Zinc-Phosphate-Free Lubricant Coating for Cold Forging
In the 1930s, Friedrich Singer started using a zinc-phosphate coating as a lubricant for semi-finished products in cold-forming processes. This practice – still common in industry – is increasingly under fire because the chemicals used must be disposed of properly and at considerable expense. Carl Bechem offers a zinc-phosphate-free lubricant coating for cold-forging applications that is less costly and more environmentally friendly than traditional products.
In the process of cold forming, lubricants are applied on the semi-finished products. This is done, without preheating, in single- or multi-stage processes after a wet chemical pre-treatment. In cold forming, tribological conditions are characterized by high relative speeds between the tooling and the workpiece, high surface pressure and enlarged surfaces. Temperatures of up to 500°C (932°F) will occur under these conditions, resulting in the need for lubricating systems to reduce direct-metal contact between tools and workpieces.
Reduced Chemical Disposal Costs
Massive cold forming became viable as an industrial process with the Singer technique, patented in Germany in 1934. By modern standards, however, this process has a number of disadvantages since the application of individual layers requires a high amount of energy and a high quantity of chemicals that must be properly disposed of after use. The system and its applications are subject to strict environmental regulations.
Environmental protection, global competition and the pressure to control costs require energy-saving and raw-material-efficient manufacturing processes. The use of zinc-phosphate coatings on semi-finished parts has to be reconsidered particularly because of its environmental implications. However, the replacement of zinc-phosphate layers is complicated due to demanding process requirements. Besides good separating properties between tool and semi-finished part, an alternative fluid has to guarantee consistent surface quality, high load-carrying capacity and good emergency-running properties.
Zinc Phosphate and Soap
At present, chemical zinc phosphating is still the industrial standard for cold forming. Zinc phosphate is frequently used in conjunction with soap (sodium stearate) to form a lubrication layer in cold forging. The soap reacts with the phosphate crystal to bond to the metal surface. This reacted soap layer forms a base for additional lubricant to be deposited on top of it so that a three-part coating of zinc phosphate, reacted soap and unreacted soap is formed. Furthermore, the resulting coating adheres tenaciously to the metal surface even under extreme deformation. In this system, the zinc phosphate is an abrasive and the soap actually lubricates. Consequently, the soap layer must be thick enough to prevent substantial contact between the metal-forming dies and phosphate crystal.
Coatings Free of Zinc Phosphate
The processability of the semi-finished part by means of conventional forming technologies and easy removability of the lubricant from the formed parts are essential features for any alternative lubricant system. One application is the coating of wire in a chemical bath prior to its being drawn to the appropriate diameter and, for this example, cold forged into fastener products.
The change to zinc-phosphate-free coating systems offers high economic and ecological potential. Within the context of a joint research project, lubricant manufacturer Carl Bechem GmbH has tested several alternative lubricant systems that meet the previously mentioned requirements. The result of this research was a series of products we feel are superior to other conventional zinc-phosphate-free systems.
The new product, called Beruforge 150, eliminates the need for zinc-phosphate coatings on semi-finished wires – even for complex forming operations. The product is suitable for easy application on all non-phosphatised wire surfaces, as well as on stainless steel and aluminum. The ease of application for Beruforge 150 is the result of the original product concept.
As a result, instead of the traditional multilayered application of zinc-phosphate/soap lubricant (which was prepared in a process requiring up to 10 steps), the new lubricant coating allows the user to coat the workpiece in one step and shorten the coating process to three steps – descaling, cleaning and coating.
The zinc-phosphate layer is chemically fixed on the surface, which makes a complicated cleaning crucial. The Beruforge system is physically located on the material. Therefore, the cleaning of the produced products is much easier. It allows the user to clean his parts under significantly milder conditions. The combination of a very easy application with outstanding performance properties makes the newly developed lubricant concept attractive to the user.
Coating manufacturers, wire producers and end users have provided positive feedback to our zinc-phosphate-free system. For example, based on their experience to date, Schrauben Betzer of Lüdenscheid, Germany, a project partner and manufacturer of precision cold-forged parts, has already changed part of their production to zinc-phosphate-free wires, especially for small screw diameters. By doing so, they do not have to remove the zinc-phosphate layer and eliminate the costs related to doing so.
We have also received positive feedback from wire-producing companies. According to Bechem, the change from a conventional multiple-layer coating to the single-layer process results in a considerably lower drag-in of solids during the forming process, which results in improved cleanliness of the machine. The significantly reduced contamination of the whole system guarantees an extended oil lifetime.
Cleaner Machines and Excellent Corrosion Protection
According to Jürgen Schoppe, an engineer in Bechem’s R&D department who helped develop Beruforge 150, one advantage to using zinc-phosphate-free lubrication is the increase in the service life of tooling.
“The forming tools’ service life will increase by up to 20%,” Schoppe said.
This premise is also confirmed by machine fabricator and development partner Jankowski GmbH of Horhausen, Germany. The new chemical system has been successfully applied in various industrial applications, including the manufacture of raw wire or rod and in the production of screws, bolts and other fasteners. This demonstrates the product’s flexibility.
The new lubricant can be used to coat individual wire sections, to dip-coat complete wire coils or within the drawing box during wire drawing. In wire drawing, the annealed wire is coated with the lubricant in the final draft. The special mixture of organic and inorganic components offers excellent corrosion protection, which in turn allows long storage life without quality loss. Intermediate storage is, however, no longer necessary.
“For the first time a reliable in-line coating process is conceivable,” said Dr. Jens Ostrowski, head of R&D oil/forming technology at Bechem.
In this process, untreated semi-finished parts are mechanically cleaned. After a calibration process using Beruforge 150 coating, the parts undergo their forming process. After the forming process, the lubricant residues can be easily removed.
“Besides the technical advantages, the biggest potential is process optimization,” Dr. Ostrowski said.
In addition to energy savings, this also means considerable reductions in disposal costs in many cases.
Considering the huge volume of steel wire used to form semi-finished parts for various industries and industrial trends toward environmental sustainability and resource efficiency, the enormous market potential of zinc-phosphate-free coatings becomes evident. The potential applications for this lubricant in forming technology are manifold.
New applications will be developed gradually. Besides the cold forging of semi-finished wires, massive cold forming of steel bars and cold calibration of forgings will play a major role in the future. The new Beruforge 150 series can increasingly replace conventional zinc-phosphate coatings for these applications.