Zinc Phosphate Coating Prior to Tube Drawing
Where phosphating is used for the cold drawing of seamless tube, the requirements are somewhat different from those applicable to welded steel tube. Until about 30 years ago, phosphate coating weights of 20-40 g/m² were used allowing the largest possible number of successive draws. Such coatings were formed with nitrate-accelerated systems at 90-95 °C.
Over the years, in search for increased operational efficiency, the number of draws for a given reduction of seamless tubing, has decreased. In order to maintain the dimensional tolerances and quality of surface finish, tubes in the mid-range of available sizes are now reduced in a single or, at most, two draws. At the same time, drawing rates have been increased. For these reasons phosphate coating for drawing of seamless tubing are now formed with weights of 4-10 g/m². This has improved the efficiency of the surface treatment and, at the same time, avoided the adverse effects which act in the firs drawing stage where coarser-crystalline phosphate coating are found. The most suitable coating is based on nitrate/nitrite accelerated zinc phosphate, formed at 40-75 °C. At the upper end of this temperature range, the option exists to use self dosing nitrate type systems. Chlorate accelerated zinc phosphate baths are also found. In all cases, the preferred form of the phosphate for cold drawing of tube and section is strongly adherent but soft structured.
In the drawing of welded tubing, the seam must first be ground down. In the case of smaller diameter tubing, this is not possible inside the welding machine. In some cases, there may be a deformation to give a particular cross-section. Since, as a rule, less severe deformations can be tolerated by welded, as opposed to seamless tubing, the use of phosphating is widespread, coating weights being of the order 1.5 - 5 g/m². These are mostly based on zinc phosphate baths operated between 50 and 75 °C with additives used to promote thinner coatings.
Phosphating is also used for tubing of un-alloyed or low-alloyed steel with chromium content up to 4-6%. Such coatings offer a number of advantages, all arising from reduced metal-to-metal contact between tube and die. Thus, cold welding damage, leading to grooving or crack formation, is minimised, tool and die life is extended and higher drawing rates may be used. Zinc phosphate coating also allows a greater degree of reduction per pass and an increased number of passes without intermediate heat-treatment.
Phosphating is carried out by immersion along the following lines:
- Alkaline degreasing.
- Water rinse.
- Pickling in sulphuric or hydrochloric acid.
- Water rinse.
- Neutralising pre-rinse.
- Water rinse.
- Neutralising rinse.
- Drying and storage.
In earlier times, the surface treatment of tubes took place using bundled tubes lifted by a manually-operated hoist from one tank to the next. More recently, automatic conveyors have been introduced and, while retaining the sequence of operations listed above, the whole operation takes place automatically.