Süperalaşımlardan nimonic 80 alaşımına kromun etkisi

Başlık çevirisi mevcut değil.

Tez No: 55766
Yazar: MÜMİN DENİZ
Danışmanlar: PROF.DR. NİYAZİ ERUSLU
Tez Türü: Yüksek Lisans
Konular: Metalurji Mühendisliği, Metallurgical Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1996
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Belirtilmemiş.
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 49

Özet

ÖZET Nikel esaslı süperalaşımlardan Nimonic 80 alaşımlarında krom miktarının yapıda ve mekanik özelliklerde meydana getirdiği değişiklikler incelenmiştir. Nikel esaslı alaşımlar yüksek sıcaklık uygulamalarında yaygın bir şekilde kullanılmakta olup kompleks süperalaşımlardır. Krom ilavesi en yüksek kullanım sıcaklığım arttırmış ve bilhassa uçak motorları, ağırlıkça %50 nikel esaslı alaşımlardan oluşurlar. Bunların fiziksel özellikleri çok karmaşık ve komplekstir. Mamafih 650- 1100°Cnin üzerindeki kullanımlar için yapı ve özellik ilişkisi en iyi bilinen malzemelerdir. Kimyasal yapısı açısından KYM östenit y alaşımın matriksi, y* alaşımın tane sınırlarım kaplayacak şekilde çökelen esas bir faz ve karbürler ikincil faz olarak kabul edilir. Tane sınırlan boyunca şerit (M^Cö) veya küresel (MC tipi) şekilde teşekkül ederler. Mikro yapıda oluşabilecek değişik fazların oluşumu ile beraber mukavemet de artar. Bunlar, y* hacim oranının artması, y* boyutunun önce büyümesi sonrada 1 mikron dolayında sabit kalışı, y"ın kübik boyutlara meyledişi, tane sınırlarında karbür partiküllerinin v* ile çevrilmesi ile karbürün verdiği özelliğin yok olması, yapıda y-y* ötektiği olarak adlandırılan küremsi tanelerin meydana çıkması, soğutma ile uygun dağılımda y* ikincil çökelmesinin ortaya çıkmasıdır. Bazı alaşımlar istenmeyen fazlar oluşturabilirler. Böylece daha düşük kopma mukavemetine sahip olurlar. Bu gibi dövelebilirliği azaltan ve levha tipindeki fazlar ekseriyetle o\ n ve laves fazlarıdır. Süperalaşımlarm yüksek sıcaklıklarda kararlı olmayan dinamik bir kimyasal yapı gösterirler. Mevcut fazlar sürekli olarak reaksiyona girip, birbirlerini etkilerler. Nikel alaşımları ergime sıcaklığının %85'i kadar kullanılabilir. Esas reaksiyon aşağıda belirtilen ve dayanıklılık kazandıran işlemlerdir.1- Kararsız fazlar oluşturmaksızın alaşımlandırma için geniş bir toleransa sahip olan nikel bunu hemen hemen doldurulmuş 3. elektron kabuğuna borçludur. 2- Krom ilavesinde oluşan Cr203 tabakası metalik kısmın dışarıya doğru difüzyonunu önlerken, atmosferdeki O2, N2, S ve diğer istenmeyen elementlerin bünyeye dahil olmalarını sınırlar. 3- Yüksek sıcaklıklarda ilavenin gayesi oksidasyona karşı A1203 oluşumu ile direnç sağlamaktır. Bu tez çalışmasında Nimonic 80 alaşımının krom miktarının artmasıyla sertliğinin düştüğü ve darbe enerjisinin de arttığı görülmüştür. Malzemenin mikroyapısında tane boyutunun büyüdüğü görülmüştür. %25 Cr miktarına gelindiğinde tane yapısının ortadan kaybolduğu gözlenmiştir. Aynı zamanda tane sınırlarında toplanan y* fazının azaldığı görülmüştür. Y fazının yüksek oranda olması malzemenin daha güvenli olmasını sağlamaktadır.

Özet (Çeviri)

THE EFEECT OF CHROMIUM ON NIMONIC 80 SUPERALLOY SUMMARY Superalloys are a group of nickel, iron- nickel and cobalt base materials that exibit outstanding strenght and surface stability at temperatures up to 85% of their melting points. (0.85 Tm) They are generally used at temperatures above 540°C. Superalloys were initially developed for use in aircraft piston engine turbosuper charger, and their development over the last 50 years has been paced by the demands of advancing gas turbine engine tecnology. Nickel-base and nickel-iron-base superalloys owe their high temperature strenght potential to their gamma prime (y*) (Nİ3A1.Tİ) content. The first reference to aluminum or titanium additions to the 80-20 Ni-Cr system occured in a patent filed by Heraeus Vacuumschmelze A.G. in 1926, in which as much as 6% Al was added to a nickel-chromium-iron alloy for increased tensile yield strenght. Not until later in the decade, however, did a French patent application recognize the occurrence of precipitation hardening in nickel-chromium alloys. In 1929, Pilling and Merica concurrently filed a number of patent applications in the United States for precipitation-hardening nickel-base alloys containing aluminum and titanium additions to nickel-base alloys were filed. The first commercial nickel-base alloy developmental work, undertaken by the British in the early 1940s, led to the wrought Nimonic 75 and 80 alloys. Increased operating, temperature erquirements for U.S. aircraft engines resulted in the use of aluminum plus titanium strengthened wrought materials during the same period of time. Component forgeability problems, however, led to the use of cast Vitallium until the shortages of cobalt supply experienced during the Korean War caused further research on nickel-base alloys. Cast nickel-base alloy developments outpaced cobalt-base developmental work by the late 1950s because of their superior strengthening potential, that is, IXstable, coherant intermetallic compound y* phase introduction. The introduction of commercial vacuum induction melting (VIM) and vacuum investment casting in the early 1950s provided further potential for y* exploitation. Many nickel-base alloy developments resulted, continuing through the 1960s. Nikel-base superalloys have microstructures consisting of an austenitic face- centered cubic (fee) matrix (y) dispersed intermetallic fee y' Nİ3(Al,Ti) precipitates coherent with the matrix, and carbides, borides, and other phases distrubited throughout the matrix and along the grain boundaries. These complex alloys generally contain more than ten different alloying constituents. Various combinations of carbon, boron, zirconium, hafnium, cobalt, chromium, aluminum, titanium, vanadium, molybdenum, tungsten, niobium, tantalum, and rhenium result in the commercial alloys used in today's gas turbine engines. Some alloying elements have single-function importance, whereas other provide multiple functions. For example, chromium is primarily added to nickel-base alloys for sulfidation resistance ( Cr2Û3 protective scala formation), whereas aluminum not only is a strong y' former but also helps provide oxidation resistance when present in sufficient quantity by forming a protective AI2O3 scale. Commercial vacuum induction melting was developt in the early 1950s, having been stumilated by the need to produce superalloys containing reactive element within an evacuated atmosphere. The process is relatively flexible, featuring the independent control of time, temperature, pressure, and mass transport through melt stirring. As such, VTM offers more control over alloy composition and homogeneity than all other vacuum melting process. The Melt Process. Base charge materials are layered in the relatively warm furnace, in a maner which recognizes and accomodates the elemental melting point of the material and bridging tendency. Only those materials with oxides that are relatively easily redoced for the encountered melt conditions are placed in the initial fümece charge along with a small, controlled carbon addition. Also, those elements that have a particularly strong affinity for nitrogen may be witheld from the base charge because they lower the activity of the dissolved nitrogen. Following furnace evacuation and particular heat-up cycles that ensure proper closure of any refractory lining cracks prior to metal liquation, optimum temperature and vacuum pressure, consistent with promoting a somewhat vigorous CO boil, is attained. Bath refining is undertaken at a temperature and duration longenough to reach the so-called system equilibrium conditions the assurance of which is provided by the attaintment of consistent furnace leak-up rates. At this point, those elements that were held from the base charge because of their relative reactivity toward oxygen, for example, aluminum, titanium, zirconium, and hafnium, are added with an associated solutioning and homogenization procedure. Dip sample alloy chemistry is checked in a relatively short time period with any necessary chemistry adjustment undertaken. A similar analytical check is undertaken prior to pouring. Filters. One of the most critical stages with respect to cleanliness is the pouring of the melt. Ceramic foam filters are used in some master metal operations to remove relatively large melt inclusions by means of entrapment. Foam filters are most effective where extremely high pour rate conditions and gross cleanliness problems prevail. Filters performance often varies because of the occasional use of filters with pour mechanical stregth and thermal shock resistance. Foam cell particle breakage often results from handling during shipment or tundish installation and, if undetected, results in filter particulate in the alloy bar stock and subsequently cast components. Optimized VIM technology and practice without filters provide a clean alloy, without the inherent risks associated with filter use when applied to master metal production. A number of casting prosses can provide near-net shape superalloy cast parts, but essantially all components are produced by investment casting. The characteristic physical and mechanical properties and complex, hollow shape-making capabilities of investment casting have made it ideal for amplifying the unusual high- temperature properties of superalloys. Cast superalloys are made in a wider range of compositions than are wrought alloys. Creep and rupture properties of a given siperalloy composition are maximized by the casting and heat-treatment procsses. Ductility and fatigue properties of polycrystalline materials are generally lower in castings than in their wrought counterparts of similar composition. The gap, however, is being reduced by new technological developmants to eliminate casting defects and refine grain size. Pattern, Cores, and Molds. The firs step in the investment casting process is the produce an exact replica or pattern of the part in wax, plastic, or a combination thereof. Pattern dimensions must compensate for wax, mold, and metal shrinkage during processing. If the product contains internal passages, a preformed ceramic XIcore is inserted in the die cavity, around which the pattern materials is injected. Except for large or complex castings, a number of patterns may be assembled in a cluster and held in position in order to channel the molten metal into the various mold cavities. Design and positioning of the runners and gating is critical to achieving sound, metallurgically acceptable castings. Today the molds are produced by first immersing the pattern assembly in an equeous ceramic slurry. A dry, granular ceramic stucco is applied immediately after dipping to strenghthen the shell, and the mold is fired to increase substantially its strength for handling and storage. An insulating blanket is tailoret to the mold configuration to minimize heat loss during the casting operation and to control solidification. The Casting Process. Most superalloys are cast in vacuum to avoid the oxidation of reactive elements in their compositions. Some cobalt-base superalloys are cast in air using induction or indirect are rollover furnaces. The vacuum casting of equiaxed-grain products is usually done in a furnace divided into two major chambers, each held under vacuum and separated by a large door or valve. The upper chamber contains an induction-heated reusable ceramic crucible in which the alloy is melted. Zirconia crucibles are commonly employed; single-use silica liners may be specified when alloy cleanliness is especially critical. The preweighed charge is intruduced through a lock device and is melted rapidly to a predetermined temperature, usually 85 to 165°C above the liquidus temperature. Precise optical measurement of this temperature is crucial. Metal temperature during casting is much more critical than mold temperature in controlling grain size and orientation; it also strongly affects the presence and location ofr micrishrinkage. When the superheat condition is satisfied, the preheat mold is rapidly transferred from the freheat furnace to the lower chamber, which is then evacuated. The mold is raised to the casting position, and the molten superalloy is quickly poured into the cavity; speed and reproductibility are essential in order to achieve good fill without cold shuts and order related imperfections. Precise mold positioning and pour rates also are imperative. For maximum consistency, melting and casting are automated with programmed closed-loop furnace control. The filled mold is olwered and removed from the furnace. The solidification of investment cast superalloy components is precisely controlled so that the microstructure, which ultimately determines mechanical properties, remains consistent. For example, once the process for a particular component has been defined, the production of these components does not deviate from the agreed-upon steps for the entire production run, which may last many years without proper approval. If steps are changed, it must be shown that the new steps do not cause a degradation in the properties of the component. xnTo control the solidification of equiaxed-grain castings, the investment caster has been several tools at his disposal: facecoats that encourage grain nucleation, pour temperature of the metal, preheat temperature of the shell, shell thickness, part orientation, part spacing, gating locations, insulation to wrap the shells, pouring speed, and shell agitation. However, the investment caster must firs fill the shell cavity, prevent hot tears or other cracks, and niinimize porosity. If the first two objectives can be met, the investment caster has some freedom to produce the desired structure. If the desired structure still cannot be made, other more complex techniques may be employed, including changing the termal conductivity of the shell. Dendrites are probably the most visible microstructural feature in superalloy castings. Primary and secondary dendrite arm spacing are controlled by the cooling rate. As the dendrite arm specing is reduced, segregation in the dendrite core and interdendritic regions is also reduced, thereby benifitting mechanical properties. Carbides conventional equiaxed-grain nickel-base superalloys typically have 0.05 to 0.20 wt% C, while cobalt-base alloys contain up to about 1.0%C. Both alloy systems may use carbon to increase grain-boundary strength. Eutectic segregation. By the very nature of solidification, segregation is introduced into the component. Important segregants of interest in cast superalloys are eutectics, which often are found in interdendritic or intergranular regions. In niçkel-base alloys, eutectic pools are the last constituents to solidify and have a cellular appearance. The composition of the eutectic pools varies, but they typically contain excess /, carbides, borides, and low melting point phases. Control of the eutectic pool is done primarily through composition. However, it has been shown that while the volume fraction of eutectic remained constant near 0. 10 vol% in IN- 713, the size of the eutectic pool increased from 11 to 19 um as the cooling rate decreased from 0.56 to 0.036 °C/s. Porosity is important to minimize the porosity in castings because the pores serve as initiation sites for fracture, especially fatigue cracks. There are three primary sources of porosity in superalloy investment casting: undissolved gas, microshrikage caused by poor feeding between dendrites, and macroshrinkage caused by inadequate gating. Undissolved gas is gas that has come out of solution but, with today's vacuum technology, it is seldom experienced. Usually made up of oxygen, nitrogen, or hydrogen this gas can form spherical voids up to two or more times the diameter of the dendrite arm spacing. Gas porosity can be essentialy eliminated by maintaining a vacuum during remelting and casting. xinMicroshrinkage is inherent to castings that experience dendritic solidification. The pores are spherical, but they typically have a diameter less than the dendrite spacing. Microshrinkage forms just ahead of the advancing solidus interface because liquid metal feeding is impeded by the tortuous path through and around the secondary dendrite arms. The control of grain size is an impotant means for developing and maintaining both physical and mechanical properties. Generally, a number of randomly oriented equiaxed grains in a given cross section is preferred to provide consistent properties, but often this is difficult to achieve in this sections. To meet this objective, mold facecoat nucleants, mold and metal temperature, and other parameters are choosen the accelerate grain nucleation and solidification. Finer grain size generally improves tensile, fatigue, creep properties at low- to-intermediate temperatures. The finer grain size produced by relatively rapid solidification is accompanied by a finer distribution of/ particles and a tendency to form blocky carbide particles. The letter morphology is preferred to the script-type carbides produced by slow solidification rates, particularly for a fatigue sensitive environment. Under these conditions, the carbide particles do not contribute to superalloy properties. As the service temperature increases, they impart important grain-boundary strengthening, provided that continuous films or necklaces are avoided.

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