Östemperlenmiş gri dökme demirin yüksek sıcaklık aşınma davranışının incelenmesi
Investigation of wear behaviour of austempered gray cast iron at elevated temperature
Yazar: ALİ ULAŞ MALCIOĞLU
Danışman: DOÇ. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Aşınma dayanımı = Wear resistance ; Brinell sertliği = Brinell hardness ; Gri dökme demir = Gray cast iron ; Mikrosertlik = Microhardness ; Ostemperleme = Austempering

Özet:
Gri dökme demirler; demir-karbon-silisyum alaşımı olmakla beraber, ötektik sıcaklık noktasında östenitin katı eriyik halinde ihtiva edebileceğinden fazla karbonu olan alaşımlardır. Yapılarındaki bu fazla karbon, grafit lamelleri halinde çökelmektedir. Gri dökme demirlerin mekanik özelliklerini iyileştirmek amacıyla östemperleme işlemi yapılmaktadır. Gri dökme demirlerde östemperleme işleminin amacı; ösferritik bir yapı oluşturarak sertlik, mukavemet ve aşınma direncini yükseltmektir. Östemperlenmiş gri dökme demirler; östemperleme işlemi sonrasında yüksek mukavemet, yüksek aşınma direnci, sönüm kabiliyeti gösteren beynitik mikroyapıya sahip dökme demir grubudur. Bu çalışmada perlitik gri dökme demirin, 900°C östenitleme işleminin ardından 250°C, 300°C, 350°C ve 400°C olmak üzere dört farklı sıcaklıkta östemperleme işlemine tabi tutulmuş, her östemperleme sıcaklığında numuneler; 5 dakika, 20 dakika, 40 dakika, 60 dakika ve 120 dakika olmak üzere 5 farklı östemperleme süresi sonrası havada soğutulmuşlardır. Östemperleme işlemi sonrası mikroyapısal incelemeler gerçekleştirilmiş, 250°C ve 300°C östemperleme sonrası alt ösferritik yapı görülürken, 350°C ve 400°C östemperlenmiş numunelerde ise üst ösferritik yapılar görülmüştür.Östemperleme işlemi sonucunda, mikroyapının östemperleme sıcaklığı ve süresine bağlı olarak değişen oranlarda beynitik ferrit ve kalıntı östenitten oluştuğu belirlenmiş ve oluşan bu yapı ösferrit olarak adlandırılmıştır. Östemperlenmiş gri dökme demir numunelerinin ve ısıl işlem görmemiş döküm halindeki numunenin mekanik özelliklerini incelemek amacıyla, sertlik ve aşınma deneyleri yapılmıştır. İncelemeler süresince; östemperleme işleminin Brinell sertlik cihazıyla yapılan makrosertlik sonucunda sertliğinin arttığı görülmüştür. Bununla birlikte mikrosertlik ölçümlerinde matrisin sertliği de ölçülmüş ve ısıl işlemsiz numuneye göre farklı östemperleme sıcaklığı ve sürelerinde östemperlenen numunelerin mikrosertliğininde arttığı görülmüştür. Östemperlenmiş gri dökme demirler; ball-on test metoduyla oda sıcaklığında ve 200°C yüksek sıcaklıkta aşınma testine tabi tutulmuş, oda sıcaklığında yapılan aşınma testi sonucu aşınma direnci 3 ile 5 kat arasında artış gösterirken, yüksek sıcaklıkta yapılan testlerde ise östemperlenmiş numuneler döküm halindeki numuneye gore aşınma direncinde iyileşme görülmüş ancak yüksek sıcaklıkta farklı östemperleme koşullarında elde edilen aşınma deneyi sonuçları, oda sıcaklığında yapılan test sonuclarına göre daha geniş aralıklarda farklılık göstermiştir. 250°C 5 dakika süresince östemperlenen numunenin yüksek sıcaklıkta aşınma direnci döküm halindeki numuneye göre yaklaşık 20 kat artarken, 400°C 120 dakika süresince östemperlenen numunede ise 2 kat kadar aşınma direnci artmıştır.

Abstract:
The term "cast iron" identifies a large family of ferrous alloys. Cast irons propose a wide selections of strength and other engineering applications and it has many manufacturing and engineering advantages compared to cast steel such as better wear resistance, vibration damping and less manufacturing costs. Gray cast irons are the most used of the general purpose engineering irons. The name comes from the characteristic gray colour of the metal on a fractured surface. Gray cast irons are used in automotive industry such as clutch plates, brake discs and flywheels due to lower cost and superior combination of properties like thermal conductivity, good vibration damping properties with good wear resistance and mechanical properties. The high damping capacity of gray cast iron is one of the most precious qualities of this material and damping capacity of gray cast iron is greater than other cast irons. Besides pearlitic gray cast iron is preferred because of it's high young modulus and good wear resistance rather than ferritic gray cast iron. Cast iron's mechanical properties change depending on metallographic structure. For example ferrite is low carbon alpha-phase. Also ferrite's tensile strength is low. Next to that pearlite is consists of ferrite and sementite. Although pearlite possesses higher hardness rather than ferrite, ductility is low. Cementite is known as eutectoid carbide and also cementite is the brittle intermetallic compound of iron and carbon. Gray cast iron's mechanical and physical properties are also influenced by some factors and these properties are controlled by the shape, amount and distribution of lamellar graphite flakes. There are some lamellar types which are accepted from ASTM and AFS standards. In gray cast irons, uniformly distributed A type lamellars are generally preferred. B type lamellars are unsufficient for high strength. C type lamellars have good at thermal shock resistance. D type lamellars show low wear resistance. And E type lamellars are found in low carbon hypoeutectic compositions. Alloying elements are so essential for determine gray cast iron's mechanical properties. In this regard chromium increases oxidation resistance. Molibden has an important role in quenching. Besides cupper increases corrosion resistance. Many attempts were made for to increase the strength properties in gray cast iron such as alloying. Although gain in strength, production cost increased undesirebly. Austempering heat treatment proposes increase in strength for a relatively small cost. Besides, the many useful applications of austempered ductile iron for improving mechanical properties, suggested a study on austempered gray iron. The aim of the present study is to investigate dry sliding wear behaviour of austempered gray cast iron at room and elevated temperatures. For investigating optimal wear behaviour, different variation of austempering temperatures and times are used. Besides as-cast gray cast iron's microstructure consists of pearlite. A traditional austempering heat treatment is started with austenitising the matrix between 850 to 930oC than holding in a salt bath above Ms temperature between 230 to 430°C for enough time followed by cooling in air for attain the ausferritic microstructure. In this study as-cast pearlitic gray cast irons were austenized at 900°C for 60 minutes and subsequently austempered for 5, 20, 40, 60, and 120 minutes at 250°C, 300°C, 350°C and 400°C. Microstructural characterizations were carried out by optical microscopy researches. Austenitizing temperature and time have significant effect on austempered gray iron's mechanical properties. Austenitizing temperature controlls austenite's carbon content. With austenite's carbon content, austenite's hardenability and stability can change. If austenitizing temperature reduces, structure becomes more stable and less martensite occurs. Besides that in austenitizing kinetic, perlite content in matrix structure is so important parameter. With increasing of perlite content, austenitizing kinetic accelerates. Austempering temperature and time have effect on mechanical properties and wear resistance. Austempering takes place in 2 stages. In first stage, ausferritic transformation begins and austenite decomposes to ferrite and high carbon austenite. In short austempering times carbon content is insufficient to stable retained austenite so during cooling to room temperature martensite take places. But when austempering time increases austenite enriches with carbon and ausferrite structure becomes stable. In high austempered temperatures such as 350 and 400°C ferrite aciculars nucleation rate slow down, and retained austenite in matrix increases because of the carbon enrichment in austenite. Nevertheless, in long austempering times such as 3 hours retained austenite starts to decrease because high carbon austenite decomposes to ferrite and carbide. So results showed that retained austenite changes with different austempering conditions and it effects mechanical properties. After all if austempering time extensions; although impact resistance and ductility increase, hardness decreases. Also increasing austempering time, decreases martensite content. As a result of austempering, it is concluded that microstructure is comprised of bainitic ferrit and retained austenite which changes according to the temperature and time of the austempering. Besides chemical composition is so essential for austempering. Silicon has a significant role in the improvement of the ausferritic matrix in grey iron. Silicon reduces the carbon solubility in austenite and accelarates carbon diffusion. Next to that sulfur generates strong carbon diffusion barrier and slows down carbon diffusion between lamellar graphite and grey cast iron matrix. Besides that, this structure which consists retained austenite and bainitic ferrite is called as ausferrite. After austempering in 250°C and 300°C lower ausferrite structure is seen, while in 350°C and 400°C upper ausferrite structure is seen. Lower ausferrite structure is acicular but upper ausferrite structure is feathery and coarse. The hardness of pearlitic gray iron, increased with austempering process. Brinell hardness performed due to investigating the hardness behaviour. Besides Vickers microhardness is done for researching hardness in the matrix of the samples. With increasing austempering temperature and time, hardness started to decrease. In austempered microstructure; austenite is soft phase so while austempering time proceeds retained austenite increases and consequently hardness decreases. Brinell hardness and Vickers microhardness give information about microstructural transformations. The highest Brinell and Vickers hardness were measured in 250°C austempered along 5 minutes and the lowest hardness were measured in 400°C austempered along 120 minutes. After all, Vickers microhardness show that in short austempered times microhardness is so high because of the presence of martensite in matrix phase. Wear is the loss of material with material transfer between of surfaces. Wear happens in some criterions such as mechanic effects and friction. Besides there are 4 main wear mechanisms. These are abrasion, adhesion, erosion and surface wear. Abrasion wear happens because hard particles forced against throughout the solid surface. Although metallic particles can cause abrasion, unmetallic particles mostly cause this
problem. For preventing abrasive wear there are some surface treatments such as; case hardening, wear plates, and weld hardfacing coatings. Next to that, adhesive wear occur from transference of material between of surfaces during frictional contact under loadings. For preventing adhesive wear there are some recommendations. For example lubrication reduces wear. Also avoiding sliding similar materials can prevent wear. Also there are some parameteres which effects wear. One of these paramaters are based on the materials, such as material crystal structure, modulus of elasticity and surface roughness. And also environment effects such as temperature, atmosphere and moisture.
The wear tests were carried out using ball-on-disc type apparatus under dry sliding conditions in room temperature and 250oC high temperature. Results show that in room temperature, wear resistance increased between 3 and 5 times after austempering heat treatment according to as-cast sample. Besides friction coefficient values decreased after austempering. In 250oC, wear resistance increased between 2 and 20 times in different austempering conditions. According to high temperature results, the highest wear resistance were measured in 250°C austempered along 5 minutes and the lowest wear resistance were measured in 400°C austempered along 120 minutes. Results show that increasing austempering temperature and time are so essential for wear resistance behaviour. Especially in 250°C austempered samples show high wear resistance because of high hardness and low retained austenite. On the other hand in 400°C along 120 minutes austemperd sample, hardness is low and retained austenite is high. In this regard wear resistance decreases.