In this article we will discuss about:- 1. Cleaning Methods of Metals 2. Mechanical Finishing of Metals 3. Burnishing Machines 4. High Pressure and Steam Jet Cleaning Machines.
Cleaning Methods of Metals:
The removal of rust and scale is the problem one often encounters for removal from mill products, forgings, castings and fabricated metal parts.
The cleaning methods for these are discussed as follows:
(i) Abrasive Blasting:
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With a wide choice of abrasives, it is suited for all sizes and shapes, and all metals without affecting their heat treatment. However it may abrade corners, alter dimensions, cause surface etching/roughness in extreme cases.
(ii) Tumbling:
It is the least expensive process. Part configuration and size are the primary limitations of the process. Tumbling in dry abrasives (deburring compounds) is effective for removing scale and rust from small parts of simple shape. Complex shaped parts with recesses and irregularities require more time.
(iii) Brushing:
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It is used for removing light rust or loosely adhering scale. It is least expensive method suited for workpieces made from tubing.
(iv) Pickling:
It is used for complete removal of scale from mill products of fabricated parts in hot, strong solution of sulphuric or hydrochloric acid. By adjusting formulations, it can be used for removing scale from ferrous and non-ferrous parts.
It is adaptable to products of virtually any size or shape. In case of high carbon alloys, it can be potential source of hydrogen embrittlement. Some metal (upto 3%) may be lost in pickling.
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Necessary care of dispose off fumes and spent is required. Sometimes excessive pitting may occur on cast steels and irons. This process is usually used as supplementary treatment following abrasive blasting or salt bath descaling.
(v) Electrolytic Pickling:
It can remove scale twice as fast as pickling process, but is costly. It utilises 30% HCl at 55°C and 3 to 6 volts for 2 to 3 minutes.
(vi) Salt Bath Descaling:
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It is an effective means of removing or conditioning scale. The reduction or oxidation of scale in almost instantaneous when workpiece attains the bath temperature (385° to 520°C). There is no loss of metal or danger hydrogen embrittlement. High bath temperature relieves stresses but may result in cracking or warping of complex workpieces.
(vii) Alkaline Descaling:
It is more costly and slower in its action than acid pickling, but there is no loss of metal by this method. Bath contains about 60% sodium hydroxide and is maintained at room temperature and in some cases upto 95°C. Immersion time is dependent on thickness of scale to be removed. The use of current increases the rate of oxide removal.
(viii) Alkaline Cleaning:
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This method is used for removal of oily, semi-solids from metals before they are electroplated. The solutions used depend on their detersive properties for cleaning action and effectiveness. Agitation of the solution and movement of workpieces also helps.
The principal methods employed for alkaline cleaning are soak, spray, electrolytic, barrel, steam-gum, spray rotary etc. The cleaning effectiveness of alkaline compounds is attributed mainly to the action of builders (compounds to provide alkalinity and other desirable properties at low cost), which are the principal bulk components of the formulation.
(xi) Emulsion Cleaning:
It is a process for removing soils from the surfaces of metal by the use of common organic solvents dispersed in an aqueous medium with the aid of an emulsifying agent. The cleaning is done at room temperature to 80°C depending on the solvent used. Emulsion system is generally a hydrocarbon and water. Emulsion cleaning is used when rapid superficial cleaning is required and when some protection by a light residual film of oil is desirable.
(x) Solvent Cleaning:
It is used to remove oil, grease, loose metal chips, and other contaminants from the surfaces of metal parts by common organic solvents. Cleaning is usually performed at, or slightly above, room temperature.
Parts are cleaned by being immersed and loaded in the solvent, with or without agitation. Large parts are sprayed or wiped with the solvent. Higher operating temperature will invariably increase the cleaning efficiency of a solvent. Good agitation usually eliminates the need for prolonged soaking.
(xi) Vapour Degreasing:
It is a cleaning process that employs the hot vapours of a chlorinated solvent to remove oils, waxes and greases.
Degreasing is possible by:
(a) Vapour only,
(b) Vapour-Spray-Vapour system,
(c) Warm liquid-vapour system,
(d) Boiling liquid-warm liquid-vapour system.
(xii) Acid Cleaning:
It is a process in which a solution of mineral acid, organic acid or acid salt, in combination with a wetting agent and detergent, is employed to remove oxide, shop soil, oil, grease and other contaminants from iron and steel surfaces.
The difference between acid cleaning and acid pickling is a matter of degree. Various methods of application are wipe on-wipe off, spray cleaning, immersion barrel acid cleaning and electrolytic cleaning.
(xiii) Pickling:
It is the process of chemical removal of surface oxides (scale) from metal by immersion in an acid solution. Wide variations are possible in the type, strength and temperature of the acid solutions used. It can be adapted to continuous operations. Sulphuric acid is commonly used for pickling, because in comparison to HCl, it has lower cost, produces less fume and has less volume to be handled.
However H2SO4 produces darker surfaces and smelt on high- carbon steel, and requires to be heated to high temperature. Inhibitors are added to acid pickling solutions primarily to protect the steel being cleaned, by retorting or stopping the chemical action of the acid on the basic metal.
(xiv) Electrolytic Pickling:
It overcomes some of the difficulties encountered in still pickling. While rust Fe2O3 gets removed by still pickling, black magnetic oxide Fe2O3, is removed by electrolytic pickling. This process is much rapid because of greater evolution of hydrogen.
(xv) Salt Bath Descaling:
These processes could be either reducing, or oxidising or electrolytic. Salt baths for the oxidising process are most stable and least costly. The reducing and electrolytic processes are move effective in attacking heavy, tightly adhering scale, Electrolytic process usually provides more thorough scale removal than the reducing process. Since it operates at a lower temperature, it is advantageous for descaling metals that undergo a change in properties at higher temperatures.
(xvi) Abrasive Blast Cleaning:
This process entails the forceful direction of abrasive particles either dry or suspended in a liquid—against the surfaces of metal or products, to remove contaminants or to condition the surfaces for subsequent finishing.
This process is used for removal of scale, rust etc., or roughening of surface in preparation for bonding, painting or other coating, or removal of burrs. The abrasives used are grit (angular metallic articles,) short (spherical particles), sand etc. Metal cleaning methods may be classified as stated in Table 8.1.
Mechanical Finishing of Metals:
Very often parts have to be finished by mechanical means. The mechanical finishing processes include—polishing and buffing, barrel finishing, shot peening and power brush cleaning and finishing.
(i) Polishing:
Polishing is an abrading operation employed for the removal or smoothing out of grinding lines, scratches, pits mould marks, parting lines, tool marks, stretcher strains, etc. Polishing is performed with either a belt or a wheel to which an abrasive is bonded. The process removes metal, and causes some plastic working of the surface. Emery and corundum are the natural abrasives commonly used on polishing wheels. Polishing wheels are usually made of muslin.
(ii) Buffing:
Buffing is a process for producing smooth, reflective, scratch-free surfaces on workpiece by bringing them into contact with revolving cloth or buffs charged with a suitable compound. The action of the wheels, together with the of the compound, either cuts or flows the metal, thus removing minor defects and imparting a smooth, lustrous finish.
Most buffing compounds consist of an abrasive that is immersed in a binder-carrier. The abrasive serves as the principal cutting medium, the binder provides lubrication, prevents overheating of the work, and firmly cements the abrasive to the wheel face. Binders must not chemically etch, corrode or mar the metal surface.
(iii) Barrel Finishing:
Barrel finishing involves the tumbling or rolling of parts in rotating barrels, or the agitation of parts in shaker containers, vibratory tubs for cleaning, descaling, deburring, shine rolling and burnishing. It can be performed either dry or wet and can be applied to parts ranging from rough castings to delicate parts.
Water is usually used as the liquid in wet barrel finishing. A finishing medium, usually metallic or non- metallic materials in bulk or chunks, is used in the barrel. Cleaning, descaling, deburring, burnishing and shine rolling can be performed in a barrel depending on the compounds used in the barrel.
Shot peening is a process in which a stream of shots is impinged at the metal surface at high velocity and under controlled condition to induce compressive stresses in the exposed surface layers of metallic objects. It not only cleans the surface being peened, but also increases fatigue strength.
It also relieves tensile stresses that contribute to stress- corrosion cracking and tests the adhesion of silver plating on steel. The peening action (rounded depressions in the surface) improves the distribution of stresses in surfaces that have been disturbed by grinding, machining or heat treating. Shot peening is especially effective in reducing stress concentration effects of notches, fillets, forging pits, surface defects, and decarburisation.
A higher residual stress, approaching yield strength, can be obtained by strain peening, which consists of peening the surface while it is being strained in tension as in the case of springs.
(iv) Power-Drive Wire:
Power-drive wire or non-metallic-fibre-filled rotary brushes are widely used for cleaning, deburring, edge blending and surface finishing of metals. The brush may be constructed using straight wire, twisted wire, crimped wire, or natural or synthetic fibre.
Trim (length or fill material extending from the retaining member to the brush face) may be short, medium, or long. Density of fill may be heavy, medium or light. The selection of a power brush for a specific cleaning or finishing operation depends primarily on selection of the proper combination of variables in brush construction and operating procedure.
Burnishing Machines:
External burnishing machines are used for finishing the external diameters of soft, cylindrical components. The process of burnishing being a process of cold-rolling, does not produce the peaks or valleys but actually removes these by turning-over the peaks into the valleys. This increases the bearing area of the component, and its tolerances remain stable. This process has the advantages that it is remain stable.
This process has the advantages that it is extremely fast; requires no consumables; having practically no scrap; produces a surface of very high surface-finish; and provides greater life to the component. It substitutes finish-grinding.
This process is most economical and is suitable for a wide variety of components, including fan-shafts; piston-bar and pressed-bar of sewing machines; etc. The burnished surface accepts electroplating as a finishing process.
High Pressure and Steam Jet Cleaning Machines:
These machines are robust, mobile and ideally suited for the removal of grease, sticky and hard deposits, oils, rust and old paint from vehicles and vehicle components, plant and machinery, shop floors, containers and pipes, hot water or steam is ejected through the equipment under pressure which can be increased upto 150 kg/cm2. Water temperature and flow are also adjustable and chemicals/ detergents stored in inbuilt tanks can be added in controlled quantities to further accelerate the cleaning process.
The equipment consists of a reciprocating type slow speed piston pump and an oil-fired burner with a heat exchanger. Safety devices to safeguard the operator and machine are provided. The most obstinate deposits in inaccessible areas can be removed with the utmost of ease.
These cleaners are extremely useful in automobile, defence and railway workshops and industries like chemical, cement, earthmoving equipment, breweries, dairies, mining, food processing and fertiliser plants.