Table of Contents
18.104.22.168.1 The Influence of Cooling Lubricants
Cooling lubricants are used during machining and forming processes in order to ensure the highest possible production output at low costs with optimal integrity of the work surface. The following focuses on cooling lubricants used in machining (material removal). Similar conditions apply to forming processes, but they differ in some aspects, such as thermal conductivity.
Cooling lubricants can be categorized as follows (Refs. 22.214.171.124-23 and 126.96.36.199-5):
- Those containing petroleum with more than 60% mineral oil in the concentrate
- Semi-synthetic with 5-60% mineral oil in the concentrate
- Mineral oil-free synthetics
- Salt solutions
- Grinding oils
The cooling lubricant has a decisive influence on the machining process and its environment (Fig. "Risks from coolong lubricants"). The following deals especially with the positive qualities of cooling lubricants. If these are not utilized, problems can be expected (Fig. "Boring tool fracture identified by eddy current").
Lubricating effect: The goal is to minimize the friction in the mixed friction area between the tool, part, and chips. This eases the cutting process and minimizes the resulting friction energy, i.e. heat development. In order to attain a low coefficient of friction, cooling lubricant concentrates are mixed with extreme pressure additives (EP additives). These react with the metallic contact surfaces under the conditions of mixed friction. These chemical reactions are supported by high temperatures and the shear stress on the fluid. This results in the formation of reaction coatings that act as dry lubricants. Conventional extreme pressure additives are organic sulfur, chlorine, and phosphorus compounds (Refs. 188.8.131.52-22 and 184.108.40.206-28). These can form metal sulfides, chlorides, or phosphates. The reaction with the chips is especially intensive due to their small volume and relatively large reactive surface areas. This effect removes the EP additives from the cooling lubricant, lowering their concentration. In contrast, evaporation of the cooling lubricant has the opposite effect. For this reason, testing of the EP concentration must be done in time intervals that are suitable for the machining process and machining performance.
Lower friction means less friction forces and friction heat in the machining process, which has several advantages:
- Less heat development in the tools and parts
- Less tool wear, i.e. longer tool life with constant dimensional accuracy and surface quality (roughness, residual stresses, hardening)
- Less deformation through heat strain and cutting forces
- Lower required machine performance
- Less heat to be removed
Cooling/ heat removal from the machining process: The cooling effect of a cooling lubricant depends on its thermal capacity, thermal conductivity, and heat of vaporization. The relatively low vaporization temperature of water at about 100°C has an additional positive effect on cooling. This property is the reason why water cools with the greatest intensity. Mineral oils and mineral oils with additives and emulsifiers are considerably less effective in this regard. One possible advantage of oils/grinding oils is superior wettability. In general: small surface tension = good wettability = good cooling (Ref. 220.127.116.11-23). In watery emulsions, the water has the most intensive cooling effect. For this reason, from the point of view of cooling, the water content in cooling lubricants should be relatively high. However, if it is possible to lower the heat development considerably through good lubrication, grinding with oils and EP
additives, for example, may have advantages over watery emulsions in spite of their less positive physical properties.
Reduced foam formation: Foam formation is highly dependent on water quality. Soft water promotes foam formation. If there is too much foam formation, silicon-based (silicon oil) anti-foaming agents are sometimes used. However, the use of these defoamers may be very problematic with regard to penetrant testing (Ills. 18.104.22.168-13, 22.214.171.124-3 and 17.3.1-8).
Figure "Problems with cooling lubticants": The continuous text on page 126.96.36.199-28 already explains important positive qualities of cooling lubricants. For this reason, the following text focuses on the problematic aspects.
The primary concern is the safety of the parts. From this point of view, the most important are the influences on the strength properties, especially dynamic strength. Temperatures and plastic deformations during the cutting process depend especially on the lubricating and cooling properties of the cooling lubricants, as well as the material. This affects, among other aspects, the temperature reached by the part during cutting, and the wear on the tool. There is also an influence on undesired tension residual stresses in the part, possible strength losses (e.g. solution annealing in Ni alloys, annealing in heat-treated steel), and embrittlement (e.g. due to oxygen absorption in titanium alloys). Chipping processes with defined cutting edges can overstress the surface and tear it open. This can be observed, for example, near chatter marks (Fig. "‘Plucking’ damages by a machining process") and as comma cracks (Ill. 188.8.131.52-2) in nickel alloys. Grinding processes with insufficient cooling lubrication are especially prone to unallowable heat development, tension residual stresses, and crack initiation (hot cracking, Fig. "Chipping surface metallographic section") in the machined part.
The surface topography of the part, including roughness, grooves, and chatter marks (Fig. "‘Plucking’ damages by a machining process"), affects its dynamic strength. These factors are clearly influenced by the cooling lubricants. During machining with undefined cutting edges, primarily grinding, the depth of roughness and shape of grooves (Ills. 184.108.40.206-1 and 220.127.116.11-7), and therefore also the dynamic strength, are determined mainly by the material hardness and the grains on the grinding disk. The purity of the cooling lubricant, i.e. the removal of the chips, is also important. Residual grinding grains in the inflowing cooling lubricant jet can create small marks (peppering, not to be confused with comma cracks, Ref. 18.104.22.168-23). In order to prevent this, the cooling lubricant must be continuously suitably cleaned during its circulation in the machine. The lower the viscosity of the cooling lubricant, the more effective this process is. In this regard, a more viscous grinding oil has drawbacks.
Tool wear influences the dimensional accuracy and also the machining costs through shortened tool life (also see Fig. "Risks from coolong lubricants").
Corrosive attack on the part can already occur during the machining process (Ref. 22.214.171.124-23). Corrosion can be caused or accelerated by cooling lubricants in various ways.
The first of these is an attack on the part occurring when it is stored for a longer period with aggressive cooling lubricant residue. Depending on the material/material conditions (e.g. sensitized), any element formation, and residual stresses, this attack may be laminar, selective (grain boundaries), pitting corrosion, or even stress corrosion cracking (Volume 1, Chapters 126.96.36.199 and 188.8.131.52).
Synthetic and semi-synthetic cooling lubricants are especially capable of damaging lacquers on raw part surfaces. This effect can be increased by an increase in concentration due to evaporation.
There is also a danger of damage to synthetics, as well as insulating and sealing materials of the machine or parts. In parts with organic components (abradable coatings, lacquers, sealants, adhesive connections), there may be an increased damage risk due to the above effects within the framework of repairs and reconditioning. If the cooling lubricant acts as an electrolyte, in which case a pH value of greater than 8 can certainly be expected, element formation can have a corrosive effect. It is possible that light metals with (threaded) inserts made of steel (e.g. housings made from Al and Mg alloys) may be attacked. Corrosion around brass parts in machining equipment is a part of this problem.
Damages can also originate from fouling resulting from remnants of the cooling lubricant or reaction layers.
These types of fouling are of considerable, yet frequently underestimated, importance for the behavior of the part in subsequent finishing and quality assurance processes, as well as in later operation.
If the part is heated after machining, such as during heat treatment or welding, cooling lubricant residue and/or reaction layers (especially with sulfur and chlorine) can cause intercrystalline attack and/or stress corrosion cracking (Fig. "Stress corrosion cracking by process baths and hand sweat"). In Ni alloys, sulfur accumulation can lead to sulfidation. This can compromise other processes (such as coating application) and/or act as premature damage that shortens the operating life of the part.
Especially dangerous fouling is residue of Cl compounds on titanium alloys. If the temperatures exceed 450°C and there are sufficiently high tension residual stresses, which cannot always be avoided, stress corrosion cracking is to be expected (also see Fig. "Stress corrosion cracking by process baths and hand sweat").
Cooling lubricants containing sulfur cause dark coatings to form on non-ferrous heavy metals, bronzes (e.g. bushing materials), as well as silver. This can lead to jamming in cases with tight tolerances (Ref. 184.108.40.206-23). Unlike metallic silver, silver sulfide has no lubricating effect. Therefore, the tightening torque for threaded connections and the performance of sliding surfaces are worsened. Insufficient pre-stressing of bolts, galling, and catastrophic failures of moving surfaces (e.g. in fuel pumps with axial pistons) are possible results.
A special problem occurs when using silicon as an anti-foaming agent. If silicon residue remains on the parts, this can considerably worsen their wettability. This makes penetrant testing results questionable at best. This type of residue can be transferred to other parts via cleaning baths in which it floats on the surface as a film (Ills. 220.127.116.11-3 and 17.3.1-8). This can have a pronounced effect on the finishing process and additional costs for intensive cleaning.
Figure "Risks from coolong lubricants": Tool life is determined by the wear the parts and chips put on the metallic cutting edge of the tool. Nickel alloys and, especially, titanium alloys are prone to galling (Ref. 18.104.22.168-24) and are therefore highly wear-intensive. The wear is also dependent on the cutting speed and infeed pressure (top right diagram, Ref. 22.214.171.124-2). There appears to be an optimal median value for the infeed. In order to attain the highest possible cutting speed with correspondingly high cutting performance with acceptably long tool life, there are frequent reports in technical
literature recommending the use of cooling lubricants with additives containing chlorine (top left diagram, Refs. 126.96.36.199-2 and 188.8.131.52-24). As described earlier, under cutting conditions, these additives form reaction layers (bottom right diagram, Ref. 184.108.40.206-21, Ill. 220.127.116.11.1-10) that act as dry lubricants and minimize wear. Due to the danger of stress corrosion cracking during subsequent heating of the part (Ref. 18.104.22.168-25, Ills. 22.214.171.124-16 and 126.96.36.199-8) and/or during operation, these cooling lubricants should certainly not be used when working with titanium alloys. On the contrary, one must be aware of the very low threshold concentrations (Ref. 188.8.131.52-1, see Note). One must also be cautious when using cooling lubricants containing sulfur. It is possible that sulfuric reaction layers could cause sulfidation during later operation, for example in nickel alloys in connection with subsequent heat treatment. As the incubation period decreases, the operating life is shortened correspondingly.
During grinding, alloyed cooling lubricants, especially grinding oil, can be used to decisively reduce the wear on the grinding disk (bottom left graph, Ref. 184.108.40.206-23). In this situation, as well, it is important to avoid using Cl, S, and other reactive additives that could compromise the operating behavior of the part.
Only approved cooling lubricants are to be used. Changes in products must be approved by the responsible specialist department.
Be especially careful with
- Chlorine content: the Cl content must be below 0.01 %
- Sulfur content
- Silicon additives (anti-foaming agents)
Figure "Cooling lubricant ignition during grinding": In turbine engine construction, explosive materials such as titanium or magnesium alloys are machined using chipping processes. These materials are flammable as chips in air, and explosive in dust form. Collections of dry magnesium chips (chip buildup, ignition temperature above 480°C) can ignite at the machining temperatures. Titanium or magnesium dust can have an explosive, highly exothermic reaction with CO2 or nitrogen to form carbides or nitrides (Ref. 220.127.116.11-27). For this reason, these gases are problematic with regard to use as shielding gas in machine tools. Moisture is also unable to prevent fires. Attempts to extinguish fires with water are dangerous. They can result in the formation of hydrogen, which combined with air, is a highly explosive gas mixture.
An additional possibility for fires and deflagrations is the ignition of the cooling lubricant. This is especially applicable to cooling lubricant mist (cutting oil). The result is deflagrations with pressure increases. The fire danger is determined by the distribution of the oil mist and the ignition temperature. It is independent of the material being machined. However, it can be assumed that titanium sparks pose a high ignition risk due to their pronounced heat development in flight.
The lower the flashpoint of the cooling lubricant (often below 150°C), the easier it is to ignite. Flammable liquids with flashpoints below 100 °C require special safety measures. The flashpoint can be lowered further by an aging process during machining. Once ignition has occurred, even cooling lubricants with considerably higher flashpoints (>200°C) will burn.
Water/cooling lubricant mixtures are only a fire risk above double-digit mix rates of the cooling lubricant.
The most common ignition sources are tool fractures, failure of the cooling lubricant supply, or programming problems with the machine tool.
There are successful strategies for preventing explosions and fires in machine tools (Fig. "Cooling lubricant ignition during grinding", Ref. 18.104.22.168-27).
Figure "Cooling lubricant ignition during grinding" (Ref. 22.214.171.124-27): The potential danger of a machining process causing fires and/or explosions is dependant on several factors (top frame). In addition to the risks of the machined material (Ill. 126.96.36.199-13), the ignition source, and the distribution of flammable material (bottom right frame), the machining process (bottom left frame) also plays a role. The greatest danger is posed by flammable cooling lubricants such as mineral oils or native oils.
The CE mark on the machine states that: “The machine must be designed in such a way that there is no danger of fires, overheating, or explosions from any of the gases, liquids, vapors, dusts, or other substances released or used by the machine itself.”
Conforming to these guidelines only guarantees the minimum safety standards, and there is a residual risk that is tolerated.
The risk of explosions is limited through the use of preventive measures. Beyond this, it is possible to take constructive design measures.
Small fire-extinguishing systems are integrated into machine tools. They react fast enough (milliseconds or seconds) to minimize damages. Venting times for gas extinguishing systems are about 30 seconds in order to prevent reignition.
In order to ensure the fastest reaction (extinguishing) times, it is important to have an automatic detection system integrated into the extinguishing system.
Experience has shown that bench-scale model approaches are not sufficiently realistic.
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