Interaction of chemicals used in forming and fabricating
Knowing how chemicals tend to react with one another and how each type of lubricant and coolant commonly used during steel processing tends to behave is a boon when you're trying to fabricate products properly.
Lubricants, coolants, corrosion inhibitors, cleaners, coatings, fluxes, and other chemicals all are used-and thus intermixed-in metal manufacturing, forming, fabrication, welding, and finishing.
As a result, as manufacturers become more conscious of quality, safety, productivity, and disposal considerations, they scrutinize more closely the chemicals that are introduced throughout the supply chain in an effort to improve the bottom line.
Cross-contamination Right from the Start
The potential for cross-contamination of chemicals through the supply chain begins in the steel production and preparation process, where we start with a hot-rolled sheet product. Dry steel is either coiled dry or acid-pickled, rinsed, dried, and coiled. Coated steel is either acid-pickled, rinsed, dried, coated, and coiled or cold-rolled and coated.
Dry Steel. Uncoated dry coils, as the name implies, should have no surface chemicals and thus should not cause any process contamination.
However, without proper packaging and care, these coils are susceptible to premature oxidation or corrosion in transit, processing, and storage, particularly in wet or humid conditions. Surface oxidation on the coil and edges comes off the coil, which can cause a variety of problems in forming, fabrication, and finishing. These products, therefore, have limited use in specialized applications in which proper care can be ensured.
Coated Steel. Mill oils and other specialty coatings commonly are used for forming and fabricating steels because they provide surface protection during storage and handling. These oils can be described as follows:
- Corrosion-inhibiting Oils. Corrosion-inhibiting oils generally are referred to as mill oils and are the most common type used.
Specialty Lubricants and Coatings. Specialty coatings often are supplied for specific customer needs to provide process compatibility. Usually, only the largest consumers of steel are able to demand this individualized attention. However, process-compatible coatings are gaining interest and popularity.
Thus, most consumers of steel use a sheet product that is coated with mill oil that can and will affect fabrication, finishing, and disposal. In fact, inconsistency in mill oil chemistry and coating thickness can cause as much variability in processing as the steel itself.
Typically, these oils are applied lightly to protect products during shipment and storage. A light coating is applied under good packaging and storage conditions; a much heavier coating normally is used on materials that will be held for long periods of time in less than ideal conditions. Coating type and the method of application provide for a variety of conditions at the point of use.
Oil and Water Do Mix!
Chemists generalize and assume that materials with like chemical structures will. In fact, this is the basic principle that the development and design of most industrial chemicals is based on. Like dissolves like!
Water is a polar molecule; thus, water-miscible liquids contain polar molecules and water-insoluble liquids contain primarily nonpolar molecules. Following this form of logic, water-emulsifiable and water-dispersible compounds contain chemicals that are polar and nonpolar and thus have limited solubility in both water and oil-type liquids.
As such, industrial lubricants, coatings, and cleaners typically are categorized according to their varying degrees of solubility in water:
- Miscible-mixes easily with water in all proportions.
- Emulsifiable-mixes to become reasonably stable in water with proper agitation and water conditions.
- Dispersible-mixes with water, but soon separates without continued agitation.
- Insoluble-does not mix easily with water. Highly insoluble materials that can be chemically modified to provide quick and complete phase separation from water are called demulsifying.
At the one end of the forming and fabrication scheme, mill oils are water-insoluble and even water-demulsifying to provide the best protection against the elements. At the other end of the process, cleaners are water-based as a matter of safety.
Clearly, these two are on opposite ends of the water solubility chart. By integrating chemical technology and process design, we can greatly improve washability of the finished product by removing oils in the process, minimizing surface contamination, or integrating chemicals that improve washability as a part of the process.
A material safety data sheet (MSDS) typically describes the water solubility of a given chemical. The MSDS also identifies materials that will react chemically; however, this typically is not an issue in today's mature industrial work environment. Materials that are insoluble or dispersible only in water must be removed or minimized through the process, if possible.
From Coil to Finished Product
Let's use welded tube as an example of steel moving from coil to finished product. A chart showing the ideal use of chemicals used in the production of a welded steel tube might look similar to that in Figure 1.
|Oils, Coolants, and Coatings for Tube & Pipe|
|Product||Solubility in Water||Special Considerations|
|Mill Oil||Insoluble||Removed by mill coolant and skimmed from top or reservoir.|
|Slitting Oil||Insoluble||Removed by mill coolant and skimmed from top or reservoir.|
|Mill Coolant||Miscible|| Used to remove oils and dirt from metal during the forming, welding, and sizing processes.|
Typically drained and removed after use.
|Cutoff Oil||Emulsifiable|| Welding process turns residual oils into fumes.|
Minimize use; apply to blade only.
|Storage Coating||Emulsifiable|| Welding process turns coating into fumes.|
Displaces water used to remove coolant.
|Saw Coolant||Miscible||Apply to blade only.|
|Mandrel Lubricant||Emulsifiable||Minimize use; apply to tooling only.|
As we can see, at each stage of the process we contend with the chemicals introduced in earlier stages of the supply chain. Removing lubricants or coolants applied in the process or minimizing use of them improves productivity in finishing, minimizes worker contact, and reduces chemical usage. Further, an unknown change in the solubility of a process chemical directly affects the characteristics of the next one used.
Back to the Start
To understand how the amount and type of oil applied to a coil affects it as it moves through a welded tube mill, consider the following scenarios:
- Excessive Quantity of Mill Oil. Excessive use of mill oils causes slipping of the coil as it is fed into the mill. The excessive oil builds up in the forming coolant solution to the point that it creates smoke in the mill welding and fabrication welding processes. Subsequent cleaning of the finished product becomes harder as the amount of insoluble oil left on the finished product increases.
- Mill Oil Made Emulsifiable. Tube mills today typically utilize synthetic coolants that disperse insoluble mill oils from the strip so that they can be skimmed from the top of the coolant tank after separation. Emulsifiable oils do not readily separate from mill coolants and become concentrated in the mill coolant.
Left unnoticed, these oils cause excessive smoke at the weld site, slippage in the mill, and bacteria in the coolant. Many times these synthetic coolants become more likely to disperse small solids that are hard to filter and eventually redeposit on the tube surface. Again, cleaning the finished product becomes harder as the process becomes more contaminated.
As we can see, two simple and seemingly harmless changes in mill oil characteristics directly affect the productivity and safety of the process. With each additional change in process chemistry comes increased unpredictability in the finished product.
Water or Oil
Water-diluted chemicals continue to be used widely in a variety of forming and fabricating operations for a variety of reasons. A few of these are as follows:
- Weld compatibility. Smoke is reduced, flame is eliminated, and welding is more consistent with lower residues with higher flash points than oil type counterparts.
- Ease of removal. Water-soluble residues are easily removed and can reduce loading of wash systems.
- Safety improvements. Extending with water provides a drier work environment with the least operator contact. When properly applied, the water evaporates, leaving only minuscule residues on parts and machinery.
- Economics. Water is virtually free (as compared to oil) and can be plumbed as necessary to minimize freight and handling costs.
These advantages do not come without sacrifices, because water-carried chemistries do not come without faults. Water-diluted materials are much less tolerant of surface contamination at the point of application and their resistance to corrosion from the elements is limited. In addition, water actually can cause corrosion in the presence of certain chemistries, and the dilution process itself can be unpredictable.
With the nearly total elimination of solvent cleaners in many finishing operations, the use of water-based chemistries in forming and fabricating continues to grow. Therefore, eliminating or reducing the use of water-insoluble chemistries throughout the process where possible also will continue to gain momentum.
These combined trends will lead to continued improvements in chemical and application technology, from coil to finished product.