Introducing cold pilger mill technology - Ningbo New-era steel Tube Co.,Ltd.

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Introducing cold pilger mill technology

The cold mill pilgering process uses ring dies and a tapered mandrel to reduce tube cross sections by up to 90 percent. Because the process relies on large number of small forming steps, the result is tube or pipe that has nearly homogenous material characteristics. It is suitable for every metal.

Cold pilgering is a longitudinal cold-rolling process that reduces the diameter and wall thickness of metal tube in one process step. Depending on the material, the cold pilger process achieves cross-section reductions of more than 90 percent in a single working cycle.

In the dead-center positions of the mill saddle, the dies briefly disengage the tube. During these end-of-stroke periods, the hollow advances and rotates. Both the forward and return strokes form the tube.

Depending on the groove design, the forming process requires more than 10 steps, feeding and rotating the tube to different positions in relation to the forming die pass. The large number of small forming steps helps to ensure a constant wall thickness and nearly homogeneous material characteristics in the rolled tube.

The feed increment on the input side is an indication of the overall efficiency of the cold pilger mill operation. The length of finished tube per stroke of the mill saddle is calculated as the productOptimizing the process is a matter of matching the feed increment (n) and the rotation angle (a) in the two dead-center positions with the tube material and quality requirements. In some cases, the process can achieve variations smaller than 0.5 microinch for medium ODs and wall thicknesses. The roughness value of the finished tube usually is less than that of a drawn tube. With stainless steel tubes, for example, cold pilgering can achieve Ra values smaller than 0.02 microinch.

of the feed increment on the input side and the amount of elongation (the finished length minus the original length). The annual output can be estimated as the product of finished tube per stroke and the number of strokes per year, whereby the output depends on the tube material, the required dimensional tolerances, the tool design, tooling quality, and lubricant.

Materials and Applications

In most applications, the cold pilgered tubes are immediately ready for finishing or shipment. In other cases, additional cold pilgering or drawing operations are required. When additional cold-working steps are necessary, the tubes often require intermediate annealing.

The cold pilgering process is suitable for all metals. Typical materials are mild steel, stainless steel, ferritic steel, low-alloy steel, copper and copper alloys, titanium alloys, zirconium alloys, and nickel alloys (see Figure 4). Using cold pilgering to reduce precious metals is conceivable, because practically no material loss occurs. The deformation strengths of the cold pilgered tubes range from 400 N/mm² for copper to more than 1,500 N/mm² for special alloys.

Some applications, such as baseball bats and golf clubs, rely on cold pilgering to create the intermediate, tapered shape. Other applications are lightning poles, finned tubes, and nonround tubes with internal or external longitudinal ribs.

Characteristics of the Cold Pilger Process

This special forming process is indispensable for numerous applications, and cost-effective for many others, for a variety of reasons.

The cross-section reductions attained are higher than those achieved by other processes. Because the cold pilgering process applies pressure from all sides, it can achieve reductions up to 90 percent for copper; 80 percent for stainless steel, nickel alloys, and zircaloy; and 75 percent for high-strength titanium alloys.

The large cross-section reductions help to limit process-related conversion costs, because cold pilgering eliminates additional processes such as cleaning, annealing, pickling, cutting, handling, and straightening between drawing operations.

The homogenizing material flow in a circumferential direction facilitates substantial reductions in eccentricity. Experience has shown that the higher the eccentricity of the starting tube, the greater is the improvement in eccentricity by cold pilgering.

The many forming steps improve roundness, stress homogeneity, and surface roughness. Surface defect depths decrease in proportion to the amount of the wall thickness reduction.

During cold pilgering, practically no material loss occurs. Only the end faces of the finished tubes are out of shape and have to be cut off. The advantage of eccentricity improvements during cold pilgering is better material utilization. Reduced wall thickness deviations allow more footage of finished product to be produced from each ton of raw material.

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