Handbook Heat Treating (1991) WW Part 13 potx

• 980 °C (1800 °F) for 4 h with air cooling • 650 °C (1200 °F) for 24 h with air cooling • 760 °C (1400 °F) for 8 h with air cooling The 1105 °C (2020 °F) anneal is a partial solution treatment below the γ' solvus that retains some of the γ' to limit grain growth. The subsequent treatments precipitate carbides and γ'. The two-step exposures of first 870 °C (1600 °F) and then 980 °C (1800 °F) are designed to maximize first the nucleation of precipitates and then the rate of growth of the precipitates. The average grain size of the structure produced is about 11 μm with a γ' volume fraction of about 35%. The fine-grained structure has better mechanical properties at turbine disk application temperatures than that from coarse- grained heat treatment, which is designed for higher-temperature applications. The effect of the amount of cold working on the recrystallization and grain growth during subsequent solution treating of the nickel-base superalloy Nimonic 90 is shown in Fig. 11. The effect is similar to the behavior shown for A- 286 in Fig. 1. The critical amount of deformation that leads to abnormally large grains is in the range of 2 to 10% reduction in thickness, and the grain growth accelerates rapidly at temperatures above 1100 °C (2010 °F). Fig. 11 Effect of cold work and annealing on grain size for Nimonic 90 sheet cold rolled in steps from 1.8 to 0.9 mm (0.072 to 0.036 in.) thick and annealed at five temperatures The precipitation-hardened superalloys that undergo extensive deformation processing, as in sheet forming, usually require in-process annealing to maintain temperatures, relieve forming stresses, and enhance microstructural changes. The annealing practice can also have a marked effect on response to solution treating and aging. This is illustrated by the following two examples for René 41. Like solution-treatment temperatures (Fig. 9), high annealing temperatures can dissolve M 6 C carbides, which are useful in preventing formation of M 23 C 6 grain boundary films during aging. Example 4: Effect of Annealing Temperature on the Grain-Boundary Carbides and Ductility of René 41 Sheet. In one case, parts formed from René 41 sheet showed strain age cracking after solution treatment at 1080 °C (1975 °F) for 1 2 h, air cooling, and then aging at 760 °C (1400 °F) for 16 h. Cracking has been attributed to a carbide network in the grain boundaries. Cause of the carbide network was traced to in-process annealing at 1180 °C (2150 °F). At 1180 °C (2150 °F) the M 6 C carbide was dissolved. Subsequent exposure to temperatures between 760 and 870 °C (1400 and 1600 °F) produced an M 23 C 6 carbide network in the grain boundaries that reduced ductility to an unacceptable level. If the annealing temperature is kept below 1095 °C (2000 °F), M 6 C does not dissolve (Fig. 9) and ductility can be improved. A similar effect can occur in weldments of nickel-base alloys if they are annealed at temperatures above 1095 °C (2000 °F). Example 5: Effect of Thermomechanical Processing on the Grain-Boundary Carbides and Ductility of René 41 Bar Stock. A problem similar to that described in the preceding example occurred in René 41 bar stock. Grain-boundary carbide network reduced ductility and caused difficulty (sometimes cracking) during forming and welding. Investigation of the cause of the grain-boundary network indicated that the bar stock was produced with a final rolling temperature of 1180 °C (2150 °F). Light reductions were taken during final rolling to ensure proper size for the finished bar stock and to eliminate the possibility of surface tearing. This high rolling temperature, coupled with relatively light reductions (in the range of 2 to 3%), produced grain-boundary network because: • The M 6 C carbides were dissolved at the rolling temperature • Slow cooling through the range of 870 to 760 °C (1600 to 1400 °F) produced M 23 C 6 in an unfavorable morphology (grain-boundary carbide film) Rolling temperatures of 1150 °C (2100 °F) maximum, coupled with a final reduction in rolling of at least 10 to 15%, eliminated the grain-boundary carbide film and produced bars that could be welded and formed. Solid-Solution-Strengthened Iron-, Nickel- and Cobalt-Base Superalloys Solid-solution-strengthened iron-, nickel-, and cobalt-base superalloys are generally distinguishable from the precipitation-strengthened superalloys by their relatively low content of precipitate-forming elements such as aluminum, titanium, or niobium. There are, of course, some exceptions to this, particularly as regards niobium content. Typical compositions for precipitation-strengthened and solid-solution-strengthened superalloys are given in Table 1. As their classification implies, these alloys derive a significant proportion of their strength from solution strengthening, most typically associated with a high content of refractory metals, such as molybdenum or tungsten. Not to be overlooked, however, is the equally significant contribution of carbon, which serves both as a potent solution- strengthening element, and as a source of both primary and secondary carbide strengthening. Primary carbides, carried over from final melting operations, serve to control grain structure and thus contribute somewhat to alloy strength; however, the formation of secondary carbides, which is critical to developing the best strength, is also the key issue in formulating and performing alloy heat treatments. Solid-solution-strengthened superalloys are usually supplied in the solution-heat-treated condition, where virtually all of the secondary carbides are dissolved, or "in solution." Microstructures generally consist of primary carbides dispersed in a single-phase matrix, the grain boundaries of which are reasonably clean. This is the optimum condition for good elevated- temperature strength and generally best room-temperature fabricability. When the carbon is mostly in solution, exposure at elevated temperatures below the solution temperature will result in secondary carbide precipitation. In service, where the alloy component is subjected to operating stresses, this carbide precipitation will occur both on grain boundaries and intragranularly on areas of high dislocation density. It is the latter that provides for increased strength in service. When exposure to temperatures below the solution temperature occurs during component heat-treating cycles, the result is usually to precipitate secondary carbides only on grain boundaries. This is not normally beneficial for subsequent fabrication, and it reduces the capability of the alloy to develop in-service strengthening by depleting carbon from solution. Generally speaking, then, solid-solution-strengthened alloy components will exhibit highest strength when placed in service in the fully solution-heat-treated condition; however, the reality of modern complex component designs dictates what can and cannot be done in terms of final heat treatments. Quite often the compromise between component manufacturability and performance will mean something less than optimal alloy structure. Annealing and Stress Relieving. In the case of solid-solution-strengthened superalloys, heat treatments performed at temperatures below the secondary carbide solvus or solutioning temperature range are classified as mill annealing or stress-relief treatments. Mill annealing treatments are generally employed for restoring formed, partially fabricated, or otherwise as-worked alloy material properties to a point where continued manufacturing operations can be performed. Such treatments may also be used in finished raw materials to produce structures that are optimum for specific forming operations, such as fine grain size structure for deep drawing applications. Mill-annealed products may also be used in preference to solution heat treatments for final components where properties other than creep and stress-rupture strength are vital. For example, where low-cycle fatigue properties are important, mill annealing may be used to produce a finer grain size. A finer grain size from mill annealing may also be useful in applications where yield strength instead of creep strength is the limiting design criterion. Finally, mill annealing may be selected in preference to solution annealing because of external constraints, such as avoidance of component distortion at full solution annealing temperatures, or limits to temperature imposed by the melting point of component braze joints. Because mill annealing is performed below the secondary carbide solvus temperature, some decoration of grain boundaries can be expected in the microstructure. Depending upon the annealing temperature, the particular alloy, and the nature of the secondary carbide involved, this decoration may take the form of either discrete, globular particles or a more continuous film-like morphology. Cooling rates will markedly influence the appearance of this carbide precipitation, as most alloys of this type exhibit the most significant amount of precipitation in the temperature range from about 650 to 870 °C (1200 to 1600 °F). It is always recommended that components be cooled as rapidly as is feasible through this range, within the constraints of equipment used and with due consideration to avoiding component distortion from thermal stresses. Typical minimum mill annealing temperatures for various alloys are given in Table 22. These temperatures vary significantly from alloy to alloy. They are based principally upon the ability of the treatment to develop a recrystallized grain structure starting from a cold-worked or warm-worked condition and to produce low enough yield strength and high enough ductility for subsequent cold forming operations. Grain size would be expected to increase somewhat, although perhaps not markedly, when higher mill annealing temperatures are used. Table 22 Minimum mill annealing temperatures for solid-solution-strengthened alloys Approximate minimum temperature for mill annealing Alloy °C °F Hastelloy X 1010 1850 Hastelloy S 955 1750 Alloy 625 925 1700 RA 333 1035 1900 Inconel 617 1035 1900 Haynes 230 1120 2050 Haynes 188 1120 2050 Alloy L-605 1120 2050 Alloy N-155 1035 1900 The same basic temperatures would apply for mill annealing hot-worked material, although solution annealing is more common. Hot-worked material is usually dynamically recrystallized during the hot-working operation, and the main effect of mill annealing is to promote uniformity of the structure throughout the piece. Times at temperature required for mill annealing are governed by several factors. Sufficient furnace time should be allowed to ensure that all parts of the piece are at temperature for the requisite time. The requisite time should be long enough to ensure that structure changes, such as recovery, recrystallization, and carbide dissolution (if any), are essentially complete. Generally, about 5 to 20 min at temperature is sufficient, particularly in thin sections. In continuous thin-strip annealing operations, as little as 1 to 2 min will often suffice. Excessive time at temperature for mill annealing is not necessarily deleterious, but is most often not beneficial. Use of a thermocouple on the actual piece undergoing annealing is always appropriate. Stress Relief. Unlike mill annealing, stress-relief treatments for solid-solution-strengthened superalloys are not well defined. Dependent upon the particular circumstances, stress relief may be achieved with relatively low-temperature annealing, or may require the equivalent of mill or even solution annealing. In any case, such treatments represent a major compromise between the effectiveness of stress relief and the harm done to the structure or dimensional stability of the component. Strictly speaking, stress-relief annealing should be considered only if the material is not recrystallized by the treatment. If the intent is to relieve stresses in a piece or component that would otherwise be mill annealed or solution treated, then the first choice is the equivalent of a solution heat treatment or mill annealing to accomplish the required stress relief. Temperatures below the mill annealing temperature range, particularly in the range of 650 to 870 °C (1200 to 1600 °F), will likely result in significant carbide precipitation, or other phase formation in some alloys, which may significantly impair alloy performance. Treatments below 650 °C (1200 °F) may be less deleterious, but are likely to be less effective in relieving residual stresses. To relieve stresses in a partially cold- or warm-worked piece or component (that is, a finish-formed component that cannot be mill- or solution-annealed), then the stress-relief treatment should be restricted to a temperature less than that which will induce recrystallization. In this class of material, that temperature will vary with the particular alloy and degree of cold or warm work, but will generally be less than about 815 °C (1500 °F). In some materials (such as Inconel 625 and Haynes alloy 214), age-hardening reactions occurring at these lower temperatures must be considered in addition to the more general carbide precipitation encountered in other alloys. Times at temperature required to effect a significant amount of stress relief are equally ill-defined. For the equivalent to mill and solution annealing, similar times should be used. For lower-temperature stress-relief treatments, no specific guidelines are offered, but excessive times should be avoided for obvious reasons. Solution heat treating is the most common form of finishing operation applied to solid-solution-strengthened superalloys. As mentioned earlier, a solution treatment places virtually all the secondary carbides into solution. The temperatures at which all secondary carbides are dissolved vary somewhat what from alloy to alloy, and can differ as a function of the type of secondary carbide involved and the carbon content. Typical solution treatment temperatures for various alloys are given in Table 23. For some alloys the temperature range is broader than others; in most cases, such as Haynes 230, this is related to desired flexibility in controlling the grain size in the solution-treated piece. In Haynes 230, for example, an 1175 °C (2150 °F) solution treatment might produce an ASTM grain size between 7 and 9, while a solution treatment at 1230 °C (2250 °F) could be expected to yield a grain size of ASTM 4 to 6, assuming starting material in a sufficiently cold-reduced condition. Table 23 Typical solution annealing temperatures for solid-solution-strengthened alloys Typical solution annealing temperatures Alloy °C °F Hastelloy X 1165-1190 2125-2175 Hastelloy S 1050-1135 1925-2075 Alloy 625 1095-1205 2000-2200 RA 333 1175-1205 2150-2200 Inconel 617 1165-1190 2125-2175 Haynes 230 1165-1245 2125-2275 Haynes 188 1165-1190 2125-2175 Alloy L-605 1175-1230 2150-2250 Alloy N-155 1165-1190 2125-2175 Haynes 556 1165-1190 2125-2175 Recrystallization and Grain Size. A major function of the solution annealing treatment is to recrystallize warm- or cold-worked structure fully and to develop the required grain size. Aspects such as heating rate and time at temperature are important considerations. Rapid heating to temperature is usually desirable to help minimize carbide precipitation and to preserve the stored energy from cold or warm work required to provide recrystallization and/or grain growth during the solution treatment itself. For much the same reason that re-solution-treating an already annealed piece often does not coarsen grain size without increasing the temperature, slow heating of a cold- or warm-worked material to the solution- treating temperature can produce a finer grain size than may be desired or required. Time at temperature considerations for solution heat treatments are similar to those for mill annealing, although slightly longer exposures are generally indicated to ensure full dissolution of secondary carbides. For minimum temperature solution treatments, heavier sections should generally be exposed at temperature for about 10 to 30 min, thinner sections for somewhat shorter times. Solution treatments at the high end of the prescribed temperature range can be shorter, similar to mill annealing. Although very massive parts, such as forgings, may benefit from somewhat longer times at temperature, in no case should any component be exposed to solution treatment temperatures for excessive periods (such as overnight). Long exposures at solution treatment temperatures can result in partial dissolution of primary carbides, with consequent grain growth or other adverse effects. The effects of cooling rate upon alloy properties following solution heat treatment can be much more pronounced than those related to mill annealing. Because the solution treatment places the alloy in a state of greater supersaturation relative to carbon, the propensity for carbide precipitation upon cooling is significantly increased over that for mill annealing. It is therefore even more important to cool from the solution treatment temperature as fast as possible, bearing in mind the constraints of the equipment, and the need to avoid component distortion due to thermal stresses. The sensitivity of individual alloys to property loss from slower cooling down to about 650 °C (1200 °F) varies, but most alloys will suffer at least some property degradation as a result of secondary carbide precipitation. This is shown by the data in Table 24, in which the effects of various cooling practices on the low-strain creep properties of three alloys are described. Table 24 Cooling rate effects on time to 0.5% creep at 870 °C (1600 °F) with 48 MPa (7 ksi) load Time to 0.5% creep, h Solution treat at 1175 °C (2150 °F) and cool at the rate indicated Hastelloy X Haynes 188 lnconel 617 Water quench 8 148 302 Air cool 7 97 15 Furnace cool to 650 °C (1200 °F) and then air cool 6 48 9 Solution Treating Combined with Brazing. Unlike mill annealing, which is usually performed as a manufacturing step itself, solution treating may sometimes be combined with another operation, which imposes significant constraints upon both heating and cooling practices. A good example of this is vacuum brazing. Often performed as the final manufacturing step in the fabrication of components, such a process precludes subsequent solution treatment by virtue of the limits imposed by the melting point of the brazing compound. Therefore, the actual brazing temperatures are sometimes adjusted to allow simultaneous solution heat treating of the component. Unfortunately, the nature of vacuum brazing furnace equipment specifically, and vacuum furnace equipment in general, is such that relatively slow heating and cooling rates are a given. In these circumstances, even with the benefit of advanced forced gas cooling equipment, the structure and properties of alloy components are likely to be less optimal than those achievable with solution treatments performed in other types of equipment. Relationship of Processing History to Heat Treatment. As for most other alloy materials, the response of solid- solution-strengthened superalloys to heat treatment is very much dependent upon the initial material condition. Generally speaking, when the material is not in the cold- or warm-worked condition, the principal response to heat treatment is a change in the amount and morphology of secondary carbide phases present. Relief of minor residual stresses, or relaxation of internal strains, either of which may influence alloy properties to some degree, may also occur. Grain structure, however, may often be substantially unaltered by heat treatment when cold or warm work is absent. Hot-worked products, in particular those produced at high finishing temperatures, undergo recovery, recrystallization, and grain growth during the working operation itself. If finish working temperatures are too high relative to the final mill- annealing or solution-treatment temperatures, a significant degree of control over the structure resides in the working operation, rather than in the heat treatment. Similarly, if the final hot-working reductions are small, the piece to be heat treated often is initially nonuniform and responds nonuniformly to heat treatment. Material finished at a very high temperature may be best heat treated at temperatures near the high end of the allowable range, and almost always at a temperature above the finish hot-working temperature. For cases with small finish reductions, temperatures at the low end of the range would probably be advisable to minimize the nonuniformity in structure. This last approach might be particularly advisable for pieces with very heavy section thickness, such as large forgings, large-size bars, and thick plates. Fortunately, solid-solution-strengthened superalloys as a group exhibit relatively wide hot-working ranges, which allow finishing temperatures low enough to produce a warm-worked condition. They are also readily manufactured using cold working processes. In the warm-worked or cold-worked condition, grain structure control resides basically in the heat treatment, but results can be significantly influenced by the amount of work in the piece. As an example of this, the data presented in Table 25 show the influence of initial cold work on the grain size of final heat-treated Haynes 556 sheet. Table 25 Effect of cold reduction and annealing temperature on grain size of 556 alloy 5-min subsequent annealing temperature Cold reduction, % °C °F Degree of recrystallization ASTM grain size 0 None None . . . 5.0-6.0 10 1010 1850 Incomplete . . . 20 1010 1850 Incomplete . . . 30 1010 1850 Partial . . . 40 1010 1850 Partial 7.5-9.5 50 1010 1850 Full 9.0-10.0 10 1065 1950 Incomplete . . . 20 1065 1950 Incomplete . . . 30 1065 1950 Full 7.5-9.5 40 1065 1950 Full 8.0-9.5 50 1065 1950 Full 8.5-10.0 10 1120 2050 Full 5.0-5.5 20 1120 2050 Full 7.5-8.5 30 1120 2050 Full 7.0-7.5 40 1120 2050 Full 7.5-9.0 50 1120 2050 Full 8.0-9.5 10 1175 2150 Full 5.0-5.5 20 1175 2150 Full 6.0-6.5 30 1175 2150 Full 4.5-6.5 40 1175 2150 Full 4.5-6.5 50 1175 2150 Full 5.5-6.0 The particular sequence of cold-work/annealing cycles used in multistep material manufacturing or component fabrication can also affect the structure and properties of these alloys. One general guideline is to keep the temperatures used for intermediate annealing steps at or below the final annealing temperature. Intermediate annealing at temperatures above the final annealing temperature can reduce the degree of structure control possible in the alloy. The minimum level of cold work shown in Table 25, 10%, is an important rough dividing line between normal recrystailization behavior and possible abnormal grain growth in these alloys. Introduction of small amounts of cold or warm work prior to solution heat treating should be avoided where possible to minimize the potential for abnormal grain growth phenomena. The effects of very small amounts of cold work on the grain size response to annealing for Hastelloy X are shown in Table 26. The samples used to generate these data were carefully strained tensile test specimens, subsequently exposed to the annealing temperatures shown. Strains from 1 to 8% produced little effect for mill annealing temperatures up to 1120 °C (2050 °F); however, for solution annealing at 1175 °C (2150 °F), abnormal grain growth was observed for strains of 1 to 5%. Table 26 Effect of small strains on abnormal grain growth of Hastelloy X 5-min subsequent annealing temperature Prior cold work,% °C °F ASTM grain size 0 None None 4.5-6.5 1 1120 2050 4.5-6.5 2 1120 2050 4.0-6.5 3 1120 2050 4.0-6.0 4 1120 2050 3.5-6.0 5 1120 2050 3.5-6.0 8 1120 2050 3.5-6.0 1 1175 2150 5.0 + 0 at surface 2 1175 2150 5.0-5.5 + 0 at surface 3 1175 2150 00-4.5 4 1175 2150 4.5-5.0 + 1.0-1.5 5 1175 2150 3.0-3.5 + 1.0-1.5 4.5-5.0 8 1175 2150 (recrystallized) Unfortunately, in everyday fabrication of complex components, it is difficult if not impossible to avoid situations where such low levels of cold work or strain are present. Some alloys are more tolerant of this than others, but virtually all will exhibit abnormal grain growth under some conditions. Procedures that may be effective for minimizing the problem are: • Solution treating at the low end of allowable temperature ranges • Mill annealing in preference to solution annealing for intermediate heat treatments during component fabrication • Stress-relief annealing directly prior to final solution annealing References cited in this section 1. D.D. Krueger, The Development of Direct Age 718 for Gas Turbine Engine Disk Applications, in Proceedings of Superalloy 718 Metallurgy and Applications, EA Loria, Ed., The Metallur gical Society, 1989, p 279-296 2. E.E. Brown et al, Minigrain Processing of Nickel-Base Alloys, in Superalloys Processing , American Institute of Mechanical Engineers, 1972, section L 5. E.E. Brown and D.R. Muzyka, in Superalloys II, C.T. Sims, N.S. Stol off, and W.C. Hagel, Ed., John Wiley & Sons, 1987, p 180 6. H. Hucek, Ed., Aerospace Structural Metals Handbook, MPDC, Battelle Columbus, 1990, Section 4107, p 5-8 7. H. Hucek, Ed., Aerospace Structural Metals Handbook, MPDC, Battelle Columbus, 1990, Sec tion 4105, p 7 8. J.W. Brook and PJ Bridges, in Superalloys 1988, The Metallurgical Society, 1988, p 33-42 9. E.E. Brown and D.R. Muzyka, in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, Ed., John Wiley & Sons, 1987, p 185 10. H. Hucek, Ed., Aerospace Structural Metals Handbook, MPDC, Battelle Columbus, 1990, Section 4103, p 16 11. O.A. Onyeiouenyi, Alloy 718 Alloy Optimization for Applications in Oil and Grease Production, in Proceedings of Superalloy 718 Metallurgy and Applications, E.A. Loria, Ed., The Metallurgical Society, 1989, p 350 12. J. Kolts, Alloy 718 for the Oil and Gas Industry, in Proceedings of Superalloy 718 Metallurgy and Applications, EA Loria, Ed, The Metallurgical Society, 1989, p 332 13. W. Betteridge, The Nimonic Alloys, Edward Arnold, Ltd., 1959, p 77 14. E.W. Ross and C.T. Sims, in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, Ed., John Wiley & Sons, 1987, p 127 15. E.W. Ross and C.T. Sims, in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, Ed ., John Wiley & Sons, 1987, p 927 16. F. Schubert, Temperature and Time Dependent Transformation: Application to Heat Treatment of High Temperature Alloys, in Superalloys Source Book, M.J. Donachie, Jr., Ed., ASM International, 1989, p 88 Cast Superalloy Heat Treatment Heat treatment of cast superalloys in the traditional sense was not employed until the mid-1960s. Before the use of shell molds, the heavy-walled investment mold dictated a slow cooling rate with its associated aging effect on the casting. As faster cooling rates with shell molds developed, the aging response varied with section size and the many possible casting variables. These factors, coupled with significant γ' alloying additions, provided the opportunity to minimize property scatter by heat treatment. The combination of hot isostatic pressing (HIP) plus heat treatment has also greatly enhanced properties. Generally, heat treating cast superalloys involves homogenization and solution heat treatments or aging heat treatments. A stress-relief heat treatment may also be performed to reduce residual casting, welding, or machining stresses. Cobalt-base alloy heat treatments may be done in an air atmosphere unless unusually high-temperature treatments are required, in which case vacuum or inert gas environments are used. Conversely, nickel-base alloys are always heat treated in a vacuum or in an inert gas medium. Detailed information can be found in Ref 17. Like wrought superalloys, the solution heat-treating procedures of cast superalloys must be optimized to stabilize the carbide morphology. High-temperature exposure may cause extensive carbide degeneration, resulting in grain-boundary carbide overload and compromised mechanical properties. Unlike wrought superalloys, however, many polycrystalline materials are used in the as-cast plus aged condition without any specific solution step. Cast cobalt-base superalloys, for example, are not usually solution treated (although they may be given stress-relief and/or aging treatments). When required, cast cobalt-base superalloys are generally aged at 760 °C (1400 °F) to promote formation of discrete Cr 23 C 6 particles. Higher-temperature aging can result in acicular and/or lamellar precipitates. Precipitation-strengthened nickel- or iron/nickel-base superalloys are cast using the investment casting process. The resultant casting comprises a large number of grains and is referred to as a polycrystalline or conventional casting. If the casting is solidified under a thermal gradient, a columnargrained directionally solidified casting will result. Directionally solidified (DS) airfoil castings are used in the turbine sections of gas turbine engines to enhance durability and performance. Additional benefits can be achieved using directional-solidification investment casting to cast turbine airfoils as single crystals. Precipitation-strengthened nickel-base superalloys are primarily utilized for turbine airfoils, while iron-nickel alloys are employed as large investment-cast structural castings. Superalloys are heat treated to control the morphology of the precipitating phases (γ', γ'', carbides, and δ) that are responsible for the mechanical properties of the alloy. Three basic heat treatment steps are used: • Solution • Stabilization • Aging Representative heat treatments for several alloys are listed in Table 27. Table 27 Typical heat treatments for precipitation-strengthened cast superalloys Alloy Heat treatment (temperature/duration in h/cooling) (a) Polycrystalline (conventional) castings B-1900/B- 1900+Hf 1080 °C (1975 °F)/4/AC + 900 °C (1650 °F)/10/AC IN-100 1080 °C (1975 °F)/4/AC + 870 °C (1600 °F)/12/AC IN-713 as-cast IN-718 1095 °C (2000 °F)/1/AC + 955 °C (1750 °F)/1/AC + 720 °C (1325 °F)/8/FC + 620 °C (1150 °F)/8/AC IN-718 with HIP 1150 °C (2100 °F)/4/FC + 1190 °C (2175 °F)/4/15 ksi (HIP) + 870 °C (1600 °F)/10/AC + 955 °C (1750 °F)/1/AC + 730 °C (1350 °F)/8/FC + 665 °C (1225 °F)/8/AC [...]... create an awareness of the difficulties and risks of heat treating these materials to avoid the repetition of costly past errors The major risk is loss of vacuum at a temperature that results in the extremely costly destruction by oxidation, not only of parts being heat treated, but also of the furnace shielding and heating elements Principles of Heat Treating of Nonferrous Alloys Charlie R Brooks, University... 2100-2190 Nb 752 10 W, 2.5 Zr 130 0-1400 2370-2550 C 129Y 10 W, 10 Hf, 0.1 Y 900 1650 1150-1250 2100-2280 FS85 28Ta, 11 W, 0.8 Zr 1150 2100 130 0-1400 2370-2550 C103 10 Hf, 1 Ti, 0.7 Zr 1250 -137 5 2280-2510 Niobium alloys (a) Powder metallurgy; all other compositions arc-cast Molybdenum and Tungsten Annealing Practice Furnace atmosphere considerations are important when chosing heat- treating equipment for molybdenum... elements during heat treating, appropriate surface-cleaning procedures, furnace maintenance, and operational practices are essential Furnace Selection Because these alloys are easily contaminated during annealing, special care must be exercised in furnace selection, cleanliness of work, and annealing practice Cold-wall radiant-heated furnaces with refractory metal heating elements, primary heat shields,... °C °F Molybdenum alloys Mo(a) None 850-950 1560-1740 1000-1200 1830-2190 Mo-TZM(a) 0.5 Ti, 0.1 Zr, 0.03 C 1100 -130 0 2010-2370 135 0-1475 2460-2690 Mo-MHC(a) 1 Hf, 0.05 C 1100 -135 0 2010-2460 1400-1600 2550-2910 Mo-30W(a) 30 W 1150-1200 2100-2190 130 0-1450 2370-2640 Doped Mo 0.07 Si, 0.05 K 1250 -135 0 2280-2460 1400-1600 2550-2910 Tungsten alloys (a) Arc-cast or powder metallurgy; all other compositions... purpose of this heat treatment is to optimize the γ' size and morphology and to assist decomposition of the coarse, as-cast MC carbides into fine, grain-boundary carbides With nickel-base alloys used for turbine airfoils, the stabilization heat treatment is often combined with the heat treatment used to bond or diffuse a coating onto the alloy substrate In iron-nickel-base alloys a stabilization heat treatment... solution heat treatment, the stabilization heat treatment is carried out in a protective atmosphere, such as argon, helium, hydrogen, or vacuum, to prevent excessive oxidation of the casting Retorts and conventional furnaces are used to provide the stabilization heat treatment under a protective atmosphere Cooling rates equivalent to air cooling or faster are normally used As the stabilization heat treatment... heat treatment, many of these alloys do not require any heat treatment and are often used in the as-cast condition Representative heat treatments for several alloys are listed in Table 28 Table 28 Typical heat treatments for solid-solution-strengthened cast superalloys Alloy Heat treatment Hastelloy C 1220 °C (2225 °F)/0.5 h/air cool Hastelloy S 1050 °C (1925 °F)/1 h, air cool Hastelloy X As-cast Inconel... surface coating, welding, or brazing, may impose additional heat treatment requirements Heat treatments to bond coatings to a cast superalloy substrate are usually performed at temperatures of 980 to 1090 °C (1800 to 2000 °F) Stress relief, following joining or for other purposes, can be carried out over a broad range of temperatures The particular temperature represents a compromise between the effectiveness... prevent oxidation of the casting When a vacuum furnace is employed a partial pressure of an inert gas, such as argon, is used rather than a hard vacuum to prevent surface depletion of chromium and aluminum from the castings When the solution heat treatment temperature is very close to the incipient melting temperature of the alloy, varied heating rates are used to homogenize the castings during the time... Stress-relief Recrystallization °C °F °C °F Tantalum alloys Ta None 850 1560 1000-1250 1830-2280 Ta None(a) 1000 1830 1200 -135 0 2190-2460 FS63 2.5 W, 0.15 Nb 1000 FS61 (KBI-6) 7.5 W (a) Ta-10W (FS60, KBI-10) 10 W 1100 T111 8 W, 2 Hf T222 1830 1200 -130 0 2190-2370 1400-1550 2550-2820 2010 130 0-1600 2370-2910 1100 2010 1400-1650 2550-3000 9 W, 2.4 Hf, 0.01 C 1100 2010 1400-1650 2550-3000 Nb None 800 1470 900-1200 . minimize property scatter by heat treatment. The combination of hot isostatic pressing (HIP) plus heat treatment has also greatly enhanced properties. Generally, heat treating cast superalloys. solution heat treatments or aging heat treatments. A stress-relief heat treatment may also be performed to reduce residual casting, welding, or machining stresses. Cobalt-base alloy heat treatments. 0.03 C 1100 -130 0 2010-2370 135 0-1475 2460-2690 Mo-MHC (a) 1 Hf, 0.05 C 1100 -135 0 2010-2460 1400-1600 2550-2910 Mo-30W (a) 30 W 1150-1200 2100-2190 130 0-1450