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Küresel grafitli dökme demirlerin borlanması ve özellikleri

The boriding of some ductile irons and their properties

  1. Tez No: 68883
  2. Yazar: UĞUR ŞEN
  3. Danışmanlar: PROF. DR. FEVZİ YILMAZ
  4. Tez Türü: Doktora
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  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: Belirtilmemiş.
  13. Sayfa Sayısı: 202

Özet

ÖZET Borlama, yüksek sıcaklıkta ana metalin yüzeyinde borür tabakası oluşturulması işlemidir. Borlama günümüzde yalnızca metallere değil, sermet ve seramik mâlzemelerede uygulanmaktadır. Bor kaplama, yüzeyde bileşik oluşturacak şekilde bor atomlarının difüzyonu olarak da bilinmektedir. Borlama işlemi, yüzeyi iyi temizlenmiş malzemelere 700-1000°C sıcaklık aralığında, 1-10 saat sürelerde katı, pasta, sıvı veya gaz gibi çeşitli ortamlarda uygulanabilmektedir. Son teknolojik gelişmeler sonucu gaz ortamında termo-kimyasal borlama, plazma borlama ve akışkan yatakta borlama gibi yeni olan teknikler de kullanılmaktadır. Demir esaslı malzemelerin termo-kimyasal olarak horlanması üzerine bir çok çalışma yapılmış ve son 30 yıl içerisinde bu çalışmalar oldukça hızlanmıştır. Araştırmalar neticesinde elde edilen sonuçlarla bir çok gelişmiş- ülke, borlama- işlemini sanayilerine aktarmıştır. Borlama işlemi endüstriyel olarak daha çok çelikler üzerine uygulanmaktadır ve bir çok çelik grubu üzerinde detaylı araştırmalar gerçekleştirilmiştir. Ancak, dökme demirler üzerinde yapılan araştırmalar oldukça sınırlıdır ve az sayıda çalışma yapılmıştır. Bu çalışmanın amacı, termo-kimyasal olarak horlanan küresel grafitli dökme demirlerin(KGDD); mikroyapı ve kimyasal bileşim, oluşan borür tabakasının morfolojisi ve mekanik özellikleri üzerine etkisini araştırmaktır. Bu amaçla kullanılacak malzemeler için bileşimler, GGG-50 (%5 Perlit), GGG-60 (%70 perlit) ve GGG-80 (%100 perlitik) olacak şekilde seçilmiş ve benzer şartlarda horlamanın mukayeseli olarak bu KGDD'ler üzerindeki etkileri araştırılmıştır. KGDD 'lerde matris yapışım belirleyici element olarak bakır kullanılmıştır. Çeliklerle kıyaslandığında, literatürde KGDD'lerin horlanması ile ilgili oldukça sınırlı sayıda araştırma mevcut olup, kapsamlı araştırmalar henüz yapılmamıştır. Bu çalışmada DÖKTAŞ A.Ş. tarafından üretilen DİN 1693 normuna uygun GGG-50, GGG-60 ve GGG-80 türü küresel grafitli dökme demirlerin horlanması ve borür tabakalarının kalınlıkları, morfolojileri, sertlikleri, kırılma toklukları ve aşınma özellikleri; optik mikroskob, mikrosertlik cihazı, x-ışmlan difraktometresi, taramalı elektron mikroskobu(SEM) ve CSEM tribometre aşınma cihazı kullanılarak incelenmiştir. Yapılan incelemeler sonucunda KGDD'lerin yüzeyinde oluşan borür tabakalarının çeliklere göre farklılıklar içerdiği, oluşan borür tabakasının içerisinde dağılmış bulunan grafit kürelerinin varlığı ve ayrıca kaplama matris arayüzeyinde karbon ve silisyum esaslı bir birikimin oluştuğu görülmektedir. KGDD malzemelerin içerisinde bakır konsantrasyonunun artışına bağlı olarak, C ve Si esaslı birikimin oluşmadığı ve homojen olarak malzeme yüzeyinde oluşan Fe2B fazından oluşan borür tabakası elde edilmiştir. Aşınma deneyleri sonucunda bor kaplanmış KGDD malzemelerin sürtünme katsayısı değerlerinin çeliklere göre daha düşük, aşınma hızının ise yaklaşık olarak çeliklerle eşit olduğu görülmüştür.

Özet (Çeviri)

SUMMARY Definite limits are being approached, or have been reached in single materials development. A solution is to produce a composite material that combines the best properties of different metals or ceramics. This may be a coating of one material on another. Coatings enable the attributes of two or more materials( the substrate and the coating(s)) to be combined to form a composite having characteristics not readily or economically available in a monolithic material - Examples are tribelogieal^ properties and wear resistance coupled with corrosion resistance. In the high temperature fields economic and technical pressures to achieve extended lives and greater reliability plus a need to conserve certain relatively scarce (and hence expensive) or strategic alloying elements, have dictated increasing recourse to coatings. The need for surface stabilization led to the growing need for coatings and rapid developments in the field of surface engineering. In general resistance to wear is directly proportional to hardness. However, if hardness of a material increase, its brittleness also increases, on the other hand its toughness decreases. For this reason, it is aimed to combine hardness and toughness properties by the surface treatment. Thus, a layer resistance to wear and hard is obtained on the surface of substrate material by the surface treatment, and also the cores of substrate resist to stress by its softer (tough) structure. Various coating methods are developed provide surface properties required. Thermo- chemical (boriding, nitriding) and termoreactive diffusion processes (TRD), pack cementation techniques, plasma spray, physical vapour deposition (PVD), chemical vapour deposition (CVD), laser operations, electrochemical deposition etc. are some of them. While some of the layers obtained by these techniques form physical bond, some others form strong chemical bond with substrate by diffusion. Boriding or boronizing is a thermo-diffusion surface hardening process in which boron atoms diffused into the surface of a work piece to form borides with the base material. The boriding process can be applied to a wide variety of ferrous, non- ferrous and cermet materials. The process involves heating of well-cleaned material in the range of 700 to 1000°C, preferably for 1 to 12h in contact with a boronaceous solid powder or boronizing compound, paste, liquid or gaseous medium. During boriding, the diffusion and subsequent absorption of boron atoms into the metallic lattice of the component surface form interstitial boron compounds. The resulting layer may consist of either a single-phase boride or a polyphase boride layer. The single-phase boride layer consists of Fe2B, while the double-phase layer consists ofan outer dark-etching phase of FeB and inner bright-etching phase of Fe2B. The hardness of FeB is higher than that of Fe2B boride due to more boron atom content of FeB. The property of the bonded layer depends strongly on the composition and structure of the boride layer and the composition of the base (substrate) material. When applied to the appropriate materials, boronizing provides wear and abrasion resistance, corrosion and oxidation resistance comparable to sintered carbides. Boronizing process a number of characteristics features and benefits as follows; - Boride layers have extremely high hardness of 1800-2 100HV. Hardness is retained up to subcritical temperatures. - Heat treatable materials can be fully hardened after boronizing to optimize performance. - Boronizing increases resistance to acids, HC1 in particular. -It «an be uniformly applied to irregular shapes.- Boride layers have good resistance to abrasive, sliding and adhesive wear, also low coefficient of friction. Industrial boriding can be carried out on most ferrous materials such as structural steels and cast steels as: Armco iron, gray and ductile iron. In recent years a considerable attention about boriding of case-hardened, tempered, tool and stainless steel has been given. But, little work has been carried out about the boriding of ductile irons comparison with the boriding of steels. However, no sufficient experimental data was found in the literature on the boride coating caharacteristics of cast irons, especially ductile irons. For this purpose, the boride coatings of ductile irons were investigated. In this study, boronizing practice was intended to use for surface modification of ductile irons. Boronizing is frequently used to increase surface hardness to obtain a layer resisting wear and chemical attacks. At the same time, boride coatings used for high temperature applications up to 700°C. The aim of the present work is to investigate the effect of the boriding temperature, time and composition of ductile irons on the coating microstructure, mechanical properties and phase composition. The substrate materials chosen in present work were three groups of ductile iron. The chemical compositions of these materials are given in Table 1. Table 1 The chemical composition of ductile ironsThe samples were produced from these materials in the form of keel blocks in DÖKTAŞ A.Ş. (Orhangazi) and experiment were carried out in The Department of Metallurgy of Sakarya University. Experimental specimens (A, B and C series ductile irons) were cut and machined from the keel blocks to the dimensions of 10x1 Ox 15mm and 10x15x20mm and then, borided at 850°C and 950°C for 2, 4, 6 and 8 hours in the bath consisted of calcined borax 60%, calcined boric acid 15%, ferro-silicon (70% Si) 20% and aluminum 5%. Liquid bath boronizing can be used easily and controlled atmosphere is not necessary for this techniques. Salt bath can be used at higher temperatures for ferrous alloys, to developed the desired boride layer thickness. It is known that, this technique is cheaper than others. Since we have borax mineral reserves in TÜRKİYE, study on boronizing is very attractive. Borax bath materials can be supplied easily in Türkiye. All heat treatment companies can apply easily these techniques like cementation or nitriding. After boronizing, properties of each group of materials were examined by using classical metolographic techniques. Optical metallography was carried out on the polished boride coating surfaces of all specimens and the thickness of boride layers on coating surfaces was measured. More detailed examination of boride layers were carried out by using scanning electron microscope back scattered image analysis (SEM-BEI), linear and electron microprobe analysis (EMA), x-ray diffraction analysis. Mechanical tests such as hardness, wear and fracture toughness were also carried out. Microhardness measurements were conducted under the load of lOOg. Wear tests of C series ductile irons were carried out by using a ball-on disc machine under 2N, 5N and ION normal loads with 200 cycle/min. rate of rotor. Wear rates, volumetric losses and coefficient of frictions were measured. Fracture toughness of C series specimens was measured via Vickers indenter under 300g. to lOOOg. loads. Metallographic studies showed that coating layer has columnar shape. Coatings of borided ductile irons consist of two layers; one of them is outer parts of coatings which have two phases of FeB and Fe2B. Other inner layer is based on carbon and silicon. In addition to these, we frequently observe ferrite phase on heat effected zone. Boride layer thickness of coated ductile irons depends on boriding time, temperature and composition of base material. The boriding time and temperature increase the boride layer thickness as expected. Whereas, the increasing of copper concentration of ductile iron decreases the boride layer thickness. In addition of these, copper concentration of ductile iron affects the morphology of boride coatings. If copper concentration of ductile iron increases, coatings do not have the second layer based on carbon and silicon under FeB and Fe2B phases. In addition to this boride phase consist of Fe2B only. However, if boriding temperature and time increases, FeB phases and the second layer on the surfaces of ductile irons, which is based on carbon and silicon, were observed.The hardness levels of the boride layer also depend on composition of base material, bonding time and temperature. By hardness measurements from the surface until the original matrix, boriding time and temperature affect the hardness of boride layers and boride layer thickness. When the boriding time and temperature increase, the hardness and depth of boride layer increase. However, increasing of copper concentration in ductile iron decreases the hardness of boride layers. Since borided C series ductile irons have not gotten FeB phases. Therefore, C series coatings are softer than A and B series. When copper concentration of ductile iron decreases, the rate of boron diffusion and the boride layer thickness on ductile iron surfaces increase. One of the important fact is the stabilising effect of copper on the occurrence of Fe2B phase in the boride layer. X-ray diffraction analysis showed that borided A and B series ductile irons have FeB and Fe2B phases. However, borided C series ductile irons have only Fe2B phase at the temperature and time ranges studied. Scanning electron microscopy back scattered image (SElvPBEI) studies showed that, A and B series ductile irons have some cracks between FeB and Fe2B phases that is parallel to sample surfaces. However, borided C series ductile irons have not got any FeB phases and no cracks. Microprobe micro analysis studies showed that, carbon and silicon enrich between boride layers and matrix. In this study, elemental distribution in boride layers and sublayers of coating were searched. If copper concentration of ductile irons increases, the layers of carbon and silicon between boride layers and matrix decreases. If copper concentration of ductile iron increase up to 1 % by weight, the layers of carbon and silicon between boride layers and matrix don't form for the low temperature and short times. All findings support observations made by microscobe. and fulfil phase composition relations. The formation of a single Fe2B phase is more desirable than a double-phase layer with FeB. The boron-rich FeB phase is considered undesirable in part, because FeB is more brittle than Fe2B. Also, because FeB and Fe2B are formed under tensile and compressive residual stresses, respectively, crack formation is often observed at or in the neighbourhood of the FeB-Fe2B interface of a double-phase layer. Because borided C series ductile irons do not have FeB phases and carbon and silicon rich layers between boride phase and matrix, fracture toughness and wear test was applied only C series ductile irons. Borided A and B series ductile irons have some cracks between FeB and Fe2B phases. These cracks generally can not be seen by optical metallographic techniques. However, SEM-BEI micrographes clearly shows FeB and Fe2B layers and cracks between them. The fracture toughness of borided ductile irons slowly goes down by increasing boriding time and temperature. Time changing is much more effective than temperature. The fracture toughness value is changing from 4.2 MPa.m1/2 to 2.5 MPa.m1/2. These values are better than many of ceramics. For example, these values are approximately same as Sİ3N4.It has been reported that, the tribological properties depend on the microstructure of the boride layer. Wear tests carried out using the ball-on disc techniques showed that coefficient of friction of borided C series ductile iron is measured about 0.15. Whereas, coefficient of friction of boride layers on the surfaces of steel substrate was not very low. It was vary nearly among 0.38 to 0.63. The boride layers of ductile irons have spheroidal graphites that cause the dry lubrication on wear test conditions. Because of this, coefficient of friction of borided ductile irons is very low. The wear rate of boride layers on C series ductile iron was found very low like borided steel substrate. If the boriding temperatures, time and wear load increase, the rate of wear also increase. Metallographic examination of the wear surfaces showed that the wear was of an oxidative and abrasive type. Wear tracks colours which are red, grey and yellowish white indicate the presence of Fe203, Fe304 and B;03 on the surfaces, respectively.

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