Yüksek karbonlu bir çeliğin mikroyapı ve mekanik özelliklerine izotermal tavlamanın etkisi
Effect of isothermal annealing on microstructure and mechanical properties of a high carbon steel
Yazar: MUSTAFA SERDAR KUZYAKA
Danışman: DOÇ. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Çelik, bir demir-karbon alaşımıdır. Çelikte karbon dışında farklı oranlarda alaşım elementleri ve empürite elementler bulunur. Değişik özelliklerde çelik üretmek için farklı oranlarda alaşımlama yapılabileceği gibi, çeşitli ısıl işlemler uygulanarak (östemperleme, normalizasyon, vb. ) mikroyapı da değiştirilebilir.
Çelikte ana ısıl işlem prosedürleri genelde östenit dönüşümünü içerir. Bu dönüşüm ürünlerinin özelliği çeliklerin mekanik ve fiziksel özelliklerini belirler. Bu nedenle, çeliğin ısıl işleminin ilk aşaması çoğu kez, malzeme mikroyapısını östenite dönüştümek olmaktadır. Çeliğin östenitlenmesindeki amaç; daha sonra yapılacak olan soğutma işleminde istenilen mikroyapıyı sağlamak içindir. Östenit oluşumu karbonun difüzyon hızına bağlı olarak belirli bir zaman aralığında gerçekleşmektedir.
Yüksek karbonlu çeliklere uygulanan ısıl işlemler, genel olarak, akma ve çekme dayanımını arttırmak, süneklik ve tokluk özelliklerini iyileştirmek, aşınma dayanımını arttırmak, tane yapısını inceltmek ya da malzemenin işlenebilirliğini arttırmak için yumuşatmak amacıyla uygulanmaktadır.
Bu çalışmada C70 kalite çelik saca, östenitleme sonrası, farklı sıcaklıklardaki (200, 250, 300 ve 350C) tuz banyosunda izotermal tavlama ve ardından temperleme ısıl işlemleri uygulanmış ve bu işlemlerin, malzemenin, mikroyapı, sertlik ve eğme davranışına etkisi incelenmiştir. Çalışmada kullanılan ısıl işlem sıcaklıklarını belirlemede, literatür bilgileri, demir-karbon denge diyagramı ve TTT diyagramları esas alınmıştır.
Bu işlemler sırasında, yukarıda sıralanan her bir izotermal tavlama sıcaklığında 5 saniye ile 90 dakika arasında 11 farklı tavlama süresi esas alımıştır. Böylece belirli bir izotermal tavlama sıcaklığında, farklı tavlama süresinin etkisinin görülmesi mümkün olmuştur. Ayrıca ürünlere uygulanan temperleme ile temperleme sonrası mekanik özelliklerin nasıl değiştiği de belirlenmiştir.
Deneysel çalışmaların sonuçları, izotermal tavlama ısıl işlemiyle başlangıçta ferrit ve perlitten ibaret mikroyapının çoğunlukla martensitik mikroyapıya dönüştüğünü, bazı tavlama sıcaklık ve sürelerinde ise yapıda beynitin de görüldüğünü ortaya çıkarmıştır. Temperleme işlemi sonucu ise temperlenmiş martensitik mikroyapı elde edilmiştir.
Numunelerin sertlik değerleri, tüm sıcaklıklarda, artan izotermal tavlama süresi ile birlikte düşme eğilimi göstermektedir. Temperleme işlemi, beklendiği gibi, izotermal tavlama sonrası elde edilen sertlik değerlerini azaltmaktadır. Ancak temperleme sonrası sertlik değerleri, izotermal tavlama süresine bağlı olarak önemli bir değişim göstermemektedir.
Uygulanan izotermal tavlama işlemi, numunelerin eğme dayanımını arttırırken, eğme birim şekil değişimini azaltarak yapının gevrekleşmesine neden olmuştur. Temperleme sonrasında ise, eğme dayanımı, izotermal tavlama sonrasına göre bir miktar azalırken, orijinal durumdaki numunenin eğme dayanımına göre artmış, eğme birim şekil değişimi de artarak orijinal durumdaki numunenin değerine yaklaşmıştır.
Elde edilen deneysel sonuçlar, numunelere izotermal tavlama ve ardından uygulanan temperleme işlemi ile, orijinal hale göre yaklaşık aynı süneklik fakat daha yüksek eğme dayanımı kazandırılabildiğini dolayısıyla incelenen C70 kalite çeliğin tokluğunun izotermal tavlama ve temperleme işlemleriyle arttırılabildiğini göstermektedir.

Abstract:
Steel is an iron-carbon alloy. It contains various alloying additions in different ratios and impurity elements in addition to carbon. In order to produce steel with different properties, alloying elements can be added into chemical composition or various heat treatments procedures (austempering, normalization etc.) can be followed to alter the microstructure.
Main heat treatment procedures of steels usually involve transformation of austenite into various transformation products depending on cooling rate and temperature. Inherent properties of these transformation products determine the mechanical and physical properties of steel. Therefore the first step in the heat treatment of steel is to transform the microstructure into austenite. The main objective of austenitizing is to provide a completely austenitic structure which will be transformed into various products in the next steps of the heat treatment. Transformation of the initial microstructure into austenite is completed in a time period depending on the diffusion rate of carbon.
Heat treatments applied to high carbon steels, generally have an objective to increase yield and tensile strength, to improve ductility and toughness properties, to increase wear resistance, to reduce grain size and to improve machinability by decreasing hardness of the material.
There are several heat treatment types for steels including, quenching, tempering, normalizing, stress relieving, recrystallization, austempering and martempering. In the quenching heat treatment, parts are heated to the austenite region above the upper critical point varying as a function of carbon content of the steel. They are hold at this temperature for a time enough to transform initial microstructure into austenite. This is the necessary stage of all heat treatment process which aim to produce a different microstructure after the heat treatment. Because all non-equilibrium transformation products such as martensite or bainite can be produced by the transformation of austenite. After the whole parts are transformed into austenite, which generally requires holding the parts at the respective temperature for one hour per one inch of part thickness, the parts are cooled down to room temperature at a cooling rate depending on the type of product. For example, when a martensitic structure after heat treatment is desired, the part must be rapidly cooled down to room temperature to avoid the formation of other transformation products such as pearlite or bainite. Such a high cooling rate can be determined from the TTT diagram of the steel employed and is called as critical cooling rate. This means the minimum cooling rate to be used to obtain a completely martensitic structure. In fact, it is very difficult to perform such a high cooling rate for low carbon and low alloyed steels because transformation curves on TTT diagrams of those steels are very close to the left side of TTT diagram and in practice it is impossible to cool the specimen without intersecting the transformation curves. In order to be ensure the completely martensitic structure, steel must contain higher carbon and/or higher amount of alloying elements, which shift the transformation curves rightwards and allow to apply a cooling rate higher than the critical cooling rate. These types of steels which can be hardened by quenching are called as heat treatable steels and generally contain 0.4-0.6% carbon in their compositions. After quenching heat treatment, parts are very hard and have limited usage in service due to highly brittle nature of martensite. In order to improve toughness, a post heat treatment following quenching must be perform, which is called tempering. By the application of tempering heat treatment just after the quenching, toughness can be significantly improved at the expense of hardness. However, even though the parts lost their hardness during tempering, they still have enough hardness to be used in service.
Another common heat treatment is normalizing applied to steels which have been subjected to some previous thermal and/or deformation processes such as, casting, plastic deformation, welding etc. These type of production techniques result in a coarser grain structure and the grain distribution throughout the microstructure as well. In these cases, normalizing heat treatment refines the grain structure and makes them more uniform. Normalizing is similar to quenching heat treatment in that heating and holding stages of the heat treatment are the same but the cooling stage is different. The cooling rate must be slower with respect to that of quenching. Therefore, the parts are generally cooled in air in normalization.
Stress relieving heat treatment does not lead to microstructural transformation but relieves the internal stresses produced by casting, welding or plastic deformation processes. Stress relieving heat treatment consists of heating the parts to a temperature lower than the lower critical temperature (A1), holding at this temperature for a while and then cooling the parts to the room temperature in air. Recrystallization heat treatment is similar to stress relieving heat treatment in that the achieved temperatures during heating. It also requires heating the parts to a temperature lower than the lower critical temperature (A1), holding at this temperature for a while and then cooling the parts to the room temperature in air. This temperature should be higher than the recrystallization temperature of the part which is generally higher than 40% of absolute melting temperature of the part in Kelvin. Recrystallization removes the deformed grain structure by producing new and undeformed grains within them.
There are two most commonly used isothermal heat treatments for steels such as austempering and martempering. Austempering involves cooling the part which has been already austenitized to a temperature between the nose of TTT diagram and Ms temperature. The parts are soaked at these temperatures for a time enough to complete the transformation. It means that parts must be soaked for a time which is higher than the time corresponding the second transformation curve on TTT diagram. The resulting microstructure is upper bainite or lower bainite depending on the isothermal transformation temperature. Temperatures close to the nose of TTT diagram produce upper bainite and temperatures close to Ms temperature produce lower bainite. Bainitic structures generally have lower hardness but higher toughness with respect to the martensitic structures.
Another isothermal heat treatment is called as martempering. It involves rapid cooling the austenitized parts to a temperature just above the Ms temperature or between the Ms and Mf temperatures, soaking the parts at this temperature for a time and cooling the parts to room temperature before the first transformation curves are crossed. Martempering produces a martensitic microstructure and therefore requires a tempering heat treatment afterwards. The main objective of applying martempering instead of conventional quenching is to decrease the temperature difference between the surface and center which is usually the case in large section size parts. As a result of decreasing temperature difference, more uniform martensitic structure can be obtained.
In this study, isothermal annealing was performed to C70 quality steel sheet in a salt bath at 200, 250, 300 and 350C after austenitizing performed at 850C for 150 s and the samples were then tempered at 415C for 150 s. Effect of these heat treatments on microstructure, hardness and bending properties was then investigated. Heat treatment temperatures used in this study are based on information from the literature and TTT diagrams.
For the experimental procedure performed in this study, steel samples were supplied from Ekobant Metal San. Tic. Ltd. Şti. in the form of 0.8 mm thick sheets. Chemical composition of the steel was as follows (in. wt%): 0.73C, 0.27Si, 0.55Mn, 0.014P, 0.008S, 0.21 in total for Cr, Ni, Mo, Cu, Al, Ti and V and the balance Fe. According to the information given by the supplier, the sheets were first procuced by continiuous casting in the form of slab with a thickness of 200 mm. The slabs were then further rolled into a thickness of 65 mm by a slab press first and then into a thickness of 4 mm by series of hot rolling process, and the hot rolled sheets were obtained. Followed by acid cleaning and trimming of the hot rolled sheets, the thickness were reduced into 4 mm, 2 mm and 0.85 mm by successive cold rolling and intermediate recrystallization steps, and the sheets were finally subjected to temper rolling reducing the thickness of the sheets into 0.8 mm as the final thickness.
For microstructural examinations, samples mounted in bakelite were prepared by the standard metallographic procedure and examined by a Leica DM750M optical microscope. Hardness were measured by Rockwell A hardness scale (HRA) by using a Zwick/Roell ZHR hardness tester. At least five measurements were taken on the wide surfaces of the samples and the results were averaged.
For bending tests, samples with the dimensions of 80 mm (l) x 22 mm (w) and 0.8 mm (t) were used. The tests were performed in 3-point bending apparatus by Shimadzu AGS-J universal testing system with a load cell of 10 kN. In the 3-pont bending tests diameter of the mandrel and supports was 5 mm and the length between the supports was 25 mm.
Three successive heat treatment process were used in this study: Austenitizing, isothermal annealing and tempering. For austenitizing, samples cut from the sheets in the dimensions given above were loaded in a Kerr 915 model furnace by hanging on a steel frame. After austenitizing performed at 850C for 150 s was completed, the samples were rapidly removed from the furnace and transferred to a salt bath for isothermal annealing. Isothermal annealing was performed in a salt bath established in a stainless steel container containing AS135 annealing salt. The isothermal annealing furnace was a vertical Nabertherm HO 60/E model furnace. After the predetremined isothermal annealing times at a given isothermal annealing temperature, the samples were removed from the salt bath and then air cooled to room temperature. After cooled to room temperature in air, the samples were then loaded to tempering furnace. Tempering as the final heat treatment was performed in the same furnace with austenitizing.
In the isothermal annealing, 11 annealing durations at a given isothermal temperature in between 5 seconds and 90 minutes were employed. This made possible to see the effect of annealing time at a given annealing temperature. In addition, tempering performed just after isothermal annealing revealed effect of tempering heat treatment on the properties of the samples.
Experimental results showed that the initial ferrite and pearlite microstructure generally transformed into martensite. And some temperature and times produced a bainitic microstructure. Tempered martensitic microstructure was obtained after tempering heat treatment.
Hardness of the samples has a decreasing tendency with increasing annealing times at a given annealing temperature. Tempering heat treatment decreases the hardness values obtained after isothermal annealing, as expected. However, hardness values after tempering did not exhibit a significant variation with respect to isothermal annealing time.
The applied isothermal annealing increases bending strength and decreases bending strain and thus made the material more brittle. On the other hand, bending strength values significantly decrease with respect to those obtained after isothermal annealing and increase with respect to the original specimen. Bending strain also increases getting closer to values exhibited by the original sample.
Experimental results showed that a higher bending strength and almost the same bending ductility with respect to the original sample can be obtained by proper selection of isothermal annealing and tempering heat treatment. Finally results also indicated that toughness of C70 quality steel could be improved by isothermal annealing and tempering.