Lubricants and Lubrication Part 12 docx

61715.3 Lubricants for Rolling 15.3.3.3 Finest Sheet Cold Rolling Finest sheet has a thickness range of 0.15 to 0.35 mm and is rolled as wide strip on five or six-high stand tandem lines and galvanized afterwards to provide so-called tin sheet. This is why this thickness range is generally referred to as tin plate. Rolling is carried out with semi-stable high fat content emulsions by the so-called direct application of fat oil dispersions (Fig. 15.51). Before tin plating the remains of lubricants are removed in an electrolytic degreasing plant. In the case of direct appli- cation, natural fats such as palm oil or animal tallow are often prepared as a 10 to 20 % dispersion in completely softened water in large mixers. The drops of fat oil are approx. 50 lm in size. The fat and water are separated during the return run, treated and used again. Recovered fat can be used for rolling until an acid number of approx. 20 mg KOH g –1 is given. Where semi-stable emulsions are used one works with initial concentrations of 3 to 6 % in fully softened water (Fig. 15.52). The size of the drops in such emulsions is between 10 and 50 lm. The treatment of semi-stable emulsions is less expensive than dispersions used in direct application. Fig. 15.51 Finest sheet cold rolling on a five- stand four-high tandem mill; principle of the so-called direct application. 1, preparation of an dispersion out of water and fatty oil (15–20 %); 2, dispersion feed to the rolling mill; 3, addition of water to the mixing tank; 4, addition of fresh fatty oil to the mixing tank; 5, addition of reconditioned fatty oil to the mixing tank; 6, mixing tank; 7, return tank (separation of fatty oil and water); 8, water feed (containing max. 1 % fat) to the rolling mill; 9, reconditioning of the fatty oil; 10 return of cooling water and dispersion. 618 15 Forming Lubricants 15.3.3.4 Cold Rolling of High Alloy Steel Sheet Higher alloyed steels, more especially high grade stainless steels, are rolled on mul- tiroll stands with a preference for lubricants which do not mix with water. Although high grade stainless steels generally produce scale to a considerably lesser extent, the steel is pickled before cold rolling to keep roll wear low. Austenitic steels are generally pickled with a mixed acid without hydrochloric acid content, but ferritic steel is pickled with hydrochloric acid. The rolling speeds, which depend upon the alloy and material thickness, can be up to 500 m min –1 , in which case, for example, high grade stainless steel hot strip be reduced by up to 85 % in eleven passes without intermediate annealing. Fre- quently, however, annealing, pickling and re-rolling have to take place in order to achieve the required strip quality. The selection of the rolling oils has a decisive influence on the surface finish of the sheet. Mat surfaces are produced when the viscosity is too high even when using ground or polished work rolls. This is almost always undesirable in the case of high grade stainless steels. This is why in practice preference is given to mineral oils at approximately 15 to 20 mm 2 s –1 at 20 C which have the closest possible boiling range [15.88]. Oils with lower viscosities often cause problems by vapor bubbles being developed through the reduced heat dissipation. 3 4 2 1 Fig. 15.52 Finest sheet cold rolling on a five-stand four-high tandem mill using a semi-stable emulsion (without filtering station). 1, emulsion tank (3–6 %); 2, emulsion feed; 3, emulsion return; 4, stirrer. 61915.3 Lubricants for Rolling Because rolling oils used on multiroll stands also take over the lubrication of the roll bearings, the additives must cater for both operations. In the case of a twenty-high roll stand the flow rate for strip lubrication is, for exam- ple, 600 to 1000 m 3 h –1 and for bearing lubrication 200 to 250 m 3 h –1 with an oil tem- perature of approx. 40 C. The required oil pressure for strip lubrication is approx. 12 bar and for bearing lubrication 6 bar. Oil cleaning is carried out generally by floating filtration because of the high quality surface finish required. The achievable filter fine- ness is generally < 2 lm and the residue contamination content < 10 mg l –1 (solid for- eign matter). Figure 15.53 shows a multi roll stand, type Sendzimir. Fig. 15.53 Rolling of stainless steel sheet on a multi-roll stand using rolling oil. (a) oil circulation: 1, oil tank with clean oil (e.g. 100 m 3 ); 2, oil feed to lubricate the strip (12 bar); 3, oil feed for the lubrication of the roller bearings (6 bar); 4, oil return; 5, return tank with uncleaned oil (e.g. 169 m 3 ); 6, oil reconditioning by hyperfine filtration (0.5–2 lm fineness, < 10 mg L –1 load of solid matter. (b) arrangement of spray-nozzles at the rolling stand [15.88]: direction A, nozzles 1, 2, 3, 4, 5 in action; direction B, nozzles 2, 3, 4, 5, 6 in action. 620 15 Forming Lubricants 15.3.4 Rolling Aluminum Sheet Cast blocks from 200 to 650 mm thick are used which are rolled down on hot roll stands to 10 to 20 mm after milling off the oxide skin and heating by reversing to be subsequently reduced to 6 to 2 mm on two to five-high stand tandem lines. Rever- sing stands can be two-high or four-high in construction. Hot rolling is only carried out with emulsions. Sheet and strip are rolled down from 3.0 to 0.1 mm and foil down to 5 lmby cold rolling. Cold rolling is almost exclusively carried out with low viscous, water- immiscible oil. 15.3.5 Aluminum Hot Rolling Depending upon the alloy and required reduction in thickness the blocks are pre- heated to 450 to 580 C before hot rolling. With a rolling speed between 180 and 300 m min –1 in the reversing process and appropriately higher speeds in tandem stands, the material is rolled with stable emulsions in an initial concentration between 2 to 6 % and an upstream temperature of 35 to 60 C. The temperature in the last pass, depending upon the alloy, is still 280 to 230 C. The concentrates comprise mineral oil hydrocarbons or synthetic hydrocarbons with polar additives (ester, fatty acids, fatty alcohols), tensides and further anti-wear substances, where needed. Fatty acids form aluminum soap which can cause too high slip and tarnishing. This is why it is only to be used in low amounts. Decisive for the lubrication and quality of the surface finish is the development of a uniform, fine coating of aluminum pick-up (roll coating) on the work rolls. If the coating is too thin the friction conditions are unfavorable and frequently unstable. On the other hand a roll coating which is too thick causes uncontrolled release of parts of this coating which, in turn, leads to pick-up rolling and surface faults. To prevent this, excessive pick-up is removed from the work rolls during the rolling process [15.89] by means of brush rolls. Exact control of the temperature of the work and back-up rolls is accomplished by means of specific emulsion dosing to obtain a favorable hot strip profile. Emulsion analysis, emulsion care and cleaning is carried out in the same way as in other applications. Here again the particle size distribution is considered as well as the stability tests. In the case of aluminum rolling, even greater attention has to be paid to the compatibility of the machine lubricants and hydraulic fluids with the emulsion in order to avoid detrimental patches. 62115.3 Lubricants for Rolling 15.3.6 Aluminum Cold Rolling After soft annealing at temperatures from 370 to 430 C, finish rolling is carried out on cold wide strip lines at speeds up to 2700 m min –1 . The sheet thickness is between 0.1 and 3.0 mm but the major amount of rolled strip is about 0.6 mm thick and achieved in two to seven passes. Despite the advantages of better heat dissipation, lower fire risk and less expen- sive health protection water mixable rolling oils [15.90] have been unable to establish themselves for this application , because of, for example, patch forming as a result of residue moisture in the round coil, and hydrogen embrittlement of the work rolls can only be controlled to a deficient degree. The demands put on cold rolling oils are summarized in Table 15.15 according to their importance. Tab. 15.15 Demands on cold rolling oils. High surface quality finish Large reduction in thickness No spotting High roll service life No tendency to gum Good filtering ability Low tendency to burn Fulfillment of hygienic working requirements When making up strip rolling oils, the main attention has to be paid to the selection of the base oil. Particularly suitable are paraffinic hydrocarbon substances with a viscos- ity which is not too high, in order to avoid mat surface finishes. Consequently, closely cut base oils with flash points above 85 C and a viscosity from 2 to 4 mm 2 s –1 at 20 C are mainly considered. The most popular additives are straight-chained alcohols, acids and esters with chain lengths from 10 to 14 carbon atoms as well as oxidation inhibitors. The activation level seldom exceeds the 5 % mark. The selection of base oils and finish formulated rolling oils is based on a can test at 300 C. It is essential for users who have a distillative rolling oil preparation system avail- able that the rolling oil additives have approximately the same boiling point as the base oil, so that they can be distilled together. The higher boiling proportions remain in the distillation residue together with the abrasion and contamination. If no distillative cleaning system is available floating filtration is necessary using kieselguhr with added bleaching earth where needed. Other cleaning methods with- out filtration aids have also been tested [15.91]. In principle, foil rolling oils differ from strip rolling oils only through a lower additive content, occasionally also through lower viscosity and preferably with a very low aromatic content to ensure compliance with the stipulations for use in the food sector. 622 15 Forming Lubricants Doubling, i.e. rolling two foils together on the top of each other, is the only suc- cessful way of rolling foils down to approx. 5 to 6 lm without the rolling stock tear- ing. To ensure that the two foils do not weld to each other, a low boiling point hydro- carbon substance without aromatic substances is applied by dripping as the foils run into the rolls. After rolling, the two foils are separated again in a separating operation. Double rolled foils can be recognized since they have one bright side (from the work roll) and one mat side (from the other foil). As is the case with hot rolling, to avoid spotting synthetic or barely-synthetic spe- cial lubricants and hydraulic oils are also used when cold rolling on the roll stand and in the rolling operations. A further problem in the case of cold rolling is the enrichment of the working area air with oil vapor and oil spray. The air-pure process [15.92] has proved to be suitable for effective air de-oiling. 15.3.7 Rolling Other Materials Copper and its alloys are generally rolled in a hot roll process with water, i.e. without special lubricants, although occasionally with, stable slightly fatty emulsions. Generally stable emulsions with fat oils and synthetic esters are also used for cold rolling. Copper and copper alloys are nobler metals than iron and aluminum. This also has an effect on the tribological conditions [15.93]. Under all circumstances, staining of the bright, mostly decorative surfaces by the emulsion or its contents has to be avoided. Consequently special attention must be paid to ensure the absence of substances containing sulfur [15.94]. Moreover, lubricant residue must be volatilized under inert gas to leave no residue at the annealing temperatures (310 to 480 C for copper and 540 to 650 C for brass). Titanium is hot-rolled, dry. Fatted oils are used for cold rolling. Zinc is rolled in a semi-hot state at 200C, very often dry, or cold with a slight lubrication using low viscosity oils with polar substances or emulsions. By heating to between 120 to 150 C when cold rolling, a large part of the lubricant is volatilized so that no further cleaning is necessary. Tin and lead are generally cold rolled. Low viscous special lubricants are used for special alloys (solders and similar materials). Tungsten and molybdenum rolling has to be carried out at high temperatures under inert gas due to sensitivity to oxidi- zation at high temperatures. Alternatively coating with glazes is also possible. Semi- hot or cold rolling is carried out with solid matter content or strongly polar oils with additives. Nickel and cobalt alloys are dry rolled. The cold rolling process on multiroll stands is similar to the cold rolling of stainless steels as far as the use of lubricants is concerned. 62315.4 Solid Metal Forming Lubricants (Solid Forming, Forging and Extrusion) 15.4 Solid Metal Forming Lubricants (Solid Forming, Forging and Extrusion) Theo Mang and Wolfgang Buss Under this heading, a large number of forming methods are included. The forming processes in this group are not precisely defined. In this section we will review in particular extrusion and forging. We can see here, especially in the case of cold forming in the production of small parts in large series, a continuous increase to the detriment of cutting processes. The material savings, and the resulting savings in energy if the fusion heat is included, are of a particular advantage. Apart from this there are a number of technical advantages compared with workpieces produced by cutting, especially the utilization of strain hardening and the favorable course of the fibers as a result of the material flow. Frequently there are transitions and combina- tions between the processes and forming technologies so that, for example, the exact classification laid down in DIN 8583 for differentiating between the lubrication techniques does not offer a suitable basis. On the other hand the temperature of the workpieces and tools is particularly significant for the selection of lubricants. Impression die forging is an excellent method economically. For example, the parts manufactured mainly by independent forges attained a turnover of approxi- mately US$ 4 billion in USA in the year 1997. The most important buyers are the automobile industry (48 %), aerospace (23 %) and the manufacturers of off-highway equipment (6 %) [15.95]. 15.4.1 Processes Mainly, processes are classified on material flow and the type of the tool used. How- ever, there are frequently no clear differences between forging and extrusion. 15.4.1.1 Upsetting To be understood under this is a process with compression between flat dies. Cold heading is also to be mentioned as a cold forming process for the production of con- necting elements (screws and rivets). Fig. 15.54 Some variations in the extrusion process. (a) hollow backwards (can extrusion); (b) hollow forward; (c) solid forward/ hollow backwards; (d) cross solid forward and hollow backwards. 624 15 Forming Lubricants 15.4.1.2 Extrusion During extrusion processes, the workpiece is placed in a container and compressed by means of ram movement. Figure 15.54 shows the various types of extrusion pro- cesses. Extruded products can be hollow or solid. According to the material flow, one differentiates between forward (direct), backward (reverse) and cross procedures. 15.4.1.3 Impression Die Forging This operation is shown in Fig. 15.55. Economically, it is the dominant hot forging method. This is also reflected in the significance of this group of lubricants. Closed die forging is a special form of impression die forging. Here the filling of the die is not supported by the development of a flash. There is no possibility for excessive material to escape, and vent holes are provided for vapor generated from the lubricant to escape. 15.4.1.4 Open Die Forging Open die forging is different from impression die and closed die forging in that the metal is never completely enclosed as it is being shaped by the dies. 15.4.2 Forming Temperatures The surface temperatures of forged workpieces and tools are a decisive factor in the selection of lubricants. A difference is made between three general areas as far as both forging and extrusion are concerned: 15.4.2.1 Cold The parts are formed at ambient temperature without preheating. It goes without saying that we must also consider that considerable heat is generated at high form- ing speeds. Fig. 15.55 Impression die forging. 62515.4 Solid Metal Forming Lubricants (Solid Forming, Forging and Extrusion) 15.4.2.2 Warm In this case the workpieces (billets) and/or the tools are heated to facilitate material forming. The billet temperature is below the recrystallization temperature of the material. Warm and cold extrusion methods can be combined with each other [15.96]. 15.4.2.3 Hot The best forming properties and lowest forces are given at temperatures above recrystallization temperature. In the case of hot steel forging the workpiece tempera- tures are between 1100 and 1200 C. 15.4.3 Friction and Lubrication with Cold Extrusion and Cold Forging The best possible use of the material, good quality surface finish and dimensional accuracy, the use of strain hardening and extensive rationalization are the main feat- ures of the most important cold massive forming methods. In the foreground stand the cold extrusion presses with specific variations in methods as far as material flow and die movement are concerned. Figure 15.54 shows a few significant variations of the extrusion method. Upset- ting, embossing and reducing are methods related to the type of lubrication and are frequently combined with extruding. Numbering amongst the cold massive forming methods are also thread and profile rolling which are either follow-on process of extrusion methods, closely associated with the extrusion lubrication technology or are used in conjunction with cutting operations. The ironing type forming method can be applied as a follow-on to either a sheet- metal deep drawing operation or be used for the cup produced by backwards extru- sion. The focal point in cold extrusion for ferrous materials lies in both non-alloyed and low alloyed steels, as well as in the case-hardened and annealed steels used in auto- mobile production and the automobile industry which are particularly important processors of cold extrusion parts. Steels with higher carbon content are no longer extruded or only when the degree of forming is low; where stainless steels are con- cerned the ferrite materials can be formed better than austenitic materials [15.97]. Choosing the correct material for the extrusion tools is essential due to the extreme pressure loads; in many cases the limits to forming by extrusion are deter- mined solely by the tensile strength of the tool. Tool steels and high speed steels, as well as carbide metals, play an important role. The preferred utilization of high speed steels as a die material has initiated developments to improve the resistance to wear; especially worth mentioning here is the possibility given for TiC-coating. New developments in the chrome steel sector (e.g. 12 % Cr, 1.2 % C, 1.4 % Mo, 2.5 % W, V), which can be further improved by the CVD method are also worth mentioning. The treatment processes such as nitration and boronization also play an essential role. Hard chromium plating has not been able to establish itself in massive forming because of the low adhesion of the chromium coating applied by 626 15 Forming Lubricants electrolysis. Applicable for forming tools are limit loads of 2500 N mm –2 , maximum 3000 N mm –2 as a general rule; the limits for armored matrices with steel or carbide metal are 2000 to 2500 N mm –2 . 15.4.3.1 Friction and Lubricant Testing Methods Compared with the other forming processes covered here, the maximum surface pressures occur in cold forging and cold extrusion (up to 3500 N mm –2 ). This leads to a particularly high tool load and, as a result, also to especially difficult tribological situations. To this must be added the very high surface expansion, to some extent, which has to be followed by the lubricant or the lubricant carrier. In the case of hollow backwards extrusion the surface expansion can, for example, be ten times the initial surface area (Fig. 15.56). Table 15.16 shows the stress profile for the friction and lubrication in respect of surface pressure, relative speed and surface expansion for five processes. This also includes ironing type forming. A better comparison is possible by formulating spe- cific values (sizes without set dimensions). The maximum surface pressure P max refers to the initial stress yield k fo , the relative speed between tool and workpiece V R , and the ratio between final and initial surface A 1 /A 0 applies for the surface expan- sion. This allows conclusions about the suitability of friction and lubricant testing machines [15.98, 15.99]. Tab. 15.16 Stress profile for friction and lubrication in respect of surface pressure, relative speed, and surface expansion for five processes [15.98]. Ironing Upsetting Hollow forward extrusion Solid forward extrusion Hollow backward extrusion P max /k f0 V R /V WZ A 1 /A 0 2.1 2.3 2.2 5.9 2.4 4.5 5.5 5 4 6.4 5.7 4 9 6.3 11 The sliding movement between tools and workpiece, which takes place under high specific pressure and with large surface expansion, generates high friction Fig. 15.56 Lubricant stress through surface expansion with cup backwards extrusion. 1, coated surface before forming; 2, surface formed by a stamper after forming. [...]... surface roughness of the workpieces [15.103] Alkyl and arylphosphoric acid esters and even phosphoric acid partial esters are also used [15.104] Because the thermal stress is considerable when using chlorinated products particular attention must be paid to their stability and the risk of corrosion on machines and extruded parts A degreasing of the parts directly after extrusion is recommended when... no longer necessary and the main consideration for the selection of a lubricant lies more on a lower coefficient of friction and higher resistance to temperature In many cases lubrication is necessary for the warm forming process because of the high production speed, high tool stress and the required precision of the parts and the workpieces (billet coating) and the tools (die lubrication) Billet coating... measurement according to E Doege and R Melching [15 .122 , 15 .123 ] (f) Assessment of lubricants by die filling; measurement of the fill height, h point of view Colloidal preparations in oil and in water as well as pasty graphite lubricants are used Graphite preparations have replaced lubricants such as saw dust, heavy oils or powdered carbon to a very wide extent in lubrication technology Where graphite... C, b-titanium alloys between 700 and 850 C Considerable demands are put on the thermal stress of the lubricants, since the lubricating film is subject to high temperature both on the workpiece and on the tool Apart from the demand for thermal stability the used lubricants must not cause high temperature corrosion on the surface of the tool or workpiece The risk is particularly great as a result of... spread (both on the tool and workpiece side), for example, in screw production spread has been observed between 15 000 and 60 000 parts which was not caused by oil For the most important areas of application, extrusion oils are classified by type and amount of additive into four classes [15.105] standard screws and high tension hexagonal bolts: oil with polar additives and EP additives on a phosphorus... sized parts, structure-effective solid lubricants, especially molybdenum sulfide (MoS2), are used for smaller parts As a general rule the solid lubricants are applied to a phosphate layer as dry powder lubricants by tumbling or from aqueous suspensions by immersion or spraying Application by immersion follows in dipping baskets or rotating screen tubs [15.105, 15.109–15. 112] In a few cases, solid lubricants. .. lubricants and solid lubricants are concerned In the large scale production of nuts, screws, bolts and similar parts on multistage presses, the extrusion oils in oil circulation systems have an important lubricating and cooling function In principle, coating the wire surface is not necessary for simple operations, especially in the case of wire parts mass produced by upsetting Frequently the lime and dry... through tool design, and only influenced to a certain extent by oil selection The viscosity of extrusion oils is between 30 and 120 mm2 s–1 at 40 C The preferred viscosity range is between 35 and 65 mm2 s–1 at 40 C Selection criteria for 15.4.3.3 629 630 15 Forming Lubricants the initial viscosity are the workpiece temperatures, size of the extruded parts, machine pumping facilities and the specific... toxicological point of view and that white forging lubricants must not have a detrimental effect on workplace conditions as a result of toxic decomposition of substances in the product [15 .125 –15 .129 ] 15.4.7 Aluminum Forging Gaining increasing significance are parts produced by massive forming from aluminum and aluminum alloys Playing a role in this respect are low weight and the excellent mechanical... some years to measure the friction and assess lubrication for cold extrusion After the appropriate pretreatment of the plane fric- Testing lubricants for solid forming by cup backwards extrusion [15.102]: (a) blank and cup geometry; (b) maximum related cup height for lubricants 1, 2, and 3 are related to the temperature of the blanks Fig 15.58 627 628 15 Forming Lubricants tion surfaces a cylindrical . m 3 h –1 and for bearing lubrication 200 to 250 m 3 h –1 with an oil tem- perature of approx. 40 C. The required oil pressure for strip lubrication is approx. 12 bar and for bearing lubrication. tempera- tures are between 1100 and 120 0 C. 15.4.3 Friction and Lubrication with Cold Extrusion and Cold Forging The best possible use of the material, good quality surface finish and dimensional accuracy,. hot roll stands to 10 to 20 mm after milling off the oxide skin and heating by reversing to be subsequently reduced to 6 to 2 mm on two to five-high stand tandem lines. Rever- sing stands can be