GGG-60 küresel grafitli dökme demirde östemperleme ısıl işlemin çekme, yorulma ve aşınma özelliklerine etkisine

Abstract

Östemperlenmiş Küresel Grafitli Dökme Demirler (ÖKGDD), Küresel Grafitli Dökme Demirlere (KGDD) östemperleme işleminin uygulanmasıyla üretilen yüksek mukavemet, yüksek süneklik ve aşınmaya karşı direnç gösteren beynitik mikroyaprya sahip bir dökme demir grubudur. ÖKGDD'nin üretim sürecinde karşılaşılan sorunlar, östenitleme sıcaklığı ve süresi, östemperleme sıcaklığı ve süresi, kimyasal bileşim ve kesit kalınlığı gibi östemperleme kinetiğini ve mekanik özellikleri önemli ölçüde etkileyen parametlerin etkisini araştırmak amacıyla 1970 yılmdan bu yana pek çok bilimsel arastama yapılmıştır. Bu çalışmada, % 3.72 C, % 2.51 Si, % 0.43 Mn, % 0.0237 P, % 0.01 S, % 0.07 Cr, % 0.021 Sn ve % 0.047 Mg bileşimine sahip GGG-60 (DİN 1693) smıfi KGDD, 900 °C'de 100 dakika süreyle östenitlenmiş ve 300 °C'de 45, 60, 90 ve 180 dakika süreyle östemperlenmiştir. Östemperleme süresinin mekanik özelliklerini araştırmak amacıyla, sertlik, çekme, çift yönlü eğmeli yorulma, disk üzerinde pim yöntemiyle metal-metal aşınma deneyleri yapılmışta. Çahşmal r sonunda 300 °C'de östemperleme işlemiyle beynitik ferrit ve yüksek karbonlu östenitten ibaret iğnesel ÖKGDD mikroyapısı elde edilmiştir. Östemperleme işlemiyle döküm halindeki KGDD'nin serdiği, çekme ve akma mukavemeti ve aşınma direnci önemK ölçüde artmışta. Öte yandan östemperleme süresiyle sertlik, çekme ve akma mukavemeti ve yorulma mukavemeti önemli bir değişme göstermemiş, kopma uzaması ve kesit daralması değerlerinin ise östemperleme süresinin artmasıyla arttığı belirlenmiştir. Östemperlenmiş numunelerin aşmma direncinin ise deney yüküne bağlı olarak değiştiği gözlenmiş; Düşük deney yüklerinde (10 ve 20 N) ' önemli bir değişim gözlenmediği halde, yüksek deney yüklerinde (40 ve 80 N) östemperleme süresinin artmasıyla birlikte artmaktadır.

Austempered Ductile Iron (ADI) is a group of high strength, high ductility and high wear resistant engineering materials which are produced by applying two step heat treatment to a conventional ductile iron. The first step of heat treatment in producing ADI is austenitisation in the temperature range 825 - 950 °C to change the as-cast matrix structure into austenite and the second step is rapid cooling to the isothermal temperature (250-400°C) in a salt bath and holding at this temperature for a certain time. After this isothermal treatment, the cast component is removed from the bath and allowed to cool to room temperature in air. Final microstructure would consist of bainitic ferrite needles, carbide and retained austenite, namely lower bainite or carbide free ferrite laths and retained austenite, namely upper bainite depending on the selected austempering temperature. In austempering of ductile iron, austenitisation temperature and time, austempering temperature and time as well as chemical composition of ductile iron have a strong influence on the kinetic of heat treatment and in turn the mechanical properties of ADI. 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. XIV When the cast component is immersed into the isothermal sakh bath after austenitisation, austempering process progressives through a two stage reaction. The first stage is given by the reaction, y-Kx+yhc (1.1) At this stages, bainitic transformation starts at the places close to the graphite nodules and progressives as the carbon in ferrite is rejected into austenite. This reaction continues until austenite is saturated by carbon. If the reaction is interrupted at the early stage of austempering, some amount of austenite not having been saturated by carbon transforms to martensite during cooling to room temperature. As a result, final microstructure would consist of bainitic ferrite needles, retained austenite and some amount of martensite. After austenite has been saturated by carbon, it would be stable enough not to transform to martensite. If the austempering time is prolonged, carbon saturated austenite starts to decompose into ferrite and carbide due to the second stage of the transformation expressed as; yhc->a+Fe3C (1.2) The amount of retained austenite increases with increasing the austempering time, reaches a plateau between the time ti and t2, then decreases. The tin <5 interval ti and ti which the amount of retaired austenite is maximum, is described processing window, and optimum mechanical properties are achieved within the processing window. Austempering temperature, has a strong influence on the final microstructure of ADI. At the temperature range 250-330°C, the diffusion rate of carbon is low but the nucleation of ferrit plateless is high. As a result, carbon entirely cannot be rejected into austenite and precipitates as carbide in ferrite. Since bainitic ferrite platelets have a high nucleation but low growing rate, there are a lot of fine platelets in a lower bainitic microstructure, and retained austenite appears like a film adjacent to the ferrit. Due to the restricted effect of carbides and fine nature of bainitic ferrite needles against dislocation movement, lower bainite has a higher hardness and strength values but martensite formed due to the XV transformation of austenite at early stages of austempering causes in a reduction ductility and toughness values. At the temperature range 330-400 °C, diffusion rate of carbon and the nucleation rate of ferrite laths are high while growing rate of ferrite laths is low. Final microstructure obtained by austempering at this temperature range would consist of carbide-free and coarser ferrite laths and more retained austenite occurs in blocky morphology. As a result of this microstructure, upper bainite is responsible for higher ductility and toughness values but less hardness and strength compared with lower bainite. Austenitising temperature influences the carbon content of the austenite, austenite grain size and chemical homogenity of the matrix an in turn particularly the first step reaction during austempering. As an overview, increase of the austenitising temperature reduces the elongation and impact energy particularly at higher austempering temperatures. For a given austempering temperature, selection of a lower austenitising temperature accelerates the first reaction of transformation and requires less time to reach processing window and optimum mechanical properties. Tie aim of this work is to determine the static and dynamic strength, ductility and wear resistance of an unalloyed ADI produced by austempering at 300°C and to examine the effects of austempering time on these mechanical properties. A GGG-60 grade (DIN 1693) ductile iron utilized in this work was supplied from Döktaş AŞ. in the form of Keel Block. The chemical composition of ductile is (wt.%) 3.7 C, 2.51 Si, 0.43 Mn, 0.29 Cu, 0.07 Cr, 0.047 Sn, 0.037 P, 0.01 S, 0.047 Mg and the balance iron. Standart cylindrical tensile, bending fatique and wear testing specimens were machined from the Keel Blocks. These specimens were austenitised at 900 °C for 100 minutes, and then quickly transferred into salt bath (50 % NaN03 - 50 % KN03) at 300 °C and held at this temperature for 45, 60, 90 and 180 minutes. After removal from the salt bath xvi the specimens were allowed to cool to room temperature in air. The surfaces of the heat treatment specimens were finished by gentle grinding with a 600 grid emery paper. Dublicate tesile, bending fatigue and pin-on-disk wear test specimens each austempering condition were tested on an Losenhausen tensile testing machine, Denison fatigue testing machine and a conventional pin-on-disk apparatus. Bending fatigue test were performed with a stress ratio of R = -1 and the frequency was 1420 cycle/min. Dry sliding wear tests were carried out using a pin-on-disc type wear tester. The counterface disc material was AISI/SAE 8620 carbonized steel with the hardness of 63 HRC. During the wear tests wear specimens were pressed vertically against the rotating disc with four different loads (10-40 N). The sliding distance of the wear specimens on the disc was 3600 m. and sliding velocity was lm/sec. Weight losses of the specimens were obtained by measuring precisely the weights of the specimens before and after wear test. Average weight loss values were converted into volume loss by taking into account the density of the ADI measured as 7.26 g/cm3. Wear rate (Wr, mm3/mm) representing each austempering condition was calculated as volume loss per sliding distance. Cross sections machined from tensile sample grip ends were utilized for hardness tests and optical microscopy examinations. Metallographic specimens were prepared in the standart manner and etched with 3 % NitaL Hardness measurements were performed on Wolfpert hardness tester with a diamond pyramid indenter using 20 kg load. Increasing of austempering time from 45 to 180 minutes did not significiantly effect the hardness, proportional limit and ultimate tensile strength of the investigated ADI. On the other hand, ductility exhibited by elongation at fracture and reduction of area increased with increasing austempeing time at 300 °C. Similar behavior of ductility with respect to austempering time observed in an Mn alloyed ADI is attributed to decrease in the amount of martensite that is present at sma^ quantities in the microstructure. Fatigue behaviours of ADIs were investigated using S-N aproach for a stress ratio R = -1. Since ADI does not have a definite endurance limit, fatigue strength was therefore determined at fixed 107 cycles for all austempering conditions. Fatigue strength of ADIs xvii was almost constant in the investigated austempering time range similar to hardness and strength. During SEM investigations cracks were observed around graphite nodules an the fracture surface of these fatigue test specimens. The wear rate of ADI decreased with increased austempering time for higher loads (20- 40 N) while was not significantly different for lower loads (10-20 N). It is concluded that strain induced martensite formation during wear is responsible for the good wear resistance of ADI. The following conclusions can be drawn on the basis of experimental results performed on unalloyed ADI produced by austempering at 300°C after austenitisation at 900°C for 100 minutes. As-cast ductile iron consist of spheroidal graphite nodules surrounding ferrite and pearlite namely bull's eye microstructure. After austempering, it is obtained that an acicular microstructure consisting of bainitic ferrite needles and retained austenite. Matrix hardness values of ductile iron has been increased from 270 to 580 HV 0.2 by austempering. After austempering, an increase of 15 % in hardness, 50 % in tensile strength and i 5 % in proportional limit were achived compared with as-cast ductile iron. Hardness, tensile strength and proportional limit has not changed significantly due to the austempering time. Percent elongation and reduction of area of austempered specimens has increased with increasing austempering time. Endurance limit has increased approximately 10 % after austempering compared with as-cast ductile iron. However with increasing austempering time, endurance limit has not shown a significant change. It is thought that it may result from that hardness and strength values are constant during austempering. xvni Wear resistance has increased at least three times compared with as-cast ductile iron. On the other hand with increasing austempering time wear resistance has not changed significantly. For each austemering time, the tests performed by the load of 80 N have caused less or equal wear rate to that of the tests performed by 40 N. It is thought that, this is caused by the transformation of high carbon retained austenite in ADI to stress induced martensite under the effect of test load. Optimum mechanical properties have been achieved by the austempering time of 180 minutes.