Inconel 713 LC süper alaşımının akışkan yataklı fırın sistemi ile nitrürlenmesi
Nitriding of inconel 713 LC superalloy with fluidized bed
Yazar: MİNE YÜCEL
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Uçak ve uzay endüstrisindeki türbin motorları başta olmak üzere yüksek sıcaklıklarda yüksek performans gerektiren yerlerde kullanılmak üzere geliştirilen alaşımlara süper alaşımlar denilmektedir. Uçak ve uzay endüstrisindeki gaz türbin motorları için kullanılan malzemelerin büyük çoğunluğunu oluşturdukları gibi, sahip oldukları yüksek mekanik özellikler ile marin edüstrisi, endüstriyel enerji üretim motorları havacılık için motor harici parçalarda da yaygın kullanım alanına sahiptirler. Süper alaşımlar sıcaklığın ve mekanik etkilerin yüksek olduğu koşullarda dahi yüksek sertlik dayanımı, aşınma, yorulma, sürünme ve korozyon direnci özellikleri göstermektedirler. Sahip oldukları yüksek mekanik özellikler, içerdikleri kompozisyon elementleri ve miktarları ile sağlanmaktadır. İçerdikleri ana elementler dikkate alınarak demir esaslı, nikel esaslı ve kobalt esaslı süper alaşımlar olmak üzere üç ana grupta incelenmektedirler. Ana elementler ile birlikte süper alaşımlarda alaşımlandırma elementleri olarak krom, alüminyum, titanyum, molibden, niyobyum ve tungsten elementleri de bulunmaktadır. Nikel esaslı süper alaşımlar en yaygın kullanım alanına sahip süper alaşım gruplarıdır. Sahip oldukları yüksek ısıl dayanım, korozyon direnci ve yüksek mukavemet gibi değerler ile uçak, marin ve endüstriyel enerji üretim motorlarının çok büyük bir kısmını oluşturmaktadırlar. Nikel esaslı süper alaşımlar, oksit dağılımları ile sertleşme, katı- eriyik ve çökelti serleşmesi mekanizmaları ile sertleştirilmektedirler.
Inconel 713 LC, en yaygın kullanılan nikel süper alaşım grubu Inconel'in bir çeşididir. Yapısal özelliklerini etkileyen başlıca elementlere; kimyasal kompozisyonundaki alüminyum, titanyum, molibden, krom ve niyobyum örnek verilebilir. Sektörde yaygın olarak kullanılan Inconel 718 nikel süper alaşımından en büyük farkı; Inconel 718 yüksek oranda demir içerirken Inconel 713 LC'de demir oranı çok tutulmuştür. Sahip olduğu düşük demir oranı ile korozyon direncinde artış sağlanmıştır. Ayrıca Inconel 713 LC'nin diğer önemli özelliği, karbon oranı diğerlerine kıyasla çok daha düşüktür. Bu sayede; yapıda oluşan ve gevrek yapısıyla kırılma, sürünme gibi mekanik özellikleri olumsuz yönde etkileyen metal karbür yapılarının oluşumu azaltılmıştır.
Ana yapıyı değiştirmeden yüzeyde sertlik ve aşınma direnci artışı gibi başlıca özellikler sağlayan nitrürleme işlemi ilk ve yaygın olarak çeliklere uyulanmıştır. Düşük sıcaklıklarla yüzey özellikleri gelişimi sağlayan nitrürleme yönteminin, nikel süper alaşımlara farklı nitrürleme yöntemleri ile uygulanması ve malzemede yarattığı değişimler araştırılmaya devam etmektedir.
Bu çalışmada Inconel 713 LC nikel süper alaşımının, düşük sıcaklıkta gaz nitrürleme tekniği olan akışkan yataklı fırın ile farklı sıcaklık ve sürelerde nitrürlenmesi sonucu meydana gelen yapı ve başlıca mekanik özelliklere etkisi incelenmiştir. Sistemde azot kaynağı olarak %60 amonyak ve %40 azot gazı karışımı kullanılmıştır. Nitrürleme işlemi için kullanılan sıcaklık ve süreler 400°C 6 saat, 400°C 7,5 saat ve 375°C 7,5 saat koşullarıdır.
Farklı sıcaklık ve süre koşullarında nitrürlenmiş numunlerin yapısal karakterizasyonu optik mikroskop, taramalı elektron mikroskobu (SEM) ve XRD analizleri ile gerçekleştirilmiştir. Mekanik açıdan yüzeyde meydana getirilen değişimlerin analizi için 250 mN, 500 mN ve 1000mN yükleri altında yüzey mikro sertlik ölçümleri alınmıştır. Ayrıca 600°C ve 700°C sıcaklıklarda yüksek aşınma analizi yapılarak yüzey profillerinin çıkarılması ile akışkan yatak sistemiyle nitrürlenen numunelerde aşınma kaybı ve aşınma derinliği değerleri bulunarak nitrürleme sonrası aşınma direncindeki değişim gözlemlenmiştir.
Yapılan çalışma ile Inconel 713 LC süper alaşımının yüzeyinde nitrürleme ile elde edilen mekanik özellikleri geliştiricek ince bir difüzyon bölgesi oluştuğu ve ana göbek yapısının korunduğu tespit edilmiştir.

Abstract:
Alloys that are developed in order to use in places where high performance is required at high temperatures, especially in turbine engines in the aeronautics and astronautics industry, are called superalloys. As they constitute the vast majority of materials used for gas turbine engines in the aeronautics and astronautics industry, they also have a widespread use in non-engine components for aerospace, industrial power generation engines and in the marine industry thanks to their high mechanical properties. At these usage areas, superalloys show high strength, good resistance to erosion, to fatigue, to friction and to corrosion, even under high temperatures and rough conditions with high mechanical effects. The high mechanical properties they have are provided by the composition elements and amounts they contain. Because the containing elements determine the phases of the structures and mechanical properties take form due to these phases. Production method and the secondary processes, like heat treatments, also plays role of the phase formation. Cast, wrought and powder metallurgy are the product forms of superalloys. Investments casting under vaccum and the single crsytal methods are the most common production methods of superalloys. They provide to control the grain boundaries of the crystal structure and hereby the mechanical properties of the structure like creep resistance can be increased. 
Considering the main elements they contain, they are analyzed in three main groups consisting of iron-based, nickel-based and cobalt-based superalloys. Along the main elements in superalloys, chromium, aluminum, titanium, molybdenum, niobium and tungsten are also included as alloying elements. The iron-based superalloys are the developed from austenitic stainless steel. They also contains high amount of nickel. Cobalt-base superalloys are mainly known with their high temperature properties. They are generally used at combustor parts of the engine which are exposed to high temperature with high stress gradients. Nickel-based superalloys are the most commonly used superalloy groups. With their high thermal resistance, corrosion resistance and high endurance levels, they constitute a great deal of aircraft, marine and industrial power generation engines. Another important criteria for the usage at industrial engines is weldability of the material. Nickel-based superalloy parts are convenient to be welded. The strenghtening mechanism of nickel-based superalloys are oxide dispersion strengthening, solid-solution and precipitation hardening mechanisms. 
Inconel 713 LC is a type of Inconel which is the most commonly used nickel superalloy group. Vanes of the endustrial turbines, first stage blade in jet aircraft engines, wheels of diesel turbocharges, rotor blades of gas turbines are the examples of the usage areas of Inconel 713 LC. Titanium, molybdenum, chrome, niobium, aluminum in chemical composition are some of the main elements that affect its constitutional properties. The largest difference from Inconel 718 nickel superalloy, which is commonly used in the sector, is that while Inconel 718 contains a high amount of iron, it is kept very low in Inconel 713 LC. An increase is ensured in the corrosion resistance through its low iron ratio. Moreover, another important feature of Inconel 713LC is its very low carbon ratio compared to the other ones. By this way, a reduction is ensured in the formation of carbide structures that negatively affect mechanical properties like fatigue and creep with its brittleness, formed in the structure. The main strengthening mechanisim is precipitation hardening and solid solution hardening. These are mainly generated with gamma prime phases that are exist with gamma phase at the austenitic fcc structure. Aluminum, niobium or titanium are lead the formation of the gamma prime phases during the heat treatment processes. Gamma primme phases that are formed with aluminum and titanium are called intermetallic gamma prime. Niobium is one of the key elements for precipitation. It provides the formation of gamma double prime phases. The crystal structure of the gamma prime phase is simple cubic and the gamma double prime is centered tetragonal. Gamma double prime has more effect on strengthening mechanism at the structre. The other phase that increase the hardness of the material at the structure is metal carbide phases. Carbon cluster with titanium or niobium and leads the formation of the metal carbides. 
The nitriding process that provides main features like an increase in strength and corrosion resistance on the surface without changing the framework is initially and commonly used in steel. The factors that makes the nitriding process more advantages are low process temperature, providing oppurtunity of controlling the process parameters, saving the properties of the core part during the process and no need to apply quenching after the process in order to decrease the distortion.
The basis of the nitriding process types is the diffusion of the atomic nitrogen in to the material surface and the with concentration difference, it increase the resistance of the area where the diffusion of nitrogen occurs. In that way, it provides to change the surface properties of the material. After the nitriding proccess, some layers can occur at the material surface like compound layer and diffusion layer. Coumpond layer is also called as white layer. It has high corrosion resistance and high strength due to its structure but it is also brittle. With this brittle structure, cracks can easliy occur at that surface and it can cause more damage at wear if cracks exist at the surface. Also, it can prevent the diffusion of the nitrogen into the surface. 
Nitriding can be considered in three groups as liquid (salt bath), gas and plasma. Liquid nitriding is made at salt baths and the nitrogen source is the salts that contains nitrogen, they can be molten salts that includes cyanide or cyanate. At plasma nitriding, the process takes under vacuum and at high voltage at 400-560°C temperature. High voltage and vacuum increase the velocity of the nitrogen ions. With ion bombarding, material is heated, the surface is cleaned and it provides active nitrogen for the nitriding process. At gas nitriding, the process temperature can be 350°C - 570°C. The nitrogen source is ammonia. The reason of not using nitrogen directly to the nitriding process is, molecule nitrogen is hard to decompose in to the atomic nitrogen. Ammonia is at unstable state at the nitriding temperature and when it reaches at the heated surface, it decompose into the hydrogen molecule and nitrogen. Then, atomic nitrogen starts fill in to the voids at the microstructure and in that way, diffusion takes place on the surface of the material. The thickness of this diffusion layer can be controlled by changing the temperature and time of the nitriding process.
Studies are being maintained on the application of different nitriding methods that provide improvements in surface characteristics in low temperatures on nickel superalloys and the changes in material it causes. In this study, structures formed following the nitridation of Inconel 713LC nickel superalloy with nitriding at various temperatures and durations with fluidized bed furnace - which is a gas nitriding technique performed at low temperatures - and the effects on main mechanical features are investigated. A mixture of 60% ammonia and 40% nitrogen gas was used in the system as nitrogen source. Temperatures and durations used for the nitridation procedure are 400°C 6 hours, 400°C 7.5 hours and 375°C 7.5 hours.
Structural characterization of the nitrided samples under various temperatures and time conditions was performed with optical microscope, scanning electron microscopy (SEM) and XRD analysis. Microstrength measurements of the surface were obtained under loads 250 mN, 500 mN and 1000 mN for the analysis of mechanical changes caused on the surface. In addition, surface profiles were removed by performing high corrosion analysis at 600 °C and 700 °C temperatures, and the change in corrosion resistance in samples nitrided with fluidized bed was observed after corrosion loss and depth values were found.
The study showed that a thin diffusion zone formed on the surface of Inconel 713 LC superalloy with nitridation, which will improve the mechanical properties, and that the main core structure was preserved.