Ggg 50 Ve Ggg 80 Sınıfı Küresel Grafitli Dökme Demirlerin Abrasiv Aşınma Davranışına Ostemperleme İşleminin Etkisi

Abstract

Küresel grafitti dökme demirlerin (KGDD) mekanik özelliklerini arttırmak amacıyla yapılan çalışmalarda, ostemperleme ısıl işlemi ile, KGDD'e beynitik bir mikroyapı kazandırılmış ve mukavemet, süneklik, aşınma direnci gibi mekanik özellikleri çok yüksek olan "ostemperlenmiş küresel grafitti dökme demir"ler (OKGDD) ortaya çıkarılmıştır. Ostenitleme ve ostemperleme sıcaklıkları ve süreleri ile alaşım elementlerinin hem mikroyapı hem de mekanik özellikler üzerinde önemli etkileri olması nedeniyle, bu konuda yapılan çalışmalar halen sürmektedir. KGDD'lerin özelliklerini daha da geliştirmek amacıyla klasik ostemperlemenin yanısıra basamaklı ve özel ostemperleme konularında da araştırma yapılmaktadır. Bu çalışmada, GGG 50 veGGG 80 (DİN 1693) sınıfı KGDD numunelere klasik ve özel ostemperleme işlemi uygulanmıştır. Klasik ostemperleme işlemi, döküm yapısındaki numunelerin 900°C'de 100 dakika ostenitlenmesi ve sonra 250°C'deki bir izotermal tuz banyosunda GGG 50 ve GGG 80 sıma numuneler için sırasıyla, 150 ve 210 dakika tutularak havada soğutulmaları şeklinde gerçekleştirilmiştir. Özel ostemperleme işlemi ise, döküm halindeki GGG 50 ve GGG 80 sınıfı KGDD numunelerin 900°C'de 100 dakika ostenitlendikten sonra, su verilmesi, daha sonra 730°C'deki çift faz bölgesinde (a+y) (60 dakika) süre ile bekletilerek, bu süre sonunda 250°Cdeki bir izotermal tuz banyosunda GGG 50 veGGG 80 sınıfı KGDD numuneler için sırasıyla 150 ve 210 dakika bekletip havada soğutmak şeklinde yapılmıştır. Klasik ve özel ostemperleme işlemlerinin aşınma davranışı üzerindeki etkilerini araştırmak için, döküm halindeki numuneler ile klasik ve özel ostemperlenmiş numuneler üzerinde serttik ve AI2O3 şerit zımpara aşınan pim yöntemine göre abrasiv aşınma deneyleri yapılmıştır. Metalografik çalışmalarda, döküm halindeki GGG 50 ve GGG80 sınıfi KGDD'lerin sırasıyla ferritik ve fenitik+perlitik mikroyaprya sahip olduğu anlaşılmıştır. Klasik ostemperleme ile her iki sınıf KGDD alt beynitik bir mikroyapı kazanmıştır. Özel ostemperleme sonucu ise, GGG 50 sınıfı KGDD'de hücre sınırlarında bir miktar beynitik dönüşüm ile birlikte genelde ferritik bir mikroyapı elde edilirken, GGG80 sınıfı KGDD'de grafit kürelerinin ferrit halkaları ile çevrelendiği ve bunlar arasında yine beynitik dönüşümün görüldüğü bir mikroyapı elde edilmiştir. Her iki sınıf KGDD numunede serttik; klasik ostemperleme işlemi ile döküm batine göre artarken, özel ostemperleme işlemi ile düşmüştür. Farklı boyutlardaki zımpara ile yapılan aşınma deneylerinde de aşınma direnci sertlik deneyine benzer bir davranış göstermiştir. Aşınma direnci her iki sınıf numune için klasik ostemperleme işlemi ile, döküm haline göre artarken, özel ostemperleme işlemi sonucu düşmüştür. Aşınma yüzeylerinin Taramak Elektron Mikroskobu (SEM) ile incelenmeleri sonucu, genel olarak "ploughing" aşınma mekanizması karekteristikleri taşıdığı ve aşındırıcı zımpara boyutu arttıkça aşınma yüzeylerinin de kabalaştığı görülmüştür.

Austempered Ductile Irons (ADI) are the new members of cast irons family at present. Although ADI has been available for several years, there remains a lack of awareness as to what austempered ductile iron really is, how it is made, what its properties are, and how it is being used in service. ADI offers a special combination of properties: much higher strength, greater toughness, through hardenability, superior wear resistance, with the same design flexibility as ductile iron (DI). The austempering process which is applied to ductile irons has two steps. The first step in the austempering process is, austenitisation in the temperature range 850- 950°C to change the as-cast matrix structure into austenite with a carbon content that is controlled mainly by the choice of austenitising temperature and iron composition. The second step is, rapid cooling to an austempering temperature in the range from 250 to 400°C and holding for a time between 1 and 3 h. The carbon level is high enough to make the austenite thermally stable so that, it is retained during the third heat treatment step of air cooling to room temperature. Depending on the temperature employed, austempering produces a matrix structure of bainitic ferrite or bainitic ferrite-carbide in a high carbon austenite namely lower bainite or carbide free ferrite laths and retained austenite, namely upper bainite depending on the selected austempering temperature. Lower bainitic morphology is predominant at the lower temperature range (250-330°C) and responsible for high hardness and strength values but low ductility and toughness. On the other hand, the predominant morphology is upper bainite at higher temperature range (330-400 °C) and responsible for higher ductility and toughness values but low hardness and strength values. Special austempreing process consists of prequenching the ductile iron followed by reaustenitizng in the (a + y) temperature range and an isothermal treatment in the bainitic transformation range. Toughened ADI produced by this process shows high fracture toughness compared with the ordinary ADI. This process provides a fine ausferritic microstructure and stable retained austenite around both the graphite/matrix interfaces and the eutectic cell boundary, which are the crack initiation sites in the ductile iron. The martensitic microstructure introduces by prequenching has a large number of precipitation sites for the acicular ferrite to form, and thus, a uniform ausferritic microstructure without the unstable retained austenite is obtained. Moreover,the stabitilty of retained austenite in the final microstructure is increased due to the fact that the alloying elements concentrate in the austenite phase during holding in the (a + y) temperature range. It is generally recognized that, the structure and the mechanical properties of the toughened ADI are determined by the composition, heat treatment conditions and microstructure of ductile iron before austempering. There is a significant difference in heat treatment and mechanical property behaviour of ADI and steel. The first stage in the tempering of a quenched steel is the formation of ferrite and carbide from the martensite. The second stage is the coarsening of the carbides.The first stage determines the strength and ductility with high strengths obtained early in the tempering process. The second stage produces a simultaneous decrease in strength and increase in ductility. In austempering of ductile iron, austenitisation temperature and time,austempering temperaure and time as well as chemical composition of ductile iron have a strong influence on the kinetics of heat treatment and in turn the mechanical properties of ADI. Although wear is an important topic, it has never received the attention it deserves because wear has an important economic role in daily life. Wear may be defined as the removal of material from solid surfaces as a result of mechanical action. Cast irons are the most popular tribologic materials which can be used especially in applications where wear resistance is important. The matrix of these materials can vary from pearlite to martensit in order to support graphite. Hard phases, can change in order to resist abrasive wear. For all critical wear applications, the both the hardness values near the surface and microstructure must be controlled. Because the microstructure is important in the control of mechanical properties. The relation between the cast irons and the wear behaviour of components that form the matrix structure can be defined as follows:Pearlit improoves the adhesive wear resistance a lot. But, ferrite is not suitable in all kinds of wear parts in service. Cementite affects the wear resistance positively. In components which requires abrasive wear resistance,the martensit structure reaches to maximum hardness value. The structures of bainitic transformation also increase the wear resistance. Abrasive wear arises when a hard, rough surface slides against a softer surface, digs into it, and plows a series of grooves. The material originally in the grooves is normally removed in the form of loose fragments, or else it forms a pair of mounds along each groove. The material in the mounds is then vulnerable to subsequent complete removal from the surface. Abrasive wear can also arise in a somewhat different situation, when hard, abrasive particles are introduced between sliding surfaces and abrade material off each. The mechanism of this form of abrasive wear, seems to be that an abrasive grain adheres temporarily to one of the sliding surfaces, or else is embedded in it, and plows out a groove in the other. The two forms of wear, one involving a hard, rough surface and the other hard,abrasive grains, are generally referred to as the two-body and the three-body abrasive wear process, respectively. Abrasive wear of the two-body kind does not take place when the hard, sliding surface is smooth. Similarly three-body abrasive wear does not occur when the particles in the system are smal, or when they are softer than the sliding materials. Hence, it is usually possible to arrange matters so that a sliding system is, initially at any rate free from abrasive wear. Once sliding has commenced, however, abrasive wear may become a problem, as wear debris, often made harder by oxidation or work hardening, begins to accumulate in the system as result of other wear processes. In other cases contaminating particles may be introduced into the sliding system from the environment. Abrasive wear is very widely used in material-finishing operations. The two-body type of abrasive wear is made use of in files, abrasive paper, abrasive cloth,and abrasive wheels,whereas the three-body type of wear is used for sand blasting,lapping, and polishing. The widespread lack of interest in the importance of wear is due to a number of factors. The main one is that historically th e study of wear process was approached very late. So the elucidation of the laws of wear is relatively recent and it has not been completed. For this situation, three reasons can be told. The first is that, informer times wear was much less important in the determining the usefull lives of machines and other mechanisms than it is today. Second, there is the fact wear rates are often quite low. Indeed, until recently there was no ready way of studying any wear process continously. Third, was the attitude on the part of many engineers and scientists that wear is phenomenon so complicated and erratic. The systematic investigation was bound to be a waste of time. The aim of this work is, to determine the influence of conventional and special austempering processes on the abrasive wear behaviour of GGG 50 and GGG 80 grade ductile irons. Special austempering process was recently proposed by Kobayashi and Yameda, to improve toughness of ADI. The chemical compositions of ductile irons which are used in this work are given in Table 1. GGG 50 and GGG 80 grade ductile irons utilized in this work were supplied from DÖKTAŞ A.Ş. in the form of Keel blocks. The conventional austempering process applied in two steps. In the first step, the specimens were austenitised at 900°C for 100 minutes in a austenitising furnace. In the second step, the specimens were rapidly transferred to the isothermal salt bath (250°C). GGG 50 grade specimens were held for 150 minutes, while the GGG 80 grade specimens were held 210 minutes at 250°C. After isothermal treatment, specimens were cooled to room temperature in air. Table I: The chemical compositions of ductile irons which are used in the experiments. The special austempering process, which was proposed by Kobayashi and Yameda applied in 3 steps. In the first step, the martensitic structure was achieved by water quenching in the investigated ductile irons. Austenitising temperature and time were 900° C and 100 minutes respectively. In the second step, specimens were reheated to at730°C for 60 minutes (730°C corresponds to ferrite and austenite dual phase region in the iron -carbide diagram). By the end of this step, GGG 50 grade ductile irons were austempered at 250°C for 150 minutes, while GGG 80 grade ductile irons were austempered at 250°C but, for 210 minutes (step 3) and cooled in air. Metallografic specimens were prepared in the standart manner and etched with 3 % Nital. Hardness and abrasive wear tests were conducted on the as-cast, conventionally austempered and specially austempered specimens, beside microscopic examinations. After the austempering processes, hardness and abrasive wear tests were applied to the specimens by using title pin on flat method. Hardness measurements were performed on Wolpert hardness tester with a diamond pyramid intender. Abrasive wear tests were carried out on a pin on flat type wear tester under 34 N. During wear tests, specimens that were rubbing on abrasive band were always moving perpendicular to sliding direction. So that they always passed over fresh abrasive. The total sliding distance and the speed of abrasive band were 33 m and 0.22 m/s respectively. In the abrasive wear tests, two types of AI2O3 abrasive bands were utilized (60 mesh and 120 mesh). Wear test specimens were machined from Keel blocks with 3 mm tips before heat treatment. Weight loss of each specimen due to abrasive wear were obtained by measuring precisely the weights of the specimens. Average weight loss values were converted into volume loss by taking into account the density of the ADI grades (for GGG 50, 7.13 gr/cm3 ; for GGG 80, 7.24 gr/cm3 ). Wear rate (Wr, mm3/mm ) was calculated as volume loss per sliding distance. Wear rate and the hardness values can be seen in Table H After the wear tests, the worn surfaces of the wear specimens were investigated by Scanning Electon Microscope (SEM) Following conclusions can be drawn on the basis of experimental results performed on GGG 50 and GGG 80 grade ADI specimens produced by conventional and special austempering processes. As cast GGG 50 grade ductile iron has a ferritic microstructure, while the GGG 80 grade ductile iron has a ferritic-pearlitic microstructure. A lower bainitic structure was obtained in the both grade specimens by applying conventional austempering. As a result of special austempering, a ferritic microstructure with fine carbides and bainite at the cell boundaries was appeared in GGG 50 grade ductile iron. However, GGG 80 grade ductile iron obtained a microstructure in which graphite nodules were surrounded by ferrite rings and a bainitic structure was also present in GGG 80 grade ductile iron. Table II: The hardness and the wear rates. Conventionally austempering treatment increased the hardness of GGG 50 and GGG 80 grades with respect to as cast condition about 100 % and 50 % respectively. However, special austempering process decreased the GGG 50 and GGG 80 grades with respect to as cast condition about 10 % respectively. and 20 % Abrasive wear tests revealed that, the wear resistance of the investigated cast irons increase with increasing bulk hardness as expected. Conventionally austempering process increased, special austempering proces decreased the wear resistance of investigated cast irons with respect to as cast condition. The wear rate decreased by the increase of hardness and by the reduction in the sizes of abrasive particles. In the wear tests made on both 60 and 120 mesh AI2O3 abrasive bands, the wear rates of GGG 50 and GGG 80 grade ductile iron were found nearly the same despite they have different hardness values. Scanning Electron microscopy investigations of worn surfaces revealed wear tracks, which are extended in the sliding direction and graphites which lost the nodularity by the increase of abrasive particle size, wear surface deformation and damage is increased. Additionally, matrix deforms plastically in the direction of sliding and generally the worn surfaces have the characteristics of ploughing wear mechanism.