How to avoid common defects of cold drawn metal tubes?
In general, cold drawing is often made at room temperature with metal material pulling through the die, and this process changes its crystalline structure meanwhile creating dislocations, and the metal material emerges harder. Depending upon how many cold drawing processes you need, it often requires annealing also called heat treatment, to soften it so that it can be drawn two or more times. A high precision square or rectangular metal tube might take two or three draws, with anneals in between, to achieve the desired dimension or special shape. There are two different way of making cold drawn tubes:
√ Cold drawing
√ Cold rolling + cold drawing
The cold drawing solution has following benefits:
√ Good surface finish
√ Improved mechanical properties due to the strain hardening effect.
√ Can produce thin wall with high precision metal tubes.
√ Tight tolerance metal tubes like copper profile & waveguide tubes
√ Capable of producing irregular tubes
Cold rolling solution generally generates smoother finish, but it is somehow less precise than cold drawing, yet, you can still achieve high precision by adding one cold drawing process to achieve better surface finish. However, cold drawing process also has some disadvantages. When the metal tubing is drawn through an exterior die with a diameter smaller than the outer diameter of the tubing, it is subjected to longitudinal tensile stresses and to both radial and circumferential compressive stresses, which make cold drawn metal tubes prone to certain defects in the production process like buckling, porosity, bubbles, shrinkage cavity residues, non-metallic inclusions, segregation, white spots, cracks, and various abnormal fracture defects, and they are often spotted during online inspection, and the tubes are mainly used for mechanical structures, heavy duty bollards, hydraulic and pneumatic or hydraulic equipment and medical equipment. Therefore, detecting and finding out the root causes is critical for delivering high quality seamless steel pipes. To achieve a consistent quality, the production division needs to keep a tight production control covering mold design, cold drawing speed, annealing process, pickling, lubrication, and other conditions. There are many reasons behind of these defects which can have a large impact on the performance of seamless steel pipes, and therefore, cold drawn seamless steel pipe manufacturers must avoid these defects as much as possible during the manufacturing process. In particular, high-precision cold-drawn precision steel tube requires tighter inner and outer diameter dimensional accuracy, inner and outer surface smoothness, roundness, uniform wall thickness as well as straightness. To have a general understanding of the defects and root causes, we’ve listed the most common ones below for your reference:
1. Buckling, the inner and outer surfaces of the steel pipe present buckling in a straight line or in a helical direction, and it might be caused by:
√ Serious wear of the mold head.
√ The ratio (H/b) is not set up at a correct level.
√ Uneven heating of billet, or annealing of billet with too high or too low temperature.
√ Improper parameters of the perforation process or the perforating roller is worn out.
√ The mother tube has buckling and inclusions impurity, and it might also have serious scratches or crack defects.
Prevention Measurements
√ Tighter control of mother tube or billet quality in terms of annealing, general dimension, tube thickness, buckling conditions and surface quality.
√ Condition check on perforating machine before cold drawing process started.
2. Tube size out of tolerance, and it includes wall thickness, uneven wall thickness, diameter as well as eccentricity, and the possible causes are:
√ Improper selection of drawing mold, or improper adjustment of mandrel (inner mold)
√ Unreasonable design
√ Molds are not made in correct size or might be serious worn, or insufficient hardness resulting in deformation and wear.
√ Improper heat treatment, high temperature, or uneven heat treatment performance
√ Improper control of the law of increasing and decreasing walls
√ Pull tabulation is unqualified.
√ The steel pipe is deformed during straightening process.
Prevention Measurements:
√ Improve overall die quality.
√ Perform heat treatment with right temperature and cooling time.
√ Extra attention is required during straightening process.
√ Regular inspection on tooling is required during production period.
√ Master the rules of increasing and decreasing wall thickness of metal tubes pipes of different steel types and specifications based upon the metal material property.
3. Drawing Marks
The marks are also called scratches, and they can be long and deep in a longitudinal straight line, and most of them are ditch shaped. The drawing marks might be caused by improper annealing.
Prevention Measurement
√ Improve the die quality
√ The metal tube surface shall be pickled &cleaned thoroughly, for copper tubing, the mother tube surface needs to be cleaned thoroughly before cold drawing or cold rolling starts.
√ Make sure the size and shape of pointing is proper for smoothly going through the die.
4. Crinkling, the surface of the metal tubing has uneven ring waves or waves along the length direction, locally or through the whole length, appearing on the inner and outer surfaces of the metal tube. Below are the possible causes:
√ After pickling, the rinsing is not done thoroughly.
√ Improper phosphating process leads to uneven saponification.
√ The core of pulling rod might be too thin, and it might vibrate during the cold drawing process, which will cause unexpected deformation.
√ The entrance cone angle of the die might be too large, which makes the contact area between the metal tube and the die hole too small for a stable drawing process.
Prevention Solution
√ Thorough cleaning during pre-treatment process
√ Tight control over phosphating and saponification processes.
√ The pulling rod size needs to be justified for the drawing task.
√ Optimize the design of the entrance of the die.
5. Concave, the metal tube wall is inwardly depressed, and the possible causes are:
√ Diameter reduction is too big for cold drawing without mandrel for thin wall tubes.
√ Die surface might attached unexpected metal scraps.
Prevention Measurement:
√ When drawing thin-walled metal tubes, the diameter reduction should be made reasonably.
√ Die needs to be cleaned before cold drawing production.
6. Crack, the metal tubes show longitudinal cracks that penetrate the wall, and there are partial cracks throughout the length. The cracks might be caused by following reasons:
√ The amount of diameter reduction is too big.
√ Lack of adequate lubrication
√ Too many drawing passes with severely hardened material
√ Improper heat treatment resulting in inconsistent plasticity of tube material or incomplete elimination of hardening effect.
√ Metal types have low plasticity
Prevention Measurement:
√ Adjust size reduction based upon metal mechanical property and its diameter.
√ Apply lubrication during the cold drawing process
√ Alloy steel and copper alloy tubes require heat treatment.
7. Scarring
The surface of the steel pipe presents pinholes with different degrees of severity, and usually the outer surface condition is worse than the inner surface. It’s usually either caused by long pickling time, or the content of ferrous sulfate is too high, or the acid concentration is uneven. The prevention solution is follow the pickling operation procedure strictly, and make sure that the pickling solution has correct acid and ferrous sulfate level.
8. Pimples (Rough Surface)
Pimples appear locally or continuously on the surface of the steel tube, and possible causes are
√ The oxide scale generated by over heat treatment, and the oxide scale is then pressed into steel material during straightening process.
√ Pickling is not done properly, residual iron oxide scale or dirt in the saponification solution remain on the tube surface, which will cause rough surface during cold drawing process.
√ Steel tubes are rusted again long after pickling
The prevention solution is to make sure that pickling is done properly, and steel tubes needs to be cold drawn after the pickling process. The heat treatment needs to be carried out strictly according to the procedure to avoid over heat treatment.
Under the strict production control, cold drawn seamless metal tubes can have great properties like no oxide layer, no leakage under high pressure, high precision, good surface finish, minimum deformation during cold bending, and no cracks in flaring and flattening. These high-quality metal tubes are mainly used for mechanical structures, generators, liquid cooling transformers, hydraulic equipment and pneumatic or hydraulic components of automobiles. The high precision metal tube with tight tolerance is usually made by cold drawing process, it’s regarded as a new type of high-tech energy-saving product, and it can save metal material, improve processing efficiency, and save energy.
