Weldable

There is hardly a metal that cannot, or has not, been joined by some welding process. From a practical standpoint, however, the range of alloy systems that may be welded is more restricted. The term weldability specifies the capacity of a metal, or combination of metals, to be welded under fabrication conditions into a suitable stmcture that provides satisfactory service. It is not a precisely defined concept, but encompasses a range of conditions, eg, base- and filler-metal combinations, type of process, procedures, surface conditions, and joint geometries of the base metals (12). A number of tests have been developed to measure weldabiHty. These tests generally are intended to determine the susceptibiHty of welds to cracking. 

Copper and Copper Alloys. The coppers are divided into oxygen-bearing and oxygen-free coppers. Numerous copper alloys are of commercial importance, including those alloys with zinc (brasses), with tin (phosphor bronzes), and with aluminum (aluminum bronzes) all are weldable. 

D. Stout and W. D. Doty, Weldability of Steels, 4th ed.. Welding Research Council, New York, 1987. 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

CSPE. Chlorosulfonated polyethylene (CSPE), a synthetic mbber manufactured by DuPont, is marketed under the name Hypalon. It can be produced as a self-curing elastomer designed to cure on the roof. The membrane is typically reinforced with polyester and is available in finished thicknesses of 0.75 to 1.5 mm. Because CSPE exhibits thermoplastic characteristics before it cures, it offers heat-weldable seams. After exposure on the roof, the membrane cures offering the toughness and mechanical set of a thermoset. The normal shelf life of the membrane for maintaining this thermoplastic characteristic is approximately six months. After the membrane is fully cured in the field, conventional adhesives are needed to make repairs. 

The only a-ahoy of commercial importance is Ti—5A1—2.5Sn. This ahoy is weldable, has good elevated temperature stabhity and good oxidation resistance to about 600°C, and is used for forgings and sheet-metal parts, such as aircraft engine compressor cases because of its weldabhity. The ELI version of this ahoy is used in the cryogenic area of rocket engines. However, the ahoy is difficult to produce and has not been designed into more recent aerospace systems. 

Weldability. Welding has long been an important method of fabricating copper alloy parts as these alloys ate generally readily weldable (40). 

When both components of a threaded joint are of weldable metal, the joint may be seal-welded as shown in Fig. 10-130. Seal welds may be used only to prevent leakage of threaded joints. They are not considered as contributing any strength to the joint. This type of joint is limited to new construction and is not suitable as a repair procedure, since pipe dope in the threads would interfere with welding. This method provides tight joints with a minimum of welding labor. When threaded joints used to join materials with widely dinerent coeffi-... 

Filler metal is required to conform with the requirements of Sec. IX. Backing rings (of ferrous material), when used, shall be of weldable qiiahty with sulfur hmited to 0.05 percent. Backing rings of non-ferroiis and nonmetaUic materials may be used provided they are proved satisfactory by procediire-qnaJification tests and provided their use has been approved by the designer. 

The Underwriters Standards overlap API, but include tanks that are too small for API Standards. Undei writers Standards are, however, not as detailed as API and therefore put more responsibility on the designer. They do not specify grades of steel other than requiring weldability. Designers should also place their own limits on the diameter (or thickness) of vertical tanks. They can obtain guidance from API. 

Fixed-Tube-Sheet Heat Exchang ers Fixed-tube-sheet exchangers (Fig. 11-36Z ) are used more often than any other type, and the frequency of use has been increasing in recent years. The tube sheets are welded to the shell. Usually these extend beyond the shell and serve as flanges to which the tube-side headers are bolted. This construction requires that the shell and tube-sheet materials be weldable to each other. 

Steel Carbon steel is the most common, cheapest, and most versatile metal used in industry. It has excellent ductility, permitting many cold-forming operations. Steel is also very weldable. 

Material Moist, e.g.. chlorine below dew point F)ry, e.g., fluorine above dew point Hydrogen halides, dry,J e.g., dry hydrogen cliloride, F Available forms Cold formability in wronglit and clad form Weldability Maximum strength annejiled condition x 1000 Ib/in- Coefficient of thermal expansion, millionths per F, 70-212 F Remarks ... 

Series major additive Mg Al -r 3 Mg 0.5 Mn Strong work-hardening weldable plate pressure vessels, ship superstructures. 

Production platforms are coated only in exceptional cases or for the purposes of investigation because the life of the structure is greater than the life of the coating. Therefore in the design of the cathodic protection, only the protection potential Us of the steel need be considered. Steels with an ultimate tensile strength of up to 350 N mm are used for these structures, which are weldable even in thick sections, and the hardness of the welded material can be kept to 350 HV (see Section 2.3.4 [2,10]). Aluminum anodes with the same protection effect and life as zinc anodes have much less weight. This is a very important advantage for... 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Carbon Equivalent (CE) is an approximate measure of weldability expressed in terms of the sum of carbon content and the alloy contents divided by applicable factors to relate equivalence in carbon in effectiveness in hardening—and thereby cracking. Commonly used formulas with commonly accepted but rather arbitrarily set maximums are ... 

Extensive field experience has shown the 50 Cr/50 Ni and 60 Cr/40 Ni alloys to offer the best answer to controlling fuel oil ash corrosion. Type 446 stainless steel also shows acceptable corrosion rates but must be used judiciously due to its low strength at elevated temperatures and weldability. Since components of 50 Cr/50 Ni in contact with vanadium-sodium fuel ash melts still suffer high corrosion rates, they should be designed to minimize the amount of surface area available where ash may accumulate. 

There has been some recent interest in polymers containing very small proportions (<2000 ppm) of a second comonomer. These can interfere with crystallisation and the resulting products are claimed to have improved compression strength, electrical insulation properties, weldability and transparency compared with the unmodified homopolymers. 

Somewhat between PTFE and PFA materials is the product Hostaflon TFM, which is a copolymer of TFE and a small amount of the perfluoro(propyl vinyl ether). It has improved impact strength and weldability and has been promoted as a suitable material for forming into bottles. Yet another TFE-perfluoroalkoxy copolymer was introduced by Du Pont in 1979 as Teflon EPE. This material had a somewhat lower melting point (295 °C) than the more common PFA fluoropolymers but it is no longer marketed. 

Mild steel (<0.25% carbon) is the most commonly used, readily welded construction material, and has the following typical mechanical properties (Grade 43A in BS4360 weldable structural steel) ... 

Up to 5% Mg is added to most commercial Al to improve its mechanical properties, weldability and resistance to corrosion. 

Series Mg alloys (0.3-5%) good strength and weldability coupled with excellent corrosion resistance in... 

Packing of York-Fiberbed high efficiency mist eliminators consists of ceramic, glass, polypropylene, fluoropolymer fibers. Cages and frames are fabricated from all stainless steels and other weldable alloys as well as FRP. 

These elements that insert in a pipe or specially fabricated cylindrical holder can usually be fabricated from any workable and weldable metal or alloy. In addition, most plastic that can be fabricated by molding, cutting, heat welding, or even bolting can be used. This wide array of fabrication materials allows the units to fit an extremely wide range of corrosive applications. 

Stainless steel t>pe 304 Ni 9.0 c 0.08 Annealed cold-rolled 35 85 60 160 General purpose, weldable ... 

G[Pg.200]

Other more highly alloyed types, of which a typical example is given in Table 3.11, have the designation of precipitation hardening martensitic. Relative to the simple 13% chromium types they have a substantial nickel content and low carbon with additions from molybdenum, copper, aluminium, titanium and niobium. These offer improved corrosion resistance, strength, toughness, weldability and fabrication properties, but not always together. 

All the stainless steels can be machined in the softened states, but they may present some problems unless the correct techniques are adopted. This is especially so with the austenitic grades where the extreme ductility minimises chip breaking and the work hardening may cause difficulties unless modest cuts are made. The free-cutting grades (those with high sulphur contents or selenium additions) are much easier to machine, but it must be remembered that they have somewhat reduced corrosion resistance, ductility and weldability compared to their normal counterparts. Detailed machining instructions are readily available from steel suppliers. 

Major uses of the ferritic steels have been on motor vehicles as trim and in domestic equipment such as cutlery and hollow ware, but use has also been made in refrigerators, washing machines and on sinks and similar fittings. Some types would no doubt find much wider application in the chemical field and other fields where their superior corrosion resistance would be a considerable advantage if it was not for the fact that the austenitic types have advantages (sometimes considerable) in fabrication. However, the availability of the low interstitial weldable types and the super ferritics is increasing in scope. 

More specialised alloys are covered by the DTD and L series for aircraft applications and include the high strength Al-Zn-Mg alloys. The medium strength weldable Al-Zn-Mg compositions are finding increasing utility in engineering and a national specification may be anticipated in the near future. 

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