Östemperlenmiş küresel dökme demir (ADI) krank millerinin östemperleme ısıl işleminin sonlu elemanlar yöntemi ile incelenmesi
Investigation of austempered ductile iron (ADI) crankshafts' austempering heat treatment using the finite element
- Tez No: 885031
- Danışmanlar: PROF. DR. ŞAFAK YILMAZ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Metalurji Mühendisliği, Mechanical Engineering, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2024
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Malzeme ve İmalat Bilim Dalı
- Sayfa Sayısı: 81
Özet
Dökme demir malzemeler yapısında %2-5 arasında karbon elementi bulunduran demir-karbon-silisyum alaşımlarıdır. Endüstride çok fazla uygulamada kullanılan bu malzemelerin mekanik ve fiziksel özelliklerini iyileştirmek için çeşitli ısıl işlem yöntemleri kullanılmaktadır. Küresel dökme demir malzemelerin mekanik özelliklerini iyileştirmek için tuzda soğutma yöntemiyle yapılan ısıl işleme östemperleme ısıl işlemi ve oluşan ürüne östemperlenmiş dökme demir denir. Östemperlenmiş dökme demir (ADI) malzemeler, yüksek dayanım, süneklik ve aşınma direnci gibi özellikleri nedeniyle otomotiv ve savunma sanayi gibi endüstrilerde dövme çelik alternatifi olarak giderek daha fazla tercih edilmektedir. Ancak, ADI malzemelerin üretiminde kritik bir aşama olan östemperleme ısıl işleminin tasarımı deneysel maliyeti yüksek olduğu için zorlu bir süreçtir. Bu nedenle, östemperleme prosesinin tasarımını daha ekonomik hale getirmek için sayısal ve analitik yöntemler kullanılmaktadır. Östemperleme ısıl işleminin sonlu elemanlar yöntemi ile incelenmesi, literatürde çeliklerin su verilmesi ısıl işleminin incelenmesine göre daha az araştırılmıştır. Ayrıca, döküm işleminin malzeme özelliklerini etkilemesinden kaynaklı, dökme demir malzemelerin ısıl işlem simülasyonları için gerekli sıcaklığa bağlı sağlıklı malzeme verilerine ulaşmak oldukça zordur. Bu bağlamda, çalışmanın amacı, EN-GJS-700 malzemeden üretilmiş bir krank mili için sonlu elemanlar yöntemi kullanarak ısıl işlem simülasyonu gerçekleştirmek ve simülasyon sonuçlarını deneysel verilerle karşılaştırmaktır. Bu amaçla, ABAQUS yazılımı kullanılarak simülasyonlar gerçekleştirilmiştir. Sağlıklı malzeme verilerine ulaşmak amacıyla dilatometri, diferansiyel taramalı kalorimetre (DSC) ve lazer flaş yöntemi (LFY) analizleri gerçekleştirilmiştir. Analizler için gerekli numuneler, ENGJS-700 standardına uygun dökülmüş Y bloklardan elde edilmiştir. Analizler 10 ℃/dak ısınma hızlarıyla gerçekleştirilmiştir. Ölçülen değerler literatürdeki veriler ile karşılaştırılmış ve doğrulanmıştır. Sonlu elemanlar analizi, krank milinin 920 ℃ sıcaklıktan 280℃'e tuz banyosuna sokulması ve burada 2 saat bekletilmesi senaryosunu incelemiştir. Simülasyonda krank miline 5mm tetrahedral mesh atılmıştır. Analiz için gerekli ısı taşınım katsayıları AS-135 tuzu soğuma eğrilerinden elde edilmiştir. Sonlu elemanlar simülasyonu sonucu krank milinin kasnak, 1. ana yatak ve volan bölgesinin en yavaş soğuyan bölgeler olduğu görüşmüştür. Bu bölgelerde oluşacak fazların tahmini literatürden elde edilen sürekli soğuma diyagramını (CCT diyagramı) kullanılarak yapılmıştır. Yapılan tahminler sonucu, 920 ℃'deki östenit fazının 280℃'de 2 saat bekletilmesi sonucunda, kasnak ve 1. ana yatak bölgesinde %100, volan bölgesinde %99 ösferrite dönüşeceği öngörülmüştür. Yapılan tahminler, optik mikroskop ile alınan metalografik görüntüler ile karşılaştırılmıştır. Buna göre volan, kasnak ve 1. ana yatak bölgesinde %100 ösferrit oluştuğu gözlenmiştir. Bu sonuçlar, simülasyon çalışmasının kasnak ve 1. ana yatak bölgesi için %100 doğrulukla çalıştığını ve volan bölgesinde %1'lik hata oranıyla tahmin yaptığını belirtmiştir. Bu çalışma, literatüre EN-GJS-700 malzemenin ısıl karakterizasyonunu kazandırmakla kalmayıp, aynı zamanda karmaşık geometrili parçaların ısıl işlem süreci tasarımlarında sonlu elemanlar yönteminin kullanımını da hedeflemektedir.
Özet (Çeviri)
The materials that are manufactured of cast iron are alloys of iron, carbon, and silicon that contain between 2-5% carbon respectively. Because cast irons contain a higher percentage of carbon than steels, they are simpler to manufacture than steels. Grey cast iron, white cast iron, tempered cast iron, and nodular cast iron (ductile iron) are the four types of cast iron that are utilised extensively in the application area. Because there is a significant concentration of carbon, graphite is formed in the matrix. The shape of the graphite, which has a direct influence on the mechanical behaviour of the material, is determined by the composition of the material as well as the rate at which the liquid cast iron cools. Because of the nodular shape of the graphite, the material is able to perform better in terms of its mechanical and physical properties. A magnesium or cerium element is added right before the graphite is cast into the mould in order to create the desired graphite nodularity. This is addition is called magnesium or cerium treatment. In order to enhance the mechanical and physical qualities of ductile iron or nodular cast iron materials, which are utilised extensively in the manufacturing sector, a variety of heat treatment techniques are utilised. In order to improve the mechanical qualities of nodular cast iron or ductile, a heat treatment method known as austempering is utilised. This procedure involves quenching the ductile iron and then exposing it to salt. Because of their high strength, ductility, and resilience to wear, austempered ductile iron (ADI) materials are becoming an increasingly popular alternative to forged steel in a variety of industries, including the automotive and defence sectors. The austempering process begins with the austenitizing phase, where the material is heated to a temperature range of 870-950 ℃ and maintained for a duration of 1 to 3 hours. This essential process guarantees the full conversion of austenite throughout the material. Afterwards, the item is submerged into a salt bath that is kept at a temperature ranging from 250 to 500 ℃ for a duration of 0.5 to 3 hours. During this phase, the austenite phase undergoes a transition into the desirable ausferrite phase. The quality and result of the austempering process depend on numerous pivotal aspects. The austenitizing temperature and time are crucial factors in achieving consistent and thorough phase change. Moreover, the temperature of the salt bath and the length of time the material is immersed in it are crucial factors that determine the final microstructure and qualities of the material. The success of this heat treatment process is also significantly influenced by the quality of the casting, highlighting the significance of precise casting practices in achieving desired results. Due to the high number of parameters which influence the austempering heat treatment, which is an essential stage in the fabrication of ADI materials, design of this heat treatment process is a complex procedure where involves multi-physic interactions. Therefore, in order to make the design of the austempering process more cost-effective, numerical and analytical methodologies are utilised. The research of finite element analysis for the quenching of steels has received more attention in the literature than the finite element method analysis of the austempering heat treatment, which has received a less amount of attention. Furthermore, due to the fact that the casting process has an effect on the properties of the material, it is rather difficult to collect reliable material data that is dependant on the temperatures that are required for heat treatment simulations of cast iron materials. Taking this into consideration, the purpose of this research is not only to make a contribution to the existing body of knowledge by presenting a thermal characterization of the EN-GJS-700 material, but also to concentrate on the application of the finite element approach in the process of designing heat treatment procedures for parts with complicated shapes. This research was carried out in order to accomplish both of these goals. As a result, the purpose of this research is to simplify the process of designing heat treatment systems for engineers and to provide assistance to them in optimising heat treatment procedures in an effective manner. In order to achieve the objectives of this study, simulations were carried out with the help of the ABAQUS software. In order to determine necessary temperature dependent thermal material parameters, analyses using dilatometry, differential scanning calorimetry (DSC), and the laser flash technique were carried out. For the austenite formation temperature (AC1) and austenite finishing temperature (AC3) dilatometry analysis was deployed. For the temperature dependent heat capacity DSC analysis was deployed. For thermal diffusivity coefficient laser flash technique was used. After measuring the thermal diffusivity coefficients, by using equations temperature dependent thermal conductivity values ere obtained. Cast Y blocks that were in accordance with the EN-GJS-700 standard were used to collect the samples that were necessary for these analyses. All of the analyses were carried out at a rate of 10 ℃/min. Comparisons were made between the values that were measured and the data that was found in the literature. According to dilatometry test results, austenite formation temperature (AC1) and austenite finishing temperature (AC3) were determined as 787℃ and 867℃. These values then were compared and validated with literature results. In the comparison of temperature dependent heat capacity values, there is a deviation in the temperature between 500℃-900℃. In the comparison of temperature dependent thermal conductivity values, it is observed that measured values are similar to literature results. After obtaining the necessary temperature dependent material's thermal property values, simulation phase for the crankshafts to predict their phase transformation started. The scenario of submerging the crankshaft in a salt bath at 920 ℃ and keeping it there for two hours before allowing it to cool down to 280 ℃ was investigated using the finite element analysis. As part of the investigation, a tetrahedral mesh measuring 5 millimetres was put to the crankshaft. In order to complete the work, the appropriate heat transfer coefficients were derived from the cooling curves of AS-135 salt. The pulley, the first main bearing, and the flywheel regions were found to be the regions that cooled at the slowest rate, as determined by the finite element simulation. The continuous cooling transformation (CCT) diagram that was obtained from the literature was utilised in order to make the forecast of the phases that will result from the development of these portions. After the predictions were made, it was projected that the austenite phase would turn into 99% ausferrite in the flywheel region after being held at 280 ℃ for two hours. This transformation would take place in the pulley xxiii and first main bearing regions, where the austenite phase completely transforms to ausferrite. In addition, for the other regions of crankshaft, also fully ausferrite transformation is predicted, since these regions shows faster cooling rate which prevents pearlite formation. There was a comparison conducted between the forecasts and the metallographic pictures that were acquired using an optical microscope's capabilities. It was observed that the pulley, and first major bearing regions were the only places where ausferrite development was found to be fully complete. This was a significant finding. As a result of these discoveries, the simulation study was able to attain a total accuracy of 100% for the pulley and first main bearing regions, however it was only able to produce predictions with a 1% error rate for the flywheel zone. The limited resolution of optical microscope metallographic images in detecting small-sized phases with low volume fractions and the reliance of simulations on numerical calculations should all be taken into account collectively to clarify the %1 error observed in the flywheel zone. As a result, this study has provided the literature with temperature-dependent material values necessary for thermal analysis, such as heat capacity, critical transformation temperatures, thermal conductivity coefficient, and thermal expansion coefficient for EN-GJS-700 material. Additionally, using cooling curves obtained through the finite element method and CCT diagrams, the details of the practical application of designing and analysing a heat treatment process for a machine part are addressed.
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