Çeliklerin ısı iletim katsayısının belirlenmesi
Determination of the thermal condictivities of the steels
- Tez No: 39625
- Danışmanlar: PROF.DR. ALPİN K. DAĞGÖZ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 101
Özet
Çeliklerin ısı iletim katsayıları onların kimyasal bileşimlerine yani o çeliği oluşturan elementlerin yüzde miktarlarına bağlı olarak değişir. Her bir ilave element içeriğinin ısı iletim katsayısını ne şekilde etkilediği şimdiye kadar birçok araştırmacı tarafından yapılan çeşit li deneyler sonucu tespit edilmiştir. Aynı şekilde sıcak lık da çeliklerin ısı iletim katsayısını etkiler. Bu konuda F. Richter, çeliklerin ısı iletim katsayı larını o çeliği oluşturan elementlerin yüzde bileşimine ve sıcaklığa bağlı olarak veren genel ifadeler vermiştir. Burada yapılan çalışmada da çelikler üç ana grup altında toplanmış ve her bir grup için kendilerine özgü özel eşit likler elde edilmiştir. Bu gruplar sırasıyla alaşımsız ve hafif alaşımlı çelikler, ferritik çelikler ve ostenit çeliklerdir. Ayrıca daha doğru sonuçlar elde etmek açı sından alaşımsız ve hafif alaşımlı çelikler de kendi ara larında krom ve molibden içerenler, krom ve (veya) molib den içermeyenler şeklinde ayrılarak incelenmiştir. Bu ifadelerin belirlenmesinde Richter' in her üç _ grup çelik için derlediği tablolardan yararlanılmıştır. İlk önce her çelik grubunu oluşturan çeliklerin ısı iletim katsayılarını veren k=A+BT+CT2 veya k=A+BT şeklindeki ifa delerdeki A, B, C katsayılarını, ayrı ayrı çeliği oluştu ran yüzde bileşenîerire göre veren ifadeler elde edilmiştir.. Daha sonra bu A, B ve C ifadeleri birleştirilerek aranan ısı iletim katsayısı (k) ifadesi oluşturulmuştur. En son olarak bulunan ifade ile gerçek ifade, düşey eksen ısı iletim katsayısı k[W/mK], yatay eksen sıcaklık T[°C] olmak üzere grafik halinde çizdirilerek karşılaştırılmıştır. Sonuçta elde edilen ifadelerin Richter' in çalışmala rıyla oldukça iyi bir uygunluk gösterdiği belirlenmiştir. vıx
Özet (Çeviri)
Following the calculations of the mechanism of heat transfering it is required that specific kind of materials should be used on the machines, which are operated at high temperatures in the factories or in thermic installations. The most important fact is that these materials should have special characteristics, durable nature and thermal conductivities. In this kind of installations, usually- resistant steels are used against the harmful effects of hot gases. Thermal conductivities of steels depend on their chemical compositions. Effects of specific alloying ele ments in steel have been determined by many researchers throughout the world up to now. On this subject, F.Richter has formulated a system to the fact that the thermal conductivity equation of steels depend on their alloying element percentages and temperatures. In this study, which is named“Determination of the Thermal Conductivities of Steels”some new equations are offered. These equations determine the thermal conduc tivities of low alloy, ferritic and austenitic steels depending in their components and temperatures. Instead of formulating many equations of many kind of steels, it has been preferred to use three kind of equations which are on low alloy, ferritic and austenitic groups of steel. In comparring the curves of the graphich of F. Richter and those in this study shows that the new equations offered are in good agreement with Richter1 s work. The mechanical properties attained in the heat-trea ted steel depend mainly on its chemical composition. Steel is a combination of iron and carbon. Apart from these, it contains varying amounts of other elements, principally Mn, P, S and Si, which are always present, Vllieven if only in trace amounts. The steel containing car bon alone may not possess the desired mechanical proper ties. Addition of alloying elements, such as Mn, Ni, Cr, Mo, V, W, etc., individually and in various combinations, helps in attaining the ultimate properties and characte ristics of the particular steel. In the development of chemical compositions to obta in the desired properties in steels, it was, of course, imperative that strength be given first consideration. Since increased strength can be obtained with various combinations of alloying elements, a number of different compositions have been produced which offer interesting combinations of other properties and characteristics in addition to the required minimum strength. The mechanical properties of all steels are determi ned primarily by their microstructures. High-strength low alloy steels generally have ferrite-pearlite microst ructures and their properties are affected by changes in the microstructure. The effects of commonly specified chemical elements on the properties of the steel are discussed hereby, considering each element individually. Carbon is the principal hardening element in steels. It combines with iron to from iron carbide or cementite which is hard in nature. Carbon, which markedly increases pearlite content, is one of the more potent and economical strengthening elements. Further addition of carbon increa ses the hardness and tensile strength of steel with a corresponding reduction in impact strength. As the car bon content increases above 0,85%,itleads to a lowering of strength, and the hardness remain almost constant. Upon quenching, the maximum hardness attainable also increases with the increasing carbon content, but above a value of 0,6%, the rate of increase is very small. Commonly used constructional steel has a carbon content ranging from 0,1 to 0,6% The carbon content varies from 0,5 to 1,4% in plain carbon tool steels. Case hardening steels have a carbon content varying from 0,5 to 0,025%. The thermal conductivity on the steels with element of carbon in the amount of 0,2% increases very rapidly. Manganese is present in all commercial steels. It contributes significantly to increase the strength and hardness in the same manner as carbon, but to a lesser extent. It reduces the critical rate of cooling, thereby increasing the hardenability of the steel, and also incre ases considerably the resistance to abrasion. A steel with a higher manganese content, i.e. above 0,8%, is IXcalled a manganese-alloyed steel. High-strength low alloy steels generally have higher manganese contents than structural carbon steels. Manganese is added to high-strength low alloy steels to counteract an impairment of this property caused by some other alloying element that is present. In steel for welding applications, this element should be kept below some maximum value that depends on the over-all composition but mainly on the carbon content. Manganese-alloyed steels are sensitive as far as over heating is concerned. They tend to become coarse-grained. The presence of manganese in steel also helps to obtain a better surface quality because it combines with sulphur, thereby minimizing the formation of sulphide which is responsible for the hot-shortness or susceptibility to cracking and tearing at rolling temperatures. Manganese steels are used for springs, crossing rails, crusher and dredger parts. The presence of manganese in non-distor ting steels is 12%. In rust-proof steels manganese is in combination with chromium and nickel. Silicon and manganese are companions in steels and may be present up to 0,35% in almost all constructional steels. Silicon is used in greater amounts in some steels such as the silico-manganese steels. Silicon develops hardness and elasticity in steel,. but diminishes tensile strength and ductility. When hardened and tempered, silicon steels possess high strength combined with good ductility and shock resistance. They are used in low hysteresis steels, spring steels and in acid-resisting plants. Chromium, after carbon, is perhaps the most important element in steels. It forms several carbides, depending on the treatment and the amount of chromium present. Chromium is present in most constructional steels and in high-grade tool steels. It is one of the main elements in high-speed steels. Chromium increases the austeinitizing temperature. It increases the corrosion resistance in steel conside rably by creating a tenacious chromium-rich oxide film on the surface in stainless steels and heat resisting steels.Chromium is used primarily to increase the hardenabi- lity of steels. It increases the tensile strength, tough ness and resistance to abrasion. Nickel is one of the most important alloys for inc reasing the strength and toughness of steel, influencing to a great extent its structural transformation. When present in appreciable amounts, it improves the mechanical properties. The presence of sufficiently large amounts of nickel renders the steel austenitic even at room tem perature. Nickel lowers the eutectoid temperature of steel. Lowering of the temperature leads to a suitable alloy for effective quenching. Nickel does not form carbides and does not have a marked effect on hardenability. It impro ves corrosion resistance. Nickel alloyed steels are used for structural and engineering purposes (2-4%) in bridges, machine parts, case hardening steels, etc. Nickel is added in amounts up to about 1.0 percent in a number of high-strength' low alloy steels. This element provides a moderate increase in strength by solution hardening of the ferrite. In the case steel containing nickel the thermal conductivity shows the parabolic variation and with the amount of 30%Ni the thermal conductivity is at minimum. Molybdenum exhibits a greater effect on hardenability per unit added than any other commonly specified alloying elements except manganese. As a result of this, it incre ases the depth of hardening by reducing the critical cooling rate. Molybdenum, in combination with other elements, increases toughness. The presence of molybde num reduces the susceptibility of steel to temper brittle- ness as apparent in nickel-chrome steels, when rapid coo ling from tempering temperature is desired. In tool steels, molybdenum like tungsten is mainly used in the hot work and high speed steel grades. It forms carbides and improves the resistance to wear, tough ness and hot strength. Molybdenum alloyed steels are used in constructional steels for case-hardening, in direct hardening steels, hot-working tool and constructional steels, high speed rust-proof steels and in combination with chromium and nickel. xiTungsten and molybdenum are related elements. Tungs ten forms complex carbides. Tungsten alloyed steel is distinguished by high cutting hardness, resistance to abrasion, good hot strength and high red hardness. Tungsten renders the transformation of austenite to martensite very sluggish. It also inhibits the grain growth. Tungsten alloy steels are insensitive to over heating. In austenitic chromium nickel steels, the addition of tungsten increases the yield point. Tungsten alloyed steels are used in high speed steels, tool steels, hot work steels, magnet steels, valve steels, rust-proof steels, etc. In constructional steels vanadium increases the tensile strength and the yield point. It improves the ratio between them. It is a strong carbide-former, and its carbides are very stable. Hardenability of medium carbon steels is increased with vanadium additions up to 0,04-0,05%. Above this value, the hardenability decreases with normal quenching temperature due to the formation of insoluble carbides. Vanadium is useful when a higher austenitizing tem perature is needed. Due to its carbide-forming properties vanadium is used in tool steel. It increases the hot hardness and, when present in sufficient amount in tool steels, increases the wear resistance of tools. Vanadium with chromium, nickel and molybdeum is often used in constructional steels which are likely to be sub jected to high stresses. Vanadium is a widely used strengthening agent in high-strength low alloy steels. Vanadium-bearing steels are strengthened both by precipitation hardening of the ferrite and by refining the ferrite grain size. Precipitation of vanadium carbide and nitride particles in the ferrite can provide a marked increase in strength that is dependent on not only the thermal processing, for example, the rolling process used, but also on the base composition. An important group of present-day high-strength low alloy steels have phosphorus contents in the range of 0,01 to 0,18 percent. Additions of phosphorus markedly incre ase the strength properties of steel by entering into solid solution in the ferrite, but this increase is accom panied by a decrease in ductility. The atmospheric- xxicorrosion resistance of steel is considerably increased by the addition of phosphorus. Also, when small amounts of copper are present in the steel, the effect of phosp horus is greatly enhanced so that a given amount of phosp horus and copper together provide a greater beneficial effect on corrosion resistance than that produced by the corresponding amount either of the individual elements. The thermal conductivity varies very little, when the phosphorus content increases in the steel. Copper in limited quantities is beneficial to steels of the high-strength low alloy type. Many present-day high-strength low alloy steels contain copper in amounts ranging from 0,20 to 1,50 percent. Copper increases the strength properties of low-and medium-carbon steels to a moderate extent by ferrite strengthening with only a slight accompanying decrease in ductility. Copper up to 0,75 per cent is considered to have little effect on notch toughness or welding performance. Steels containing over about 0,60 percent copper are capable of exhibiting precipitation hardening of the ferrite. Steels containing about 0,50 percent or more of copper frequently exhibit“hot shortness”during hot working, so that cracks or extremely roughened surfaces, sometimes referred to as“checking”may develop during hot deformation. In the case of steel with alloying element of cobalt, the thermal conductivity depends on the amount of cobalt. According to the researches, when the amount of cobalt decreases, the thermal conductivities change in multiple quantities. (It increases or decreases). For example if the amount of Co 10% or 90% the thermal conductivity is at minimum, but if the amount of Co is between 50%-65% the thermal conductivity is at maximum. The elements of Si, W, Mn, Cr are effective in dec reasing of the thermal conductivity and addition of alu minium to the iron also lowered it. xin
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