SKILL BUILDER
ALUMINUM HOT ROLLING
7 Key Emulsion Parameters and How to Manage Them
Introduction
The output from a hot rolling process depends on the interactions between the mill hardware, the process parameters, and the rolling fluid
Optimizing the equilibrium between these influencing factors is critical to achieve good surface quality, meet rolled material specification, and to maintain high productivity levels and minimize operating costs.
The emulsions used to lubricate and cool the metal rolling process are subjected to high temperature, pressure and contamination, which can affect the physical and chemical properties of the fluid If not monitored and maintained, the emulsion may not perform optimally, jeopardizing the process equilibrium and output
Here are the 7 key emulsion parameters and how to manage them
1 Concentration
Most emulsions are designed to operate at 2 5-6 0% concentration, depending on the mill type, or as determined by the fluid provider and process team If the concentration drops below this level, there?s a risk of under lubrication that may result in metal sticking to the work rolls, aluminum oxide pick up, and excessive roll coating
If the concentration rises above the specified level, there?s a risk of over lubrication, slip, and slab refusal High oil concentration can often be a sign of contamination, so the source of this should be investigated, e g hydraulic oil Concentration can be reduced by a) pausing regular oil top ups, b) adding water to the system, or in some cases c) a partial dump may be required.Concentration should be measured on a daily basis
2 pH
The pH of a hot rolling fluid should be in the range of 6 0-8 0 depending on the type of chemistry that is used and is indicative of the emulsion stability and lubrication performance The emulsion stability must be at a level to ensure optimal plate-out for lubricant delivery into the roll bite. For soap-based emulsions, a low pH reading signifies a low emulsion stability, and a
7 Key Emulsion Parameters and How to Manage Them
high reading signifies a high emulsion stability.For non-soap-based emulsions, a low pH indicates a high emulsion stability; a high pH indicates a low stability
pH levels can be corrected using additives, however it?s important to investigate potential root causes For example, a low pH can indicate bacterial contamination,whereas a high pH could be the result of a change in water quality or grease contamination
3. Conductivity
Fluid conductivity indicates the level of dissolved salts and water quality, which can impact emulsion stability Conductivity should normally be in the region of 100-800 ?S/ cm If the conductivity is too low, there is a risk of foam formation with soap-based rolling fluids ? this is usually only seen with fresh emulsions and conductivity will increase during rolling
When the conductivity is too high, the stability of the emulsion is decreased, which can lead to higher consumption, more skimming, contamination on the rolled surface, and risk of corrosion This is particularly true for soap-based emulsion systems.If high conductivity correlates to high ash level, it?s likely a result of under-lubrication and metal buildup.If ash levels are normal, then the increased conductivity is the result of an external contaminant In either scenario, a partial dump should be considered
4 Viscosity
Maintaining the specified fluid viscosity is important to achieve consistent lubrication during rolling If the viscosity is too low, the process will be under-lubricated, resulting in high loads, sticking of metal to work rolls, a too-thick roll coating and aluminum oxide pick up However, low viscosity is rare and usually only seen with fresh emulsion It can be corrected by adding a high-viscosity version of the emulsion concentrate
High viscosity is a more common issue, and can lead to over-lubrication, low loads, slip and refusals.If a high viscosity is measured, the reading should first be double checked, before checking for contamination sources such as tramp oil leaks (hydraulic oil leaks are most likely, but high viscosity lubricants such as gear and bearing oils can have a strong effect), as well as examining skimming efficiency Once the root of the issue has been identified, a partial dump should be considered to correct the viscosity
5. Ash Content
Ash levels of the rolling fluid should usually be below 1000 ppm, which corresponds to the inorganic components of the lubricant Other than foaming, there is no risk associated with low ash level, which is usually seen only with fresh emulsion and solved within a short period of time by rolling with the emulsion High ash levels are
ALUMINUM HOT ROLLING
7 Key Emulsion Parameters and How to Manage Them
often caused by metal buildup as a result of under-lubrication and present both risk of contamination on the rolled surface and corrosion Once any lubrication issues are resolved, skimming devices should be checked, and a partial dump considered
6 Additive Levels
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Fatty acids and esters
Lubricity additives such as fatty acids (in soap-based emulsions) and esters become depleted over time during the normal rolling process This results in under-lubrication, with risks of high loads, a too-thick roll coating, pick up and wear of work rolls In the case of low ester or fatty acid levels,check for sources of contamination e g hydraulic oil leak, before adding appropriate additives
If a high level of ester is detected, there is a risk of over-lubrication, low loads, slip and refusals. External contamination from other ester-based lubricants should be checked, as well as calculations and practice of tank-side ester additions.Where high ester levels are negatively impacting on rolling, a partial dump is advised
Free fatty acids will react to form metallic soaps which improve lubricity, however a high level of metallic soaps risks problems from over-lubrication, sludge buildup and deposits that may cause mill corrosion Intensified skimming can reduce metallic soap level
Organic soaps
Organic soaps support plate out,surface wetting and emulsion stability.If levels drop, then
appropriate tank-side additives can be used, and a formulation adjustment can be considered to increase the organic soap level in the emulsion concentrate It?s unusual for the level of organic soaps to be too high as they are depleted in the rolling process, but this would produce an emulsion that is too tight and result in under-lubrication In this scenario a partial dump would be advised
Metal soaps
Metal soaps form during the rolling process and support lubrication.With fresh soap-based emulsion, it may be necessary to adjust the rolling process parameters (e.g.coiling temperature and speed) during the break in period However,if metal soap levels become too high there is increased risk of refusals, slip and contamination on the rolled surface, which may necessitate a partial dump Metal soaps can be efficiently reduced by a combination of sufficient filtration and skimming techniques
7 Particle Size
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The particle size distribution (PSD) is a measure of the median particle size of the emulsion Typically, the D(4,3) value is reported, which considers the volume weighted mean particle size distribution For aluminum hot rolling emulsions, the PSD is usually found in the range of 1-4 ?m, depending on the emulsion type Soap-based emulsions tend to have higher PSD values in comparison to
ALUMINUM HOT ROLLING
7 Key Emulsion Parameters and How to Manage Them
non-soap emulsions.When the PSD is too low for the respective emulsion system, there is risk of under-lubrication, high loads, sticking of metal to work rolls,aluminum oxide pick up and a too thick roll coating.In this scenario, check for external contamination as well as any recent modifications to the mill hardware (pumps, nozzles, spray bars) that may be leading to increased shear forces on the emulsion system
If the PSD is too high, there is risk of over-lubrication, low loads, slip, refusals and change in cooling and wetting behavior Smoke formation may also be observed in some cases.In this instance,check the distribution profile:in case of the appearance of a second peak, or a significant shift of the particle size distribution to higher values, check for any contamination source (such as a hydraulic oil leak) that could be contributing to the increase in concentration.To resolve the issue, consider a partial dump,
increased skimming and tank-side addition of emulsifiers
Conclusion
Through regular monitoring and management of emulsion parameters, optimal rolling conditions will be maintained, helping you achieve high productivity levels and reducing total cost of operations
Quaker Houghton is the global leader in industrial process fluids With many decades of hands-on aluminum rolling experience, our teams support you from commissioning green field mills through to running legacy equipment, and any troubleshooting along the way
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ALUMINUM HOT ROLLING