GGG 40-80 sınıfı küresel grafitli dökme demirlerde östemperleme ısıl işleminin darbe direnci ve darbe geçiş sıcaklığına etkisinin incelenmesi

Abstract

1693 normuna uygun, GGG 40-80 sınıfı standart Küresel Grafitli Dökme Demir (KGDD) keel bloklardan hazırlanan deney numuneleri, 900 °C'de 100 dakika östenitleme işlemini takiben, 250 °C, 300 °C, 350 °C ve 400 °C sıcaklıklarda 7-210 dakika zaman aralığında östemperleme işlemine tabi tutulmuş ve östemperlenmiş Küresel Grafitli Dökme Demir (ÖKGDD) elde edilmiştir. Daha sonra (- 60 °C) - (+ 100 °C) sıcaklık aralığında çentiksiz Charpy darbe testi uygulanmıştır. Böylelikle, bir yandan oda sıcaklığı kırılma davranışlarını, diğer taraftan da sünek-gevrek davranış biçimlerini gözlemlemek mümkün olmuştur. Oda sıcaklığı darbe enerjisi dökümlerin sınıfina bağlıdır; GGG-40 sınıfi dökümlerde maksimum darbe enerjisi 13.5 Kg-m olarak ölçülmekte ve bu değere 350 °C sıcaklığında 80 dakikalık bir işlem ile ulaşılabilinmektedir. GGG-50 sınıfı dökümlerde ise, maksimum darbe enerjisine yine 350 °C deki östemperleme sıcaklığında ulaşılırken, 11.6 Kg-m darbe enerjisi 30 dakikada elde edilmiştir. GGG- 60, GGG-70 ve GGG-80 sınıfi dökümlerde de maksimum darbe enerjisi 300 °C deki östemperleme şartında elde edilmekte ve maksimum değerlere de GGG-60 sınıfi dökümlerde 150 dakika (14 Kg-m), GGG-70 sınıfi dökümlerde 150 dakika (13.5 Kg- m) ve GGG-80 sınıfi dökümlerde 120 dakikada (12 Kg-m) ulaşılmaktadır. KGDD' lerde, 330 °C sıcaklığının altındaki östemperleme çalışmalarında alt beynit yapısı elde edilmekte, 330 °C sıcaklığının üzerinde ise, önce karbür içermeyen ferrit çıtaları ve yüksek karbonlu östenit ile karakterize olunan sünek ve yüksek tokluğa sahip ausferritik yapı ve daha sonra II. reaksiyonun oluşumu ile kırılgan üst beynitik yapı meydana gelmektedir. Böylelikle ausferritik fazın üst beynit oluşumu sırasında bir geçiş fazı olduğu kanısına varılmıştır. 250 °C ve 400 °C östemperleme sıcaklıklarında hazırlanmış olan numunelerde oda sıcaklığının altında sünek -gevrek davranış biçimi görülmemekle birlikte, oda sıcaklığının üzerinde 250 °C deki östemperleme işleminde artış meydana gelmekte ve bu artışa yapıdaki martensit fazının temperlenmesinin sebep olduğuna inanılmaktadır. 400 °C de ise herhangibir değişim meydana gelmemekte ve buna sebep olarak da bu sıcaklıkta karbür oluşumunun getirdiği zararın martensit temperlenmesinin getirdiği yarardan daha yüksek olduğu düşünülmektedir. 300°C ve 350 °C de östemperlenmiş malzemelerin darbe enerjileri ise test sıcaklığındaki azalma ile sürekli bir düşüş sergilemektedir. Oda sıcaklığının üzerindeki artışa temperlenmiş martensit fazının sebep olduğu, Oda sıcaklığının altındaki düşüşlere ise, matriks yapıdaki var olan arda kalmış östenitin, martensite dönüşümünden kaynaklandığına inanılmaktadır.

Austempered ductile iron (ADI) has emerged as a new engineering material in the past ten years, with the properties comparable, or in some cases superior to those of forged steel, and its use for automobile components offers many advantages such as higher tensile strength, better machinability, and significantly lower manufacturing cost. In the austempering process, the first step is austenitisation in the temperature range 850-900 °C to change the as cast matrix structure into austenite with a carbon content that can be controlled by the choice of austenitising temperature as well as the iron composition. The next step is the rapid cooling to and holding at the austempering temperature in the range 250-400 °C for a period of time between I and 3 hours. Although the application of this process is now well in use, the process is still under development. There are significant differences and conflicts among the findings of the numerous researchers; the main problem with the process technology stems from the lack of understanding of kinetics during heat treatment and,in particular, the parameters which control the kinetics. The aim of the present work is to investigate the effect of ductile-brittle transition temperature and the basic heat treatment parameters on the microstructure of different grades of spheroidal graphitic iron. For this purpose five different, standard classes of ductile iron have been used. The chemical composition of these materials is given table 1. Table 1: Chemical Compositions of Castings VI The castings were produced from these materials in the form of keel blocks in DÖKTAŞ AŞ. (Orhangazi) and experiments were carried out in the Metallurgy Department of Sakarya University. Metallographic and statistical analyses carried out on the castings gave the characteristics of experimental materials as shown table 2.. Table 2: Characteristics of experimental materials Experimental specimens were cut and machined from the keel blocks to the dimensions of 55x10x10 mm, and then austenitised at 900 °C for 100 min. Austenisation of the specimens were carried out under a cover of cast iron chips to minimize decarburisation. Austempering was performed at temperatures 250 °C, 300 °C, 350 °C and 400 °C by rapid quenching the specimens from the austenisation temperature into salt bath pre-heated to a selected austempering temperature, for varying times from 7 to 210 minutes. After austempering, the samples were allowed to cool in air down to the room temperature. In ductile irons, at austempering temperatures below about 330 °C bainite starts to form by growth of ferrite plates into austenitic matrix, and at these temperatures carbon diffusion is so slow that carbon can not be easily rejected from the ferrite and precipitates as carbides in the ferrite. Resulting microstructure from such transformation is a mixture of ferrite and carbide and called lower bainite. It is observed that the appearance of this structure closely resembles to that formed in steels. At higher temperatures between 330-400 °C the transformation mechanism of the austenite during the austempering is different in that as the ferrite lathes grow into austenitic matrix, carbon is rejected easily from the ferrite due to the higher diffusion rates. As a result, as the ferrite laths thicken the carbon content of the austenite increases to form a microstructure consisting of ferrite and austenite with high carbon content. This structure is frequently termed as upper bainite and characterized as.a + yhc. However this structure is different than upper bainite identified in steels in which upper bainite is a mixture of ferrite and carbide. In ductile iron, however, high Si content prevents carbide formation in the early stage of transformation. It is, therefore, thought as Kovacs stated that in the first stage of the transformation during the austempering of ductile iron, the structure developed is better named as ausferrite rather than the upper bainite. vn The austempering of cast iron is a two stage process depending on austempering time as shown in Fig. 1. In the first stage of transformation, the amount of high carbon austenite increases until a time of tj, and start to decrease in stage II after a time of t2 is reached when the high carbon austenite starts to form more stable ferrite and carbide phases. This behavior exposes a well defined process window between the austempering times of t2 and ti. Between this time interval, the ideal austempering microstructure is obtained and iron exhibits optimum mechanical properties since the presence of martensite in stage I and carbides in stage II is detrimental to the mechanical properties particularly ductility and tougness. Time t2 Fig. 1 :Schematic plot of the per cent high carbon austenite against austempering time for homogenous matrix It is a well known fact that the alloying elements in ductile iron do not spread out homogeneously and give rise to a severe segregation in the microstructure. Of these, elements, Si, Cu and Ni concentrate mainly in the matrix around the graphite spheres causing the carbon concentration to decrease in these areas. The others, carbide forming elements such as Mn, Mo, Cr and V concentrate along the cell boundaries and increase the carbon concentration in these regions. Therefore, there are a considerable composition differences between the matrix next to the graphite spheres and the matrix in the cell boundaries. Bainitic ferrite nucleates in the regions around the graphite spheres which have a lower carbon concentration and grows towards the cell boundaries which have a higher carbon concentration. As a result, the stage II reaction of the austempering transformation is reached in the earlier times of the transformation in the regions around the graphite spheres while the stage I reaction is still in progress in the cell boundary locations with high carbon concentration. This may cause the two stages of the transformation to overlap and the process window to shrink(Fig. 2a) or vanish completly (Fig. 2b) vin Time a) Time b) Fig. 2 Schematic plot of the per cent high carbon austenite against austempering time for non-homogenous matrix The ideal austempering conditions to produce the optimum mechanical properties should be selected after the stage I reaction is completed and before the stage II reaction initiates. In figure I, which depicts an ideal situation, it is possible to obtain an ideal austempering condition to produce a full austempered matrix with the highest carbon content in the process window. However often measurements of the volume fraction of high carbon austenite as a function of austempering time show a short plateau as shown in Fig. 2a or even a maximum below the plateau value as shown in Fig. 2b. Austempering times selected in the plateaus of the Fig. 1 and Fig. 2a produce a structure in a full ausferritic character, whereas austempering time corresponding to the maximum high carbon austenite in Fig. 2b produce a structure which is a mixture of ausferrite and untransformed austenite. The latter product transforms the martensite upon cooling to the room temperature after the austempering process. In this case, tougness and ductility of the material depend on the amount, and the distribution of the martensite phase in the structure. The maximum below the plateau value depends on the severity of segregation of the alloying elements. A homogenous distribution of the alloying elements in the matrix causes the maximum below the plateau to shift towards the plateau value and therefore, the amount of martensite to decrease in the structure. Vice-versa, the increased degree of segregation lowers the maximum point and, therefore increases the amount of martensite in the structure. In these cases, there is no a direct correlation between the time to produce maximum high carbon austenite and that to produce maximum ductility and toughness. Maximum values in these properties are reached at austempering times after the time that yields the maximum high carbon austenite. The evidences to support this view can be found in the quantitative work of Darwish and Elliott in which it was shown that the maximum values in ductility and impact strength were obtained at the longer austempering times than that yielded the maximum high carbon austenite. It is, therefore, thought that the amount of martensite in the structure has more detrimental effect than the products resulted from the stage II reaction on ductility and toughness. rx The results obtained from the present work are summarized as follows: 1. The impact energy values obtained from the experiments were found to depend on the austempering time and temperature 2. The highest impact strength values for castings of Dl and D2 were obtained at the austempering temperature of 350 °C, and for those D3, D4 and D5 at that of 300 °C. The reason for this must be due to the fact that the alloying element, Cu present in the composition of the castings of D3, D4 and D5 lowers the Aj temperature. 3. The lowest impact energy values were obtained at the austempering temperature of 400 °C, but these were higher than those of the castings with full pearlitic structure. 4. The reason for the lowest impact energy values obtained at 400 °C were thought to be due to the higher degree of the segregation of the alloying elements. 5. The impact energy-austempering time relations obtained from the experiments established the ideal austempering conditions for maximum toughness and ductility possible to obtain from the ductile irons, but do not accurately demonstrate their austempering behavior. 6. The experimental samples austemperised at 250 °C and 400 °C did not show any ductile-brittle transition behaviour when tested at temperatures below the room temperature. 7. The ductile-brittle transition behavior of the lower and upper bainitic structures obtained at 250 °C and 400 °C were found to be similar to that observed in steels 8. The impact energies of the materials austempered at 300 °C and 350 °C showed a gradual decrease with decreasing test temperature.