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Silisyum katkısının döküm CrNiCo süperalaşımının özelliklerine etkisi

Başlık çevirisi mevcut değil.

  1. Tez No: 75054
  2. Yazar: HAVVA KAZDAL ZEYTİN
  3. Danışmanlar: PROF. DR. ADNAN TEKİN
  4. Tez Türü: Doktora
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Metalurji Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Malzeme Bilim Dalı
  13. Sayfa Sayısı: 158

Özet

Bir yüksek sıcaklık malzemesi olan CrNiCo süperalaşımı, çelik kütüklerin tavlandığı firınlarda kullanılmaktadır. Hem yüksek oranda içerdiği Cr, Co, Ni gibi stratejik ve pahalı alaşım elementleri, hem de kullanıldığı alandaki ekonomik değeri bakımından, bu malzemenin servis ömrünün uzun olması istenmektedir. Standart bileşimi ile yaklaşık olarak 6 ay servis ömrüne sahip olan bu alaşımın, çeşitli nedenlerle servis dışı kalması önemli maddi kayıplara neden olmaktadır. Bu çalışmada, 30Cr 22Nİ 22Co 16Fe 5Mo 4W 0.1 C bileşimine sahip CrNiCo süperalaşımının servis ömrünün, silisyum ile modifiye edilerek artırılması hedeflenmiştir. Normal ergitme ve döküm koşullarında yaklaşık olarak bileşiminde ağ.%0.5 Si bulunan bu alaşıma ağ.% olarak 2.16, 3.05, 4.20 ve 5.43 Si ilave edilmiştir. Beş değişik silisyum bileşiminde hazırlanan numuneler döküm halinde ve çalışma sıcaklıklarına yakın sıcaklıklar olan 900, 1100 ve 1250 °C'de ısıl işlem gördükten sonra karakterize edilmiştir. Mikroyapısal incelemeler için optik metalografi, elektron metalografi (SEM, TEM), x-ışınları, enerji ve dalga boyu dispersif spektrometrik (EDS,WDS) analiz metodları kullanılmıştır. Mekanik özelliklerin belirlenmesi için döküm halindeki ve ısıl işlem görmüş alaşımlarda mikro ve makro sertlik ölçümleri, aşınma deneyleri ile 900, 1000 ve 1100 °C'de yüksek sıcaklık çekme deneyleri yapılmıştır. Döküm halindeki CrNiCo süperalaşımının mikroyapısı, ostenitik matris, blok sigma fazı, sigma içine yerleşmiş chi fazı ile düşük silisyumlu bileşimlerde matris/sigma arayüzeyinde, yüksek silisyumlu bileşimlerde hem matris/sigma hem de sigma/chi arayüzeyinde oluşmuş hücresel M23C6 fazlarından oluşmaktadır. Silisyum arttıkça katılaşma sırasında oluşan geniş yüzeyli sigma ve chi fazının miktarı artmaktadır. Yüksek silisyumlu bileşimlerde hücresel yapı bozularak M23C6 lamelleri kısalmakta ve sürekliliğini kaybetmektedir. Ağ.% 5.43 Si bileşiminde mikroyapı son derece kırılgan rozet görünümlü bir hal almaktadır. 900, 1100 ve 1250°C'de yapılan ısıl işlemler sonrasında, primer sigma fazı etrafında çökelmeler olmakta, silisyum miktarı arttıkça aynı ısıl işlem süre ve sıcaklıklarında bu çökeltilerin miktarı da artmaktadır, i 100 °C'de matriste çökelen fazlar bir miktar kabalaşmakta ve düşük silisyumlu bileşimlerde sigma tazı R fazına dönüşmektedir. 1250 °C'de primer fazlar çözünmekte, matriste çökelme olmamaktadır. Silisyumun en önemli etkisinin, aşınma özellikleri üzerinde olduğu belirlenmiştir. Döküm halindeki alaşımda, mikroyapıdaki primer fazların artışı ile aşınma hızı düşmekte, ancak çekme dayanımı olumsuz etkilenmektedir. Silisyum artışı ile, alaşımın sertliği artmış, matrisin sertliği ise değişmemiştir. Bütün ısıl işlem sıcaklıklarında, silisyum arttıkça sertlik artıp, çekme dayanımı düşerken, süneklik sıcaklığa bağlı olarak önemli ölçüde yükselmektedir. Isıl işlem görmüş alaşımın aşınma hızı, çökelti fazlarının morfolojilerine bağlıdır. En düşük aşınma hızı, matriste geniş yüzeyli ve bir miktar kabalaşmış çökeltilerin yoğun olduğu 1 100 °C'de elde edilmiştir.

Özet (Çeviri)

Superalloys contain Group VIIA elements and they are described as alloys which are used at elevated temperatures because of their high resistance to stress and high surface stability. Development of superalloys has gone parallel with the developments in the aerospace and land based vehicles, especially in the gas turbine engines. This development is attributed to the interdisciplinary studies (metallurgy, pyhsics, mechanical eng., design, chemical eng. and marketing) carried out in the last 50 years. Along with new alloy developments, processing and production techniques have played a key role in the evolution of superalloys. Considering their uses, superalloys are expected to have the following properties:. High resistance to creep at elevated temperatures,. Structural stability,. High resistance to thermal fatigue,. Good wear and corrosion resistance at high temperatures. These properties can be obtained by selection of alloy composition, mechanical and heat-treatment parameters. Superalloys, in general, are classified into three major groups: Nikel, cobalt and nickel/iron based alloys. Superalloys also contain alloying elements of Cr, Mo, W, Nb, Ta, Al, Ti, C, B, Zr, Hf, La, Yt and Th in various concentrations. 80Ni-20Cr alloy is a typical Ni-based superalloy. This alloy has good thermal conductivity and high resistance to oxidation. Creep strength of these alloys can be improved with small additions of Al and Ti. Precipitation hardening is the main hardening mechanism in the nickel-based superalloys. Al and Ti forms y' Ni3(Al, Ti) precipitates. High temperature life time of these super alloys depends on y' phase. The improvements in the properties are based on the followings:. Higher Al and Ti, higher y' phase,. Higher Co, higher solubility temperature for y' phase,. Addition of solid solution strengthener such as W, Mo. This group of superalloys is used at the temperatures in the range 650-1100 °C. Some Ni-based superalloys are used up to 1200 °C for the moving parts of turbine engines. Cobalt-based superalloys are used at extreme temperatures. Cobalt, a group VIIIA element, transforms from F.C.C. to H.C.P. structure at 417 °C. Cobalt-based alloys are hardened by classical methods. Solid solution strengtheners, such as W, Mo, Ta and Cr are added to the solubility limit. Precipitation hardening takes place bycarburization Cr, Hf, Zr, Ta, W, Nb, and Ti. Carbide precipitates form during either production step or during service temperatures. Cobalt-based alloys were first used at 1900s after some developments in the systems of Co-Cr and Co-Cr-W. Especially, Stellite and Vitallium alloys are important and they are used for cutting tools and biomedical applications. As-cast and forged cobalt-based alloys have been used because of the following reasons:. They have high melting points and can be used at higher temperatures.. They have good hot corrosion resistance in hostile gas turbine atmospheres due to their alloying element content.. Compared to nickel-based superalloys, cobalt-based alloys exhibit higher thermal fatigue resistance. Cobalt-based superalloys have complex chemical composition and crystallographic structure. The microstructure consists of austenitic matrix and carbide and intermetallic precipitates in the matrix. Intermetallic phases are geometric closed packed (GCP) and topologik closed packed (TCP) electron compounds. The main carbides formed in alloys are M3C2, M7C3 and M23C6 and M6C and MC. The evolution of nickel/iron superalloys goes parallel to that of stainless steels. First alloy named Tinidur was developed by Krupp in Germany in 1935. The alloy is high in Fe and low in Ni. The composition of this alloy is 30%Ni, 15%Cr, 1. 7%Ti and 0.1 %C and it is strengthened by Nİ3Tİ (y') precipitates. Ni/Fe-based alloys contain generally 25-35% Ni and 15-25%Cr. Iron-nickel based alloys have high temperature strength because of solid-solution hardening, grain boundary hardening and alloying effects. This is attributed to the following structural properties:. Iron and nickel based austenitic matrix,. Alloying additions that are soluble in austenite,. Alloying additions that form precipitates (intermetallics, carbides, and borides),. Alloying additions for grain boundary strengthening. In this group the. most important phases are v' and v" precipitates and MC and M6C carbides. In addition, refractory solid solution strengtheners such as W, Mo improve properties of the alloys. Forged and rolled alloys can be used for high temperature applications. Iron/nickel-based alloys with medium room temperature and high temperature strength levels are cheaper than nickel-cobalt based alloys. Developments in both alloying and processing are crucial for obtaining desired properties. Chemical compositions of the alloys have been optimized using various alloying elements which provide structural and surface stability at elevated temperatures. As for the process development, vacuum melting process is important for high strength requirements. In addition, adding reactive elements to the alloys is possible only when melting is carried out in vacuum or protective atmospheres.Contamination of the alloys can also be minimized. With vacuum melting process, end products can be obtained. Single crystal turbine blade can also be produced. Since 1970, oxide dipersion strengthened alloys (ODS) obtained by a mechanical alloying technique have received a great attention. Both, high temperature strength and good corrosion resistance requirements are met by these alloys. Surface protection of superalloys has been an important area. Diffusion coatings of Al, Cr and Si are obtained by PVD or CVD techniques. For the last 40 years, improvements both in alloying and processing have led to high creep properties. Improvements are summarized below.. Hot deformation,. Conventional casting,. Directional solidification,. Single crystal growth. In the future, possible parameters to be optimized:. Composition,. Casting processing,. Heat treatment. For the development of superalloys, the following properties must be considered:. Mechanical properties,. Density,. Defect-free product,. High temperature corrosion resistance,. Economics. In this study, a high temperature material with composition of 30Cr22Ni22Col6Fe5Mo4W and 0.1 C is used. It is employed for steel annealing furnaces as support materials on which steel bars roll. This material is used at temperatures about 900-1200 °C because of its low thermal conductivity and high temperature wear strength. Classical CrNiCo superalloy has a service life of 6 months. In order to improve service life of the alloy, effects of Si addition on the material properties were studied. Si was added during casting as a deoxidant in the amonuts of 0.48%w, 2.16%w, 3.05%w, 4.2%w and 5.43%w. Alloys melt in open-air induction furnace were cast into pre-heated molds. As-cast alloys were heat treated at temperatures of 900, 1100 and 1250 °C to study service performance. Characterization studies were carried out on both as-cast and as-heat-treated alloys. Electron and optical microscopies were used for microstructural analysis. X-ray diffraction studies were carried out to determine the phases present in the alloys. Wear, hardness and tensile tests were done to determine mechanical properties.Studies on as-cast alloys revealed that the alloys consisted of the following phases: intermetallic sigma in austenitic matrix, chi formed in sigma phase and M23C6 phase which grew around both sigma and chi. The amounts of these phases depend on Si additions. At low Si contents, total amount of phases is about 10 vol.%, while it is 50% at high Si contents such as 5.43%w Si. At low Si contents, cellular M23C6 carbides form as a result of eutectoid reaction. The cellular carbide grains become small spherical particles located around sigma and chi phases. M23C6 carbides grow at the sigma/chi and sigma/austenite interfaces. The amount of chi phase present inside the sigma phase increases with increasing Si content. Si favors formation of intermetallics in CrNiCo superalloys. The microstructure at 5.43% Si has a rosette like morphology and it is very brittle. Wear properties are affected by increased intermetallic formation. Wear rate of the as-cast alloy fall drastically as Si content increases from 0.48% to 2. 16%. At higher Si contents, wear rate decreases further at a slower rate. This behavior may be explained by a mechanism involving intermetallics with a high surface area. It is found that wear properties are related rather to properties of phases present in the alloys, not to the hardness of the alloys. Morphology and size of the phases are very important. İn CrNiCo superalloys, intermetallic phases have high surface area and hardness. These phases increase with Si content and reduce wear rate. Macrohardness of CrNiCo superalloy increase as a function of Si. The most important findings obtained from hardness measurements are following: macrohardness increase from 250HV to 650HV. The increase in hardness depends on phases. Hardness of the matrix phase does not change. These observations indicate that Si does not affect the hardness of the matrix phase, while Si increases the amount of intermetallics and hardness of these phases. An increase in Si content or in the amount of phases degrade the tensile strength of the as-cast alloys. Above 3.05% Si content, toughness is reduced drastically because of increased hardness and brittle intermetallic phases. Optimum hardness, tensile stregth and wear rate are obtained from the alloys of 4.2% Si. Microstructures of the CrNiCo superalloys heat-treated at service temperatures were examined by optical and electron microscopes and X-ray diffractometer. Heat treatments were carried out at 900, 1 100 and 1250 °C for 0.5, 5, 25 and 200 hours. At 900 °C, precipitates were observed around the sigma phase after 30 mins heat treatment. As the heat-treatment time was increased, precipitates were seen to be formed in matrix phase, as well. With increasing Si content, precipitation rate increase and in 30 mins the matrix phase was observed to be full of precipitates. After 25 hours heat treatment, the samples contained rod-like and lameller phases. Cellular M23C6 phase was seen in the samples heat treated at 900 °C because this phase was dissolved in a solution. X-ray diffraction (XRD) analyses on this sample revealed that massive sigma, chi and Cr6.5Ni2.5Si (sigma) were present. After 25 hours, Laves phase was detected by XRD. The most important finding was that at low Si contents, massive sigma phase transformed to R phase, while at high Si contents, sigma phase was stable. Si increased the stability of intermetallic phases formed at high temperatures.Microstructures of the samples heat-treated at 1100 °C were observed to be similar to those of the samples at 900 °C. Precipitates were, however, more spherical and precipitation rate was higher at 1100 ° C. Also, primary chi phase formed during casting started to be dissolved. At 1 100 °C, for low Si contents acicular Laves phase and Cr6.5Ni2.5Si phase were observed around massive sigma phase. More equiaxed particles in matrix were seen at higher Si contents. At this temperature, sigma phase transformed to R phase. Chi phase was still observed at higher Si contents. Si additions led to more stable intermetallic phases. These secondary phases precipitated in the matrix were observed to be coarsened at 1 100 °C. At a heat treatment temperature of 1250 °C, majority of the phases was dissolved. At low silicon contents, austenitic matrix, sigma and M23C6 phases were seen. At higher Si contents, the phases formed were more stable and at 5.43% partial melting was observed. This result indicates that the melting point of the alloy is decreased as a result of new eutectic formation. Therefore, CrNiCo superalloys that contains 4.2% Si and above are not recommended for the use at a service temperature of 1250 °C. Microstructures are the most important features that affect the high temperature properties and life-time of the alloys. Effects of heat-treatment at near service temperatures on the hardness, wear properties and tensile strength of CrNiCo superalloys were investigated. Maximum hardness values were obtained at 900 and 1100 °C. The matrix phases of the samples heat-treated at these temperature were observed to be full of hard secondary precipitates. The hardnes decreases with increasing temperatures. For a given heat-treatment temperature, hardness increases with an increase in Si content. At 1250 °C, hardness becomes lower due to dissolution of phases. Tensile tests at 900, 1000 and 1100 °C showed that tensile properties were not significantly affected by Si content. Tensile strength, however, decreased with increasing temperature. Ductility increased strongly with increasing temperature. Si additions lowered ductility at all temperatures studied. Wear tests on the heat-treated samples showed that lowest wear rate was obtained from the samples of 4.2%Si heat-treated at 1100 °C for 25 hours. At 1100 °C, the phases precipitated in the matrix were coarser and had wide surfaces. It is known that hard intermetallic phases with wide surfaces reduce wear rate. At 1250 °C, wear rate generally increased. For the Si content above %4.2, wear test results were not meaningful because of partial melting of the alloys. Wear rate, generally, decreased with increasing Si content at all temperatures studied here. Depending upon Si content, CrNiCo superalloys heat-treated at near service temperatures and as-cast CrNiCo superalloys exhibited good wear properties. Si increased hardness of as-cast and heat-treated alloys. Tensile strength and ductility were reduced as the Si content increased. Considering service conditions, optimum composition of this alloy was determined to be the composition with 4.2%Si which yielded a good combination of wear, tensile strength and ductility.

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