Metalotermik yöntem ile NiB master alaşımlarının üretimi
Production of NiB master alloys via methallothermic method
Yazar: ÖMÜR CAN ODABAŞ
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Pirometalurji = Pyrometallurgy ; Üretim metalurjisi = Production metallurgy

Özet:
Bu yüksek lisans tez çalışması çerçevesinde NiB master alaşımlarının metalotermik redüksiyon proseslerinden kendi kendine ilerleyen yüksek sıcaklık sentezi (SHS) yöntemi kullanılarak geleneksel üretim yöntemlerinden çok daha kısa sürede ve çok daha ekonomik olarak ve daha basit bir proses ile üreterek yüksek katma değer sağlayabilmektir. Hedeflenen nihai ürüne ulaşabilmek amacıyla oksit tozları ve redükleyici olarak seçilen metalin toz formunu içeren karışım stokiyometrik oranlarda hazırlandı. Karışımın bir pota içerisine yerleştirilip tetiklenmesi ile aşırı ekzotermik reaksiyonun başlaması sağlanmıştır. Karışım üzerinde bir dalga şeklinde ilerleyen reaksiyon çok kısa sürede tamamlanmasıyla istenilen ürüne ulaşılmasına imkan vermektedir. Böylelikle klasik üretim yöntemlerinin uzun işlem süresi, maliyetli ekipman gereksinimleri ve çevresel zararları gibi dezavantajları elimine edilebilmektedir.Bu çalışmada Ni85B15 ve benzeri bor içeren nikel alaşımlarının metalotermik yöntem ile üretimi gerçekleştirebilmek için çalışmalar yapılmıştır. Yapılan deneyler çerçevesinde reaksiyona giren toz miktarı ve oranları FactSage 6.4 termokimyasal simülasyon programı ile hesaplanmıştır. Bu sayede oksit bileşenler ve redükleyici metal istenilen kompozisyonda son ürüne ulaşmak amacıyla daha doğru olarak hazırlanabilmiştir. Reaksiyonun adyabatik sıcaklığı, redükleyici miktarının nihai ürüne etkisi ve stokiyometrik hesaplamalar simülasyon program ile hesaplanmıştır. Termodinamik hesaplamalar sonucunda NiB oluşumu için planlanan reaksiyonun 298 K'deki entalpi değeri (ΔH298) -372kJ olarak tespit edilmiştir. Adyabatik sıcaklık değeri ise 3380 oC olarak hesaplanmıştır ve bu değer reaksiyonun kontrollü olarak ilerleyebileceği sınır değerler aralığında olduğu görülmüştür. Bir diğer parametre olan spesifik ısı değeri (SI) hesaplamalar sonucunda 4018 J/g olarak tespit edilmiştir. Elde edilen termodinamik verilerden hareketle metalotermik uygulama şartları sağlandığı belirlenmiştir.Deneylerde nikelli hammadde olarak nikel oksit (NiO), bor kaynağı borik asitin 2 saat süreyle 500oC'de fırın içerisinde bekletilmesiyle elde edilen bor oksit (B2O3), redükleyici olarak alüminyum (Al), cüruflaştırıcı olarak ise fluşpat (CaF2) kullanılmıştır. Yapılan ilk grup denemelerde oda sıcaklığında bakır pota içerisinde gerçekleştirilen SHS çalışmalarında metal eldesi gerçekleşmemiştir. Bu sebeple ön ısıtma uygulaması denenmiş 500oC'de 20dk ısıtılan seramik potalarda yapılan çalışmalarda % 49,1 NiO, %22 B2O3, %28,9 Al ve %5 CaF2 ilavesi ile yapılan çalışmalarda metalotermik redüksiyon gerçekleşmiş ve metal-cüruf ayrımı gözlenmiştir. Bu koşullarda elde edilen ürünlerde ticari Ni15B bileşimine sahip alaşımı oluşturabilmek için çalışmalar yapılmıştır.SHS uygulamaları sonucunda elde edilen ürünlere karakterizasyon çalışmaları amacıyla XRD ve AAS analizleri uygulanmıştır.

Abstract:
The aim of this study was to produce NiB master alloy via SHS method instead its current industrial production method, carbothermic reduction, in an electric arc furnace. With the increased environmental concerns, primarily on carbon emissions and energy saving,  the SHS process with its lower energy costs and lower environmental impact may become a favourable/preferable production method of NiB. In industry, NiB is u used as master alloys. Master alloys are alloys used as additives in the preparation of complex alloys. These alloys generally contain a carrier element and other elements which may be incongruous to direct melt addition. Thus in the case of NiB master alloys, nickel (Ni) is a carrier element for boron (B). NiB master alloy should typically contain 15-20% B, 80-85% Ni and the balance of permissible contaminants. Hardfacing alloys contain 3-6% B, Ni and other elements. Hardfacing alloys are normally produced by using master alloys of higher B content. NiB master alloy is primarily used in these alloys for the preparation of complex hardfacing alloys. The B in hardfacing alloys serves to increases the hardness of the final alloy.
The relevant information about the nature of NiB production process was compiled from previous applicable works available in literature. It was determined that the carbothermic and aluminothermic reduction of boron oxide (B2O3)and boric acid (H3BO3) is mainly used to produce NiB with a B content ranging from 15-20% in a D.C. arc furnace. Charcoal or wood chips, and other carbonaceous materials used as reductants for the production of NiB obviously present a competitive price advantage over common metallic reductants such as Al, Mg, Ca and Si. Notwithstanding the low Al content of the final alloys produced by the carbothermic process, it is evident that the slags formed are predominantly ash type slags inherent of carbothermic reduction processes.The metallothermic reduction of metal oxides to form metals and alloys is an alternative process to the carbothermic reduction smelting. The metallothermic reduction of oxides is generally an exothermic process, unlike the carbothermic process. Some nickel based alloys such as NiB are commercially produced by the reduction smelting of their metal oxides with carbon, either as coke or charcoal, in an electric arc furnace. The carbothermic reduction reactions are generally endothermic and therefore require large thermal energy input when producing liquid products. The exothermic nature of metallothermic processes is commonly managed to reduce the thermal energy input.The thermal energy requirements of carbothermic process necessitate high power rating furnaces as previously mentioned. High power rating furnaces require proportionally high capital expenditure. In contrast, some metallothermic reductions are sufficiently exothermic to be autothermic. That is, the reaction generates adequate heat energy to propagate to completion without any external heat input post ignition. In general, autothermic reactions reach high temperatures that yield well-separated metal and slag. The additional energy required to promote self-propagation of metallothermic reaction can be obtained through several methods. In certain cases, the reactants are preheated to the relevant temperature to effect self-propagation/autothermicity. In other instances, the use of highly oxidic compounds which are simultaneously reduced with the charges are introduced to the bulk charge. These oxidic compounds are commonly termed reaction boosters as they generate high energy per mol reductant and therefore boost the overall reaction enthalpy. The principle of combustion without heating, which is the basis of SHS, has eliminated the main drawback of traditional production processes: high energy capacity and low productivity. Owing to this, it has become possible to create highly effective procedures for refractory nonoxygen and oxide compounds and materials, semiconducting compounds, hydrides, intermetallics, and other materials and products based on them. In some cases, the technology is completely waste-free. A feature of SHS is the high purity of the products, which is due to the complete use of reagents and self-purification to remove impurities during synthesis. The technical potential of SHS is determined by the variety of the materials used and the possibility of specific combinations of reactants and products not available for other methods. The chemical nature of the reactions of combustion used for the synthesis is also variable. There are a number of reaction parameters, which may influence the SHS reactions. Establishing the optimum reaction parameters for synthesizing a material is based on obtaining a fundamental understanding of the controlling reaction mechanisms in each SHS reaction system. The specific heat and the thermal conductivity are important thermal properties influencing the propagation rate. It is expected that increasing the thermal conductivity of the mixture will increase its burning rate due to the pre-heating effects. The greater the thermal conductivity, the more rapidly heat is transferred from the reaction zone into the unreacted composition, so the smaller the fraction of heat loss. The width of the reaction zone should increase. The burning rate increases, but the ease of ignition decreases. In the case of the specific heat, the greater the specific heat of the composition, the greater the heat required to raise the next layer of composition to the ignition temperature. The temperature of the reaction zone is reduced, and the burning rate lowered. At highest thermal conductivity and heat capacity, ignition difficulty is achieved, and normally an increased intensity or duration of the ignition stimulus is required. To obtain the desired finish product, oxides powders and aluminum as a reductant, were mixed in the stoichiometric ratio. The mixture poured in a preheated crucible then the reaction was triggered by tungsten wire and exothermic reaction occured. Due to reaction time is very short, it provides to get products in a short time. In this master thesis study, experiment sets are prepared to produce Ni85B15 master alloy via metallothermic method and some nickel boron compounds which include similar boron with Ni85B15 alloy. Within the bounds of experiments, amounts of initial powder ratio calculated using with FactSage 6.4 thermochemistry simulation program.
Thermochemical evaluations were made to estimate the adiabatic temperatures and possible product compositions in the final product by using FactSage 6.4 thermochemical software. The aluminothermic reduction process was performed in ceramic alumina crucibles which were pre-heated in an oven at 500oC for 20 min. before powder mixtures were poured into it. Under normal gravity and air environment, oxides of metals  ( NiO, B2O3 ) and fluorspar (CaF2 ) as a flux for good slag metal separation and Al as a metallic reductant were used. To obtain optimum flux ratio for the best slag metal separation 5 different experiment organized and the optimum result obtained from 5 %wt. CaF2 value. By changing the parameters such as reductant aluminum content and molar ratios of other initial components, we expected to change the yield of the target product.