Volume 15 - Casting Part 13 pot

In addition to creep strength and corrosion resistance, two other properties stability, and resistance to thermal fatigue are important considerations in the selection of nickel-base heat-resistant casting alloys. Thermal-fatigue resistance is partially controlled by composition, but it is also significantly affected by grain-boundary area and alignment relative to applied stresses. The crystallographic orientation of grains also influences thermal stresses because the modulus of elasticity, which directly influences thermal stresses, varies with grain orientation. The stability of property values is directly influenced by metallurgical stability; any microstructural changes that take place during long-term exposure at high temperatures under stress cause attendant changes in properties. For example, if the γ' phase coarsens, strength decreases. Also, potentially deleterious topologically close-packed (tcp) secondary phases, such as σ, Laves, and , may form. Coarsening of γ' can be controlled to some degree by adjusting alloy additions. Formation of tcp phases is controlled by adjusting the composition of the matrix to minimize the electron vacancy number, N v . A high N v indicates a tendency toward the formation of tcp phases. In general, an N v value below 2.4 indicates minimal formation of deleterious phases; however, this relationship varies with base-alloy composition. The metallurgical structures of both cast and wrought heat-resistant alloys are discussed in greater detail in Metallography and Microstructures, Volume 9 of ASM Handbook, formerly 9th Edition Metals Handbook. Alloys 713C and 713LC are closely related investment casting alloys used principally for low-pressure turbine airfoils in gas turbines. Intended for operation at intermediate temperatures from 790 to 870 °C (1450 to 1600 °F), these alloys are generally used in uncooled airfoil designs. Alloy 738X is an investment casting alloy similar in strength to Alloy 713C and Udimet 700 but with outstanding sulfidation resistance. It is used principally for latter-stage turbine airfoils and for hot-corrosion-prone applications such as industrial and marine engines. Udimet 700, although primarily a wrought alloy, is also used in investment cast high-pressure turbine blades. In cast form, it is similar in strength to Alloy 713C but offers better hot-corrosion resistance. It is designed for operation at intermediate temperatures from 730 to 900 °C (1350 to 1650 °F). A stability-controlled version of U-700 is known as René 77. Alloy 100 is designed for use at metal temperatures up to about 980 °C (1800 °F) in cooled and uncooled airfoils. A stability-controlled version of Alloy 100 is known as René 100. B-1900, to which 1% Hf is usually added to improve ductility and thermal-fatigue resistance, is designed for use at metal temperatures up to about 980 °C (1800 °F) in cooled and uncooled airfoils. René 80 offers excellent corrosion resistance in sulfur-bearing environments. It is designed for use at metal temperatures up to about 950 °C (1750 °F). Alloy 792 is designed for use in applications similar to those of René 80. It is one of the most sulfidation-resistant nickel alloys available. MAR-M 246 and MAR-M 247 are designed for use at metal temperatures of about 980 to 1010 °C (1800 to 1850 °F) in cooled and uncooled airfoils and radial and axial wheels (Fig. 3). Fig. 3 Various radial and axial turbine wheels made from Mar-M-247 alloy. Courtesy of Howmet Corporation, Whitehall Casting Division. DS MAR-M 200 + Hf is produced by directional solidification (see discussion below) and is designed for metal temperatures of about 1010 to 1040 °C (1850 to 1900 °F). It is used in cooled airfoils. Other alloys (such as Udimet 500) are occasionally used in turbine airfoil applications, and Alloy 718 has been cast into large static structures for gas turbines. Additional information on the applications and processing of investment cast nickel-base heat-resistant alloys can be found in the articles "Classification of Processes and Flow Chart of Foundry Operations" and "Investment Casting" in this Volume. Alloys for directional and single-crystal solidification possess high elevated-temperature strengths. Directionally solidified turbine blades have high strength in the direction of principal stress (the longitudinal direction) because grain boundaries are aligned parallel to this direction. Thus, the effect of grain boundaries on properties is minimized. Single-crystal alloys have no grain boundaries and therefore require no grain-boundary strengthening elements. For this reason, they can be solution heat treated at higher temperatures than conventional alloys, precipitating a greater amount of the γ' strengthening phase. The lack of grain boundaries also enhances the corrosion resistance of these materials. Table 2 lists several DS/SC alloy compositions. A turbine vane made from CM-247-LC DS alloy is shown in Fig. 4. Properties and performance of DS/SC alloys are detailed in Ref 1, 2, 3, and 4. Fig. 4 Directionally solidified turbine vane made from CM-247-LC alloy. Courtesy of Thyssen Guss AG. References cited in this section 1. K. Harris, G.L. Erickson, and R.E. Schwer, "Development of the Single-Crystal Alloys CM SX- 2 and CM SX-3 for Advanced Technology Turbine Engines," Technical Paper 83-GT- 244, American Society of Mechanical Engineers 2. K. Harris, G.L. Erickson, and R.E. Schwer, "Directionally Solidified DS CM 247 LC Optimized Mechanical Properties Resulting From Extensive γ ' Solutioning," Paper presented at the Gas Turbine Conference and Exhibit, Houston, TX, March 1985 3. K. Harris, G.L. Erickson, R.E. Schwer, J. Wortmann, and D. Froschhammer, "Development of Low- Density Single-Crystal Superalloy CMSX-6," Technical Paper, Cannon-Muskegon Corporation 4. K. Harris, G.L. Erickson, and R.E. Schwer, "CMSX Single Crystal, CM DS & Integral Wheel Alloys Properties and Performance," Paper presen ted at the Cost 50/501 Conference, High Temperature Alloys for Gas Turbines and Other Applications, Liège, Oct 1986 Melting Practice Electric induction furnaces have become the mainstay of the foundry industry for small heat sizes, especially when a number of different alloys are produced. They are also the least expensive of the major furnace types to install. The foundry industry uses these furnaces in sizes ranging from 9 kg to 18 Mg (20 lb to 20 tons); however, most electric induction furnaces are in the 25 to 1350 kg (50 to 3000 lb) range. Figure 5 shows a cross section of an induction furnace. The furnace shell rests on trunnions, which tilt the furnace during tapping. A copper coil surrounds the furnace lining and the charge materials inside. The metal charge is melted by its resistance to the current induced by a magnetic field when current flows through the coil. More detailed information on induction furnaces can be found in the article "Melting Furnaces: Induction Furnaces" in this Volume. Fig. 5 Cross section of a coreless electric induction furnace. Vacuum Melting. Nickel-base alloys containing more than about 0.2% of the reactive elements aluminum, titanium, and zirconium are not suitable for melting and casting in oxidizing environments such as air. At the higher alloying levels, these elements readily oxidize, resulting in gross inclusions, oxide laps, and poor composition control. Consequently, such alloys generally require inert gas injection or vacuum melting and casting methods. Extralow gas contents, which can be obtained by vacuum melting, are also required for certain nickel-base alloys. Vacuum melting processes, which are described in the article "Vacuum Melting and Remelting Processes" are always used for directional solidification and single-crystal casting alloys. Metal Treatment Argon Oxygen Decarburization (AOD). Some foundries have recently installed AOD units to achieve some of the results that vacuum melting can produce. The AOD unit looks very much like a Bessemer converter with tuyeres in the lower side-walls for the injection of argon or nitrogen and oxygen. These units must be charged with molten metal from an arc or induction furnace. About 20%, but usually less, cold charge consisting of solid virgin material can be added to an AOD unit. The continuous injection of gases causes a violent stirring action and intimate mixing of slag and metal, which can lower sulfur values to below 0.005%. The gas contents (hydrogen, nitrogen, and oxygen) may be even lower than those of vacuum induction melted alloys. More information on AOD processing is available in the section "Argon Oxygen Decarburization" of the article "Degassing Processes (Converter Metallurgy)" in this Volume. Electroslag remelting furnaces represent another type of equipment that may see some use in the high-alloy foundry in the next decade. Electroslag remelting machines have been used for many years by the wrought steel companies to produce refined ingots. In the Soviet Union, electroslag remelting is being used to cast shapes, and the technology is being evaluated in the United States as well. The process works by taking an ingot (which becomes the electrode), remelting it in stages under molten slag to refine it, and then resolidifying the metal in a water-cooled mold. See the section "Electroslag Remelting (ESR)" in the article "Vacuum Melting and Remelting Processes" in this Volume. Plasma Refining. Steadily increasing requirements for alloy cleanliness have led producers to adopt several novel refining technologies and process routes, many involving increased use of the ladle as a refining vessel. Such procedures require longer holding times in the ladle, which necessitate either increased superheats in the furnace or external heating in the ladle to avoid early solidification. Higher superheat, in addition to requiring excessive energy expenditure, can contribute to the problem of melt contamination. The preferred solution is to supply heat to the ladle, maintaining the alloy at minimal superheat during refining. This can be accomplished by the transferred arc plasma torch, with the added benefit of enhanced refining reactions that aid in the production of clean metal with low levels of residual elements. In this work, experiments have been carried out in an induction furnace equipped with a gas-stabilized graphite electrode to investigate the control of oxygen and induction levels and the enhancement of desulfurization afforded by the transferred arc plasma. See the section "Plasma Heating and Degassing" in the article "Degassing Processes (Ladle Metallurgy)" in this Volume. Foundry Practice Foundry practice for nickel-base alloys is for the most part similar to that used for cast stainless steels (see the article "High-Alloy Steels" in this Volume). Specific aspects of foundry practice discussed here include pouring, gating and risering, cleaning, welding, and heat treatment of conventional corrosion-resistant nickel-base alloy castings. The processing of investment cast and DS/SC alloys is reviewed in the articles "Investment Casting" and "Directional and Monocrystal Solidification", respectively, in this Volume. Pouring Practice Three types of ladles are used for pouring nickel-base castings: bottom pour, teapot, and lip pour. Ladle capacity normally ranges from 45 kg to 36 Mg (100 lb to 40 tons), although ladles having much larger capacities are available. The bottom-pour ladle has an opening in the bottom that is fitted with a refractory nozzle (Fig. 6). A stopper rod, suspended inside the ladle, pulls the stopper head up from its seat in the nozzle, allowing the molten alloy to flow from the ladle. When the stopper head is returned to the position shown in Fig. 6, the flow is cut off. The position of the stopper head is controlled manually by the slide-and-rack mechanism shown at the left in Fig. 6. Bottom pouring is preferred for pouring large castings from large ladles, because it is difficult to tip a large ladle and still control the stream of molten steel. Also, the bottom-pour ladle delivers cleaner metal to the mold. Inclusions, pieces of ladle lining, and slag float to the top of the ladle; thus, bottom pouring greatly reduces the risk of passing nonmetallic particles into the mold cavity. On the other hand, it is impractical to pour molten metal into small molds from a large bottom-pour ladle. The pressure head created by the metal remaining in the ladle delivers the molten metal too fast. Also, the time required to fill a small mold is short, thus requiring that a large bottom-pour ladle be opened and closed many times in order to empty it. This may cause the ladle to leak, although special nozzles have been developed to minimize leakage. Despite the fact that the size of bottom-pour ladles could be scaled down for pouring smaller castings, this is unnecessary because of the almost equal ability of the teapot ladle to deliver clean metal. The teapot ladle incorporates a ceramic wall, or baffle, that separates the bowl of the ladle from the spout. The baffle extends almost four-fifths of the distance to the bottom of the ladle (Fig. 7). As the ladle is tipped, hot metal flows from the bottom of the ladle up the spout and over the lip. Because the metal is taken from near the bottom of the ladle, it is Fig. 6 Typical bottom-pour ladle used to pour large castings. free of slag and pieces of eroded refractory. The teapot design is feasible in various sizes, generally covering the entire range of casting sizes that are below the minimum size for which the bottom-pour ladle is used. Lip-pour ladles resemble the teapot type but have no baffles to hold back the slag. Because the hot metal is not taken from the bottom of the ladle, this type of ladle pours a more contaminated metal and is seldom used to pour high-alloy castings. Nevertheless, it is widely used as a tapping ladle (at the melting furnace) and as a transfer ladle to feed smaller ladles of the teapot type. Pouring Time. Ideally, the optimum pouring time for a given casting would be determined by the weight and shape of the casting, the temperature and solidification characteristics of the molten metal, and the heat transfer and thermal stability characteristics of the mold. However, most foundries are required to pour may different castings from one heat or even from one ladle. Therefore, rather than attempting to control pouring time directly, foundries control the speed with which molten steel enters the mold cavity. This control is achieved through the design of the gating system. Gating Systems An effective gating system for pouring nickel-base alloys, as well as other metals, into green sand molds is one that fills the mold as rapidly as possible without developing pronounced turbulence. It is essential that the mold be filled rapidly to minimize temperature variations within the metal; this results in optimized control of solidification. Turbulent metal flow is harmful because it breaks up the metal stream, exposing more surface area to air and forming metallic oxides. The oxides can rise to the top of the mold cavity, resulting in a rough surface of inclusions in the casting. In addition, turbulent flow erodes the mold material. These eroded particles also float to the top of the mold cavity. Preferred Metal Flow. According to preferred practice, the pourer directs the metal stream toward the pouring cup at the top of the mold, controlling the pouring rate to keep the cup full of molten metal throughout the pouring cycle. The opening in the bottom of the cup is directly over the sprue, or downgate, which is tapered at the bottom, thus reducing the diameter of the stream of descending metal. The taper prevents the stream from pulling away from the walls and drawing air into the gating system. The descending metal impinges on the sprue well at the bottom of the sprue, and the direction of flow changes from vertical to horizontal, with the metal flowing along runners to gates (ingates), and then to the main body of the casting. A gating system that incorporates these features is shown in Fig. 8. Fig. 7 Typical teapot ladle used to pour small- to medium-size castings. Fig. 8 Gating system for good metal flow. Gating system design largely determines the manner in which molten metal is fed into the mold, as well as the rate of feeding. The number of gates influences the distribution of the flow between gates. A good design has even distribution between gates both initially and while the mold is filling. The distribution of flow in the gating system affects the type of flow that occurs in the main body of the casting. A large difference in the flow between gates creates a swirl of metal in the mold about a vertical axis, in addition to that occurring about a horizontal axis. The gating system shown in Fig. 8 is an example of a so-called finger-type parting line system, in which the fingers feed metal to the casting just above the parting line. Other major types of gating systems used in alloy foundries include the bottom gate, which feeds metal to the bottom of the casting, and the step gate, which feeds metal through a number of stepped gates, one above another. In the system shown in Fig. 8, the ratio of the cross-sectional area of the choke of the sprue to that of all of the runners emanating from the sprue basin and to all of the gates is 1:4:4. As shown in Fig. 8, the runner area decreases progressively by an amount equal to the area of each gate it passes. This practice ensures that, once the system is filled with metal, it remains full during the pouring cycle and feeds equally to each gate. Furthermore, the gates are located in the cope, while the runner, which extends beyond the last gate, is located in the drag. Extension of the runner serves as a trap for the first, and usually the most contaminated, metal to enter the system. The entire runner must fill before the metal will rise to the level of the gates. Thus, each gate begins feeding at the same time. The runners and gates are curved wherever a change in direction occurs. This streamlining reduces turbulence in the metal stream and minimizes mold erosion. In contrast to the ratio of the system shown in Fig. 8 (1:4:4), if the total cross-sectional area of the gates is less than that of the runners (1:2:1, for example), the result is a pressurized system. The metal squirts into the mold cavity and flows turbulently over the mold bottom, which can cause roughening of the bottom surfaces. Conversely, if the total cross-sectional area of the gates is significantly greater than that of the runners (1:2:3, for example), the gating system will be incompletely filled, and flow from the gates will be uneven. This condition increases the likelihood of mold erosion. When this type of system is required, complicated additions to gating systems are used, including whirlgates, horn gates, strainer cores, tangential gates, and slit gates. However, any addition to the gating system usually increases the cost of the casting because all gating must be removed. More detailed information on gating practice can be found in the article "Gating Design" in this Volume. Mold Erosion. In addition to the contribution of gating design to a reduction in mold erosion, further reduction can be achieved by making the gating system out of tile, which is superior to green sand in erosion resistance. However, the use of tile is generally limited to gating systems for large castings, where the quantity and speed of molten metal passing through the gating system would seriously erode green sand and where precise control of the flow rate is less critical. Thus, gating systems for smaller castings are rammed in sand, usually with a semicircular or rectangular cross section for the gates and runners. Risers Molten nickel-base alloys contract approximately 0.9% per 55 °C (100 °F) as they cool from the pouring temperature to the solidification temperature. They then undergo solidification contraction of 3% during freezing, and finally the solidified metal contracts 2.2% during cooling to room temperature. Therefore, when casting nickel alloys, an ample supply of molten metal must be available from risers (reservoirs) to compensate for the volume decrease, or shrinkage cavities will develop in the locations that solidify last. Because feed from the riser occurs by gravity, risers are usually located at the top of the casting. Riser forms are placed on the pattern and molded into the cope half of the mold. The riser cavity is usually open to the top of the mold, although blind risers are sometimes used. Riser Size and Shape. As described in the article "Riser Design" in this Volume, formulas based on surface area, volume, and freezing time of the casting are used to determine riser size. Most risers are cylindrical in shape, with their heights approximately equal to their diameters. This configuration provides a low ratio of surface area to volume, which prolongs the time the metal in the riser remains liquid. Placement of a riser, in conjunction with its size, determines its effectiveness. The thicker sections of a casting act as reservoirs for feeding the thinner sections, which solidify first. Thus, risers are placed over thick sections that cannot be fed by other areas of the casting. Demonstrating this principle, the gear blank casting shown in Fig. 9 is provided with a large riser over the central hub and six smaller risers spaced equally around the rim of the gear to ensure adequate feeding. Metal enters the mold at the two gates, which are 180° apart. Feeding Distance. Castings of uniform thickness present a different problem. Studies have established the feeding distances of a riser for various rectangular shapes in both the horizontal and vertical planes, with and without an end effect (that is, the extra cooling provided by the sand cover of an end surface). For a uniform section, the maximum feeding distance can be extended by adding a taper. The progressively thicker section solidifies in a progressively longer time, so that a favorable temperature gradient is established from the end of the section to the riser. A tapered pad of exothermic material placed in the mold along the length of the casting will also produce a favorable temperature gradient. Welding Cast Nickel. Alloy CZ-100 can be readily repair welded or joined to other castings or to wrought forms by using any of the usual welding processes with suitable nickel rod and wire. Joints or cavities must be carefully prepared for welding because small amounts of sulfur or lead cause weld embrittlement. Nickel-Copper Alloys. The weldability of the nickel-copper alloys decreases with increasing silicon content, but is adequate up to at least 1.5% Si. Niobium can enhance weldability, particularly when small amounts of low-melting residuals are present. Careful raw material selection and proper foundry practice, however, have largely eliminated any difference in weldability between niobium-containing and niobium-free grades. The higher-silicon compositions ( ≥ 3.5% Si) are not considered weldable. They can be brazed or soldered, as can the lower-silicon grades. Nickel-Chromium-Iron Alloys. The CY-40 castings can be repair welded or fabrication welded to matching wrought alloys by any of the usual welding processes. Rod and wire of matching nickel-chromium contents are available. Postweld heat treatment is not required after repair welding or fabrication, because the heat-affected zone is not sensitized by the weld heat. Nickel-Chromium-Molybdenum Alloys. Alloys CW-12MW and CW-7M can be welded by any of the usual welding processes, using wire or rod of matching composition. For optimum weldability, carbon content should be as low as practicable. The usual practice is to solution treat and quench after repair welding. Heat treatment after welding is generally necessary because these alloys are subject to sensitization in the heat-affected zone and because intermetallic precipitates may form in the heat-affected zone. Nickel-Molybdenum Alloys. Alloys N-12MV and N-7M can be welded by using any of the usual welding processes with wire or rod of matching composition. Postweld heat treatment is usually performed because these alloys are subject to the precipitation of intermetallic compounds in the heat-affected zone. Heat Treatment Cast nickel (alloy CZ-100) is used in the as-cast condition. Some other alloys are also used as-cast, but most require some type of thermal treatment to develop optimum properties. Fig. 9 Gating and feeding system used to cast gear blanks. Nickel-copper alloys are used in the as-cast condition. Homogenization at 815 to 925 °C (1500 to 1700 °F) may, under some conditions, improve corrosion resistance slightly, but in most corrosive conditions, alloy performance is not affected by the minor segregation present in the as-cast alloy. At about 3.5% Si, silicon begins to have an age-hardening effect. The resultant combination of aging and the formation of hard silicides when the silicon content exceeds about 3.8% can cause considerable difficulty in machining. Softening is accomplished by a solution heat treatment, which consists of heating to 900 °C (1650 °F), holding at temperature for 1 h per 25 mm (1 in.) of section thickness, and oil quenching. Maximum softening is obtained by oil quenching from 900 °C (1650 °F), but such treatment is likely to result in quench cracks in castings with complex shapes or varying section thickness. In the solution heat treatment of complicated or varying-section castings, it is advisable to charge them into a furnace below 315 °C (600 °F) and heat to 900 °C (1650 °F) at a rate that limits the maximum temperature difference within the casting to about 56 °C (100 °F). After being soaked, castings should be transferred to a furnace held at 730 °C (1350 °F), allowed to equalize in temperature, and then oil quenched. Alternatively, the furnace can be rapidly cooled to 730 °C (1350 °F), the casting temperature can be equalized, and the castings can be quenched in oil. Solution heat treated castings are age hardened by placing them in a furnace held at 315 °C (600 °F), heating uniformly to 595 °C (1100 °F), holding at 595 °C (1100 °F) for 4 to 6 h, and air or furnace cooling. Nickel-Chromium-Iron Alloys. Alloy CY-40 is used in the as-cast condition because it is insensitive to the intergranular attack encountered in as-cast or sensitized stainless steels. A modified cast nickel-chromium-iron alloy for nuclear applications with 0.12% C (max) is usually solution treated as an additional precaution. Sensitization in the heat-affected zone is not a problem with CY-40. Unless residual stresses pose a problem, postweld heat treatment is therefore not required. Nickel-Chromium-Molybdenum Alloys. The high chromium and molybdenum contents of CW-12MW and CW-7M result in the precipitation of carbides and intermetallic compounds in the as-cast condition, which can be detrimental to corrosion resistance, ductility, and weldability. These alloys should therefore be solution treated at a temperature of 1175 to 1230 °C (2150 to 2250 °F) and water or spray quenched. Nickel-Molybdenum Alloys. Slow cooling in the mold is detrimental to the corrosion resistance, ductility, and weldability of N-12MV and N-7M. These alloys should therefore be solution treated at a minimum temperature of 1175 °C (2150 °F) and water quenched. Specific Applications Corrosion-resistant nickel-base castings are primarily used in fluid-handling systems with matching wrought alloys; they are also commonly used for pump and valve components or for applications with crevices and velocity effects requiring a superior material in a wrought stainless system. Because of their relatively high cost, nickel-base alloys are usually selected only for severe service conditions in which maintenance of product purity is of great importance and for which less costly stainless steels or other alternative materials are inadequate. Detailed information on the corrosion resistance of nickel-base alloys in aqueous media is available in the article "Corrosion of Nickel-Base Alloys" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook. In the application of heat-resistant alloys, considerations include: • Resistance to corrosion (oxidation) at elevated temperatures • Stability (resistance to warping, cracking, or thermal fatigue) • Creep strength (resistance to plastic flow) Numerous applications of cast heat-resistant nickel-base alloys were discussed earlier in this article. Information on the high-temperature corrosion resistance of these alloys is available in the articles "Fundamentals of Corrosion in Gases," "General Corrosion" (see the section "High-Temperature Corrosion"), and "Corrosion of Metal Processing Equipment" (see the section "Corrosion of Heat-Treating Furnace Accessories") in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook. Cast Nickel. Nickel castings are most commonly used in the manufacture of caustic soda and in processing with caustic (see the section "Corrosion by Alkalies and Hypochlorite" in the article "Corrosion in the Chemical Processing Industry" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook). As the temperature and caustic soda concentration increase, austenitic stainless steels are useful only up to a point. The nickel-copper and nickel- chromium-iron alloys take over as useful alloys under these conditions, while cast nickel is selected for the higher caustic concentrations, including fused anhydrous soda. Minor amounts of such elements as oxygen and sulfur can have profound effects on the corrosion rate of nickel in caustic. Detailed corrosion data should therefore be consulted before making a final alloy selection. Nickel-Copper Alloys. The principal advantages of the Ni-30Cu alloys are high strength and toughness, coupled with excellent resistance to mineral acids, organic acids, salt solution, food acids, strong alkalies, and marine environments. The most common applications for nickel-copper castings are in the manufacture of, and processing with, hydrofluoric acid and the handling of salt water, neutral and alkaline salt solutions, and reducing acids. Nickel-chromium-iron alloys are commonly used under oxidizing conditions to handle high-temperature corrosives or corrosive vapors where stainless steels might be subject to intergranular attack or stress-corrosion cracking. In recent years, the CY-40-type alloy has found large-scale application in handling hot boiler feedwater in nuclear plants because of a greater margin of safety over stainless steels. More information on this application is available in the article "Corrosion in the Nuclear Power Industry" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook. Nickel-chromium-molybdenum alloys are probably the most common materials for upgrading a system in which service conditions are too demanding for either standard or special stainless steels because of severe combinations of acids and elevated temperatures. These cast alloys can be used in conjunction with similar wrought materials, or they can serve to upgrade pump and valve components in a wrought stainless steel system. Nickel-molybdenum alloys have specialized application areas, primarily in the handling of hydrochloric acid at all temperatures and concentrations. Applications should not be based on upgrading in areas where stainless steels are inadequate, because the nickel-molybdenum alloys are unsuitable for handling most oxidizing solutions for which stainless steels are used. Alloys for directional and single-crystal solidification are used as blades for aircraft and some land-based turbines (Fig. 1 and 4). Under elevated temperatures, they have very high strength in the direction of primary stress. References 1. K. Harris, G.L. Erickson, and R.E. Schwer, "Development of the Single-Crystal Alloys CM SX- 2 and CM SX-3 for Advanced Technology Turbine Engines," Technical Paper 83-GT- 244, American Society of Mechanical Engineers 2. K. Harris, G.L. Erickson, and R.E. Schwer, "Directionally Solidified DS CM 247 LC Optimized Me chanical Properties Resulting From Extensive γ ' Solutioning," Paper presented at the Gas Turbine Conference and Exhibit, Houston, TX, March 1985 3. K. Harris, G.L. Erickson, R.E. Schwer, J. Wortmann, and D. Froschhammer, "Development of Low- Density Single-Crystal Superalloy CMSX-6," Technical Paper, Cannon-Muskegon Corporation 4. K. Harris, G.L. Erickson, and R.E. Schwer, "CMSX Single Crystal, CM DS & Integral Wheel Alloys Properties and Performance," Paper presented at the Cost 50/501 Conference, High Temperature Alloys for Gas Turbines and Other Applications, Liège, Oct 1986 Selected References • W.J. Jackson, Ed., Steel Castings Design Properties and Applications, Steel Castings Research and Trade Association, 1983 • J.D. Redmond, Selecting Second-Generation Duplex Stainless Steels, Chem. Eng., Oct 1986 and Nov 1986 • Steel Castings Handbook, Supplement 8, High Alloy Data Sheets, Corrosion Series, Steel Founders' Society of America, 1981 [...]... 0. 015 0.006 0 .15 Corrosion resistance Ti-6Al-2Sn-4Zr-2Mo 2% 0.10 0.010 0.006 6 0 .15 2 2 4 Elevated-temperature creep Ti-6Al-2Sn-4Zr-6Mo . Ti-6Al-4V, annealed 855 124 930 135 12 20 Ti-6Al-4V-ELI 758 110 827 120 13 22 Ti-6Al-2Sn-4Zr-2Mo, annealed 910 132 1006 146 10 21 Ti-6Al-2Sn-4Zr-6Mo, STA 1269 184 134 5 195 1 1 Ti-3Al-8V-6Cr-4Zr-4Mo,. are the metastable β alloys Ti-3Al-8V-6Cr-4Zr-4Mo (Beta C) and Ti-15V-3Al-3Cr-3Sn (Ti 1 5- 3). Originally developed as a highly cold-formable and subsequently age-hardened sheet material, these. Cryogenic toughness Ti-3Al-8V-6Cr-4Zr-4Mo <1% 0.10 0. 015 0.006 3.5 0.2 8.5 6 . . . 4 4 Strength Ti-15V-3Al-3Cr-3Sn <1% 0.11 0. 015 0.006 3 0.2 15 3 3 . . . . .