Küresel grafitli dökme demirlerin yüksek çevrimli yorulma davranışına silisyum oranının etkisi
The effect of silicon content on high cycle fatigue behavior of spheroidal graphite cast iron
Yazar: MERVE DEMİRLEK
Danışman: DOÇ. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Küresel grafitli dökme demirler = Nodular graphite cast irons ; Mekanik özellikler = Mechanical properties ; Yorulma davranışı = Fatigue behavior

Özet:
Dökme demirler %2?den fazla karbon içeren demir alaşımlarıdır. Ergimiş demir-karbon-silisyum alaşımında silisyum grafit oluşumunu desteklerken, karbür oluşumunu engelleyici etkisi vardır, bu ergimiş alaşıma Ce veya Mg eklenerek,  karbonun nodüler grafit halinde çökelmesi sağlanır ve küresel grafitli dökme demirler elde edilir.
Küresel grafitli dökme demirleri güvenli bir şekilde kullanabilmek için, mekanik özelliklerini; çekme mukavemeti, süneklik, sertlik, yorulma özellikleri hakkında ve bunları etkileyen parametreler hakkında bilgi sahibi olunmalıdır.
Küresel grafitli dökme demirlerin mekanik özelliklerini etkileyen faktörler mikroyapı(matris, grafit yapısı ve sayısı), kimyasal bileşim ve soğuma hızıdır.
Küresel grafitli dökme demirler, yüksek mukavemet ve tokluk, iyi işlenebilirlik ve düşük maliyet gibi özelliklere sahiptir. Dökülebilirlik avantajının yanı sıra bu iyi mekanik özelliklere sahip olması küresel grafitli dökme demirlerin birden fazla endüstride tercih edilmesine sebep olmaktadır. Otomotiv endüstrisi bunlardan sadece biridir, örneğin otomotiv endüstrisinde krank mili, fren ve aks gibi parçaların üretiminde küresel grafikti dökme demirler tercih edilir. Bilindiği gibi bu otomotiv parçaları değişken tekrarlı yüklere maruz kaldığı için yorulma dayanımı bu endüstri için önemli bir mekanik özelliktir. Küresel grafitli dökme demirlerin yorulma dayanımı, grafitlerin yapısı ve sayısı, matris yapısı ve sertliği, yapıdaki inklüzyonlar, döküm parçasının boyutu ve yüzey özelliklerine bağlıdır. 
Bu çalışmada iki farklı kalite küresel grafitli dökme demir, EN-GJS-500-7 (%2,4 Si) ve EN-GJS-500-14 (%3,6 Si) malzemeleri incelenmiştir. Mikroyapı karakterizasyonu yapılmış, mikroyapısı ve kantitatif analizi gerçekleştirilmiştir. Dönel eğmeli yorulma testi ile yapılan deneylerde yüksek çevrimli yorulma davranışı incelenmiştir. İki farklı kalite küresel grafitli dökme demirin S-N eğrileri çizilmiştir. Yüksek silisyum oranının; geleneksel küresel grafitli dökme demirlerin çekme, akma mukavemeti, sünekliği, sertliği gibi mekanik özelliklerine etkisi ve yorulma sınırına etkisi incelenmiştir. Yorulma kırılma yüzeyleri taramalı elektron mikroskobunda incelenmiştir.
Geleneksel EN-GJS-500-7 KGDD ferritik-perlitik mikroyapıya sahipken, yüksek silisyum içeren EN-GJS-500-14 malzemesinin ferritik mikroyapıya sahip olduğu görülmüştür, malzemenin sünekliğini arttırmıştır. Yapıdaki fazla Si akma mukavemetini artırırken, yorulma dayanım sınırı değerlerinin benzer olduğu sonucuna varılmıştır. Dayanıklılık oranları karşılaştırıldığında ise EN-GJS-500-14 kalite malzemenin daha yüksek bir orana sahip olduğu gözlemlenmiştir.

Abstract:
Cast irons find widespread applications in automobiles, particularly in cast engine components (manifolds) and in the crankshaft. These ferrous alloys have much higher carbon content than steels, usually in the range 2.2-5 wt.%, and are widely popular because of their low cost and ease in fabrication by casting. Cast irons have lower melting points than steels and hence, can be melted in relatively cheaper furnaces. Because many of them have relatively higher amounts of silicon (about 2 3%), the liquid has good fluidity and castability. Their strength and heat resistance features have been improved considerably by alloy additions. 
Even better mechanical properties can be achieved in cast irons, without destroying the excellent casting and machining properties, by the production of a spheroidal graphite and its called ductile iron. A distinguishing feature of this widely used type  of cast iron, also known as spheroidal graphite iron or nodular iron, is that the  graphite is present in ball-like form instead of in flakes as in ordinary gray cast iron. The spheroidal nodules are roughly spherical in shape and are composed of a number of graphite crystals, which grow radially from a common nucleus with their basal planes normal to the radial growth axis. This form of growth habit is promoted in an as-cast grey iron by the addition of small amounts of Mg or Ce to the molten metal in the ladle which changes the interfacial energy between the graphite and the liquid. Good strength, toughness and ductility can thus be obtained in castings that are too thick in section for malleabilizing and can replace steel castings and forgings in certain applications. 
The internal notch effect of graphite nodules is significantly lower than that of flakes, so that the mechanical properties of cast iron with spheroidal graphite are superior to those containing flake graphite. The spherical shape is obtained by treating the melt with magnesium, calcium or cerium. However, the melt must be previously desulphurised to avoid growth of embrittling sulphides. Because magnesium evaporates easily on account of its high vapour pressure, it is more effective if added to the melt in the form of a Mg prealloy (NiMg or FeSiMg). Subsequent inoculation with 0.4 to 0.7% FeSi and other elements with an oxygen affinity (Al, Zr) increases the number of spheroidal graphite, produces an ideal graphite shape and counteracts chilling in small wall thicknesses. The effect of magnesium and inoculation gradually weakens so that large graphite nodules are often found in thicker walls.
Apart from limitations in the phosphorus (max. 0.08 %) and sulphur contents (max. 0.02 %), the basic composition of ductile cast iron corresponds to alloy concentrations in grey cast iron with Si contents of 1.7 - 2.8 %.
A saturation level close to unity is desirable to obtain a melt with good flow characteristics. The properties of ductile iron are essentially determined by the metal matrix. Its ferrite/pearlite ratio can be altered with specific alloying elements and by adjusting the cooling rate. Slow cooling increases the ferrite fraction, which is frequently found as a shell around the graphites. This type of structure is known as a bull?s eye.
The numerous, successful uses of ductile iron in critical components in all sectors of industry highlight its versatility and suggest many additional applications. In order to use ductile iron with confidence, the design engineer must have access to engineering data describing the following mechanical properties: elastic behavior, strength, ductility, hardness, fracture toughness and fatigue properties.
Mechanical properties of ductile iron depend on microstructure factors including the quantity, size and distribution of phases, number, size and shape of graphite particles and, the defects like porosities and inclusions. During solidification, graphite nodules nucleate homogenously and heterogeneously on inclusions. Then, they will be surrounded by the austenite phase. The diffusion of carbon from the liquid phase through the austenite phase leads to the growth of graphite nodules. It is obvious that the graphite morphology is responsible for good ductility and toughness of ductile iron. The nodular graphite acts as crack arrester and consequently increases the ductility. The larger graphite loses its role of crack arrester, and thus, ductility decreases. Besides, sufficient amount of graphite nodules are required to avoid the formation of carbides during solidification. The presence of carbides in the microstructure has superior effect on mechanical properties. The number of graphite nodules influences the content of ferrite/pearlite in the matrix; therefore, the graphite nodule count is an important parameter in the characterization of the microstructure in ductile. Solidification rate is the most important factor which affects the formation and the morphology of graphite because graphite in ductile irons precipitates quickly compared with that in grey cast irons. Many parameters affect the microstructure of the ductile iron and its solidification process in the permanent molds including the chemical composition such as C and Si, nodulation process, inoculation, mold preheating and pouring temperature. The accurate control of the chemical composition in the ductile cast iron without using any riser in the feding system leads to a casting specimen with no porosity Carbon and silicon contents have significant effects on the graphite formation in the ductile cast iron.
Ductile cast iron is an alternative for lots of applications on account of its excellent damping capacity, high strength, high toughness, good machinability, and low cost high tendency to distortion and poor thermal conductivity. For instance, it is used in automotive components that are subjected to predominantly mechanical loads. These include housings for disc brakes, steering gear, rear axles (particularly engine brackets), steering knuckles and brake pads for commercial vehicles. As known these machine parts and many of others are often subjected to fluctuating loads in service. For example; connecting roads are pushed and pulled in piston engines. Crankshafts are generally subjected to torsional stress and bending stress due to self-weight or weight of components or possible misalignment between journal bearings.
It is well known that fatigue endurance of ductile cast iron, similar to other Fe alloys, is affected primarily by the matrix microstructure and by notch factors or surface conditions. The graphite nodules work as shrinkage cavities. In fatigue crack initiation stage, micro cracks initiate around these graphite nodules and then form the macro-crack, which then propagates and leads to the final failure of the specimen. For this reason, ductile cast iron is particularly sensitive to geometrical size effect and technological size effect. The larger the casting thickness, the lower the cooling rate. Thus, also the nodularity and nodule count of the microstructure decrease. This yields a decrease in fatigue strength (technological size effect). Nadot et al. in their investigation on fatigue properties of nodular cast iron, observed that, in all the cases, a unique microshrinkage was at origin of the fatal crack that led the sample to failure. In such conditions, many approaches of fatigue resistance evaluation for defect containing materials consider that crack initiation stage is negligible.
A large scatter in fatigue strength of ductile irons indicates there are many variables on which the fatigue strength is dependent. These variables that influence the fatigue properties of ductile iron are graphite shape, graphite size, nonmetallic inclusions, matrix hardness and structure, specimen size, surface condition, surface degradation such as corrosion and the type of loading.
The aim of the current work is investigate the effect high silicon content on fatigue strength and other mechanical properties of conventional ferritik-pearlitic ductile iron. EN-GJS-500-14 which has %3,6 Si and is primarily used in the automotive industry where enhanced fatigue resistance is required and compare the result with ferritic-pearlitic grade ductile iron (EN-GJS-500-7 which has %2,4 Si) as it already has many properties advantages  than ferritic-pearlitic one.
The matrix of EN-GJS-500-7 consists of a mixture of pearlite and ferrite. The mixture can vary within a component, depending on wall thickness and cooling time, leading to large variations in the hardness of the material. But EN-GJS-500-14, is solution strengthened with silicon and the matrix consists only of ferrite giving the material a more even hardness distribution. Large variation in hardness makes machining hard to optimize, which gives EN-GJS-500-14 an advantage in components requiring machining. 
In this study, effect of silicon content on high cycle fatigue behavior of EN-GJS-500-7 and EN-GJS-500-14 grade ductile irons was investigated. Microstructural characterization was made by quantitative metallography and scanning electron microscopy (SEM). High cycle fatigue (HCF) tests were performed in a rotating bending fatigue tester operating at 50 Hz and using four point loading configuration. Following fatigue tests, S-N and P-S-N curves were plotted and endurance limit was determined. Fatigue fracture surfaces were examined by scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). Results revealed that higher amount of silicon encourages formation of a ferritic microstructure and provides higher toughness, higher ductility and yield strength values with respect to the lower silicon content ductile iron. Also, higher endurance ratio is achieved by the aid of higher amount of silicon.