Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Episode 12 ppt

HK40 . . . A 351 0.35-0.45 23.0-27.0 19.0-22.0 1.75 HL J94604 A 297, A 608 0.20-0.60 28-32 18-22 2.00 HN J94213 A 297, A 608 0.20-0.50 19-23 23-27 2.00 HP . . . A 297 0.35-0.75 24-28 33-37 2.00 HP-50WZ (c) . . . . . . 0.45-0.55 24-28 33-37 2.50 HT J94605 A 297, A 351, A 567, A 608 0.35-0.75 13-17 33-37 2.50 HT30 . . . A 351 0.25-0.35 13.0-17.0 33.0-37.0 2.50 HU . . . A 297, A 608 0.35-0.75 17-21 37-41 2.50 HW . . . A 297, A 608 0.35-0.75 10-14 58-62 2.50 HX . . . A 297, A 608 035-0.75 15-19 64-68 2.50 (a) ASTM designations are the same as ACI designations. (b) Rem Fe in all compositions. Manganese content: 0.35 to 0.65% for HA, 1% for HC, 1.5% for HD, and 2% for the other alloys. Phosphorus and sulfur contents: 0.04% (max) for all but HP- 50WZ. Molybdenum is intentionally added only to HA, which has 0.90 to 1.20% Mo; maximum for other alloys is set at 0.5% Mo. HH also contains 0.2% N (max). (c) Also contains 4 to 6% W, 0.1 to 1.0% Zr, and 0.035% S (max) and P (max) The three principal categories of H-type cast steels, based on composition, are: • Iron-chromium alloys • Iron-chromium-nickel alloys • Iron-nickel-chromium alloys Information on the properties of H-type grades of steel castings is contained in the section "Heat-Resistant Cast Steels" in this article. Cast Stainless Steels Revised by Malcolm Blair, Steel Founders' Society of America Composition and Microstructure As shown in Table 1, cast stainless steels can also be classified on the basis of microstructure. Structures may be austenitic, ferritic, martensitic, or ferritic-austenitic (duplex). The structure of a particular grade is primarily determined by composition. Chromium, molybdenum, and silicon promote the formation of ferrite (magnetic), while carbon, nickel, nitrogen, and manganese favor the formation of austenite (nonmagnetic). For example, a cast extra-low-carbon grade such as 0.03% C (max) cannot be completely nonmagnetic unless it contains 12 to 15% Ni. The wrought grades of these alloys normally contain about 13% Ni. They are made fully austenitic to improve rolling and forging characteristics. Chromium (a ferrite and martensite promoter), nickel, and carbon (austenite promoters) are particularly important in determining microstructure (see the section "Ferrite Control " in this article). In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic. Ferrite in Cast Austenitic Stainless Steels. Cast austenitic alloys usually have from 5 to 20% ferrite distributed in discontinuous pools throughout the matrix, the percent of ferrite depending on the nickel, chromium, and carbon contents (see the section "Ferrite Control" ). The presence of ferrite in austenite may be beneficial or detrimental, depending on the application. Ferrite is beneficial and intentionally present in various corrosion-resistant cast steels (see some of the CF grades in Table 1, for example) to improve weldability and to maximize corrosion resistance in specific environments. Ferrite is also used for strengthening duplex alloys. The section "Austenitic-Ferritic (Duplex) Alloys" in this article gives further information. Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring. The intergranular cracking occurs in the weld deposit and/or in the weld heat-affected zone and can be avoided if the composition of the filler metal is controlled to produce about 4% ferrite in the austenitic weld deposit. Duplex CF grade alloy castings are immune to this problem. The presence of ferrite in duplex CF alloys improves the resistance to stress-corrosion cracking (SCC) and generally to intergranular attack. In the case of SCC, the presence of ferrite pools in the austenite matrix is thought to block or make more difficult the propagation of cracks. In the case of intergranular corrosion, ferrite is helpful in sensitized castings because it promotes the preferential precipitation of carbides in the ferrite phase rather than at the austenite grain boundaries, where they would increase susceptibility to intergranular attack. The presence of ferrite also places additional grain boundaries in the austenite matrix, and there is evidence that intergranular attack is arrested at austenite-ferrite boundaries. It is important to note, however, that not all studies have shown ferrite to be unconditionally beneficial to the general corrosion resistance of cast stainless steels. Some solutions attack the austenite phase in heat-treated alloys, whereas others attack the ferrite. For instance, calcium chloride solutions attack the austenite. On the other hand, a 10° Baumé cornstarch solution, acidified to a pH of 1.8 with sulfuric acid and heated to a temperature of 135 °C (275 °F), attacks the ferrite. Whether corrosion resistance is improved by ferrite and to what degree depends on the specific alloy composition, the heat treatment, and the service conditions (environment and stress state). Ferrite can be detrimental in some applications. One concern may be the reduced toughness from ferrite, although this is not a major concern, given the extremely high toughness of the austenite matrix. A much greater concern is for applications that require exposure to elevated temperatures, usually 315 °C (600 °F) and higher, where the metallurgical changes associated with the ferrite can be severe and detrimental. In applications requiring that these steels be heated in the range from 425 to 650 °C (800 to 1200 °F), carbide precipitation occurs at the edges of the ferrite pools in preference to the austenite grain boundaries. When the steel is heated above 540 °C (1000 °F), the ferrite pools transform to a χ or σ phase. If these pools are distributed in such a way that a continuous network is formed, embrittlement or a network of corrosion penetration may result. Also, if the amount of ferrite is too great, the ferrite may form continuous stringers where corrosion can take place, producing a condition similar to grain boundary attack. In the lower end of this temperature range, the reductions in toughness observed have been attributed to carbide precipitation or reactions associated with 475 °C (885 °F) embrittlement. The 475 °C embrittlement is caused by the precipitation of an intermetallic phase will a composition of approximately 80Cr-20Fe. The name derives from the fact that this embrittlement is most severe and rapid when it occurs at approximately 475 °C (885 °F). At 540 °C (1000 °F) and above, the ferrite phase may transform to a complex iron-chromium-nickel-molybdenum intermetallic compound known as σ phase, which reduces toughness, corrosion resistance, and creep ductility. The extent of the reduction increases with time and temperature to about 815 °C (1500 °F) and may persist to 925 °C (1700 °F). In extreme cases, Charpy V-notch energy at room temperature may be reduced 95% from its initial value (Ref 1, 2). At temperatures above 540 °C (1000 °F), austenite also has better creep resistance than ferrite. The weaker ferrite phase may lend better plasticity to the alloy, but after long exposure at temperatures in the 540 to 760 °C (1000 to 1400 °F) range, it may transform to σ or χ phase, which reduces resistance to impact. In some instances, the alloy is deliberately aged to form the σ or χ phase and thus increase strength. Austenite can transform directly to σ or χ without going through the ferrite phase. In weld deposits, the presence of σ or χ phase is extremely detrimental to ductility When welding for service at room temperature or up to 540 °C (1000 °F), 4 to 10% ferrite may be present and will greatly reduce the tendency toward weld cracking. However, for service at temperatures between 540 and 815 °C (1000 and 1500 °F), the amount of ferrite in the weld must be reduced to less than 5% to avoid embrittlement from excessive σ or χ phase. Ferrite Control. From the preceding discussion, it is apparent that ferrite in predominantly austenitic cast stainless steels can offer property advantages in some steels (notably the CF alloys) and disadvantages in other cases (primarily at elevated temperatures). The underlying causes for the dependence of ferrite content on composition are found in the phase equilibria for the iron-chromium-nickel system. These phase equilibria have been exhaustively documented and related to commercial stainless steels. The major elemental components of cast stainless steels are in competition to promote austenite or ferrite phases in the alloy microstructure. Chromium, silicon, molybdenum, and niobium promote the presence of ferrite in the alloy microstructure; nickel, carbon, nitrogen, and manganese promote the presence of austenite. By balancing the contents of ferrite-and austenite-forming elements within the specified ranges for the elements in a given alloy, it is possible to control the amount of ferrite present in the austenite matrix. The alloy can usually be made fully austenitic or with ferrite contents up to 30% or more in the austenite matrix. The relationship between composition and microstructure in cast stainless steels permits the foundryman to predict and control the ferrite content of an alloy, as well as its resultant properties, by adjusting the composition of the alloy. This is accomplished with the Schoefer constitution diagram for cast chromium-nickel alloys (Fig. 2). This diagram was derived from an earlier diagram developed by Schaeffler for stainless steel weld metal (Ref 1). The use of Fig. 2 requires that all ferrite-stabilizing elements in the composition be converted into chromium equivalents and that all austenite-stabilizing elements be converted into nickel equivalents by means of empirically derived coefficients representing the ferritizing or austenitizing power of each element. A composition ratio is then obtained from the total chromium equivalent, Cr e , and nickel equivalent, Ni e , calculated for the alloy composition by: Cr e = %Cr + 1.5(%Si) + 1.4(%Mo) + %Nb - 4.99 (Eq 1) Ni e = %Ni + 30(%C) + 0.5 (%Mn) + 26(%N - 0.02) + 2.77 (Eq 2) where the elemental concentrations are given in weight percent. Although similar expressions have been derived that take into account additional alloying elements and different compositional ranges in the iron-chromium-nickel alloy system, use of the Schoefer diagram has become standard for estimating and controlling ferrite content in stainless steel castings. Fig. 2 Schoefer diagram for estimating the ferrite content of steel castings in the composition range of 16 to 26% Cr, 6 to 14% Ni, 4% Mo (max), 1% Nb (max), 0.2% C (max), 0.19% N (max), 2% Mn (max), and 2% Si (max ). Dashed lines denote scatter bands caused by the uncertainty of the chemical analysis of individual elements. See text for equations used to calculate Cr e and Ni e . Source: Ref 1 The Schoefer diagram possesses obvious utility for casting users and the foundryman. It is helpful for estimating or predicting ferrite content if the alloy composition is known and for setting nominal values for individual elements when calculating the furnace charge for an alloy in which a specified ferrite range is desired. Limits of Ferrite Control. Although ferrite content can be estimated and controlled on the basis of alloy composition only, there are limits to the accuracy with which this can be done. The reasons for this are many. First, there is an unavoidable degree of uncertainly in the chemical analysis of an alloy (note the scatter band in Fig. 2). Second, in addition to composition, the ferrite content depends on thermal history, although to a lesser extent. Third, ferrite contents at different locations in individual castings can vary considerably, depending on section size, ferrite orientation, presence of alloying-element segregation, and other factors. Both the foundryman and the user of stainless steel castings should recognize that the factors mentioned above place significant limits on the degree to which ferrite content (either as ferrite number or ferrite percentage) can be specified and controlled. In general, the accuracy of ferrite measurement and the precision of ferrite control diminish as the ferrite number increases. As a working rule, it is suggested that the ±6 about the mean or desired ferrite number be viewed as a limit of ferrite control under ordinary circumstances, with ±3 possible under ideal circumstances. References cited in this section 1. M. Prager, Cast High Alloy Metallurgy, in Steel Casting Metallurgy, J. Svoboda, Ed., Steel Founders' Society of America, 1984, p 221-245 2. C.E. Bates and L.T. Tillery, Atlas of Cast Corrosion-Resistant Alloy Microstructures, Steel Found ers' Society of America, 1985 Cast Stainless Steels Revised by Malcolm Blair, Steel Founders' Society of America Heat Treatment The heat treatment of stainless steel castings is very similar in purpose and procedure to the thermal processing of comparable wrought materials (see the article "Heat Treating of Stainless Steels" in Heat Treating, Volume 4 of ASM Handbook. However, some differences warrant separate consideration here. Homogenization. Alloy segregation and dendritic structures may occur in castings and may be particularly pronounced in heavy sections. Because castings are not subjected to the high-temperature mechanical reduction and soaking treatments involved in the mill processing of wrought alloys, it is frequently necessary to homogenize some alloys at temperatures above 1095 °C (2000 °F) to promote uniformity of chemical composition and microstructure. The full annealing of martensitic castings results in recrystallization and maximum softness, but it is less effective than homogenization in eliminating segregation. Homogenization is a common procedure in the heat treatment of precipitation-hardening castings. Sensitization and Solution Annealing of Austenitic and Duplex Alloys. When austenitic or duplex (ferrite in austenite matrix) stainless steels are heated in or cooled slowly through a temperature range of about 425 to 870 °C (800 to 1600 °F), chromium-rich carbides form at grain boundaries in austenitic alloys and at ferrite-austenite interfaces in duplex alloys. These carbides deplete the surrounding matrix of chromium, thus diminishing the corrosion resistance of the alloy. In small amounts, these carbides may lead to localized pitting in the alloy, but if the chromium-depleted zones are extensive throughout the alloy or heat-affected zone (HAZ) of a weld, the alloy may disintegrate intergranularly in some environments. An alloy in this condition of reduced corrosion resistance due to the formation of chromium carbides is said to be sensitized, a situation that is most pronounced for austenitic alloys. In austenitic structures, the complex chromium carbides precipitate preferentially along the grain boundaries. This microstructure is susceptible to intergranular corrosion, especially in oxidizing solutions. In partially ferritic alloys, carbides tend to precipitate in the discontinuous carbide pools; thus, these alloys are less susceptible to intergranular attack. Solution annealing of austenitic and duplex stainless steels makes these alloys less susceptible to intergranular attack by ensuring the complete solution of the carbides in the matrix. Depending on the specific alloy in question, temperatures between 1040 and 1205 °C (1900 and 2200 °F) will ensure the complete solution of all carbides and phases, such as σ and χ, that sometimes form in highly alloyed stainless steels. Alloys containing relatively high total alloy content, particularly high molybdenum content, often require the higher solution treatment temperature. Water quenching from the temperature range of 1040 to 1205 °C (1900 to 2200 °F) normally completes the solution treatment. Solution-annealing procedures for all austenitic alloys require holding for a sufficient amount of time to accomplish the complete solution of carbides and quenching at a rate fast enough to prevent reprecipitation of the carbides, particularly while cooling through the range of 870 to 540 °C (1600 to 1000 °F). A two-step heat-treating procedure can be applied to the niobium-containing CF-8C alloy. The first treatment consists of solution annealing. This is followed by a stabilizing treatment at 870 to 925 °C (1600 to 1700 °F), which precipitates niobium carbides, prevents the formation of damaging chromium carbides, and provides maximum resistance to intergranular attack. Because of their low carbon content, CF-3 and CF-3M as-cast do not contain enough chromium carbide to cause selective intergranular attack; therefore, these alloys can be used in some environments in this condition. However, for maximum corrosion resistance, these grades require solution annealing. If the usual quenching treatment is difficult or impossible, holding for 24 to 48 h at 870 to 980 °C (1600 to 1800 °F) and air cooling is helpful for improving the resistance of castings to intergranular corrosion. However, except for alloys of very low carbon content and castings with thin sections, this treatment fails to produce material with as good a resistance to intergranular corrosion as properly quench-annealed material. Cast Stainless Steels Revised by Malcolm Blair, Steel Founders' Society of America Corrosion-Resistant Steel Castings As previously mentioned, various high-alloy steel castings are classified as corrosion resistant (Table 1). These corrosion- resistant cast steels are widely used in chemical processing and power-generating equipment that requires corrosion resistance in aqueous or liquid-vapor environments at temperatures normally below 315 °C (600 °F). These alloys are also used in special applications with temperatures up to 650 °C (1200 °F). Compositions The chemical compositions of various corrosion-resistant cast steels are given in Table 1. These cast steels are specified in the ASTM standards listed in Table 1. Straight chromium stainless steels contain 10 to 30% Cr and little or no nickel. Although about two-thirds of the corrosion-resistant steel castings produced in the United States are of grades that contain 18 to 22% Cr and 8 to 12% Ni, the straight chromium compositions are also produced in considerable quantity, particularly the steel with 11.5 to 14.0% Cr. Corrosion resistance improves as chromium content is increased. In general, intergranular corrosion is less of a concern in the straight chromium alloys (which are typically ferritic), especially those containing 25% Cr or more. This is attributed to the high bulk chromium contents and the rapid diffusion rates of chromium in ferrite. Iron-chromium-nickel alloys have found wide acceptance and constitute about 60% of total production of high-alloy castings. They generally are austenitic with some ferrite. The most popular alloys of this type are CF-8 and CF-8M. these alloys are nominally 18-8 stainless steels are the cast counterparts of wrought types 304 and 316, respectively. The carbon content of each is maintained at 0.08% (max). Effects of Molybdenum on Corrosion Resistance. Alloys CF-3M and CF-8M are modifications of CF-3 and CF- 8 containing 2 to 3% Mo to enhance general corrosion resistance. Their passivity under weakly oxidizing conditions is more stable than that of CF-3 and CF-8. The addition of 2 to 3% Mo increases resistance to corrosion by seawater and improves resistance to many chloride-bearing environments. The presence of 2 to 3% Mo also improves crevice corrosion and pitting resistance compared to the CF-8 and CF-3 alloys. The CF-8M and CF-3M alloys have good resistance to such corrosive media as sulfurous and acetic acids and are more resistant to pitting by mild chlorides. These alloys are suitable for use in flowing seawater, but will pit under stagnant conditions. Alloy CG-8M is slightly more highly alloyed than the CF-8M alloys, with the primary addition being increased molybdenum (3 to 4%). The increased amount of molybdenum provides superior corrosion resistance to halide-bearing media and reducing acids, particularly H 2 SO 3 and H 2 SO 4 solutions. The high molybdenum content, however, renders CG- 8M generally unsuitable in highly oxidizing environments. Molybdenum-bearing alloys are generally not as resistant to highly oxidizing environments (this is particularly true for boiling HNO 3 ), but for weakly oxidizing environments and reducing environments, Mo-bearing alloys are generally superior. Molybdenum may also produce detrimental catalytic reactions. For example, the residual molybdenum in CF-8 alloy must be held below 0.5% in the presence of hydrazine. Effects of Chromium, Carbon, and Silicon on Corrosion Resistance. In alloys of the CF type, the effects of composition on rates of general corrosion attack have been studied, and certain definite relationships have been established. Through the use of the Huey test (five 48 h periods of exposure to boiling 65% nitric acid, as described in practice C of ASTM A 262), it has been shown that, in this standardized environment, carbide-free quench-annealed alloys of various nickel, chromium, silicon, carbon, and manganese contents have corrosion rates directly related to these contents. Figure 3 shows the influences on corrosion rate exerted by various elements in a 19Cr-9Ni casting alloy. Variations in nickel, manganese, and nitrogen contents for the ranges shown have relatively slight influences, but variations in chromium, carbon, and silicon have marked effects. The relationship between composition and corrosion rate for properly heat-treated CF alloys in boiling 65% nitric acid is summarized in the nomograph presented in Fig. 4. Fig. 3 Effects of various elements in a 19Cr- 9Ni casting alloy on corrosion rate in boiling 65% nitric acid. Data were determined for solution-annealed and qu enched specimens. Composition of base alloy was 19Cr, 9Ni, 0.09C, 0.8Mn, 1.0Si, 0.04P (max), 0.03S (max), 0.06N. Fig. 4 Nomograph for determining corrosion rate in boiling 65% nitric acid for solution- annealed and quenched type CF casting alloys Iron-Nickel-Chromium Alloys. For some types of service, extensive use is made of iron-nickel-chromium alloys that contain more nickel than chromium. Most important among this group is alloy CN-7M, which has a nominal composition of 28% Ni, 20% Cr, 3.5% Cu, 2.5% Mo, and 0.07% C (max). In effect, this alloy is made by adding 20% Ni and 3.5% Cu to alloy CF-8M, which greatly improves resistance to hot, concentrated, weakly oxidizing solutions such as sulfuric acid and also improves resistance to severely oxidizing media. Alloys of this type can withstand all concentrations of sulfuric acid at temperatures up to 65 °C (150 °F) and many concentrations up to 80 °C (175 °F). They are widely used in nitric- hydrofluoric pickling solutions; phosphoric acid; cold dilute hydrochloric acid; hot acetic acid; strong, hot caustic solutions; brines; and many complex plating solutions and rayon spin baths. Results of in-plant corrosion testing of CF-8, CF-8M, and CN-7M alloys are shown in Table 3. These tests give the specific effect of molybdenum on 19Cr-9Ni alloys in reducing selective attack and pitting, and the overall corrosion rate computed from loss in weight. The higher nickel plus copper and molybdenum in the CN-7M alloy reduces the rate of corrosion to a rate lower than that of the CF-8M alloy. Table 3 Results of in-plant corrosion testing of CF-8, CF-8M, and CN-7M alloys Temperature of solution, Metal loss on surface Type and composition of corroding solution °C °F Alloy μm/yr mils/yr Surface condition by visual examination Remarks CF-8 665 26.2 Very heavy etch (a) CF- 8M 28 1.1 Light tarnish (b) Neutralizer after formation of ammonium sulfate: ammonium sulfate plus small excess of sulfuric acid, ammonia vapor, and steam 100 212 CN- 7M 18 0.7 Bright CF-8M was installed for low corrosion tolerance equipment in this service and performed satisfactorily CF-8 385 15.2 Very heavy etch (a) CF- 8M 10 0.4 Slight tarnish Settling tank after neutralizer: ammonium sulfate plus excess of sulfuric acid 50 122 CN- 7M 2.5 0.1 Bright (b) CF-8 in service showed excessive corrosion rate plus heavy concentration cell attack CF-8 685 27.0 Heavy etch CF- 8M 175 6.8 Moderate etch Ammonium sulfate processing solution: ammonium sulfate at pH of 8.0 50 122 CN- 7M 50 2.0 Light etch CF-8M had too high a corrosion rate in service for good valve life, although suitable for equipment of greater corrosion tolerance. CN-7M was installed in this service 99 to 100% fuming nitric acid 20 68 CF-8 245 9.6 Moderate etch CF-8 was satisfactory except for low-tolerance equipment such as valves. CN - 7M valves performed CN- 7M 79 3.1 Light etch acid CF- 8M 345 13.5 Moderate etch valves. CN-7M valves performed satisfactorily in service CF- 8M 2.5 0.1 Bright Saturated solution of sodium chloride plus 15% sodium sulfate; pH of 4.5 60 140 CF-8 240 9.5 Concentration cell corrosion at various small areas of specimen CF-8M was installed for valves in service (a) Concentration cell attack under insulating washer. (b) Slight concentration cell attack under insulating washer Corrosion From Chlorine. The influence of contaminants is one of the most important considerations in selecting an alloy for a particular process application. Ferric chloride in relatively small amounts, for example, will cause concentration cell corrosion and pitting. The buildup of corrosion products in a chloride solution may increase the iron concentration to a level high enough to be destructive. Thus, chlorine salts, wet chlorine gas, and unstable chlorinated organic compounds cannot be handled by any of the iron-base alloys, creating a need for nickel-base alloys. Microstructures Although corrosion-resistant cast steels are usually classified on the basis of composition, it should be recognized that classification by composition also often involves microstructural distinctions. Table 1 shows the typical microstructures of various corrosion-resistant cast steels. As noted previously, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic. Martensitic grades include Alloys CA-15, CA-40, CA-15M, and CA-6NM. The CA-15 alloy contains the minimum amount of chromium necessary to make it essentially rustproof. It has good resistance to atmospheric corrosion, as well as to many organic media in relatively mild service. A higher-carbon modification of CA-15, CA-40 can be heat treated to higher strength and hardness levels. Alloy CA-15M is a molybdenum-containing modification of CA-15 that provides improved elevated-temperature strength. Alloy CA-6NM is an iron-chromium-nickel-molybdenum alloy of low carbon content. Austenitic grades include CH-20, CK-20 and CN-7M. The CH-20 and CK-20 alloys are high-chromium, high carbon, wholly austenitic compositions in which the chromium content exceeds the nickel content. The more highly alloyed CN- 7M, as described earlier in the section "Iron-Nickel-Chromium Alloys," has excellent corrosion resistance in many environments and is often used in sulfuric acid environments. The CN-7MS alloy has a corrosion resistance similar to that of CN-7M. The CN-7MS alloy has outstanding resistance to corrosion from high-strength (>90%) nitric acid. Ferritic grades include CB-30 and CC-50. Alloy CB-30 is practically nonhardenable by heat treatment. As this alloy is normally made, the balance among the elements in the composition results in a wholly ferritic structure similar to wrought AISI type 442 stainless steel. Alloy CC-50 has substantially more chromium than CB-30 and has relatively high resistance to localized corrosion in many environments. Austenitic-ferritic (duplex) alloys include CE-30, CF-3, CF-3A, CF-8, CF-8A, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M. The microstructures of these alloys usually contain 5 to 40% ferrite, depending on the particular grade and the balance among the ferrite-promoting and austenite-promoting elements in the chemical composition (see the section "Ferrite Control" in this article). Duplex alloys offer superior strength, corrosion resistance, and weldability. The use of duplex cast steels has focused primarily on the CF grades, particularly by the power generation industry. Strengthening in the cast CF grade alloys is limited essentially to that which can be gained by incorporating ferrite into the austenite matrix phase. These alloys cannot be strengthened by thermal treatment, as can the cast martensitic alloys, not by hot or cold working, as can the wrought austenitic alloys. Strengthening by carbide precipitation is also out of the question because of the detrimental effect of carbides on corrosion resistance in most aqueous environments. Thus, the alloys are effectively strengthened by balancing the alloy composition to produce a duplex microstructure consisting of ferrite (up to 40% by volume) distributed in an austenite matrix. It has been shown that the incorporation of ferrite into 19Cr-9NI cast steels improves yield and tensile strengths without substantial loss of ductility or impact toughness at temperatures below 425 °C (800 °F). The magnitude of this strengthening effect for CF-8 and CF-8M alloys at room temperature is shown in Fig. 5. Table 4 shows the effect of ferrite content on the tensile properties of 19Cr-9Ni alloys at room temperature and at 355 °C (670 °F). Table 5 shows the effect of ferrite content on impact toughness. Table 4 Effect of ferrite content on tensile properties of 19Cr-9Ni alloys Tensile strength Yield strength at 0.2% offset Ferrite content, % MPa ksi MPa ksi Elongation in 50 mm (2 in.), % Reduction in area, % Tested at room temperature 3 465 67.4 216 31.3 60.5 64.2 10 498 72.2 234 34.0 61.0 73.0 20 584 84.7 296 43.0 53.5 58.5 41 634 91.9 331 48.0 45.5 47.9 Tested at 355 °C (670 °F) 3 339 49.1 104 15.1 45.5 63.2 10 350 50.8 109 15.8 43.0 69.7 20 457 66.3 183 26.5 36.5 47.5 41 488 70.8 188 27.3 33.8 49.4 Table 5 Charpy V-notch impact energy, ferrite content, and Cr e /Ni e ratio of duplex cast steels Alloy Charpy V-notch energy Ferrite content, % Cr e /Ni e ratio (c) [...]... HD 0.2 5- 1.2 5- 1.2 5- 0.6 3- 0.6 3- 0.6 3- 0.6 3- HE 0.1 3- 0.6 3- 0.8 8- 0.6 3- 0.6 3- 0.6 3- 0.6 3- HF 0.1 3- 1.25+ 2.5 1.25+ 2.5+ 1.25+ 6.25 HH 0.1 3- 0.6 3- 1.25 0.6 3- 0.63 0.63 0.6 3- HI 0.1 3- 0.25+ 0.8 8- 0.6 3- 0.6 3- 0.6 3- 0.6 3- HK 0.2 5- 0.2 5- 0.8 8- 0.6 3- 0.6 3- 0.6 3- 0.6 3- HL 0.25+ 0.6 3- 0.88 0.6 3- 0.6 3- 0.63 0.6 3- HN 0.13 0.25+ 1.2 5- 0.6 3- 0.6 3- 0.63 0.63 HP 0.6 3- 0.63 1.25 0.6 3- 0.6 3- 0.6 3- 0.6 3- HT 0.1 3- 0.25+... higher-chromium CB-30 and CC-50 alloys, on the other hand, are fully ferritic alloys that are not hardenable by heat treatment These alloys are generally used in the annealed condition and exhibit moderate tensile properties and hardness Like most ferritic alloys, CB-30 and CC-50 possess limited impact toughness, especially at low temperatures Three chromium-nickel alloys, CA-6NM, CB-7Cu, and CD-4M Cu, are... 0.63 HP 0.6 3- 0.63 1.25 0.6 3- 0.6 3- 0.6 3- 0.6 3- HT 0.1 3- 0.25+ 1.25 0.63 0.6 3- 0.63 2.5 HU 0.1 3- 0.2 5- 0.8 8- 0.6 3- 0.6 3- 0.6 3- 0.63 HW 0.1 3- 0.2 5- 0.88 0.63 0.6 3- 1.2 5- 6.25 HX 0.1 3- 0.2 5- 0.8 8- 0.6 3- 0.6 3- 0.6 3- 0.6 3- Source: Ref 11 Unbalanced compositions are possible within the standard composition range for HK alloy, and hence some ferrite may be present in the austenitic matrix Ferrite will transform... (195 0-2 050 °F) CF-20 WQ from above 1095 °C (2000 °F) CF-8M, CF-12M WQ from 106 5-1 150 °C (195 0-2 100 °F) CF-8C WQ from 106 5-1 120 °C (195 0-2 050 °F) CF-16F WQ from above 1095 °C (2000 °F) CH-20 WQ from above 1095 °C (2000 °F) CK-20 WQ from above 1150 ° (2100 °F) CN-7M WQ from above 106 5-1 120 °C (195 0-2 050 °F) Fig 7 Mechanical properties of various cast corrosion-resistant steels at room temperature (a) Tensile... resistance compared to CH-20 CN-7M Highly resistant to H2SO4, H3PO4, H2SO3 salts, and seawater Good resistance to hot chloride salt solutions, nitric acid, and many reducing chemicals General Corrosion of Martensitic Alloys The martensitic grades include CA-15, CA-15M, CA-6NM, CA-6NM-B, CA-40, CB-7Cu1, and CB-7Cu-2 These alloys are generally used in applications requiring high strength and some corrosion... and some mild acids These alloys also have good atmospheric-corrosion resistance The CB-7Cu alloys are hardenable and offer the possibility of increased strength and improved corrosion resistance among the martensitic alloys General Corrosion of Ferritic Alloys Alloys CB-30 and CC-50 are higher-carbon and higher-chromium alloys than are the CA alloys mentioned above Each alloy is predominantly ferritic,... properties are usually duplex-structure alloys with much more chromium than nickel The addition of copper enables these alloys to be strengthened by precipitation hardening These alloys are significantly higher in strength than the other corrosion-resistant alloys even without hardening The alloys CB-7Cu-1 and CB-7Cu-2 have corrosion resistances between those of CA-15 and CF-18 They are widely used for... T at 650 °C (120 0 °F) (c) AC from 980 °C (1800 °F), T at 595 °C (1100 °F) (d) AC from 980 ° (1800 °F), T at 315 °C (600 °F) CB-30 A at 790 °C (1450 °F), FC to 540 °C (1000 °F), AC CC-50 (a) As-cast (2% Ni; >0.15% N) (c) AC from 1040 °C (1900 °F) (>2% Ni; >0.15% N) CE-30 (a) (b) As-cast WQ from 106 5-1 120 °C (195 0-2 050 °F) CF-8 WQ from 106 5-1 120 °C (195 0-2 050 °F) CF-20 WQ from above... Table 12 Approximate rates of corrosion for ACI heat-resistant casting alloys in air and in flue gas Oxidation rate in air, mm/yr Corrosion rate, mm/yr, at 980 °C (1800 °F) in flue gas with sulfur content of: 870 °C (1600 °F) Alloy 0 .12 g/m3 980 °C (1800 °F) 1090 °C (2000 °F) 2.3 g/m3 Oxidizing Reducing Oxidizing Reducing HB 0.6 3- 6.2 5- 12. 5- 2.5+ 12. 5 6.2 5- 12. 5 HC 0.25 1.25 1.25 0.6 3- 0.63+ 0.63 0.6 3-. .. chromium and molybdenum provide a high degree of localized corrosion resistance (crevices and pitting), and the duplex microstructure provides SCC resistance in many environments This alloy can be precipitation hardened to provide strength and is also relatively resistant to abrasion and erosion-corrosion Fully Austenitic Alloys Alloys CH-10 and CH-20 are fully austenitic and contain 22 to 26% Cr and 12 . elevated-temperature strength. Alloy CA-6NM is an iron-chromium-nickel-molybdenum alloy of low carbon content. Austenitic grades include CH-20, CK-20 and CN-7M. The CH-20 and CK-20 alloys are high-chromium,. nitric acid, and many reducing chemicals General Corrosion of Martensitic Alloys. The martensitic grades include CA-15, CA-15M, CA-6NM, CA-6NM-B, CA-40, CB-7Cu1, and CB-7Cu-2. These alloys are. CF-3A, CF-8, CF-8A, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M. The microstructures of these alloys usually contain 5 to 40% ferrite, depending on the particular grade and the balance