Superalloys for High Temperatures—A Primer
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© 2002 ASM International. All Rights Reserved. www.asminternational.org Superalloys: A Technical Guide (#06128G) Chapter 1 Superalloys for High Temperatures—a Primer How and When to Use This Chapter in the field, check the table of contents and index for valuable insights into what you can It is always difficult to locate concise but find in each succeeding chapter. precise information on a subject. Executives and managers, particularly in industries using few superalloys, often need just basic infor- Some History mation with the least extraneous or amplify- ing data. Purchasing agents or communica- Designers have long had a need for tions experts need a modest knowledge base stronger, more corrosion-resistant materials to do their jobs more appropriately. The en- for high-temperature applications. The stain- gineer may need more detail but still just a less steels, developed and applied in the sec- quick refresher about alloy types and design ond and third decades of the 20th century, to start. The ability to lay hands on enough served as a starting point for the satisfaction practical information to solve problems or of high-temperature engineering require- answer questions about the superalloys is the ments. They soon were found to be limited basis for this book. The ability to know in their strength capabilities. The metallurgi- enough to ask questions and/or delve further cal community responded to increased needs into the superalloy field is the basis for this by making what might be termed ‘‘super-al- chapter! loys’’ of stainless varieties. Of course, it was The primer provided in this chapter sup- not long before the hyphen was dropped and ports such needs as those described previ- the improved iron-base materials became ously by providing a concise overview of the known as superalloys. Concurrently, with the major topics considered in the book, starting advent of World War II, the gas turbine be- with a little history and then a statement came a high driver for alloy invention or ad- about the nature of superalloys. This primer aptation. Although patents for aluminum and introduces the reader simply and directly to titanium additions to Nichrome-type alloys the wide variety of topics that must be con- were issued in the 1920s, the superalloy in- sidered in the application of superalloys. As dustry emerged with the adaption of a cobalt for the book, whether the user is familiar with alloy (Vitallium, also known as Haynes Stel- basic superalloy metallurgy or is a complete lite 31) used in dentistry to satisfy high-tem- novice, this book provides a single-volume perature strength requirements of aircraft en- approach to the subject of superalloys. The- gines. Some nickel-chromium alloys (the ory is kept to a minimum, with practical Inconels and Nimonics), based more or less, knowledge stressed. one might say, on toaster wire (Nichrome, a If you are new to the subject, start with nickel-chromium alloy developed in the first this primer; it may be all that you need. If decade of the 20th century) were also avail- you are somewhat or strongly knowledgeable able. So, the race was on to make superior © 2002 ASM International. All Rights Reserved. www.asminternational.org Superalloys: A Technical Guide (#06128G) 2 / Superalloys: A Technical Guide metal alloys available for the insatiable thirst timate strength. However, when temperatures of the designer for more high-temperature rise, particularly to temperatures (on an ab- strength capability. It continues yet! solute temperature scale) of about 50% of the melting point/range for an alloy, strengths must be reckoned in terms of the time over What Are Superalloys and What which they are measured. Thus, if a metal is subjected to a load considerably less than the Can You Do to Them? load (stress) that would break it at room tem- perature, but is at a high temperature, then the Superalloys are nickel-, iron-nickel-, and metal will begin to extend with time at load. cobalt-base alloys generally used at temper- This time-dependent extension is called atures above about 1000 ЊF (540 ЊC). The creep and, if allowed to continue long iron-nickel-base superalloys such as the pop- enough, will lead to fracture (or rupture, as ular alloy IN-718 are an extension of stain- it is called). Thus the creep strength of a less steel technology and generally are metal or its rupture strength (technically wrought. Cobalt-base and nickel-base super- called creep-rupture strength but more com- alloys may be wrought or cast, depending on monly called stress-rupture strength) or both the application/composition involved. are necessary components of understanding A large number of alloys have been in- its mechanical behavior just as much as are vented and studied; many have been pat- the customary yield and ultimate strengths. ented. However, the many alloys have been Similarly, the fatigue (cyclic) capability will winnowed down over the years; only a few be reduced. So, to fully validate the capabil- are extensively used. Alloy use is a function ity of a metal alloy, dependent on application of industry (gas turbines, steam turbines, temperature and load, it may be necessary to etc.). Not all alloys can be mentioned; ex- provide yield and ultimate strengths, creep amples of older and newer alloys are used to strengths, stress-rupture strengths, and appro- demonstrate the physical metallurgy response priate fatigue strengths. Related mechanical of superalloy systems (see Chapters 3 and properties such as dynamic modulus, crack 12). Figure 1.1 compares stress-rupture be- growth rates, and fracture toughness also may havior of the three alloy classes (iron-nickel-, be required. Appropriate physical properties nickel-, and cobalt-base). A representative such as thermal expansion coefficient, den- list of superalloys and compositions, empha- sity, and so on complete the property list. sizing alloys developed in the United States, is given in Tables 1.1 and 1.2. Appropriate compositions of superalloys can be forged, rolled to sheet, or otherwise Basic Metallurgy of Superalloys produced in a variety of shapes. The more highly alloyed compositions normally are Iron, nickel, and cobalt are generally face- processed as castings. Fabricated structures centered cubic (fcc-austenitic) in crystal can be built up by welding or brazing, but structure when they are the basis for super- many highly alloyed compositions containing alloys. However, the normal room-tempera- a large amount of hardening phase are diffi- ture structures of iron and cobalt elemental cult to weld. Properties can be controlled by metals are not fcc. Both iron and cobalt un- adjustments in composition and by process- dergo transformations and become fcc at high ing (including heat treatment), and excellent temperatures or in the presence of other ele- elevated-temperature strengths are available ments alloyed with iron and cobalt. Nickel, in finished products. on the other hand, is fcc at all temperatures. In superalloys based on iron and cobalt, the fcc forms of these elements thus are generally stabilized by alloy element additions, partic- A Short Review of the High- ularly nickel, to provide the best properties. Temperature Strength of Metals The upper limit of use for superalloys is not restricted by the occurrence of any allo- At ordinary temperatures, the strengths of tropic phase transformation reactions but is a most metals are measured in terms of short- function of incipient melting temperatures time properties such as yield strength or ul- of alloys and dissolution of strengthening © 2002 ASM International. All Rights Reserved. www.asminternational.org Superalloys: A Technical Guide (#06128G) Superalloys for High Temperatures—a Primer / 3 Fig. 1.1 Stress-rupture strengths of superalloys phases. Incipient melting is the melting that densities of about 0.322 lb/in.3 (8.9 g/cm3). occurs in some part of the alloy that, when Iron-nickel-base superalloys have densities of solidified, is not at equilibrium composition about 0.285 to 0.300 lb/in.3 (7.9 to 8.3 g/ and thus melts at a lower temperature than cm3); cobalt-base superalloys, about 0.300 to that at which it might otherwise melt. All al- 0.340 lb/in.3 (8.3 to 9.4 g/cm3); and nickel- loys have a melting range, so melting is not base superalloys, about 0.282 to 0.322 lb/in.3 at a specific temperature even if there is no (7.8 to 8.9 g/cm3). Superalloy density is in- nonequilibrium segregation of alloy ele- fluenced by alloying additions: aluminum, ments. Superalloys are strengthened not only titanium, and chromium reduce density, by the basic nature of the fcc matrix and its whereas tungsten, rhenium, and tantalum in- chemistry but also by the presence of spe- crease it. The corrosion resistance of super- cial strengthening phases, usually precipi- alloys depends primarily on the alloying el- tates. Working (mechanical deformation, of- ements added, particularly chromium and ten cold) of a superalloy can also increase aluminum, and the environment experienced. strength, but that strength may not endure at The melting temperatures of the pure ele- high temperatures. ments are as follows: nickel, 2647 ЊF (1453 Some tendency toward transformation of ЊC); cobalt, 2723 ЊF (1495 ЊC); and iron, the fcc phase to stable lower-temperature 2798 ЊF (1537 ЊC). Incipient (lowest) melting phases occasionally occurs in cobalt-base su- temperatures and melting ranges of superal- peralloys. The austenitic fcc matrices of su- loys are functions of composition and prior peralloys have extended solubility for some processing. Generally, incipient melting tem- alloying additions, excellent ductility, and peratures are greater for cobalt-base than (iron-nickel- and nickel-base superalloys) for nickel- or iron-nickel-base superalloys. favorable characteristics for precipitation of Nickel-base superalloys may show incipient uniquely effective strengthening phases.