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
Yazar: HAKKI BERKE SOYDEMİR
Danışman: PROF. DR. ŞAFAK YILMAZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Makine Mühendisliği = Mechanical Engineering ; Metalurji Mühendisliği = Metallurgical Engineering

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.

Abstract:
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
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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.