Östemperlenmiş küresel grafitli dökme demirlerde bakır miktarı mikroyapı ilişkisi

Abstract

3 ayrı bileşimde dökülmüş küresel grafitli dökme de mir keel bloklardan hasırlanan deney numunelerine östemperleme ısıl işlemi uygulanmıştır. Numuneler, 850 °C ve 900 °C lerde 60 dakika östenitlemeyi takiben, 250, 300, 350 ve 400 "C sıcaklıklarda 10, 25, 50, 75 ve 100 dakika olmak üzere beş farklı sürede östemperleme ısıl işlemine tabi tutulmuşlardır. Bu işlem sonucunda bakır alaşım elementinin ve temel ısıl işlem parametrelerinin mikroyapı üzerine etkileri incelenmeye çalışılmıştır. Küresel garfitli dökme demirlerin östemperlenmesi sonucunda, başlangıca göre küre sayısında önemli oranlarda azalmalar gözlenmiştir. Küre boyut ve dağılımında kullanılan görüntü analiz cinasının ise, küre sayısında ki değişimin östemperleme parametrelerine bağımlılığının belirlenmesinde yetersiz bir ölçme yöntemi olduğu sonucuna varılmıştır. östemperlenmiş küresel grafitli dökme demirlerde östenitieme sıcaklığının artması kalıntı östenit miktarını arttırmaktadır. östemperleme sıcaklığında meydana gelen artış ise, genelde kalıntı östenit miktarını arttırmakta ancak, 350 "C den sonra ani düşüşlere sebep olmaktadır. Kalıntı östenit miktarının östemperleme zamanı ile tu tarlı bir ilişkisi kurulamamıştır. Bakır içeriğinin artmasının alt beynit yapısında kalıntı östenit miktarını arttırdığı gözlenmiştir. Ayrıca bakırın, yüksek östenitleme ve östemperleme sıcaklıklarında malzemenin sertliğini kayda değer derecede arttır dığı anlaşılmıştır. östemperleme şartlarının değişmesiyle beynitik matriksin de değiştiği görülmüştür. Düşük östemperleme sıcaklıklarında, sertliği yüksek ve kalıntı östenit miktarı nisbeten düşük alt beynit yapısı oluşurken, yüksek östemperleme sıcaklıklarında sertliği düşük ve % 50 lere varan oranlarda kalıntı östenit içerebilen üst beynit yapısı oluşmaktadır.

Austempering is cess involving two Figure. The first s castings in the qenching to a lower and austempering at 4 hours. During th isothermally to g proportions of reta an isothermal heat treatment pro- main stages as shown schematically in tage (austenitising) involves heating range 850-900*.C, followed by rapid temperature in the range 235-450 °C this temperature for between 0.5 and e holding period austenite transforms ive a bainitic structure with varying ined austenite and martensite. "1-5""' 20 2 5 Time, h 35 4 0 Fig. : Scehematic of a typical austempering heat treatment sycle A qualitative understanding of the transformation of austenite to bainite in austempered ductile iron (ADD has been achieved although the mechanisim has not been widely studied. Transformation in the austempering temperature range (235-450°C) starts by the nucleation of bainitic ferrite at interphase and grain boundaries. Iron carbide pre cipitation does not necessarily occur immediately, as is the case in bainitic steels owing to the high silicon content of the cast iron. At isothermal temperatures below about 330 "C growth rates in the ferritic needless are high, and, as the rate of carbon diffusion is rela tively low this results in high carbon contents in the bainitic ferrite which can, initially have a distorted tetragonal crystal structure. At an early stage of VI austempering treatment thie carbon is rejected from the ferrite and. precipitates ae ? -carbide (Fea.iC) in ferrite needless and thie ie often referred to ae "bainitic caı-bide". Transformation can proceed eo long ae thie carbon ie rejected from bainitic carbide into residual auetenite and, depending upon to temperature, it may take from 0.5 to 3 hours for the reaction to be completed. At the and only a email quantity of auetenite is retained and this is maintained even after cooling to the room temperature. Thie structure is referred to as lower bainite. Lower bainite cone lets of fine ac.icular structure of ferrite and very fine carbides. Ductile irons having a lower bainite matrix structure have high hardness values (much than 400 HB), high tene ile strengths in the range 1,200 N/mm* to 1,600 N/mmz, but only limited ductility (1-3 % at failure). At temperatures above about 330°C a different trans formation mechanism operates resulting in the formation of upper bainite. Carbon diffusion ie more rapid so most of the carbon is able to diffuse out of the growing bainitic ferrite plates in surrounding auetenite. If the austempering process ie insufficiently long the martens ite (Ms) temperature will still be above ambient temperature and the x-esidual auetenite will transform at least briefly to martens ite during cooling. As the austempering time ie prolonged, the carbon content of the residual auetenite will in- crease and eventually reach a level (approx. 1.5-1.7 %C) where the bainitic transformation is inhibited. Further- more the high carbon content depresses the Ms tempera- ture resulting in auetenite being retained after cooling to ambient temperature. This retained auetenite can be stable down to at least -120 "C. The structure of upper bainite in ductile irons after austempering treatments consists of relatively coarse ferrite plates and up to 40 % retained austenite. Ductile irons austempered to produce an upper bainitic matrix structure have a lower hardness (280-320 HB), and a tensile strength in the range 900 to 1,050 N/mm* and considerably higher ductility with elongations at failure in the range 6-12 %. The transformation of austenite to the upper bainite structure of ferrite in austenite in ductile iron and in high silicon steels generally occuree in two stages. The first stage (stage I) involves nucleation and growth of fen-ite platelets at auetenite grain boundar ies or near graphite nodules. Growth of these ferrite platee, which appeal* to occur in microscopic bunches or colonies, continues until impingement occurs with other plates and rejection of carbon into the auetenite reduc- VII ed the driving force for further growth. (The auetenite phase is a rather passive participal in the transform ation, being reduced in amount while simultaneously undergoing an increase in carbon content). At the and of stage 1, the austenite is sufficiently stabilised not to transform into martens ite on coQling to room tempera ture and a"fully austempered" ductile iron structure results. It is this structure which is desired for optimum properties. The second stage (stage II) is the time period when slow ferrite plate thickening occurs together with nu- cleation and growth of the iron carbide at the expence of the austenite, a result which markedly deteriorates ductility. Transformation to lower bainite is similar to that of upper bainite, except that iron carbides form with the ferrite in stage I. The optimum physical properties are achi stage I is completed and before stage II start actual casting, an overlap between stage I an may be unavoidable, however if stage I is not there will be volumes of unreactedretained aus the matrix. This austenite will most likely to martensite when the part is cooled to room ture. Relatively small amounts of martensite 2 %) may cause severe embrittlement in the cas this reason, it is important that stage I completed. eved when s. In an d stage II completed tenite in transform tempera - ( less than ting. For should be In an actual casting, significant solute segregation occurs. The segregation influences the local rate of reaction and the austenite decomposition is inhomogene- ous. Much of the overlap between stage I and stage II stems from the solute segregation in the casting. The aim of the present work is to investigate the effect of copper content and basic heat treatment para meters (i.e. austenitising temperature, austempering temperature and time) on the microstructure of different grades of spheroidal graphitic iron. For this purpose three different classes of ductile iron wicth chemical compositions as is given below have been used. VIII The casting were produced in the form of keel blocks in DÖKTAŞ A.Ş. (Orhangasi) and experiments were carried out in Sakarya, Department of Metallurgy. The hardness and microstructural properties of castings used in experiments are given below. Experimental specimens in pieces were cut and machined. the form of 15x10x5 mm Heat treatments of these specimens involved austenitisation at 850 "C and 900 "C for one hour. All specimens were heat treated under a cover of cast iron chips to minimise decarburisation. Austempering was accomplished at temperatures 250 "C, 300 °C, 350 °C and 400°C by quenching into salt bath and holding at select ed temperature for varying times from 10 to 100 minutes. After austempering, saples were folowed cool at room temperature. Retained austenite content of the austempered struc tures was measured using x-ray diffraction spectra obtained from a dif fractometer with Mo Ka radiation at 40 KV and 18 Ma. The angular 29 was from 30" 26 to 40° 26, which includes bec ferrite (211), and fee austenite (220), (311) peaks. Measurement of the integrated areas under the these peaks allowed estimates of the volume fraction of retained austenite X to be made. As a result of austempering treatments carried out on experimental materials, the average nodule count has decreased by 19 % in class A, by 16 % class B and C materials. The nodularity, however, were found to have iproved slightly. In class A material the maximum amount of retained austenite was fund to be about 49 %, after austenitiaing at 900 °C for 60 min. followed by austempering 350 °C for 50 minutes. In class B and C materials, this was 31% and 47 %, respectively, after 75 minutes of austempering under the same conditions. IX Maximum hardness values, for all three classes of materials have reached after 60 minutes of austenitising at 900°C and 10 minutes of austempering at 300*C. These values were 55 Re, 56 Re and 56 Re for classes A,B and C respectively. The amount of retained austenite has increased with increasing austempering temperature and reached a maximum at 350 "C. Above that temperature, the amount of retained austenite has sharply dicreased. However, at 300 C, a decrease in the amount of retained austenite has been observed (especially for class A and B). The retained austenite has increased with increasing austenitising temperature. Unfortunately, a corralation between the percentage of austenite retained in matrix and austempering time could not have been established. With increasing copper content amount of retained austenite observed to increase in especially lower bainitie structures. Moreover, hardness values increas ed with increasing Cu percentage in higher austenitising and austempering temperatures. As a result, whilst, some aspects of the problem have been clarified, some other aspects, particularly, the ones contadictining the previous views, remaines as topics for further investigations.