Tin was one of the first metals known to man. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys and compounds, and use of the metal increased with advancing technology. Today, tin is an important metal in industry even though the annual tonnage used is much smaller than those of many other metals. One reason for the small tonnage is that, in most applications, only very small amounts of tin are used at a time.
Tinplate. The largest single application of tin is in manufacture of tin-plate (steel sheet coated with tin), which accounts for about 40% of total world tin consumption.
Since 1940, the traditional hot dip method of making tinplate has been largely replaced by electrodeposition of tin on continuous strips of rolled steel. Electrolytic tin-plate can be produced with either equal or unequal amounts of tin on the two surfaces of the steel base metal. Nominal coating thickness for equally coated tinplate range from 0.38 to 1.54 µm on each surface. The thicker coating on tinplate with unequal coatings (differential tinplate) rarely exceed 2.0 µm. Tinplate is produced in thickness from 0.15 to 0.60 mm.
Over 90% of world production of tinplate is used for containers (tin cans). Tinplate cans find their most important use in packaging of food products, beer and soft drinks, but also are used for holding paint, motor oil, disinfectants, detergents and polishes. Other applications of tinplate include fabrication of signs, niters, batteries, toys, gaskets, and containers for pharmaceuticals, cosmetics, fuels, tobacco and numerous other commodities.
Electroplating accounts for one of the major uses of tin and tin chemicals. Tin is used in anodes, and tin chemicals are used in formulating various electrolytes, for coating a variety of products. Tin electroplating can be performed in either acid or alkaline solutions. Sodium or potassium stannates form the bases of alkaline tin plating electrolytes that are very efficient and capable of producing high-quality deposits.
Hot Dip Coatings. Coating of steel with lead-tin alloys produces a material called tern plate. It is easily formed and easily soldered and is used as a roofing and weather sealing material and in construction of automotive gasoline tanks, signs, radiator header tanks, brackets, chassis and covers for electronic equipment and sheathing for cable and pipe. Hot dip tin coatings are used on wire for component leads as well as food handling and processing equipment.
Unalloyed Tin. There are only a few applications where tin is used unalloyed with other metals. Unalloyed tin is well recognized as the most practical lining material for handling high-purity water in distillation plants because it is chemically inert to pure water and will not contaminate the water in any way.
Tin in Alloys. Solders account for the second largest use of tin (after tinplate). Tin is an important constituent in solders because it wets and adheres to many common base metals at temperatures considerably below their melting points. Tin is alloyed with lead to produce solders with melting points lower than those of either tin or lead. Small amounts of various metals, notably antimony and silver, are added to tin-lead solders to increase their strength. These solders can be used for joints subjected to high or even subzero service temperatures.
Both solder compositions and applications of joining by soldering are many and varied. Commercially pure tin is used for soldering side seams of cans for special food products and aerosol sprays. The electronics and electrical industries employ solders containing 40 to 70% tin, which provide strong and reliable joints under a variety of environmental conditions. General-purpose solders (50Sn-50Pb and 40Sn-60Pb) are used for light engineering applications, plumbing and sheet metal work. Lower-tin solders (20 to 35% Sn, remainder Pb) are used in joining cable and in production of automobile radiators and heat exchangers. Low-tin solders are used in large amounts to fill crevices at seams and welds in automotive bodies, thereby providing smooth joints and contours. Solders containing about 2% tin (remainder lead) are used for can side seams to provide hermetic seals. Tin-zinc solders are used to join aluminum, while tin-antimony and tin-silver solders are employed in applications requiring joints with high creep resistance.
Alloys for Organ Pipes. Tin-lead alloys are used in the manufacturing of organ pipes. These materials commonly are named "spotted metal" because they develop large nucleated crystals or "spots" when solidified as strip on casting tables. The pipes that produce the diapason tones of organs generally are made of alloys with tin contents varying from 20 to 90% according to the tone required.
Pewter is a tin-base white metal containing antimony and copper. Originally, pewter was defined as an alloy of tin and lead, but to avoid toxicity and dullness of finish, lead is excluded from modern pewter. These modem compositions contain 1 to 8% antimony and 0.25 to 3.0% copper.
Bearing Materials. Tin has a low coefficient of friction, which is the first consideration in its use as a bearing material. Tin is structurally a weak metal, and when used in bearing applications it is alloyed with copper and antimony for increased hardness, tensile strength and fatigue resistance. Normally, the quantity of lead in these alloys, called tin-base babbits, is limited to 0.35 to 0.5% to avoid formation of the tin-lead eutectic, which would significantly reduce strength properties at operating temperatures.
Lead-base bearing alloys, called lead-base babbits, contain up to 10% tin and 12 to 18% antimony. In general, these alloys are inferior in strength to tin-base babbits, and this must be equated with their lower cost.
Bearing alloys must maintain a balance between softness and strength. Aluminum-tin bearing alloys represent an excellent compromise between the requirements for high fatigue strength and the need for good surface properties such as softness, seizure resistance and embed ability. Aluminum-tin bearing alloys are usually employed in conjunction with hardened steel or ductile iron crankshafts and allow significantly higher loading than tin- or lead-base bearing alloys.
Low-tin aluminum-base alloys (5 to 7% Sn) containing small amounts of strengthening elements, such as copper and nickel, are often used for connecting rods and thrust bearings in high-duty engines. Strict dimensional tolerances must be adhered to and oil contamination should be avoided. Alloys containing 20 to 40% tin, remainder aluminum, show excellent resistance to corrosion by products of oil breakdown and good embeddability, particularly in dusty environments. The higher-tin alloys have adequate strength and better surface properties, which make them useful for crosshead bearings in high-power marine diesel engines.
Battery-grid Alloys. Lead-calcium-tin alloys have been developed for storage-battery grids largely as replacements for antimonies lead alloys. Use of ternary lead-base alloys containing up to 1.3% tin has substantially reduced gassing, and therefore batteries whose grids are made of these alloys do not require periodic water additions during their working life. Two chief methods of grid manufacture are casting and fabrication of wrought alloys including punching, roll forging and expanded metal processes.
Copper Alloys. Copper-tin bronzes were some of the first alloys used by man, and these alloys continue to be used for structural and decorative purposes. True bronzes contain tin in amounts up to 10% as well as very small amounts of phosphorus. Quaternary bronzes containing 5% Sn, 5% Zn, 5% Pb, and remainder Cu are used for general-purpose castings for applications requiring reasonable strength and soundness, such as gears, pumps, and automotive fittings.
Dental alloys for making amalgams contain silver, tin, mercury, and some copper and zinc. The copper increases hardness and strength and the zinc acts as a scavenger during alloy manufacture, protecting major constituents from oxidation. Most dental alloys presently available contain 25 to 27% tin and consist mainly of the inter-metallic compound Ag and Sn. When porcelain veneers are added to gold alloys for high-grade dental restoration, 1% tin is added to the gold alloy to ensure bonding with the porcelain.
Titanium Alloys. Tin strengthens titanium alloys by forming solid solutions. Titanium can exist in the low-temperature alpha phase or the higher-temperature beta phase, which remains stable up to the melting point. In titanium alloys, relative amounts of alpha and beta phases present at the service temperature have profound effects on properties. Aluminum additions raise the transformation temperature and stabilize the alpha phase, but may cause embrittlement in amounts greater than 7%. However, with tin additions, increased strength without embrittlement can be obtained in aluminum-stabilized alpha titanium alloys. Optimum strength and workability can be obtained with 5% aluminum and 2.5% tin; in addition, this alloy has the advantage of being weldable.
Zinc and Zinc Alloys
Chemical specifications for zinc casting alloys are given in ASTM B86 for No. 2, No. 3, No. 5 and No. 7 alloys and ASTM B791 for ZA-8, ZA-12 and ZA-27.
Alloys covered by B86 are hypo-eutectic (i.e. they contain less aluminum than the eutectic chemistry of 5% Al) with a composition close to 4% Al. Alloys included in B791 are hypereutectic with an aluminum content greater than the eutectic chemistry. All of the zinc casting alloys have dendritic/eutectic microstructures, however, the hypoeutectic alloys solidify with zinc-rich dendrites, whereas hypereutectic alloys solidify with aluminum-rich dendrites.
The mechanical and physical properties of the castings, and to a lesser extent their corrosion properties, are closely linked to the specific alloy type, the casting process and quality of the castings produced, the amount of ageing or service life of a component casting and the level of impurities, amongst others.
Both Cu and Mg increase strength properties, reduce ductility and inhibit intergranular corrosion. Iron is present as small FeAl3 particles and does not influence mechanical properties unless it exceeds 0,1%. When limited to the specified amounts shown in ASTM B86 and B791 Pb, Sn and Cd do not cause intergranular corrosion or the lowering of physical and mechanical properties. Other impurity elements such as Cr, Ni, Mn, Ti, Sb, In, As, Bi and Hg do not normally occur in sufficient quantities in zinc casting alloys to be of concern.
Gravity Casting Alloys
A recent alloy development is generation of a family of zinc foundry alloys suitable for sand, permanent mold, plaster mold, shell mold and investment casting. Many of these alloys also can be die cast in cold chamber machines where strength and/or hardness beyond the properties of AG41A are required. Currently, two alloys are finding increasing application, a 12%-Al alloy (ILZRO 12) and a 27%-Al alloy (Zn-27Al). These two gravity casting zinc alloys have been designed for structural applications and should not be confused with die casting and slush casting alloys, which often are used for decorative gravity cast parts.
The mechanical properties of these zinc alloys make them attractive substitutes for cast iron and copper alloys in many structural and pressure-tight applications. Because zinc is less costly than copper, these zinc alloys have a distinct cost advantage over copper-base alloys. The ease of machining of zinc and its inherent corrosion resistance give it advantages over cast iron.
Zinc gravity casting alloys have attractive foundry properties. Due to their low melting temperatures (below 540°C) and casting temperatures, energy requirements are low. They are readily cast in thin sections - less than 2,5 mm in sand molds. Melting and casting of these alloys are virtually pollution free. No fluxing or degassing is required, and because of the low casting temperatures minimal pollution from the sand mold results.
The 12%- Al alloy is preferred for heavy sections and is suitable for permanent mold casting in both metal and graphite molds. Its permanent mold casting characteristics are similar to those of aluminum permanent mold alloys. The 27%-Al alloy should be specified when higher mechanical properties are required in thin section sand castings. Care should be taken to prevent hot spots in the mold, which contribute to underside shrinkage.
Zinc gravity casting alloys can be used for general industrial applications where strength, hardness, wear resistance or good pressure tightness is required. Zinc alloys often are employed to replace cast iron because of their similar properties and higher machinability ratings. The good bearing and wear characteristics of zinc alloys permit them to be used for bearing bushings and flanges. Other applications in which zinc alloys have been successfully substituted for cast iron or copper alloys include fuel-handling components, pulleys, electrical fittings and hardware components.
Wrought Zinc and Zinc Alloys
Wrought zinc and zinc alloys may be obtained as rolled strip, sheet and foil; extruded rod and shapes; and drawn rod and wire. These metals exhibit good resistance to corrosion in many types of service, and because the corrosion products that may form on them are white, other materials are not stained by them.
Wrought zinc has chemical characteristics particularly adapted to certain uses, such as dry batteries and photoengraver`s plate, and offers combinations of desirable physical and mechanical properties at relatively low cost. In common with many other metals and alloys, wrought zinc creeps under constant loads that are substantially less than its ultimate strength; that is, wrought zinc does not have clearly defined elastic module, and hence creep data from service tests must be used in designing for strength and rigidity under conditions of continuous stress.
All severe fabrication of wrought zinc should be done at temperatures above 20°C. Rolled zinc of the proper grade is readily drawn into a great variety of articles such as batter cups, eyelets, meter cases, novelties, flashlight reflectors and fruit-jar caps. Suitable grades of rolled zinc also are readily rolled, press formed, stamped or spun into items such as plates for addressing machines, buckles, ferrules, ornaments, nameplates, gaskets, weather-stripping and lamp parts.
The ordinary grades of wrought zinc can be soldered easily by conventional methods. The usual precautions should be observed regarding proper cleaning and fluxing. The metal must not be overheated to the point where it melts. Pulsed-arc welding may be used for joining; gas welding of zinc is used only for repair work.
Wrought zinc is easily machined using standard methods and tools. However, if it is necessary to machine zinc containing exceedingly coarse grains, the metal should be heated to a temperature between 70 and 100°C in order to avoid cleavage of crystals.
In general the same methods are used for finishing wrought zinc and zinc alloys as are used for finishing zinc-base die castings. However, in bake-enameling of wrought zinc, greater caution should be exercised in avoiding temperatures high enough to impair mechanical properties. For this reason air-dried finishes are preferable.
Titanium and Titanium alloys
Since the introduction of titanium and titanium alloys in the early 1950s, these materials have in a relatively short time become backbone materials for the aerospace, energy, and chemical industries.
The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications. Today, titanium alloys are used for demanding applications such as static and rotating gas turbine engine components. Some of the most critical and highly-stressed civilian and military airframe parts are made of these alloys.
The use of titanium has expanded in recent years to include applications in nuclear power plants, food processing plants, oil refinery heat exchangers, marine components and medical protheses.
The high cost of titanium alloy components may limit their use to applications for which lower-cost alloys, such as aluminium and stainless steels. The relatively high cost is often the result of the intristic raw material cost of metal, fabricating costs and the metal removal costs incurred in obtaining the desired final shape.
These titanium net shape technologies include powder metallurgy (P/M), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used titanium net shape technology. The annual shipment of titanium castings in the United States increased by 260% between 1979 and 1989.
As aircraft engine manufactures seek to use cast titanium at higher operating temperatures, Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo are being specified more frequently. Other advanced high-temperature titanium alloys for service up to 595oC, such as Ti-1100 and IMI-834 are being developed as castings. The alloys mentioned above exhibit the same degree of elevated-temperature superiority, as do their wrought counterparts over the more commonly used Ti-6Al-4V.
The wrought product forms of titanium and titanium-base alloys, which include forgings and typical mill products, constitute more than 70% of the market in titanium and titanium alloy production. The wrought products are the most readily available product form of titanium-base materials, although cast and powder metallurgy (P/M) products are also available for applications that require complex shapes or the use of P/M techniques to obtain microstructures not achievable by conventional ingot metallurgy.
Powder metallurgy of titanium has not gained wide acceptance and is restricted to space and missile applications. The primary reasons for using titanium-base products are its outstanding corrosion resistance of titanium and its useful combination of low density (4.5 g/cm3) and high strength. The strengths vary from 480 MPa for some grades of commercial titanium to about 1100 MPa for structural titanium alloy products and over 1725 MPa for special forms such as wires and springs.
Another important characteristic of titanium- base materials is the reversible transformation of the crystal structure from alpha (, hexagonal close-packed) structure to beta (, body-centered cubic) structure when the temperatures exceed certain level. This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum.
Pure titanium wrought products, which have minimum titanium contents ranging from about 98,635 to 99,5 wt%, are used primarily for corrosion resistance. Titanium products are also useful for fabrication but have relatively low strength in service.
Titanium has the following advantages:
· Good strength
· Resistance to erosion and erosion-corrosion
· Very thin, conductive oxide surface film
· Hard, smooth surface that limits adhesion of foreign materials
· Surface promotes dropwise condensation
Commercially pure titanium with minor alloy contents include various titanium-palladium grades and alloy Ti-0,3Mo-0,8Ni (ASTM grade 12 or UNS R533400). The alloy contents allow improvements in corrosion resistance and/or strength.
Titanium-palladium alloys with nominal palladium contents of about 0,2% Pd are used in applications requiring excellent corrosion resistance in chemical processing or storage applications where the environment is mildly reducing or fluctuates between oxidizing and reducing.
Alloy Ti-0,3Mo-0,8Ni (UNS R533400, or ASTM grade 12) has applications similar to those for unalloyed titanium but has better strength and corrosion resistance. However, the corrosion resistance of this alloy is not as good as the titanium-palladium alloys. The ASTM grade 12 alloy is particularly resistant to crevice corrosion in hot brines.
Titanium alloy compositions of various titanium alloys. Because the allotropic behavior of titanium allows diverse changes in microstructures by variations in thermomechanical processing, a broad range of properties and applications can be served with a minimum number of grades. This is especially true of the alloys with a two-phase, , crystal structure.
The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy. This alloy is well understood and is also very tolerant on variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminium. Alloy Ti-6Al-4V, which has limited section size hardenability, is most commonly used in the annealed condition.
Other titanium alloys are designed for particular application areas. For example:
§ Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and Ti-6Al-2Sn-4Zr-6Mo for high strength in heavy sections at elevated temperatures.
§ Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) for creep resistance
§ Alloys Ti-6Al-2Nb-ITa-Imo and Ti-6Al-4V-ELI are designed both to resist stress corrosion in aqueous salt solutions and for high fracture toughness
§ Alloy Ti-5Al-2,5Sn is designed for weldability, and the ELI grade is used extensively for cryogenic applications
§ Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.
Welding has the greatest potential for affecting material properties. In all types of welds, contamination by interstitial impurities such as oxygen and nitrogen must be minimized to maintain useful ductility in the weldment. Alloy composition, welding procedure, and subsequent heat treatment are highly important in determining the final properties of welded joints.
Some general principles can be summarized as follows:
§ Welding generally increases strength and hardness
§ Welding generally decreases tensile and bend ductility
§ Welds in unalloyed titanium grades 1, 2 and 3 do not require post-weld treatment unless the material will be highly stressed in a strongly reducing atmosphere
§ Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn have a high likelihood of fracturing with little or no plastic straining.
Titanium and titanium alloys are heat treated for the following purposes:
§ To reduce residual stresses developed during fabrication
§ To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
§ To increase strength (solution treating and aging)
§ To optimise special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.
Lead and Lead Alloys
Lead was one of the first metals known to man. Probably the oldest lead artifact is a figure made about 3000 BC. All civilizations, beginning with the ancient Egyptians, Assyrians, and Babylonians, have used lead for many ornamental and structural purposes. Many magnificent buildings erected in the 15th and 16th centuries still stand under their original lead roofs.
Compositions and Grades
Bellow is listed the Unified Numbering System (UNS) designations for various pure lead grades and lead-base alloys.
· Pure leads L50000 - L50099
· Lead - silver alloys L50100 - L50199
· Lead - arsenic alloys L50300 - L50399
· Lead - barium alloys L50500 - L50599
· Lead - calcium alloys L50700 - L50899
· Lead - cadmium alloys L50900 - L50999
· Lead - copper alloys L51100 - L51199
· Lead - indium alloys L51500 - L51599
· Lead - lithium alloys L51700 - L51799
· Lead - antimony alloys L52500 - L53799
· Lead - tin alloys L54000 - L55099
· Lead - strontium alloys L55200 - L55299
Grades of lead
Grades are pure lead (also called corroding lead) and common lead (both containing 99.94% min lead), and chemical lead and acid-copper lead (both containing 99.90% min lead). Lead of higher specified purity (99.99%) is also available in commercial quantities. Specifications other than ASTM B 29 for grades of pig lead include federal specification QQ-L-171, German standard DIN 1719, British specification BS 334, Canadian Standard CSA-HP2, and Australian Standard 1812.
Corroding Lead. Most lead produced in the United States is pure (or corroding) lead (99.94% min Pb). Corroding lead which exhibits the outstanding corrosion resistance typical of lead and its alloys. Corroding lead is used in making pigments, lead oxides, and a wide variety of other lead chemicals.
Chemical Lead. Refined lead with a residual copper content of 0.04 to 0.08% and a residual silver content of 0.002 to 0.02% is particularly desirable in the chemical industries and thus is called chemical lead.
Copper-bearing lead provides corrosion protection comparable to that of chemical lead in most applications that require high corrosion resistance. Common lead, which contains higher amounts of silver and bismuth than does corroding lead, is used for battery oxide and general alloying.
Lead-Base Alloys
Because lead is very soft and ductile, it is normally used Commercially as lead alloys. Antimony, tin, arsenic, and calcium are the most common alloying elements. Antimony generally is used to give greater hardness and strength, as in storage battery grids, sheet, pipe, and castings. Antimony contents of lead-antimony alloys can range from 0.5 to 25%, but they are usually 2 to 5%.
Lead-calcium alloys have replaced lead-antimony alloys in a number of applications, in particular, storage battery grids and casting applications. These alloys contain 0.03 to 0.15% Ca. More recently, aluminum has been added to calcium-lead and calcium-tin-lead alloys as a stabilizer for calcium. Adding tin to lead or lead alloys increases hardness and strength, but lead-tin alloys are more commonly used for their good melting, casting, and wetting properties, as in type metals and solders. Tin gives the alloy the ability to wet and bond with metals such as steel and copper; unalloyed lead has poor wetting characteristics. Tin combined with lead and bismuth or cadmium forms the principal ingredient of many low-melting alloys.
Arsenical lead (UNS L50310) is used for cable sheathing. Arsenic is often used to harden lead-antimony alloys and is essential to the production of round dropped shot.
Properties of Lead
The properties of lead that make it useful in a wide variety of applications are density, malleability, lubricity, flexibility, electrical conductivity, and coefficient of thermal expansion, all of which are quite high; and elastic modulus, elastic limit, strength, hardness, and melting point, all of which are quite low. Lead also has good resistance to corrosion under a wide variety of conditions. Lead is easily alloyed with many other metals and casts with little difficulty.
The high density of lead (11.35 g/cm3, at room temperature) makes it very effective in shielding against x-rays and gamma radiation. The combination of high density, high limpness (low stiffness), and high damping capacity makes lead an excellent material for deadening sound and for isolating equipment and structures from mechanical vibrations.
Malleability, softness, and lubricity are three related properties that account for the extensive use of lead in many applications.
The low tensile strength and low creep strength of lead must always be considered when designing lead components. The principal limitation on the use of lead as a structural material is not its low tensile strength but its susceptibility to creep. Lead continuously deforms at low stresses and this deformation ultimately results in failure at stresses far below the ultimate tensile strength. The low strength of lead does not necessarily preclude its use. Lead products can be designed to be self-supporting, or inserts or supports of other materials can be provided. Alloying with other metals, notably calcium or antimony, is a common method of strengthening lead for many applications. In general, consideration should always be given to supporting lead structures by lead-covered steel straps. When lead is used as a lining in a structure made of a stronger material, the lining can be supported by bonding it to the structure. With the development of improved bonding and adhesive techniques, composites of lead with other materials can be made. Composites have improved strength yet also retain the desirable properties of lead.
Products and Applications
The most significant applications of lead and lead alloys are lead-acid storage batteries (in the grid plates, posts, and connector straps), ammunition, cable sheathing, and building construction materials (such as sheet, pipe, solder, and wool for caulking). Other important applications include counterweights, battery clamps and other cast products such as: bearings, ballast, gaskets, type metal, terneplate, and foil. Lead in various forms and combinations is finding increased application as a material for controlling sound and mechanical vibrations. Also, in many forms it is important as shielding against x-rays and, in the nuclear industry, gamma rays. In addition, lead is used as an alloying element in steel and in copper alloys to improve machinability and other characteristics, and it is used in fusible (low-melting) alloys for fire sprinkler systems.
Battery Grids. The largest use of lead is in the manufacture of lead-acid storage batteries. These batteries consist of a series of grid plates made from either cast or wrought calcium lead or antimonial lead that is pasted with a mixture of lead oxides and immersed in sulfuric acid.
Type metals, a class of metals used in the printing industry, generally consist of lead-antimony and tin alloys. Small amounts of copper are added to increase hardness for some applications.
Cable Sheathing. Lead sheathing extruded around electrical power and communication cables gives the most durable protection against moisture and corrosion damage, and provides mechanical protection of the insulation. Chemical lead, 1% antimonial lead, and arsenical lead are most commonly employed for this purpose.
Sheet. Lead sheet is a construction material of major importance in chemical and related industries because lead resists attack by a wide range of chemicals. Lead sheet is also used in building construction for roofing and flashing, shower pans, flooring, x-ray and gamma-ray protection, and vibration damping and soundproofing. Sheet for use in chemical industries and building construction is made from either pure lead or 6% antimonial lead. Calcium-lead and calcium-lead-tin alloys are also suitable for many of these applications.
Pipe. Seamless pipe made from lead and lead alloys is readily fabricated by extrusion. Because of its corrosion resistance and flexibility, lead pipes finds many uses in the chemical industry and in plumbing and water distribution system. Pipe for these applications is made from either chemical lead or 6% antimonial lead.
Solders in the tin-lead system are the most widely used of all joining materials. The low melting range of tin-lead solders makes them ideal for joining most metals by convenient heating methods with little or no damage to heat-sensitive parts. Tin-lead solder alloys can be obtained with melting temperatures as low as 182 °C and as high as 315 °C. Except for the pure metals and the eutectic solder with 63% Sn and 37% Pb, all tin-lead solder alloys melt within a temperature range that varies according to the alloy composition.
Lead-base bearing alloys, which are called lead-base babbitt metals, vary widely in composition but can be categorized into two groups:
· Alloys of lead, tin, antimony, and, in many instances, arsenic
· Alloys of lead, calcium, tin, and one or more of the alkaline earth metals
Ammunition. Large quantities of lead are used in ammunition for both military and sporting purposes. Alloys used for shot contain up to 8% Sb and 2% As; those used for bullet cores contain up to 2% Sb.
Terne Coatings. Long terne steel sheet is carbon steel sheet that has been continuously coated by various hot dip processes with terne metal (lead with 3 to 15% Sn). Its excellent solderability and special corrosion resistance make the product well-suited for this application.
Lead foil, generally known as composition metal foil, is usually made by rolling a sandwich of lead between two sheets of tin, producing a tight union of the metals.
Fusible Alloys. Lead alloyed with tin, bismuth, cadmium, indium, or other elements, either alone or in combination, forms alloys with particularly low melting points. Some of these alloys, which melt at temperatures even lower than the boiling point of water, are referred to as fusible alloys.
Anodes made of lead alloys are used in the electrowinning and plating of metals such as manganese, copper, nickel, and zinc. Rolled lead-calcium-tin and lead-silver alloys are the preferred anode materials in these applications, because of their high resistance to corrosion in the sulfuric acid used in electrolytic solutions. Lead anodes also have high resistance to corrosion by seawater, making them economical to use in systems for the cathodic protection of ships and offshore rigs.
Nickel and Nickel Alloys
Nickel has been used in alloys that date back to the dawn of civilization. Chemical analysis of artifacts has shown that weapons, tools, and coins contain nickel in varying amounts.
Nickel in elemental form or alloyed with other metals and materials has made significant contributions to our present-day society and promises to continue to supply materials for an even more demanding future.
Nickel is a versatile element and will alloy with most metals. Complete solid solubility exists between nickel and copper. Wide solubility ranges between iron, chromium, and nickel make possible many alloy combinations.
Applications and Characteristics of Nickel Alloys
Nickel and nickel alloys are used for a wide variety of applications, the majority of which involve corrosion resistance and/or heat resistance. Some of these include:
· Aircraft gas turbines
· Steam turbine power plants
· Medical applications
· Nuclear power systems
· Chemical and petrochemical industries
A number of other applications for nickel alloys involve the unique physical properties of special-purpose nickel-base or high-nickel alloys. These include:
· Low-expansion alloys
· Electrical resistance alloys
· Soft magnetic alloys
· Shape memory alloys
Heat-Resistant Applications. Nickel-base alloys are used in many applications where they are subjected to harsh environments at high temperatures. Nickel-chromium alloys or alloys that contain more than about 15% Cr are used to provide both oxidation and carburization resistance at temperatures exceeding 760°C.
Corrosion Resistance. Nickel-base alloys offer excellent corrosion resistance to a wide range of corrosive media. However, as with all types of corrosion, many factors influence the rate of attack. The corrosive media itself is the most important factor governing corrosion of a particular metal.
Low-Expansion Alloys Nickel was found to have a profound effect on the thermal expansion of iron. Alloys can be designed to have a very low thermal expansion or display uniform and predictable expansion over certain temperature ranges.
Iron-36% Ni alloy (Invar) has the lowest expansion of the Fe-Ni alloys and maintains nearly constant dimensions during normal variations in atmospheric temperature.
The addition of cobalt to the nickel-iron matrix produces alloys with a low coefficient of expansion, a constant modulus of elasticity, and high strength.
Electrical Resistance Alloys. Several alloy systems based on nickel or containing high nickel contents are used in instruments and control equipment to measure and regulate electrical characteristics (resistance alloys) or are used in furnaces and appliances to generate heat (heating alloys).
Types of resistance alloys containing nickel include:
· Cu-Ni alloys containing 2 to 45% Ni
· Ni-Cr-Al alloys containing 35 to 95% Ni
· Ni-Cr-Fe alloys containing 35 to 60% Ni
· Ni-Cr-Si alloys containing 70 to 80% Ni
Types of resistance heating alloys con-taining nickel include:
· Ni-Cr alloys containing 65 to 80% Ni with 1.5% Si
· Ni-Cr-Fe alloys containing 35 to 70% Ni with 1.5% Si + l% Nb
Soft Magnetic Alloys. Two broad classes of magnetically soft materials have been developed in the Fe-Ni system. The high-nickel alloys (about 79% Ni with 4 to 5% Mo; bal Fe) have high initial permeability and low saturation induction.
Shape Memory Alloys.Metallic materials that demonstrate the ability to return to their previously defined shape when subjected to the appropriate heating schedule are referred to as shape memory alloys. Nickel-titanium alloys (50Ni-50Ti) are one of the few commercially important shape memory alloys.
Commercial Nickel and Nickel Alloys
The commercial forms of nickel and nickel-base alloys are fully austenitic and are used/selected mainly for their resistance to high temperature and aqueous corrosion.
Commercially Pure and Low-Alloy Nickels. Nickel is supplied to the producers of nickel alloys in powder, pellets, or anode forms. This has led to a whole series ofalloy modifications, with controlled compositions having nickel contents ranging from about 94% to virtually 100%.
These materials are characterized by high density, offering magnetic and electronic property capabilities. They also offer excellent corrosion resistance to reducing environments, along with reasonable thermal transfer characteristics. Some nickels of commercial importance include: Nickel 200, Nickel 201, Nickel 205, Nickel 270 and 290, Permanickel Alloy 300, Duranickel Alloy 301.
Nickel-copper alloys have been found to possess excellent corrosion resistance in reducing chemical environments and in sea water, where they deliver excellent service in nuclear submarines and various surface vessels. By changing the various proportions of nickel and copper in the alloy, a whole series of alloys with different electrical resistivities and Curie points can be created. Some nickel-copper alloys of commercial importance include: Alloy 400 (66% Ni, 33% Cu), Alloy R-405, Alloy K-500.
The nickel-chromium and nickel-chromium-iron series of alloys led the way to higher strength and resistance to elevated temperatures. Today they also form the basis for both commercial and military power systems. Two ofthe earliest developed Ni-Cr and Ni-Cr-Fe alloys were:
· Alloy 600 (76Ni-15Cr-8Fe).
· Nimonic alloys (80Ni-20Cr + Ti/Al).
Some high-temperature variants include:
· Alloy 601. Lower nickel (61%) content with aluminum and silicon additions for improved oxidation and nitriding resistance
· Alloy X750. Aluminum and titanium additions for age hardening
· Alloy 718. Titanium and niobium additions to overcome strainage cracking problems during welding and weld repair
· Alloy X (48Ni-22Cr-18Fe-9Mo + W). High-temperature flat-rolled product for aerospace applications
· Waspaloy (60Ni-19Cr-4Mo-3Ti-1.3Al). Proprietary alloy for jet engine applications
Some corrosion-resistant variants in the Ni-Cr-Fe system include:
· Alloy 625. The addition of 9% Mo plus 3% Nb offers both high-temperature and wet corrosion resistance; resists pitting and crevice corrosion
· Alloy G3/G30 (Ni-22Cr-19Fe-7Mo-2Cu). The increased molybdenum content in these alloys offers improved pitting and crevice corrosion resistance
· Alloy C-22 (Ni-22Cr-6Fe-14Mo-4W). Superior corrosion resistance in oxidizing acid chlorides, wet chlorine, and other severe corrosive environments
· Alloy C-276 (17% Mo plus 3.7W). Good seawater corrosion resistance and excellent pitting and crevice corrosion resistance
· Alloy 690 (27% Cr addition). Excellent oxidation and nitric acid resistance; specified for nuclear waste disposal by the vitreous encapsulation method
Iron-Nickel-Chromium Alloys. This series of alloys has also found extensive use in the high-temperature petrochemical environments, where sulfur-containing feedstocks (naphtha and heavy oils) are cracked into component distillate parts. Not only were they resistant to chloride-ion stress-corrosion cracking, but they also offered resistance to polythionic acid cracking. Some alloys of commercial importance include:
· Alloy 800 (Fe-32Ni-21Cr). The basic alloy in the Fe-Ni-Cr system; resistant to oxidation and carburization at elevated temperatures
· Alloy 800HT. Similar to 800H with further modification to combined titanium and aluminum levels (0.85 to 1.2%) to ensure optimum high-temperature properties
· Alloy 801. Increased titanium content (0.75 to 1.5%); exceptional resistance to polythionic acid cracking
· Alloy 802. High-carbon version (0.2 to 0.5%) for improved strength at high temperatures
· Alloy 825 (Fe-42Ni-21.5Cr-2Cu). Stabilized with titanium addition (0.6 to 1.2%). Also contains molybdenum (3%) for pitting resistance in aqueous corrosion applications. Copper content bestows resistance to sulfuric acid
· Alloy 925. Addition of titanium and aluminum to 825 composition for strengthening through age hardening
The 800 alloy series offers excellent strength at elevated temperature (creep and stress rupture).
Some corrosion variants in the Fe-Ni-Cr system include:
· 20Cb3 (Fe-35Ni-20Cr-3.5Cu-2.5Mo + Nb). This alloy was developed for the handling of sulfuric acid environments
· 20Mo-4 and 20Mo-6 (Fe-36Ni-23Cr-5Mo + Cu). Increased corrosion resistance in pulp and paper industry environments.
Controlled-expansion alloys include alloys in both the Fe-Ni-Cr and Fe-Ni-Co series. Some alloys of commercial importance include:
· Alloy 902 (Fe-42Ni-5Cr with 2.2 to 2.75% Ti and 0.3 to 0.8% Al). This is an alloy with a controllable thermoelastic coefficient
· Alloys 903, 907, 909 (42Fe-38Ni-13Co with varying aging elements such as niobium, titanium, and aluminum). These alloys offer high strength and low coefficient of thermal expansion
The 900 alloy series offers very unusual characteristics and properties. Alloys 903, 907, and 909 were all designed to provide high strength and low coefficient of thermal expansion for applications up to 650 °C.
Nickel-lron Low-Expansion Alloys. This series of alloys plays a very important role in both the lamp industry and electronics, where glass-to-metal seals in encapsulated components are important. The nickel alloys are chosen for a variety of reasons.
Some alloys of commercial importance include:
· Invar (Fe-36Ni). This alloy has the lowest thermal expansion of any metal from ambient to 230°C (450°F)
· Alloy 42 (Fe-42Ni). This alloy has the closest thermal expansion match to alumina, beryllia, and vitreous glass
· Alloy 426. Additions of 6% Cr are added to this alloy for vacuum-tight sealing applications
· Alloy 52 (Fe-51.5Ni). This alloy has a thermal expansion that closely matches vitreous potash-soda-lead glass.
Soft Magnetic Alloys. The nickel-iron alloys also offer an interesting set of magnetic permeability properties, which have played an important part in switchgear and for direct current (dc) motor and generator designs.
Welding Alloys. Welding products for nickel alloys have similar compositions to the base metals, although additions of aluminum, titanium, magnesium, and other elements are made to the filler metals and welding electrodes to ensure proper deoxidation of the molten weld pool and to over-come any hot-short cracking and malleability problems.
Magnesium and magnesium alloys
Application, Alloy and Temper Designation
Magnesium and magnesium alloys are used in a wide variety of structural applications include automotive, industrial, materials-handling, commercial and aerospace equipment.
The automotive applications include clutch and brake pedal support brackets, steering column lock housings, and manual transmissions housings. In industrial machinery magnesium alloys are used for parts that operate at high speeds and thus must be lightweight to minimize inertial forces. Commercial applications include hand-held tools, luggage, computer housings, and ladders. Magnesium alloys are valuable for aerospace applications because they are lightweight and exhibit good strength and stiffness at both room and elevated temperatures.
Magnesium is also applied in various nonstructural applications. It is used as an alloying element in alloys of aluminium, zinc, lead, and other nonferrous metals. It is used as an oxygen scavenger and desulfurizer in the manufacture of nickel and copper alloys, as a desulfurizer in the iron and steel industry; and as a reducing agent in the production of beryllium and titanium. Gray iron foundries use magnesium and magnesium-containing alloys as ladle addition agents introduced just before the casting is poured. Magnesium is also being used in pyrotechnics.
Designation for alloys shall consists of not more than two letters representing the alloying elements specified in the greatest amount, arranged in order of decreasing percentages, or in alphabetical order if equal percentages, followed by the respective percentages rounded off to whole numbers and a serial letter. The full name of the base metals precedes the designation, but it is omitted for brevity when the base metal being referred to is obvious.
A standard system of alloy and temper designations, according to ASTM B 275, is explained in the table bellow.
First part | Second part | Third part | Forth part |
Indicates the two principal alloying elements | Indicates the amounts of the two principal alloying elements | Distinguishes between different alloys with the same percentages of the two principal alloying elements | Indicates condition (temper) |
Consists of two code letters representing the two main alloying elements arranged in order of decreasing percentage (or alphabetically if percentages are equal) | Consists of two numbers corresponding to rounded-off percentages of the two main alloying elements and arranged in same order as alloy designations in first part | Consists of a letter of the alphabet assigned in order as compositions become standard | Consists of a letter followed by a number (separated from the third part of the designation by a hyphen) |
A-aluminum B-bismuth C-copper D-cadmium E-rare earth F-iron G-magnesium H-thorium K-zirconium L-lithium M-manganese N-nickel P-lead Q-silver R-chromium S-silicon T-tin W-yttrium Y-antimony Z-zinc | Whole numbers | Letters of alphabet except I and O | F-as fabricated O-as annealed H10 and H11- slightly strain hardened H23,H24 and H26- strain hardened and partially annealed T4-solution heat treated T5-artificially aged only T6-solution heat treated and artificially aged T8-solution heat treated, cold worked and artificially aged |
As an example of this designation system, consider magnesium alloy AZ81A-T4.
The first part of the designation, AZ, signifies that aluminium and zinc are the two principal alloying elements.
The second part of the designation, 81, gives the rounded-off percentages of aluminium and zinc (8 and 1, respectively).
The third part, A, indicates that it is the fifth alloy standardized with 8% Al and 1% Zn as the principal alloying additions.
The fourth part, T4, denotes that the alloy is solution heat-treated.
Aluminum and Aluminum Alloys
Non-Heat-Treatable Alloys
The initial strength of alloys in this group depends upon the hardening effect of elements such as manganese, silicon, iron and magnesium, singly or in various combinations. The non-heat-treatable alloys are usually designated, therefore, in the 1xxx, 3xxx, 4xxx, or 5xxx series.
Since these alloys are work-hard-enable, further strengthening is made possible by various degrees of cold working. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated temperature treatment called stabilizing to ensure stability of properties.
Heat-Treatable Alloys
The initial strength of alloys in this group is enhanced by the addition of alloying elements such as copper, magnesium, zinc, and silicon. Since these elements in various combinations show increasing solid solubility in aluminum with increasing temperature, it is possible to subject them to thermal treatments that will impart pronounced strengthening.
These treatments include solution heat treatment, quenching and precipitation or age, hardening. By the proper combination of solution heat treatment, quenching, cold working and artificial aging, the highest strengths are obtained.
Annealing characteristics
All wrought aluminum alloys are available in annealed form. In addition, it may be desirable to anneal an alloy from any other initial temper, after working, or between successive stages of working such as in deep drawing.
Effect of Alloying Elements
1xxx series - Aluminum of 99 percent or higher purity has many applications, especially in the electrical and chemical fields. Excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties and excellent workability characterize these compositions. Moderate increases in strength may be obtained by strain-hardening. Iron and silicon are the major impurities.
2xxx series - Copper is the principal alloying element in this group often with magnesium as secondary addition. These alloys require solution heat-treatment to obtain optimum properties. In some instances artificial aging is employed to further increase the mechanical properties. This treatment materially increases yield strength, with attendant loss in elongation. Its effect on tensile strength is not so significant. The alloys in this series do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions they may be subject to intergranular corrosion.
3xxx series - Manganese is the major alloying element of alloys in this group, which are generally non-heat-treatable. Because only a limited percentage of manganese, up to about 1.5 percent, can be effectively added to aluminum, it is used as a major element in only a few instances.
4xxx series - The major alloying element of this group is silicon, which can be added in sufficient quantities (up to 12%) to cause substantial lowering of the melting point without producing brittleness in the resulting alloys. For these reasons aluminum-silicon alloys are used in welding wire and as brazing alloys where a lower melting point than that of the parent metal is required.
5xxx series - Magnesium is one of the most effective and widely used alloying elements for aluminum. When it is used as the major alloying element or with manganese, the result is a moderate to high strength non-heat-treatable alloy. Alloys in this series possess good welding characteristics and good resistance to corrosion in marine atmosphere.
6xxx series - Alloys in this group contain silicon and magnesium in approximate proportions to form magnesium silicone, thus making them heat-treatable. Though less strong than most of the 2xxx or 7xxx alloys, the magnesium-silicon alloys possess good formability and corrosion resistance, with medium strength.
7xxx series – Zinc in amounts of 1 to 8% is the major alloying element in this group, and when coupled with magnesium and copper (or without copper) results in heat-treatable alloys of very high strength. Usually other elements such as manganese and chromium are also added in small quantities. The out-standing member of this group is 7075, 7050 and 7049, which is among the highest strength alloys available and is used in air-frame structures and for highly stressed parts.
Cobalt and cobalt alloys
Cobalt is useful in applications that utilize its magnetic properties, corrosion resistance, wear resistance, and/or its strength at elevated temperatures.
This article provides a general overview of cobalt-base alloys as wear-resistant, corrosion-resistant, and/or heat-resistant materials. Particular emphasis is placed on cobalt-base alloys for wear resistance, because this is the single largest application area of cobalt-base alloys. In heat-resistant applications, cobalt is more widely used as an alloying element in nickel-base alloys with cobalt tonnages in excess of those used in cobalt-base heat-resistant alloys.
Elemental Cobalt
Physical Properties. With an atomic number of 27, cobalt falls between iron and nickel on the periodic table. The density of cobalt is 8.8 g/cm3 similar to that of nickel. Its thermal expansion coefficient lies between those of iron and nickel. At temperatures below 417°C cobalt exhibits a hexagonal close-packed structure. Between 417°C and its melting point of 1493°C, cobalt has a face-centered cubic structure.
The elastic modulus of cobalt is about 210 GPa (30 x 106 psi) in tension and about 183 GPa (26.5 x 106 psi) in compression.
Uses of Cobalt. As well as forming the basis of the cobalt-base alloys discussed in this article, cobalt is also an important ingredient in other materials:
· Paint pigments
· Nickel-base superalloys
· Cemented carbides and tool steels
· Magnetic materials
· Artificial -ray sources
Of these applications, paint pigment represents the single largest use of cobalt.
In the nickel-base superalloys, cobalt (which is present typically in the range 10 to 15 wt%) provides solid solution strengthening and decreases the solubility of aluminum and titanium, thereby increasing the volume fraction of gamma prime (?,) precipitate.
The role of cobalt in cemented carbides is to provide a ductile bonding matrix for tungsten-carbide particles. The commercially significant cemented carbides contain cobalt in the range of 3 to 25 wt%. As cutting tool materials, cemented carbides with 3 to 12 wt% Co are commonly used.
Cobalt, which is naturally ferromagnetic, provides resistance to demagnetization in several groups of permanent magnet materials. These include the aluminum-nickel-cobalt alloys (in which cobalt ranges from about 5 to 35 wt%), the iron-cobalt alloys (approximately 5 to 12 wt%). and the cobalt rare-earth intermetallics (which have some of the highest magnetic properties of all known materials).
The artificial isotope cobalt-60 is an important ?-ray source in medical and industrial applications.
Cobalt-Base Alloys
As a group, the cobalt-base alloys may be generally described as wear resistant, corrosion resistant, and heat resistant (strong even at high temperatures).
As a group, the cobalt-base alloys may be generally described as wear resistant, corrosion resistant, and heat resistant (strong even at high temperatures). Many of the properties of the alloys arise from the crystallographic nature of cobalt (in particular its response to stress), the solid-solution-strengthening effects of chromium, tungsten, and molybdenum, the formation of metal carbides, and the corrosion resistance imparted by chromium. Generally the softer and tougher compositions are used for high-temperature applications such as gas-turbine vanes and buckets. The harder grades are used for resistance to wear.
Historically, many of the commercial cobalt-base alloys are derived from the cobalt-chromium-tungsten and cobalt-chromium-molybdenum ternaries first investigated by Elwood Haynes in the beginning of 20th century. He discovered the high strength and stainless nature of the binary cobalt-chromium alloy, and he later identified tungsten and molybdenum as powerful strengthening agents within the cobalt-chromium system. When he discovered these alloys, Haynes named them the Stellite alloys after the Latin stella (star), because of their star-like luster. Having discovered their high strength at elevated temperatures, Haynes also promoted the use of Stellite alloys as cutting tool materials.
Cobalt-Base Wear-Resistant Alloys
The cobalt-base wear alloys of today are little changed from the early alloys of Elwood Haynes. The most important differences relate to the control of carbon and silicon (which were imparities in the early alloys). Indeed, the main differences in the current Stellite alloy grades are carbon and tungsten contents (hence the amount and type of carbide formation in the microstructure during solidification). Carbon content influences hardness, ductility, and resistance to abrasive wear. Tungsten also plays an important role in these properties.
Chemical composition of Stellite alloys is approximately:
Cr ~ 25-30%; Mo = 1% max; W = 2-15%; C ~ 0.25-3.3%; Fe = 3% max; Ni = 3% max; Si = 2% max; Mn = 1% max; Co = rest of balance.
Types of wear. There are several distinct types of wear which generally fall into three main categories:
· Abrasive wear
· Sliding wear
· Erosive wear.
The type of wear encountered in a particular application is an important factor that influences the selection of a wear-resistant material.
Abrasive wear is encountered when hard particles, or hard projections (on a counter-face) are forced against, and moved relative to a surface. The terms high and low stress abrasion relate to the condition of the abrasive medium (be it hard particles or projections) after interaction with the surface. If the abrasive medium is crushed, then the high stress condition is said to prevail. If the abrasive medium remains intact, the process is described as low stress abrasion. Typically, high stress abrasion results from the entrapment of hard particles between metallic surfaces (in relative motion), while low stress abrasion is encountered when moving surfaces come into contact with packed abrasives, such as soil and sand.
In alloys such as the cobalt-base wear alloys, which contain a hard phase, the abrasion resistance generally increases as the volume fraction of the hard phase increases. Abrasion resistance is, however, strongly influenced by the size and shape of the hard phase precipitates within the microstructure and the size and shape of the abrading species.
Sliding Wear. Of the three major types of wear, sliding is perhaps the most complex, not in concept, but in the way different materials respond to sliding conditions. Sliding wear is a possibility whenever two surfaces are forced together and moved relative to one another. The chances of damage are increased markedly if the two surfaces are metallic in nature, and if there is little or no lubrication present.
Cobalt-Base High-Temperature Alloys
For many years, the predominant user of high-temperature alloys was the gas turbine industry. In the case of aircraft gas turbines, the chief material requirements were elevated-temperature strength, resistance to thermal fatigue, and oxidation resistance. For land-base gas turbines, which typically burn lower grade fuels and operate at lower temperatures, sulfidation resistance was the major concern.
Today, the use of high-temperature alloys is more diversified, as more efficiency is sought from the burning of fossil fuels and waste, and as new chemical processing techniques are developed.
In general, cobalt-base high-temperature alloys have the following chemical composition: Cr = 20-23%; W = 7-15%; Ti = 10-22%; Fe = 3% max; C = 0.1-0.6%; Co = rest of balance.
Although cobalt-base alloys are not as widely used as nickel and nickel-iron alloys in high-temperature applications, cobalt-base high-temperature alloys nevertheless play an important role, by virtue of their excellent resistance to sulfidation and their strength at temperatures exceeding those at which the gamma-prime- and gamma-double-prime-precipitates in the nickel and nickel-iron alloys dissolve. Cobalt is also used as an alloying element in many nickel-base high-temperature alloys.
Cobalt-Base Corrosion-Resistant Alloys
Although the cobalt-base wear-resistant alloys possess some resistance to aqueous corrosion, they are limited by grain boundary carbide precipitation, the lack of vital alloying elements in the matrix (after formation of the carbides or Laves precipitates) and, in the case of the cast and weld overlay materials, by chemical segregation in the microstructure.
By virtue of their homogeneous microstructures and lower carbon contents, the wrought cobalt-base high-temperature alloys (which typically contain tungsten rather than molybdenum) are even more resistant to aqueous corrosion, but still fall well short of the nickel-chromium-molybdenum alloys in corrosion performance.
To satisfy the industrial need for alloys which exhibit outstanding resistance to aqueous corrosion, yet share the attributes of cobalt as an alloy base (resistance to various forms of wear, and high strength over a wide range of temperatures), several low-carbon, wrought cobalt-nickel-chromium-molybdenum alloys are produced.
Chemical composition of these alloys is: Cr = 20-25%; W = 2%; Mo = 5-10%; Ni = 9-35%; Fe = 3% max; C = 0.8% max; N = 0.1% max; Co = rest of balance.
Selection and Application of Copper Alloy Castings
Copper alloy castings are used in applications that require superior corrosion resistance, high thermal or electrical conductivity, good bearing-surface qualities or other special properties. Casting makes it possible to produce parts whose shape cannot be easily obtained by fabricating methods such as forming or machining.
Composition of copper casting alloys may differ from those of their wrought counterparts for various reasons. Generally, casting permits greater latitude in the use of alloying elements, because the effects of composition on hot or cold working properties are not important.
Castability should not be confused with "fluidity", which is the ability of a molten alloy to fill a mold cavity completely in every detail. Castability, on the other hand, is the ease with which an alloy response to ordinary foundry practice, without requiring special techniques for gating, risering, melting, sand conditioning or any of the other factors involved in making good castings.
High fluidity often ensures good castability, but it is not solely responsible for that quality in a casting alloy.
Foundry alloys generally are classed as "high shrinkage" or "low shrinkage". To the former class belong the manganese bronzes, aluminum bronzes, silicon bronzes, silicon brasses and some nickel silvers. They are more fluid than the low-shrinkage red brasses, more easily poured, and give high-grade castings in sand, permanent mold, plaster, die and centrifugal casting processes. With high-shrinkage alloys, careful design is necessary to:
a. promote directional solidification,
b. avoid abrupt changes in cross section,
c. avoid notches (by using generous fillets), and
d. properly place gates and risers,
all of which help avoid internal shrinks and draws.
Turbulent pouring must be avoided to prevent formation of dross. Liberal use of risers or exothermic compound ensures adequate molten metal to feed all sections of the casting.
All copper alloys can be successfully cast in sand. Sand casting is the most economical casting method and allows the greatest flexibility in casting size and shape.
Permanent mold casting is best suited for tin, silicon, aluminum and manganese bronzes, and for yellow brasses.
Die-casting is well suited for yellow brasses, but increasing amounts of permanent mold alloys are also being die cast. Size is a definite limitation for both methods, although large slabs weighing as much as 4500 kg have been cast in permanent molds. Brass die-castings generally weigh less than 0.2 kg. The limitation of size is due to reduced die life with larger castings.
Virtually all copper alloys can be cast successfully by the centrifugal casting process. Casting of virtually any size from less than 100 g to more than 22 000 kg have been made.
Mechanical Properties
Most copper-base casting alloys containing tin, lead or zinc have only moderate tensile and yield strengths, low to medium hardness, and high elongation. When higher tensile or yield strength is required, the aluminum bronzes, manganese bronzes, silicon brasses, silicon bronzes and some nickel silvers are used instead. Most of the higher strength alloys have better-than-average resistance to corrosion and wear.
Properties of the castings themselves are almost always lower and depend on section size and process variables. Tensile strengths for cast test bars of aluminum bronzes and manganese bronzes range from 450 to 900 MPA, depending on composition; aluminum bronzes attain maximum tensile strength only after heat treatment.
Although manganese and aluminum bronzes often are used for the same applications, the manganese bronzes are handled more easily in the foundry. As-cast tensile strengths as high as 800 MPa and elongations of 15 to 20% can be obtained readily in sand castings, and slightly higher values in centrifugal castings. Stresses may be relieved at 175 to 200oC.
Lead may be added to the lower-strength manganese bronzes to increase machinability, but at the expense of decreased tensile strength and elongation. Lead content should not exceed 0,1% in high-strength manganese bronzes. Although manganese bronzes range in hardness from 125 to 250 HB, they are readily machined with proper tools.
Tin is added to the low-strength manganese bronzes to enhance resistance to dezinfication, but should be limited to 0,1% in high-strength manganese bronzes unless great sacrifices in strength and ductility can be accepted.
Manganese bronzes are specified for marine propellers and fittings, pinions, ball-bearing races, worm wheels, gear-shift forks and architectural work. Manganese bronzes also are used for rolling-mill screwdown nuts and slippers, bridge trunnions, gears and bearings, all of which require high strength and hardness.
Most aluminum bronzes contain from 0,75 to 4% Fe to refine grain structure and increase strength. Alloys containing from 8 to 9,5% Al cannot be heat-treated unless other elements (such as nickel or manganese) in amounts over 2% are added as well. They have higher tensile strength and greater ductility and toughness than any of the ordinary tin bronzes. Applications include valve nuts, cam bearings, impellers, hangers in pickling baths, agitators, crane gears and connecting rods.
The heat treatable aluminum bronzes contain from 9,5 to 11,5% Al, in addition to iron, with or without nickel or manganese. These alloys resemble heat-treated steels in structure and in response to quenching and tempering; castings are quenched in water or oil from temperatures between 760 and 925oC and tempered at 425 to 650oC depending on exact composition and required properties.
Aluminum bronzes resist corrosion in many substances, including pickling solutions. When corrosion occurs, it often proceeds by dealuminification, a form of dealloying in which aluminum is lost preferentially. Duplex alpha-plus-beta aluminum bronzes are more susceptible to dealloying than the all-alpha aluminum bronzes.
Aluminum bronzes have a high fatigue limit, considerably greater than that of manganese bronzes or any other cast copper alloy. Unlike those of Cu-Zn and Cu-Sn-Pb-Zn alloys, the mechanical properties of aluminum and manganese bronzes do not decrease much with increases in casting cross section. This is because of a narrow freezing range, which results in a denser structure when castings are properly designed and properly fed.
Whereas manganese bronzes become hot short above 230oC, aluminum bronzes can be used at temperatures as high as 400oC for short periods of time without appreciable loss in strength.
Unlike manganese bronzes, many aluminum bronzes increase in yield strength and hardness, but decrease in tensile strength and elongation, on slow cooling in the mold. While some manganese bronzes precipitate a relatively soft phase during slow cooling, aluminum bronzes precipitate a hard constituent rather rapidly within the narrow temperature range 565 to 480oC. Hence, large castings, or smaller castings that are cooled slowly, will have properties different from small castings cooled relatively rapidly. The same phenomenon occurs on heat treating the hardenable aluminum bronzes. Cooling slowly through the critical temperature range after quenching, or tempering at temperatures within this range, will decrease elongation. Addition of 2 to 5% Ni greatly diminishes this effect.
Nickel brasses, silicon brasses and silicon bronzes, although generally higher in strength than red metal alloys, are used more for their corrosion resistance.
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