2014 STLE Annual Meeting & Exhibition May 18 21 2014 Disney s Contemporary Resort Lake Buena Vista, Florida, USA THE TRIBOLOGICAL EFFECTS OF METAL REMOVAL FLUIDS DILUTED WITH PURIFIED WATER VERSUS HARD WATER Session 2K Metalworking II Authors and Institutions: John Burke Houghton International, Valley Forge, PA USA Alan Cross Houghton International, Valley Forge, PA USA Valarie Pearson Houghton International, Valley Forge, PA USA Introduction: Metal Removal Fluids (MRFs) are generally diluted with water to form emulsions, dispersions, true solutions or combinations thereof. The amount of water added to these solutions varies from 80% to 97% by volume. Water is added to MRFs to improve cooling properties, insure proper dilution to reach the point of application, and thus provide economic value to the end user in the manufacturing process. As MRFs are reused over and over to minimize waste and because the water phase is generally the largest volume product in the fluid in use, certain contaminants in the water can interfere with the MRF properties, therefore lessening the useful life of that fluid. The implementation of the United States Environmental Protection Agency Clean Water Act of 1970 placed significant restriction on the disposal of oil and grease. The Clean Water Act was an additional incentive to end users to extend the life of MRF to minimize the impact of disposal restrictions. In the early 1970 s fluids with more stability were created. These fluids easily doubled their useful life in single sump and central systems. However, the chemists pushed the stability to higher and higher levels where now the water impurities began to dominate reactions with negative results. The water impurities become more of an issue when the MRF is used for significant extended periods (years). This is due to evaporation of the water phase in the system, thus leaving the dissolved salts behind. Now the use of purified water, instead of ordinary tap water is more popular. The first attempts were generally through the use of de-ionized (DI) water where most of the cations and anions in the water are removed by the process referred to as ion exchange. As technologies advanced, the use of water purified by reverse osmosis (RO) gained in popularity as the cost of RO systems became more competitive against DI. RO uses less hazardous chemicals during membrane cleaning versus the DI regeneration process. 1,2,3 Now, in the modern manufacturing environment the use of RO water is much more common than in the 1970 s. As part of the STLE metal working education course, pure water is advocated for water mixable fluids to extend the life of such fluids. However, in the education course there is a precaution that in some cases the use of purified water may lessen tool life in some applications. Testing was conducted to see if the use of water purified by RO had reduced tool life effects on various type of MRF and on various metals. Field observations indicate that systems charged with purified water occasionally see drill, reamer, or tap breakage until the systems harden up or the emulsions loosen up due to ingress of metals being machined, tramp oil, and other chemical reactions on the fluids. Refer to the theoretical chart below:
Experimental For the purpose of this testing, the water was prepared by taking tap water supplied from the Audubon Water Company in Norristown, Pennsylvania, softening the water by removing the calcium, magnesium and iron ions with a commercial sodium ion exchange softener and then processing the water through a RO system. The water hardness before treatment was between 300 and 600 mg/l as CaCO 3 and after the two stages of treatment, the hardness was less than 1 mg/l as CaCO 3. The specific conductance of the water after treatment was less than 20 µs/cm. The purified water was then artificially hardened by adding calcium chloride hexahydrate and magnesium chloride to make a 1,000 mg/l hardness stock solution. Portions of this stock solution were then diluted with RO water to create the 350 mg/l hardness and 700 mg/l hardness levels for testing. In this water preparation process, the hardness can be duplicated in any laboratory since the water was standardized with known chemical reagents of calcium chloride hexahydrate and magnesium chloride. Fluids chosen were: a basic emulsified oil, a premium emulsified oil, two separate solution type synthetic fluids, and a vegetable oil emulsion. For the purpose of this testing, all fluids were diluted at 5% v/v or typically 50 ml of product and 950 ml of water. Fluids were mixed in a low speed stirrer (100 rpm) and allowed to stand for 4 hours before applied to the testing instrument. All mixtures were applied to the test instrument at 21 C (70 F). The basic emulsified oil was not considered to be hard water stable, the premium emulsified oil and the vegetable oil were considered safe to 800 mg/l hardness and the two synthetics fluids are considered stable to 1,000 mg/l hardness Metals to be machined were mild steel AISI 1018, cast aluminum grade 356, aluminum grade 6016, and G2 cast iron. The machining operation was tapping of precision drilled and reamed holes supplied by YMW. Taps used were designed specifically for the metal type (ferrous and non-ferrous) being tapped. The type of tap used was a form tap and not a cutting tap. Based on previous studies, the form tap produces more reliable data than that of the cutting tap. The tap diameter was 6 mm and the tap revolutions per minute were varied from 400 RPM to 900 RPM, depending on the metal specimen being tapped. Tool friction was determined by measuring and recording the average amount of torque required to tap the holes in the metal specimen. The instrument used was a Microtap brand "LabTap" instrument. Average torque values are reported in Newton-centimeters (Ncm). The metal test specimens were specifically designed and drilled for form tapping. Procedure: Three separate fluid hardness levels were chosen for each fluid group. There were: zero hardness, 350 mg/l and 700 mg/l. Three holes were tapped per fluid, per metal, and per hardness. After each hole was tapped, the tap was removed, cleaned with a soft nylon bristle brush to remove chips and wear debris and then cleaned with isopropanol. The tap was then dried with compressed air and reinstalled on the Microtap instrument.
Results: 6061 Aluminum 300.0 250.0 200.0 150.0 100.0 50.0 0.0 Water hardness, mg/l 6061 Aluminum 28.7% 37.6% 13.3% 37.7% 0.7% 4.5% 7.6% 20.1% -2.0% -2.5% Note that a lower value indicated lower tap torque and thus improved lubricity at the point of cut. On this graph for 6061 aluminum, the two synthetic fluids were least affected by the increase of water hardness whereas the oil emulsion showed improved lubricity as the water hardness increased. 300 250 356 Aluminum Microtap Torqu, Ncm 200 150 100 50 0 356 Aluminum 35.9% 42.1% 14.7% 37.7% 5.9% 5.2% 42.9% 43.6% -2.3% -0.9%
On this chart, the result are similar to the chart for the 6061 aluminum whereas the oil and vegetable products all improved in lubricity whereas the ultra-stable synthetic fluid did not improve as harness increased. 350 300 1018 Steel 250 200 150 100 50 0 1018 Steel 11.8% 33.1% 4.8% 5.4% 5.5% 2.7% 1.7% 50.6% 7.9% 7.4% The ANSI steel 1018 chart indicated slightly different values from the aluminum data. In this case the basic oil emulsion and the two synthetic fluids did not improve or decline in torque as the hardness increased. But the premium emulsified oil and the vegetable oil emulsion did improve and hardness increased. 180 Cast Iron (G2 Dura) 175 170 165 160 155 G2 Cast Iron 1.7% 0.0% -0.6% 1.2% 3.4% 2.8% 6.4% 6.4% -0.6% 0.0%
The cast iron chart shows a much different trend pattern than the other metals. There is very little change with changes in hardness. The vegetable oil emulsion shows an improvement with increased hardness; however the percent reduction is not nearly as great as the other metals show. Conclusions: This testing indicated that fluid chemistry can have a dramatic effect on the torque properties of the fluid. The results of the mineral oil and vegetable oil products on aluminum are somewhat the same as what is observed in the field. That is, lubricity improved with time, thus torque values decrease as hardness increases. In the steel tapping data, only the premium emulsion and vegetable oil emulsion improved as hardness increased. This could be due to the increased lubricity demands of tapping the 1018 steel and the fact that the basic oil emulsion had no added lubricity components. The authors acknowledge that this testing is very basic and only involved five fluid types and four metals and a range of harness from 1 to 700 mg/l. The authors further acknowledge that high water hardness can have a devastating effect on filtering the fluids and loss of fluid to chips or swarf that can lead to selective depletion of certain additives. This testing also ignored to the inclusion of foam or entrained air which can happen with the use of very low hardness water. This testing also intentionally ignored the effects of tramp oil increases or the effects of bacterial contamination. This testing then confirms that the use of low hardness water can negatively affect tool life in tapping operations. The addition of hardness by the addition of calcium and/or magnesium salts could improve tool life with certain fluids. It cannot be universally concluded that low hardness water is absolutely beneficial for all machining operations. The takeaway is that MRF formulators need to understand the machining operations and fluid chemistry to best suit the ideal water for that application. References: 1. Burke, J. 2014 Understanding and Controlling Metalworking Fluid Failure, STLE Education Couse 2014 2. Burke, J. 2008 Metal Worked and Particle Size Considerations, STLE Education Course 2009 3. Burke, J. 2012 The Effects of Water Quality on Metalworking Fluids and Manufacturing Processes, STLE Annual Meeting 2012 Keywords: Water hardness, reverse osmosis, tool life, tapping