool (as shown in Figure 4). Its main function is to control the wall thickness of the finished pipe. They require high strength and high wear resistance materials. In addition, high surface quality is also required, such as grinding and heat treatment after turning.

Principle of Cold Rolling

The principle of cold rolling is shown in Figure 5.

Figure 5

Advantages and Disadvantages of Cold Rolling

Advantages

• 1. Since the alloy is subjected to two or three directions of pressure during cold rolling which is beneficial to the deformation of the alloy tube, it can be rolled with a large amount of deformation. Its deformation elongation coefficient can reach 4~7 (while cold drawing is only 1.2~2.0), the section compression rate can reach 75%~85% (while cold drawing is only 32%~42%).
• 2. The wall thickness tolerance of the finished tube is small. This tolerance mainly depends on the original wall thickness tolerance of the tube billet (for example, when the wall thickness tolerance of the tube billet is 10% and 8%, respectively, it can be reduced to 6% and 5% after cold rolling.)
• 3. The microstructure of the tube is improved.
• 4. Due to the use of finished rolls and core rod, the inner and outer surfaces of the rolled pipes are of good quality, high dimensional accuracy, and the tolerance of the inner and outer diameters can reach ±0.1mm, or even ±0.008mm.
• 5. There is no metal loss during the cold rolling process, so the utilization is increased.

Disadvantages

The cold rolling method has problems such as high one-time investment, complex equipment, high technical requirements for operation & maintenance and high noise.

Precautions for Cold Rolled Pipes & Tubes.

• 1. Cold rolling will reduce the outer diameter and wall thickness of the pipe. Because the volume remains the same, the alloy pipe will become longer after rolling. Therefore, when cold rolling a nickel alloy seamless pipe to a fixed length, the length of the tube billet must be calculated in advance to save material and efficiency.
• 2. Since cold rolling is to squeeze the pipe blank to achieve the purpose of deformation, generally before cold rolling, cold rolling oil is applied to the inner and outer surfaces of the alloy pipe to reduce friction. Oiling can prevent burrs from appearing during cold rolling and can also extend the life of the mold.
• 3. The alloy pipe after cold rolling has residual stress due to high-strength extrusion, and the hardness and brittleness of the alloy pipe will increase at this time. Therefore, the alloy pipe needs to be annealed after cold rolling to soften the material.
• 4. If the size of the alloy pipe requires multiple cold rolling, pickling must be carried out before each cold rolling to ensure that there is no oxide scale on the surface of the pipe, otherwise it is easy to make the wall thickness uneven or even block the pipeline during the rolling process.

The Comparison Between Cold Rolling and Cold Drawing

The comparison between cold rolling and cold drawing is as follows:

Conclusion

Cold rolling is a process for changing the outer diameter and wall thickness of the pipe by extruding the pipe. It can make the alloy obtain higher strength. The pipe needs to be oiled before cold rolling. After cold rolling, the material needs to be heat treated to reduce internal stress.

AEETHER CO., LIMITED produce nickel alloy cold-rolled pipes and cold-rolled sheets. If you have any requirements, you can send inquiry to our E-mail: