Lubricants and Lubrication Part 11 pdf

55715.1 Sheet Metal Working Lubricants interactions between solid matter and workpiece surface, on the load during the forming process and on type of solid matter used. As cleaning residues may have an effect on the layer production during the chemical surface treatment and thermochemical processes, even after profound rinsing, only especially developed cleaners are used in order to avoid failures and distur- bances. In addition, the natural character of the basic metal which is to be treated (iron, aluminum, copper, brass, zinc etc.) has also to be taken into consideration. According to the pH value of the alkaline cleaning solutions, a difference is made between: . low-level alkaline cleaners (pH between 7.0 and 10.5), . medium-level alkaline cleaners (pH between 10.5 and 11.5), and . high-level alkaline cleaners (pH above 11.5). Low-level alkaline cleaners are often called neutral cleaners’, as they are characterized by a low tendency to saponify natural and synthetic esters. A standard defini- tion of the term neutral cleaners’ does not exist. Acid cleaners (pH below 7.0) are used for special applications. Apart from the degreasing effect, these cleaners show a caustic effect for the removal of rust, rust flake or other solidly-bonding layers which cannot be removed in alkaline media. Such caustic degreasers consist mainly of hydrochloric acid, sulfurous acid or phos- phorous acid added with acid-resistant tensides. In addition to the degreasing of lubricant films, acid degreasers based on phos- phorous acid are also used to produce a thin iron phosphate layer (approximately 0.1–0.3 lm), thus providing a key for a painting layer, e.g. cataphoretic immersion paintings and/or powder coatings. Application Range for Aqueous Cleaners The decisive criterion in the selection of cleaners is the applied cleaning method: . immersing-bath cleaners (dip cleaners) which are basically emulsifying and . spray cleaners which are basically demulsifying. For special applications there are: . high-pressure cleaners . steam jet cleaners, . machine cleaners. Immersion Cleaning Immersion cleaning is an old cleaning method. The cleaners used here dissolve oils and greases easily and disperse dirt particles very well. Immersion baths are recom- mended especially for small parts or flat parts with large surface areas which may be dipped into the bath in relatively thick layers. They are also suitable when more processes are carried out in immersion baths, e.g. electroplating treatments. Subsequent to the immersion method, the parts are cleaned in a bath solution at high temperatures for a relatively long treatment period. Often, one stage will not be 558 15 Forming Lubricants sufficient and multistage cleaning methods are applied in accordance with the task range. Immersion Cleaning Facilities Standard immersion facilities can be constructed simply. The easiest solution is a heatable container made of sheet iron, e.g. the decoction bath which operates at boiling temperature and is nowadays applied in few cases only. Here, the surging of the boiling solution serves as additional mechanical cleaning support. Modern facilities avoid the disadvantages connected with the decoction’ cleaning: high heat losses, problematic steam swathes, splashes and boiling-over. The cleaning effect can be mechanically supported by circulating the cleaning solution or by flooding during which the solution is re-pumped and pressed into the bath through numerous valves positioned, staggered, opposite each other. As an alternative, the bath is re-filled and drained several times with a flood pump. By using an additionally installed sprinkler system, creamed grease is rinsed off the bath surface into a spillway, thus preventing that the cleaned parts are re-oiled with demulsified oil when taking them out of the bath. In comparison with the decoction’ bath, well circulated baths have the advantage that they ensure a perfect cleaning result at a lower temperatures (60–90 C) and after a shorter period of treatment. Occasionally, the bath solution is circulated using pressurized air instead of the pumping method. Here, cleaners with a particular low foaming tendency are needed. Ultrasound Cleaning Another immersion cleaning method is ultrasound cleaning. In some industries, e.g. in the manufacture of jewelry and silver products, cutlery, optic glasses and devices, as well as in the production of high-precision fine-steel tubes, fittings and many structural components, extremely high requirements are made on the cleanli- ness of the mostly polished surface areas. Residues of the materials used and metal abrasion particles can only be removed completely with great effort because such pigment contamination cling to metal and glass surface very strongly. Mostly, a treatment with aqueous cleaning solutions alone will not be sufficient if it is not supported by additional mechanical actions. In these cases, the use of ultrasound will have more effect than the flooding of the cleaning solution or a spray treatment. Ultrasound cleaning is based on high-frequency sound vibration which is generated with special vibrator systems. The Effects of Ultrasound In the cleaning solution ultrasound is transmitted in longitudinal direction as a longitudinal wave. Due to strong pressure variations, the solution produces numerous small low-pressure bubbles which immediately collapse releasing energy. Here, the micro-roughness of the surface areas and the accumulated particles act as cavita- tion nuclei. This is particularly true for corners, drill holes, decorative embossings and other areas where conventional cleaning methods will not lead to optimal results. 55915.1 Sheet Metal Working Lubricants In clear solutions one can observe after a few seconds, how the dirt particles are removed from the surface areas. This cleaning method is based on the pressure generated by the collapsing bubbles, which can locally exceed 1000 bar. Sound frequencies of between 20 kHz and 40 kHz, as well as operating temperatures of between 50 C and 70 C will lead to the best results. Generally, the minimum cleaning period does not exceed two minutes. The ultrasound generator of a cleaning system should be set so that a level of the sound energy of between 5 and 20 W l –1 solution of the immersion bath is achieved. Spray Cleaning This modern cleaning method is based on one or more coordinated stages. The intensive mechanical effect of the spray jets has the advantage that in comparison with the immersion bath the period of the treatment is considerably shorter and the operating temperature often can be reduced to 40–70 C. Much lower quantities of cleaning solution are needed than in the immersion bath, thus reducing its concentration to 10–30 g l –1 . This means less use of chemi- cals, less consumption of heating energy and lower costs for waste-water processing and disposal. Spray-cleaning Facilities Spray cleaning facilities have a complex construction. As a result of this, they stand out due to several important advantages with their closed type of construction. Even at high temperatures, the suction and condensation systems ensure that no disturb- ing water steam will release into the workshop hall. Decisive for the intensive cleaning effect are the shape and the order of the nozzles which are positioned in nozzle connections so that during the cleaning the parts are sprayed from all directions, thus avoiding any non-sprayed zones. Nor- mally, the spray pressure is between 0.5 and 10 bar. It will, however, be considerably higher (up to 500 bar) during special cleaning procedures, especially during debur- ring and high-pressure cleaning. There are one-chamber spraying machines, in which the parts to be cleaned are sprayed from all directions on a flexible grid, as well as multi-chamber continuous spraying facilities. By using these facilities, cleaning, rinsing, pickling and passivating processes can be combined with the drying and applying of anticorrosive agents in a complete pretreatment procedure. The main advantages are a higher throughput than in the immersion bath and an improved process safety. The number and length of the individual chambers of continuos spraying facilities have to be adjusted to the respective pretreatment problem. The type of nozzle depends on the function of the various chambers. Flat-jet nozzles are used in cleaning, degreasing and pickling chambers, whereas hollow cone nozzles are used in rinsing, passivating and neutralization chambers. The number of nozzles and their order is predominantly determined according to the shape and size of the parts to be treated. 560 15 Forming Lubricants The most important prerequisite for the perfect operation of spray cleaning facilities is the use of quickly emulsifying cleaners. By skimming or filtering, it can be ensured that a clean’ cleaning solution is available over a longer period of time. Products for Spray Cleaning Spray cleaning requires a different construction of the cleaners than in immersion cleaning. The most important characteristics is the different foaming behavior: spray cleaners contain demulsifying tensides with low foaming tendency. It must be observed that the foaming behavior depends on the temperature, i.e. minimum operating temperature has to be reached in order to ensure the self-defoaming effect. This means that the water in the spray cleaners has to be heated while prepar- ing the cleaning solution, prior to adding the cleaner concentrate. The temperature has to be kept on the same level also during down-times to avoid unwelcome foaming during the restarting process. Manual Cleaning, Cleaning with Steam Jet and High-pressureMachines using Aqueous Cleaners The Cleaning Table In many workshops and smaller manufacturing companies, metal cleaning is still carried out manually using a brush. Today, however, several developments have been introduced to improve and facilitate this manual work. Instead of using the petroleum tub with brush and cleaning cloth, a cleaning table with a hose-pipe brush for the cleaning solution is used. In many cases, the petroleum can be replaced by high-performance aqueous cleaners which are often better suited for the cleaning of repair parts. Manual Sprayers Often, a spraying process is also used, e.g. in the cleaning of vehicle engines or facilities and machines which can not be transported. The cleaning solution is sprayed onto the surface and allowed to work in. Higher contaminated surface areas are treated with the undiluted cleaning concentrate. Then the dirt particles are sprayed off with a strong water jet. The surface areas are dried in the air. The drying process is accelerated by blowing pressurized air on the surface. Steam Jet or High-pressure Machines Manual cleaning can also be carried out using steam jet or high-pressure cleaning machines which will normally not work with solvent cleaners but with aqueous cleaning concentrates. The cleaning concentrate is mixed with hot steam or water. This mixture is sprayed onto the respective surface under high pressure. The temperature and jet pressure can be adjusted to the individual application. Cleaning with Organic Solvents The solvent cleaners mainly consist of chloro- and chlorofluorohydrocarbons, and hydrocarbons and solvent cleaners. 56115.1 Sheet Metal Working Lubricants Chloro- and Chlorofluorohydrocarbons In recent years, the use of organic solvents, especially halogenated hydrocarbons, was made more difficult because of legal measures covering work place protection (Chapter 9). Nevertheless, higher emissions and the immediate exposure of the staff can be avoided with the introduction of corresponding technical measures. The most important substances in this group are trichloroethylene (trichloro- ethene), perchloroethylene (tetrachloroethene) methylene chloride (dichloro- methane), 1,1,1-trichloroethane and, the chlorofluorohydrocarbons R 11 and R 113, which are also used as refrigerants. The most important criteria for their use are their high ability to dissolve polar substances, the quick drying of the degreased parts, the avoidance of fire hazard in contrast to pure hydrocarbons and the possibi- lity of regeneration by means of distillation. Distillation methods make it possible to separate foreign components. However, it has to be taken into consideration that the boiling point of the compound will increase due to the dissolved lubricant components and that an accelerated decomposition process, accompanied by the formation of hydrogen chloride, starts at about 120 C for trichloroethylene and at 150 C for perchloroethylene. The solvent decomposition accompanied by the formation of hydrogen chloride is one of the largest problems in the area of cleaning with chlorohydrocarbons. This decomposition can also start at temperatures which are clearly below the mentioned figures and can be catalytically accelerated by foreign matter, e.g. water. Conven- tional solvents for metal cleaning contain effective stabilizers in order to avoid this decomposition which is accompanied by a corrosion process. Therefore, it is important during distillation that the stabilizers mix with the distillate and do not remain in the sump. Normally, however, inhibitors are added for re-stabilizing purposes, thus neutralizing produced hydrogen chloride which has already been produced. A routine test of the halogenated solvents to determine the acid index, the pH of the aqueous extract or the chloride content is useful in order to safely avoid corrosion problems. Test kits are often used for operational monitoring. Hydrocarbons and Solvent Cleaners White spirit, petroleum and isoparaffins are the hydrocarbons most used in metal cleaning. The comparably low prices and ease of disposal are advantageous, however, when using these substances the fire hazard has also to be taken into account. Application Range of Solvent Cleaners The degreasing of pigmented forming-lubricants using solvent cleaners generally constitutes a problem. After dissolving away the lubricant film’s soluble organic substances, the solid matter lubricant remains on the surface (see aqueous degreasing). Normally, solvents are used at room temperature or in closed systems at the boiling point of the solvent–the so-called vapor phase degreasing. The vapor phase, in which the part to be cleaned is immersed, is produced above the boiling solvent bath. It then condenses highly pure solvent matter on the cold workpiece until the temperatures are the same. The condensed solvent rinses the oil film off the workpiece, which will either dry quickly or be immersed for short-term intermediate pre- 562 15 Forming Lubricants servation in a solvent bath to which 1 to 5 % of a corrosion protection concentrate is added. Solvent cleaners are a product group of their own. They are predominantly used in the cleaning of heavily contaminated or oiled repair parts, e.g. engines, gearing systems and major machines (locomotives). These products consist mainly of hydrocarbons with special additives. They are inflammable. As a product group, solvent cleaners are related to solvents. They consist of hydro- carbon mixtures or synthetic hydrocarbons and contain further additives in accordance with the respective application, e.g. to improve the corrosion protection or washability. Today, solvent cleaners which do not contain aromatic components have proven their suitability in many applications. Solvent cleaners are used either in immersion baths or to spray the parts to be cleaned. According to the composition, they can be used as pure concentrate or diluted with petroleum or, some of them, with water. After it has been allowed to react for a certain time, the solvent cleaner is carefully sprayed off with a strong water jet. When so-called separating solvent cleaners are used, the emulsion generated during water spraying will split very quickly into an oil phase and a water phase so that the oil components are safely retained in the oil separator. Emulsion Cleaners The concentrates are to some extent analogous to water-miscible cutting fluids and consist of mineral oil, synthetic hydrocarbons and other organic fluids together with emulsifying agents and anti-corrosive compounds. They are mostly used in a concentration of 2 % in water. They do not lead to a cleaning level up to water wettabil- ity, which makes them suitable for the cleaning between two operations. Suitable water-mixed coolants with a low foaming behavior are often also used for this pur- pose. The residual film protects iron materials against corrosion. Maintenance of Cleaning Baths For reasons of cost minimization, the industry focuses more and more on the service life of cleaning baths. In solvent cleaners, the solvent can be retained and kept fresh through distillation, as already mentioned. More effort is made for the maintenance in case of aqueous cleaning procedures. Apart from the development of, for example, special spray cleaners with a very good demulsifying behavior, physical methods for the prolongation of the service life of cleaner solutions are also used. These include oil separators, skimmer, filtra- tion systems or centrifuges for solid matter separation and microfiltration. The use of these methods not only ensures the separation of introduced contamination but will also sometimes cause the cleaner’s components to be separated which then have to be re-dosed. The ultrafiltration method facilitates the separation of most organic components from the aqueous phase. Inorganic salts reach the permeate. Thus, this method is also suitable for waste water treatment. 56315.1 Sheet Metal Working Lubricants When vacuum-distilling cleaner solutions (e.g. in thin film vaporizers), only the aqueous phase is regained. The non-distilled residue contains the components of the cleaner and the contamination, including the salts. 15.1.11 Testing Tribological Characteristics Various methods are used to evaluate the deep drawing performance of lubricants. The principals are described in Section 15.1.3. The most common is the strip drawing test which is performed with and without deflection. This method simulates the conditions found under the blank holder and during the flow of material into the die. To examine wear, abrasion and the special behavior ofmetal coatings (powdering, cracking, smooth- ing effects) methods with strip deflection, e.g. draw-bead test or Erichsen testare used. In the straight strip drawing test, a strip of metal is drawn between two flat dies. The drawing force required is measured against the normal force to determine the coefficient of friction. In addition, the time is also recorded. The most important measuring parameters are tool and material dimensions, drawing speed and the maximum drawing force applicable. Apart from the coefficient of friction, the uni- formity of the force time-frame is an important criteria for evaluating the tribosys- tem. Ideally, the material flow between the tool is uniform and free of stick–slip. Finally, the maximum normal force possible until the strip jams is a measure of a lubricant’s ability to separate the sheet and the tool under pressure. Evaluation of such data allows lubricants to be developed which are ideal for metal forming operations. An example for a strip testing equipment is shown in Fig. 15.18. The test stand used by the Institute for Production and Forming Technology of the Univer- sity of Darmstadt, Germany, enables tribological systems to be examined with large tool dimensions, variable drawing speeds and very high normal contact pressures. The various stations are de-coiling, cleaning and oiling. Tools are equipped with sen- sors to measure forces. Finally, the strip metal is coiled again [15.26]. Fig. 15.18 Strip-drawing tester and tools [15.5, 15.6]. 564 15 Forming Lubricants Figure 15.19 shows the results obtained from uncoated cold-rolled steel. The strip drawing test without deflection is reported on at all times. The lubricant is a mill- applied corrosion prevention oil. The forces measured are recorded against time. According to the friction law the frictional force increases with higher loads. From 5Nmm –2 onwards stick–slip occurs. The coefficient of friction as a function of the load is outlined in Fig. 15.20 where the drawing speed is varied. The coefficient of friction falls with increasing speed. The begin of stick–slip is marked. The curves end with the maximum load, when the strip jams and further drawing would cause the metal to tear. As a rule, lubricants with a higher viscosity have better lubricity. To distinguish the effect due to pure rheology from that caused by additives, some lubricants with identical formula but with differing viscosities are tested. The result is shown in Fig. 15.21 with a corrosion preventive oil as example. The viscosity is chosen Fig. 15.19 Frictional force vs time withload as parameter, mill oil/uncoated sheet metal/50 mm s –1 . Fig. 15.20 Friction coefficient vs loadwith sliding velocity as parameter, mill oil/uncoated sheetmetal. 56515.1 Sheet Metal Working Lubricants between 15 and 150 mm 2 s –1 at 40 C to cover the most common applications from blank washing to drawing oils applied by spraying. 15.1.12 Sheet Metal Forming in Automobile Manufacturing The pressing of car body parts is one of the most important sheet metal drawing processes. The corrosion protection oil applied by the steel mills plays a part in every sheet metal forming operation and also makes up the largest proportion of lubricants used. The typical part transfer sequence from the sheet metal to the body- in-white is described in Fig. 15.22. At every stage where oil is applied to the metal surface a full compatibility with upstream and downstream processes is necessary for a high production reliability at minimum overall costs. The single steps of production are discussed in the special sections below [15.27, 15.28]. 15.1.12.1 Prelubes The idea to combine the corrosion protection properties of a corrosion preventive oil with the lubricity of a drawing oil led to the development of the prelubes. Prelubes have existed on the American market for more than 20 years, they were introduced in Europe in the early 90s. Applied at the finishing lines of the steel mills, they serve as the forming lubricant in the press shops. As for the corrosion protection oils, a prerequisite for the suitability of a prelube oil is the absolute compatibility with all single processes from the coil to the body-in-white. The use of prelubes in steel mills reduces the number and quantity of spot lubricants for additional press shop oiling dramatically. But their true benefits can only be fully achieved if the compatibility principle is applied throughout the manufacturing chain. Therefore, modern prelubes systems are modular, even different viscosities can be part of the same con- cept. This results in a far-reaching multi-functionality for all applications. Fig. 15.21 Friction coefficient vs. load with oil viscosity as parameter, mill oil/uncoated sheet metal/50 mm s –1 . 566 15 Forming Lubricants Among the many specifications for mill oils, the comprehensive standard of the Association of German Automobile Manufacturers (VDA) [15.29, 15.30] describes all relevant requirements of a mill-applied lubricant, defines all important properties and lists all test procedures. The results –with the exception of lubricity –are com- pared with the standard corrosion preventive oil Anticorit RP 4107 S. According to these VDA requirements a prelube needs to be thixotropic for a reduced run-off and is applicable by electrostatic spraying. Of course it should protect the sheet metal against corrosion as effective as the standard oil. Precipitation or other chemical reactions must not occur when mixed with the standard oil. Furthermore, the pre- Fig. 15.22 Process line. [...]... in the case of the slip-type drawing process has outstanding economic significance Typical lubricants and their application are also used for drawing pipe and profile and as a result are to be studied more closely in the following Friction and Lubrication, Machines and Tools when Wire Drawing In no other forming process the lubrication, the machine and tool technology are so closely associated with each... lubricant to lubricant and depends, on the one hand, on the type and volume of electrolytes and, on the other hand, on the composition and the ingredients contained in the lubricant Experience has shown that bivalent cations (mainly earth alkalis) destabilize more strongly than the mono- 15.2 Lubricants for Wire, Tube, and Profile Drawing valent alkalis Where high electrolyte load and high emulsion temperature... between dry and wet drawing, immersion and extrusion drawing machines and drawing dies for dry and wet drawing; three different types of lubrication technology are applied: 15.2.1.2 Dry drawing: (in wire drawing this term does not mean the absence of lubricant but the use of solid, not liquid lubricants) with dry drawing soaps in the drawing box These lubricants only lubricate the drawing die and hardly... both, rod with non-circular cross section and pipe, the basic knowledge of friction, lubrication and wear is employed for better defined round cross sections In the case of wire and profile drawing the sliding friction between the surface of the workpiece and the drawing hole surface is reduced by lubrication, but when drawing pipe with an inner tool the inner wall and inner tool also have to be lubricated... post-pressing stages because oil no longer runs off during transport and storage This feature is emphasized by the lower film thicknesses of prelubes and prelubetype low-viscosity washing oils 15.1.12.8 15.1.12.9 Welding and Bonding [15.33] Welding is not influenced by today’s corrosion protection oils and pressing lubricants and therefore poses no particular requirements However, this does not apply to the... lubricants offer additional advantages 15.1.12.13 Dry-film Lubricants There is still a growing demand for even higher-performing lubricants as high and highest-strength steels are increasingly used for sheet-metal forming Complex part design in high-strength steel is a challenge for press shop managers making use of conventional mill oils and drawing lubricants As discussed in Section 15.1.12.1, the idea... most important of all the lubricants in wet drawing Apart from these, tenside solutions, free of mineral oil and fatty oil, are significant To a small extent other synthetic solutions are also used The three groups of lubricants (see Table 15.8) can be characterized on the basis of the most important ingredients 15.2 Lubricants for Wire, Tube, and Profile Drawing Tab 15.8 Lubricants for wet drawing... for each stage As a result, present day drawing plants only generally differentiate between two groups of wet drawing lubricants: lubricants for the coarse and upper medium wire sector and those for the medium and fine wire sector Occasionally special lubricants are also used for finest and super finest wire Lubricant Concentration Despite the many different types of lubricant, specific concentrations... 15.2 Lubricants for Wire, Tube, and Profile Drawing Theo Mang and Wolfgang Buss 15.2.1 Friction and Lubrication, Tools, and Machines Forming Classification According to DIN 8584 these forming processes comes under the manufacturing processes with forming under a combination of tensile and compressive conditions, or in closer sense to drawing by sliding action processes Wire and rod drawing is applied to... 15.2.2.8 15.2 Lubricants for Wire, Tube, and Profile Drawing 15.2.3 Drawing of Steel Wire 15.2.3.1 Requirements There are so many different demands put on finished wires and their production conditions that it is not possible to draw up a system which will observe all criteria Used are all types of lubricant and lubricant materials as shown in Section 15.2.2: dry drawing, lubrication drawing and wet drawing . temperature was 40 C, and the speed 10 mm s –1 , with deflection. 15.2 Lubricants for Wire, Tube, and Profile Drawing Theo Mang and Wolfgang Buss 15.2.1 Friction and Lubrication, Tools, and Machines 15.2.1.1. cleaners used here dissolve oils and greases easily and disperse dirt particles very well. Immersion baths are recom- mended especially for small parts or flat parts with large surface areas which. related problems may occur such as corrosion of stamped parts and tools, wear pick-up and growth of bacteria and fungi. Careful maintenance and special precautions are therefore necessary. Washing

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