Termal analiz yöntemiyle küresel grafitli dökme demirlerin kalitesinin belirlenmesi
The Definition of the quality of ductile iron with the thermal analysis method
- Tez No: 83023
- Danışmanlar: PROF. DR. FERİDUN DİKEÇ
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Metalurji Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 118
Özet
ÖZET Bu tez çalışmasında, termal analiz yöntemi kullanılarak küresel grafitli dökme demir parçalarda karşılaşılan çekinti probleminin önlenmesi için çalışmalar yapılmıştır. Değişen küreleştirici ve aşılayıcı oranlan ile küresel grafitli dökme demirlerin mikroyapılarmm kontrolü sağlanmıştır. Bu amaçla; sıvı dökme demire kapaklı tandiş potada değişik oranlarda küreleştirici ilave edildikten sonra, çeşitli aşılayıcıların değişen oranlarında akan metale aşılama yapılmıştır. Aşılamanın ardından termal analiz ölçümleri, metalograflk inceleme ve dökülen numunelerden çekme testi ve çekinti irdelemesi yapılmıştır. Küreleştirici alaşım ilavelerinin deneysel sonuçlarına göre, en iyi küresel grafit yapısı %0.038-%0.045 kalıcı Mg oranlarında elde edilmiştir, %0.055'in üzerindeki kalıcı Mg oranlarında mikroyapıda sementit görülmesine rağmen, küreselliği %90'ın üzerinde grafit yapısı elde edilmiştir. Aşılayıcı türünün ve aşılayıcı ilave oranlarının değişiminin etkileri incelendiğinde, Ca ve Al içeren aşılayıcının mikro çekintiye karşı en etkili aşılayıcı olduğu belirlenmiştir. Bi içeren aşılayıcı küre sayısını arttırırken, Ba içeren aşılayıcının mikroyapı üzerinde en az etkiye sahip olduğu termal analiz sonuçlarından belirlenmiştir. Çekinti oluşumunun engellenmesi, parlama sıcaklığını mümkün olduğunca düşük (P1 150°C) ve dönüşüm süresini arttırmak suretiyle grafit oluşumunu en yüksek oranda sağlamakla mümkündür. Ötektik dönüşüm sıcaklığını termal analiz yöntemiyle belirleyerek istenilen değerlere getirmek, aşılayıcı alaşımın içerdiği özel amaçlı elementin türüne (Ca, Al, Bi, La, Ba) bağlı olarak kullanılan aşılayıcı oranının değiştirilmesiyle mümkündür. IX
Özet (Çeviri)
SUMMARY THE DEFINITION OF THE QUALITY OF DUCTILE IRON WITH THE THERMAL ANALYSIS METHOD Ductile iron is defined as a high carbon containing, iron base alloy in which the graphite is present in compact (vermicular), spheroidal shapes rather than in the shape of flakes, the letter being typical of gray cast iron. In promoting ductile iron, emphasis is often given to its ductility and toughness. Yet, cast steel, and occasionally, malleable iron, surpass the ductility and toughness of ductile iron. One finds that the above properties are seldom needed or utilized. The vast majority of castings must retain their shape in service however severe this service may be. Ductility, that is the capability of the material to bend, warp or otherwise plastically deform without failure, is considered, at the best, a safety measure. For some parts, such as gears, breakage may even be the safer failure mechanisms. The one property which is valued the most in the majority of designs is strength. As a rule, not ultimate but yield strength, with a safety factor, will enter into engineering calculations. Ductility does not. This is most fortunate because ductile irons are generally superior to both gray and malleable irons as well as to unalloyed steel, regarding their yield strength. It is true that the relationship between yield strength and toughness is inverse. Chemical and metallurgical influences that increase yield strength usually reduce impact resistance. A true evaluation of the importance of the latter must consider the permissibility of plastic deformation. In Figure 1 the impact modulus (which are indicators of reliability in service) of four materials are compared. At high elongation values the greatest impact resistance is, of course, exhibited by cast steel followed by ferritic, then pearlitic ductile iron, while the impact resistance of gray iron is nil (it fails with low elongation). Up to about 3% elongation, however, pearlitic ductile iron is reliable under impact load than either ferritic ductile iron or cast steel. If, as is customary, plastic deformation must not exceed 0.2%, the order of reliability is: 1. Pearlitic ductile iron (best)2. Ferritic ductile iron 3. Steel 4. Gray cast iron (least reliable). terrific uciilt iron «longaiion percent Figure l:The reliability of four cast ferroalloys under impact load as a function of the plastic deformation (schematic). Spheroidizing treatment involves the addition of a nodulizing agent to create the conditions in the molten metal bath for the precipitate graphite to growth in nodular shape. The element most commonly used in the treatment process is magnesium (Mg). The amount of magnesium remaining after the additions (residual Mg) and the amount of magnesium added related to the sulfur content of the base iron have been observed to influence nodule count. When the magnesium additions were sufficiently low, so that residual magnesium was in the 0.02-0.05% range, the higher magnesium addition resulted in higher nodule counts. While the use of sufficient magnesium addition to attain a fully nodular structure is desirable to reduce carbides and increase the nodule count, residual magnesium over about 0.050% may increase carbides. The satisfactory production of nodular iron castings possessing the necessary mechanical properties for the grades GGG 40 to GGG 80, whether in the as-cast or heat-treated condition, requires the use of adequate process controls to regulate to final properties. Only by the use of such control is it possible today to fulfill the highest quality and reliability demands placed on such components as automobile steering and brake parts of wheel hubs. XIAnomalies in the metallurgical status lead to unexpected scrap. The most important types of scrap influenced by the melt are shrinkages, shape and type of graphite, primary and secondary carbides, bad matrix, pinholes and other types of gas blows. It is estimated that about 30 to 40% of all scrap in a foundry is directly linked to the metallurgical status of the melt. Since no measuring device has been available to monitor the process, it sometimes behaves with unexpected variations. This has forced foundries to work with large safety margins among others concerning risering of castings, which leads to low yields. Thermal analysis is based on monitoring the solidification of a standardized test piece. The temperature/time curve is influenced by the metallurgical conditions in the melt. When a phase is precipitated (e.g. austenite) heat of fusion is released which causes a temporary arrest in the temperature/time curve. This enables the identification of the liquidus and solidus temperatures from the solidification curve. The information can be used for hypereutectic alloys in order to estimate the carbon equivalent as well as carbon and silicon. Such systems have been established on the marked since many years. However, these systems have used test cups containing tellurium to cause the sample to solidify metastable i.e. with the carbon as iron carbide. The main interests have been to measure the liquidus and eutectic temperatures. Among the necessary metallurgical objectives are: 1. To obtain the narrowest possible scatter band of material properties. 2. To control the solidification morphology to obtain maximum self feeding characteristics. 3. To obtain attaining the highest values for ductility compatible with other mechanical properties. Considered in this way, thermal analysis is a particularly valuable and accurate process control method. The basic liquid iron composition can be rapidly checked by the measurement of the Carbon Equivalent Liquidus (CEL) value, the technique can also be employed the indicate the state of nucleation of a given iron by determining its lowest eutectic temperature. To determine the CEL value, it is necessary to measure the liquidus temperature TEL and the solidus temperature TEs. This temperatures are determined with a white solidifying sample, but the lowest eutectic temperature TEmjn. must be determined with a gray solidifying sample. The integration of both thermal analysis methods into the foundry production process indicates that thermal analysis is an imported supplementary technique for process monitoring In nodular iron production processes, some trace elements, such as Bi, Pb, Sb and Ti, may play an important roles in determining the final microstructure, i.e., the graphite shape present in the cast iron. To understand how and (possibly) why this Xllelements affect the manner in which an iron cools, the way to conduct accurate thermal analysis became a concurrent objective. Carbon is dissolved in the melt at temperatures above liquidus. For hypoeutectic alloys some of the carbon dissolves in the initially precipitated austenite (usually maximum 1.9%) during solidification. The remaining carbon is ideally precipitated as graphite when the eutectic temperature is reached. For hypereutectic alloys graphite is precipitated instead of austenite when liquids is reached. Graphite has a lower density than the melt (approx. 2.2 versus 6.9 grams/cm3). During the eutectic reaction it is therefore possible that the volume of the melt can increase slightly provided that enough eutectic graphite has been precipitated. Ideally the volume expansion should be equal to the volume contraction of the austenite so that micro shrinkage is eliminated. If too much graphite is precipitated suddenly i.e. a high recalescance rate, then considerable forces will act on the mold walls. If a green sand mould used it is likely that the cavity will expand slightly (fractions of millimeter) but enough to increase the risk for shrinkages provided that the increased volume is not compensated for by enough eutectic graphite at the and of freezing or by a riser. If on the other hand the amount of eutectic graphite is too low then micro shrinkage will appear. Normally a riser can only supply feed metal efficiently until the moment when the eutectic reaction has developed fully - therefore some micro shrinkages can not be avoided by using risers (unless a very steep temperature gradient and high modulus ratio is used). The key is to have a balanced precipitation of graphite. When graphite is precipitated, latent heat of fusion (875 cal/g), is released. The specific heat of iron at that temperature (1140 °C) is approximately 0.2 Cal/g and degree C. Thus a small amount of graphite can offset the temperature drop and even raise the temperature in the sample. The typical cooling curve is illustrated in Figure 2. Calculated values from the cooling curve and its derivatives are: Likuidus :TL, commences solid precipitation usually as proeutectic austeıiıa. Start Eut. :ES, commencing point for eutectic freezing Eutectic Min. :TElow, lowest eutectic temperature Eutectic Max :TEhigh, highest eutectic temperature Recalescance :R, TEh-TEl Graphite Fl : Graphite factor 1, graphite precipitation in semiliquid state, a high value indicates a high amount of eutectic graphite. Graphite F2 : Graphite factor 2, inverted heat conductivity in solid state. A low value equals high heat conductivity which is a sign of a high amount of graphite. Graphite 3 : Graphite factor 3, Analoque to F2 but just before solidus. SI, S2, S3 :Percent surface area between T1-TE1, TEl-TEh, TEh-TS. Oxid factor : Oxidation factor expressed as S2/S2+S3 Undercooling : undercooling calculated as TEgray-TElow In this study, secondary (micro) shrinkage problem of ductile iron is predicted by using the thermal analysis system. The most important problem for production of ductile iron castings is the secondary shrinkage that is the holes in the casting results from contraction of liquid metal while transform to solid state. Cast irons solidify to xiiieutectic reaction that leads to carbon precipitate from liquid to graphite phase. The density of graphite is very low (2.2 g/cm3) that gives volume expansion to the cast iron. The volume of precipitated graphite is depend on the melting process, Mg alloys, treatment process, inoculation system and inoculant that's effects are only defined by thermal analysis. When much more nodulizer alloy used, eutectic transformation temperature decreased to very low temperature, which gave meta-stable solidification that, increased the shrinkage prosity in the test component inversely. Using some more inoculant, feeding of the components are increased with the transformation time. Feeding behaviour of the ductile iron material can be under control by selection of nodulizer and inoculant materials and their quantity. noo. TOO 200 Time Sec. Dendrites Eutecticum Figure 2: Basic Cooling Curve. XIV T%XS%%T
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