New Stamping Lubricants Meet New Challenges
Beginning in the 1970s, synthetic stamping lubricants became viable options to petroleum-based products throughout North America. These water-based lubricants provided improvements to post-process operations such as welding, reduced costs associated with parts washing and housekeeping, and were environmentally friendly. Their popularity increased in 1989 when the Auto-Steel Partnership approved, for the first time, use of a synthetic lubricant (Fuchs’ Eco Draw HVE 1) by automotive OEMs.
As use of synthetics increased, suppliers continued to enhance lubricity, anticorrosion properties and residue characteristics of synthetic lubricants. Along the way, many stampers standardized on synthetic lubricants. While direct savings may or may not have always been realized in the pressroom, improvement to weld quality, increased weld-tip life, reduced housekeeping and disposal costs and improved safety typically amounted to substantial overall savings.
Water-based stamping lubricants can lead to galvanic corrosion when stamping zinc-coated steels.
In recent years, stampers, particularly those serving the automotive industry, have begun to face new challenges related to the increased use of high-strength and advanced high-strength steels (HSS and AHSS), as well as coated steels and aluminum alloys. Stamping these materials can push the capability limits of some lubricants. And, European and Asian automotive OEMs often restrict or discourage the use of water-based lubricants when stamping their components.
Meet the Enemy—Friction and Heat
The work-hardening and springback characteristics of high-strength steels can require stampers to boost press tonnage and increase ram dwell time. The resulting increase in friction and associated heat can break down stamping lubricants, deplete their boundary protection and render them ineffective. In these situations, stampers opt for lubricants with extreme-pressure (EP) additives such as chlorine, sulfur and phosphorous. Activated by heat, these additives react with metal to form metallic “salts” with low shear strength. These salts provide an additional yet temporary coating to protect the tools and workpiece.
The most robust levels of chlorine and sulfur can be contained in paraffin, olefin or fats. However, these carriers are oil-soluble and do not mix well as additives with water-based synthetics. Chlorine and sulfur can be added at low levels to synthetic lubricants in the form of chlorinated or sulfurized ester or fatty acids that are neutralized and made water-soluble. However, in the presence of heat and humidity, chlorine and sulfur can degrade to emit trace acid, which corrodes metal. Acid scavengers can be added to oil-based formulas to safely consume the acid, but these inhibitors are not water-soluble. As a result, chlorinated or sulfurized synthetic lubricants, if not removed, can become corrosive as a residue on parts and lead to corrosion and staining.
Usually, more water-soluble phosphate esters comprise the EP additive in synthetic lubricants. But in severe metalforming operations, especially with AHSS, phosphorous may not provide enough protection from galling.
Water-based stamping lubricants can lead to galvanic corrosion when stamping zinc-coated steels.
Stampers tasked with performing severe forming operations on higher-strength steels have begun to look for more robust neat or emulsifiable oil-based lubricants. These products contain high levels of chlorine, sulfur and phosphorous to provide significantly enhanced levels of protection compared to more traditional synthetics.
New technologies have been developed that reduce issues commonly associated with petroleum. Emulsifying agents added to neat and emulsifiable oils improve the cleaning characteristics in post-process wash operations. Some water-emulsifiable lubricants contain minimal amounts of petroleum, which improves the residue on parts and reduces smoke or quality issues related to welding operations. And, some lubricants replace petroleum with renewable oils derived from vegetables and seed plants. These neo-synthetic products provide many of the benefits of both oil and synthetic lubricants.
Oils, aside from the level of effective EP additives that can be blended into them, provide other benefits that prove particularly useful when forming high-strength steels. Along with the neo-synthetic lubricants designed for this type of work, oils provide superior film strength. This allows the lubricants to survive the thinning effect created by excess heat generated during forming or tight-tolerance ironing operations. The added film strength also provides consistent boundary protection that can reduce the need for EP additives, while allowing the lubricant to better adhere to the workpiece during forming stages.
The Role of Synthetics
Synthetics still represent the cleanest choice, and so remain an important lubricant option for many applications. Some synthetics “wet” or “sheet” better on oiled steel than do emulsions. However, compared to oils and emulsions, water-based synthetic lubricants may require additional application within multistation dies to compensate for their reduced film strength and carry-through.
Newly developed synthetics contain phosphorous as an EP additive and polymer additives to enhance their tenacity throughout forming processes. This enhances their performance in the press and helps prevent corrosion on uncoated steel products.
Neat Oils and Oil-Based Emulsions for Coated Steels
Zinc-coated steels require a fresh approach to stamping lubrication. Zinc protects steel in two ways: it acts as a physical barrier to oxygen and moisture and as a sacrificial anode to the iron. For stamping these steels, neat oils provide the best anti-corrosion properties because the oil residue is impervious to moisture, and oils create low conductivity.
The next-best alternative: oil-based emulsions, which can be made water-soluble with additives and emulsifiers. Emulsifiers are polar, with an affinity for water and oil. In effect, microscopic droplets of oil are dispersed and stabilized in water. As a residue on parts, water evaporates and leaves an oily film that protects against oxidation. For use with multi-metal substrates, it is important that the lubricant not contain high levels of alkaline additives.
Synthetic lubricants form solutions in which the additives are dissolved in water. As a residue, most do not create a vapor phase to protect the substrate. Typically these products rely on amine salts for corrosion protection of ferrous alloys. The alkalinity of these salts creates conductivity that can promote galvanic reaction, which accelerates the oxidation of the zinc to form zinc oxide, or white rust. If the residue is tenacious, such as with many synthetics designed for HSS, the oxidizing effect can be exacerbated from the presence of higher film coatings. Where parts are in contact, the presence of water and alkaline salts creates a galvanic reaction in which severe corrosion can occur.
In galvanealled steel, iron is present in the zinc substrate. This alloy-like substrate makes this steel even more susceptible to galvanic reaction. Eventually, zinc oxide can react with the soap complexes in synthetics to form an insoluble zinc stearate that bonds with the substrate. In this condition, it is difficult to physically remove the corrosion with Scotch Brite pads. If removed, pitting or staining to the base metal remains.
Stampers can select from a number of lubrication options to avoid corrosion concerns when stamping zinc-coated steel. Most European automotive OEMs ask their suppliers to use neat oils when stamping parts. Volkswagen, for example, mandates that suppliers use oil (with one exception, a Fuchs emulsion called Montgomery DB 4265C); BMW requires the use of oil in all new-model stampings.
Neat-oil stamping lubricants still dominate the European and Asian markets. The trend toward oil is driven by the need to eliminate corrosion, especially on zinc-coated steel. Thixotropic prelubes and hot melts are becoming more readily available. These products contain levels of microwax that prevent oil migration on coils or blanks. As a result, the coating is uniformly light and coils do not seep oil. These high-lubricity coatings usually do not require lubricant application at the press. In more difficult operations, compatible oils known as spot lubricants can be applied strategically for ancillary protection.
North American stamping companies have traditionally applied lubricant at the press. Here, new light-viscosity oils are available that are approved by most OEMs. These products provide high-performance lubricity and can be applied through traditional methods. While disposal can be more expensive than with previously developed lubricants, typically the volume of disposed lubricant is substantially reduced when compared to the use of water-miscible lubricants.
The characteristic film strength and tenacity of neat oils usually requires less-frequent and lower-volume application, and many formulations are designed to not drip from parts or presses. They eliminate any potential reactions with hard water and will not develop microbial issues. The driving feature is their ability to protect bare and zinc-coated steel from corrosion.
Tight Nestling, and Other Ways to Prevent Puddling
Historically, North American stampers use water-miscible lubricants. Here, costs can be substantially reduced by diluting these products with water. Whether the oil is derived from petroleum or plants, many of these emulsions provide substantial anti-corrosion protection. This characteristic can be increased by stacking parts in a manner that avoids lubricant puddling, or tight nestling.
Better yet, work to optimize application to avoid excessive residue. This strategy proves most important when working with synthetic lubricants. Stampers must avoid saturating parts when using these water-based products. Galvanealled steel is most prone to corrosion in this condition, but even galvanized and bare steel can corrode when overly wetted with synthetics.
Some synthetic lubricants have reduced alkalinity, and may be designed with poor wetting characteristics so that lubricant beads off the part. These conditions reduce the exposure to conductive films and improve compatibility with zinc-coated steel. However, alkalinity in synthetic lubricants provides the primary anti-corrosion properties on ferrous metal. Also, poor wetting characteristics may require excessive reapplication or may lack the film presence required for severe stampings.A select few synthetic lubricants provide synthetic vapor barriers or may provide controlled passivation of zinc. These products reduce the risk of corrosion, but none outperform neat oils or even some emulsions. MF
See also: Fuchs Lubricants Co.
Related Enterprise Zones: Lubrication
Janee - There are a number of factors that need to be considered to properly answer your question. For example, what is the storage duration and condition? What is the shape of the part and is there welding prior to storage. Does your deScription refer to a die punch or is this a piercing operation? Assuming significant forming is involved, I would bet that an emulsion with dispersed and soluble additives would work. My article refers to such a product approved by many OEMs. In a piercing operation of flat stacked material I would recommend a low viscosity neat oil similar to those used in blankwashing. Ultimately you should have a lubricant supplier assess your operations to give the best answer. Certainly Fuchs is at your avail.
What lubricant would be best suited for punching/heavily working galvanized steel, to prevent white rust and staining? Thanks!
While the author brings up some interesting points, Metalstampers should be aware that Ohio State University did extensive testing on galvanized AHSS and concluded that synthetic ETP (extreme pressure polymer) additives outperformed Chlorine which is banned in a growing list of countries. For a full report on this topic visit http://www.irmco.com/news.asp?i=33