Cladding
Manufacturing
New - Better Definition
The term weld cladding usually denotes the application of a relatively thick layer t£3 mm. or % in.) of weld metal for the purpose of providing a corrosion-resistant surface. Hard facing pro¬ducer a thinner surface coaling than a weld clad¬ding and is normally applied for dimensional restoration or wear resistance, Typical base metal components that are weld -cladded include the in¬ternal surfaces of carbon and low-alloy steel pres¬sure vessels, paper digesters, urea reactors, tubesheets, nuclear reactor containment vessels, and hydrocrackers The cladding material is usu¬ally an austenitic stainless steel or a nickel-base alloy. Weld cladding is usually performed using submerged arc welding. However, flux-cored arc welding (either self-shielded or gas-shielded), plasma arc welding, and electrolsag welding can also produce weld claddings. Figure 6 compares' deposition rates obtainable with different welding processes. Filler metals arc available as covered electrodes, coiled electrode wire, and strip elec¬trodes. For very large areas, strip welding with cither submerged arc or electros lag techniques is the most economical. Table 4 lists some of the filler metals for stainless steel weld claddings.
Application Considerations. Weld cladding is on excellent way to impart properties to the surface of a substrate that are not available from that of a base metal, or to conserve expensive or difficult to-obtain materials by using only a relatively thin surface layer on a less expensive or abundant base material. Several inherent limitations or possible problems must be considered when planning lor weld cladding. The thickness of the required surface must he less than the maximum thickness of the overlay that can be obtained with the particular process and filler metal selected.
Welding position also must be considered when selecting an overlay material and process. Certain processes are limited in their available welding positions (e.g.. submerged arc welding can be used only in the flat position). In addition, when using a high-deposition-rate process that exhibits a large liquid pool. weIding vertically or overhead may be difficult or impossible. Some alloys exhibit eutectic solidification, which leads to large molten pools that solidify instantly, with no "mushy" (liquid plus solid) transition. Such materials arc also difficult to weld except in the flat position
(Stainless steels, 1993, Joseph R. Davis, ASM International. Handbook Committee, P 110)
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Cladding
Explosive welding or cladding, as it is often called, brings together two metal surfaces with sufficient impact and pressure to bond. Pressure is developed by a high-explosive shot placed in contact with or in close proximity to the metals. In some instances a protective material such as rubber is placed over the upper panel to prevent damage to the surface. The entire assembly is placed upon a buffer plate or anvil to absorb energy generated during yje jıining operation. Of the two arrangements showing cladding or laminating of metals, the left one is preferred.
To obtain a metallurgical bond, atoms from boh surfaces must come into intimate contact. The oxides and films always present on the surface of metals are broken or dispersed by high
pressure or dissolved in the molten region. The explosive force brings the clean surfaces together and produces a sound bond.
(Amstead, B. H., Ostwald,P. F., Begeman, M. L., MAnufacturing Processes,8th Edition, pg. 189)
Pickling
Surface Cleaning
New – Better Definition
Pickling is a treatment that uses an acid or a mixture of acids to remove scale. Scale is produced in high-temperature operations like welding, heat treatment or hot working. It is important to remove the scale because scale works against the natural corrosion resistant properties in stainless steel. The acids you use and the pickling procedure you follow depend on the grade of stainless steel being treated.
It can be dangerous to handle acids as they can burn your skin and other materials. You should know how to use them and how to dispose of them.
Sulphuric acid, nitric acid and hydrofluoric acid are some of the kinds of acids that you use in the pickling process.
Pickling is usually done by using baths or pickling paste. In a pickling bath, the weld is submerged in acid and the acid then eats away the scale.This is not useful if you have welds on large workpieces or in difficult to reach spaces. For these areas, it is much easier and safer to use pickling paste, which is a specially prepared, stiff paste made of strong acids. In this form, it can be applied to vertical or overhanging surfaces and localised areas. We often use pickling paste to remove post-weld discolouration.
The pickling process also removes rust caused by the corrosion of stainless steel or contaminant materials, like iron and steel particles.
(FCS Engineering Fabrication & Sheet Metalwork L4, 2009, Brink, McNamara, Rademeyer and Kiddelin, P36-37)
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Another chemical cleaning process is pickling, which is an acid-cleaning treatment that removes oxide scale and flux residues. Pickling involves cleaning of metal parts in dilute acids by spraying or immersion. Common acids used in pickling are 10% H2SO4 at 150-185°F or HCl acid in room temperature. The part is first cleaned with an alkali to allow acid to reach all surfaces. Pickling solution does not attack certain types of oils and grease, and alternative cleaning treatments may be needed.
(Asthana R. et al., Materials Processing an Manufacturing Science, p.316)
Alkaline Surface Cleaning
Surface Cleaning
New – Better Definition
Among chemical cleaners, the alkaline base cleaners have been and continue to be the most widely used type of formulated cleaners. When formulated with synthetic agents they create an effective detergent cleaning action.
Alkaline cleaners can remove a wide range of soils including heat-treating salts; inorganic soldering, brazing and welding fluxes; lubricants and coolants; and polishing and buffing compounds. They can be applied by just about any method of application known-by soak or tank cleaning, by spray, in electro-cleaning or barrel tumbling.
The most commonly used alkali bases are carbonates (such as sodium carbonate or soda ash), phosphates (such as trisodium phosphate or TSP), silicates (such as sodium orthosilicate or metasilicate), and hydroxides (such as sodium hydroxide). Another common alkali base is the borates.
Applications of Alkali Bases
Each alkali base serves a specific purpose. The carbonates, for example, serve as buffers, as low-cost alkalinity sources, and as water softeners.
The phosphates serve primarily as water softeners. In hard water areas, that is, where there are relatively large proportions of calcium and magnesium ions in the water, these ions will combine with ingredients in the cleaning solution to form insoluble materials. This formation of insoluble materials can be combatted by a sequestering agent in the formulated cleaner which, in effect, ties up the calcium and magnesium ions. And the phosphates are effective sequestering agents. They also impart alkalinity, rinsability, some buffer action, and are fair emulsifiers.
The silicates are excellent emulsifiers, good buffers (where pH is over 9). will hold soils in suspension, and provide active alkalinity. Hydroxides supply the necessary alkalinity, increase electrical conductivity of the solutions, and improve saponification.
Deficiencies
However, pure raw alkalies have serious deficiencies as far as cleaners are concerned -for example, they can form insoluble residues with hard water salts, and will not rinse freely; they can corrode or pit metal; they can be dangerous to personnel—and these deficiencies must be overcome. To this end, the alkali bases are mixed with surface active agents, which, in essence, utilize the desirable features of the alkalies even as they tone down the undesirable features. At the same time, the surface active agents or surfactants add certain benefits of their own.
(Cleaning Stainless Steel, 1973, ASTM International, P17)
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Alkaline Surface Cleaning:
The most used industrial cleaning method is alkaline surface cleaning, the action which is basically physical as well as chemical, aided by combination of surfactants, emulsifiers, separating agents, saponifiers, and wetting agents all attacking the part to be cleaned.The solution may be heated or agitated in motion by stirring.
Dissovable particles of dirt are washed away. Solid particles are seperated from the part and allowed to either settle in the form of sludge to the bottom or be floated away and removed from the solution by means of filtering similar devices.
(Ivana Suchy, Handbook of Die Design 2nd Edition, p668)
Fillet Welding
Manufacturing
New - Better Definition
A (did weld is the type of weld nude on the lap joint and lee joint It should be build up equal to the thickness of tin-plate, figure 1 13-30. On thick plates the fillet must be nude up of several passes as with a groove weld. The difference with a fillet weld is that a smooth transition from the plate surface to the weld is required If this transition is abrupt, it can cause stresses lh.it will weaken the joint
I he lap joint is made by overlapping the edges of the plates They should be held together tightly before tack welding them together A small tack weld may be added in the center to prevent distortion during welding, Figure I VII. ( hip the lacks he lore you start to weld
The lee joint is made In tack welding one piece of metal on another piece o( metal at a right angle. Figure I 3-32. After the joint is tack welded together, the slag is chipped from the tack welds If the slag is not removed, it will cause a slag inclusion in the ltn.it weld.
Holding thick plates tightly together on tee joints may cause underbad cracking or lamellar tearing. Figure 13-33. on thick plates the weld shrinkage can be great enough to pull the metal apart well below the head or its heat-affected zone In production welds cracking can be controlled by not assembling the plates tightly together The space between the two plates can be set by placing a small wire spacer between them. Figure 13-34
A fillet welded lap or tee joint can be strong if it is welded on both sides, even without having deep penetration Figure 1V-3-35. Some tee joints may be prepared for welding by cutting cither a bevel or a j-groove in the vertical plate This cut LS not required for strength but may he necessary because of design limitations unless otherwise instructed, most fillet welds will he equal in size to the plates welded A fillet weld will be as strong as the base plate if the sue of the two welds equals the total thickness of the base plate The weld bead should have a flat or slightly concave appearance to ensure the greatest strength and efficiency. Figure 13-36.
The root of fillet welds must be melted to ensure a completely fused joint A notch along the root of the weld pool is an indication that the root is not being fused together. Figure 13-37. To achieve complete root fusion, move the arc lo a point as close as possible to the leading edge of the weld pool. Figure I3-38. If the arc strikes the unmelted plate ahead of the molten weld pool, it may become erratic, which will increase weld spatter
(Welding: principles and applications, 2002, Larry F. Jeffus, P308-309)
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A fillet weld is used to fill in the edges of plates created by corner, lap and tee joints.Filler metal is used to provide a cross section approximately the shape of a right traingle.It is the most common weld type in arc and oxyfuel welding because it requires minimum edge preparation- the basic square edges of the parts are used.Fillet welds can be single or double (i.e., welded on one side or both) and can be continuous or intermittent. (i.e., welded along the entire length of the joint or with unwelded spaces along the length.)
(Fundamentals of modern manufacturing,materials,processes and systems,3rd edition Mikell P.Groover, p.694)
Organic Solvent Cleaning
Surface Cleaning
New – Better Definition
Organic solvent cleaners use organic solvents, solvent blends, or their vapors to rénove water-Insoluble soils such as grease, oils, waxes, carbon deposits, fluxes and tars from metal, plastic, fiberglass, printed circuit boards, and other surfaces. Organic solvent cleaning is performed prior to processes such as painting, plating, inspection, repair, assembly, heat treatment, and machining. The same type of machine that 1s used in cleaning applications can also be used for drying wet parts (by displacing surface moisture with solvent and evaporating the solvent) and for conditioning the surface of plastic parts. Both nonhalogenated and halogenated solvents may be used 1n solvent cleaning. Examples of the nonhalogenated solvents typically used are mineral spirits, Stoddard solvents, and alcohols. The five commonly used halogenated solvents used are methylene chloride (MC), perch!oroethylene (PCE), trichloroethylene (TCE), 1,1,1-trlchloroethane (TCA), and trichlorotrlfluoroethane (CFC-113). These five solvents can be used alone or in blends which contain two or more halogenated solvents and sometimes alcohols.Organic solvent cleaning does not constitute a distinct industrial category but rather is an Integral part of many major industries. The five 2-digit Standard Industrial Classification (SIC) codes that use the largest quantities of halogenated solvents for cleaning are: SIC 25 (furniture and fixtures), SIC 34 (fabricated metal products), SIC 36 (electric and electronic equipment), SIC 37 (transportation equipment) and SIC 39 (miscellaneous manufacturing Industries). Additional industries that use halogenated solvents in cleaning include SIC 20 (food and kindred products), SIC 33 (primary metals), SIC 35 (nonelectric machinery), and SIC 38 (instruments and clocks). Nonmanufacturing industries such as railroad, bus, aircraft, and truck maintenance facilities; automotive and electric tool repair shops; automobile dealers; and service stations (SIC 40, 41, 42, 45, 49, 55, and 75, respectively) also use organic solvent cleaners.
(Halogenated solvent cleaners: emission control technologies and cost analyses, 1990, Radian Corporation, P12)
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Organic solvent cleaning consists of an application of solvents to the organic contaminants such as oils or grease, in an attempt to remove them from the surface of parts. Sometimes this cleaning method has to be followed by an alkaline wash, in order to remove the solvent itself from the part surface. This type of cleaning may also be used for removal of water from electroplated parts.Solvents may be either petroleum-based or chlorinated hydrocarbons or alcohols. Other solvents include but are not restricted to benzol, acetone, and toluene.
The mechanism of cleaning is applicable mainly to contaminants of organic origin, such as grease or oils. These impurities may be easily solubilized and removed, or washed off the part's surface
(Ivana Suchy, Handbook of Die Design 2nd Edition, p669)
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