Ferritik ve perlitik KGDD'lerde östemperleme sürecinde grafit partiküllerinde değişimler

Abstract

Ferritik ve Perlitik KGDD'lerde 800°C, 850°C, 900°C, 950°C sıcaklık; 30 dk, 60 dk, 90 dk zaman aralıklarında östenitleme ve 250°Ğ, 300°C, 350°C, 400°C, 450°C sıcak lık aralıklarında İS dk» 30 dk, 45 dk, 60 dk, 90 dk, 120 dk östernperleme sonucunda grafit küre boyut ve sayısında meydana gelen değişiklikler yarı - kant i tat if bir yakla şımla incelenmiştir. Küre boyut ve sayısında değişimin östenitleme süre cinde başladığı ve östernperleme aşamasıyla devam ettiği gözlemlenmiştir. Değişimin, ferritik dökümlerde perlitik dökümlere göre daha yüksek oranlarda gerçekleştiği ölçülmüştür.

Austempering is an isothermal heat treatment pro cess involving two main stages as shown schematically in figure. The essential features of the austempering cycle are as explained below: Austenitisation at a temperature within the range 850 C - 950 C for a minimum period of 1 hour. This is usually carried out in a gas - or electrically- heated furnace, preferably with an inert or controlled atmosphere to minimize surface scaling and decarburisa- tion. Alternatively, a high temperature chrol i de-based salt bath or a fluidised bed may be used. Removal of the components from the furnace and their rapid quenching into a molten salt bath main tained at a temperature within the range 235 C - 450 C Alternative quenching media include hot oil which is limited to a maximum operating temperature of about 260 C, and fludisied beds, which have the disadvantage of providing a lower quenching rate. The rate of quen ching must be fast enough to prevent the formation of pear lite during cooling to the lower temperature. Where this can not be achieved in heavier section components, alloying additions of molybdenum, nickel or copper must be made to the iron. Air cooling or water quenching to room tempe rature VII 1000 Austenftize Quench IsothermaJly transform Air cool to ambient temperature 1.0 1.6 2.0 Time. h. Fig. : Austempering heat treatment cycle. For an iron of given composition, the austenitising temperature and time and the quenching rate to the aus tempering temperature have some influence on the resul ting mechanical properties but the latter are determin ed to a major extent by the austempering temperature and time. The austempering temperature is the more important of these latter variables. Its effects can be related to the bainitic matrix structure formed. At low austempering temperatures (approximately 335°- 330°C) lower bainite is formed. This consists' of an acicular or neddle - like fer r i tic phase containing very fine carbides, having a lower bainite matrix structure. (ii> At high austempering temperatures (approxima tely 370 - 450 O, upper bainite is formed and consists of mixture of a feathery, carbide-free fer rite inter dis persed with a high carbon retained austenite. At intermediate austempering temperatures (approx imately 330°C - 370°C) it is often difficult to make a clear demarcation between two forms of bainite since the morphology and coarseness of the fer r i tic platelets change gradually with change of austempering temperature. The transformation of austenite to the upper bainite structure of ferrite in austenite in ductile iron general ly takes place in two stage. VIII The first stage deals with nucleation and growth of fer rite platelets at austenite grain boundries or near graphite nodules. Growth of these fer r i tic plate lets, which appear to occur in microscopic bunches or colonies, continues until impingement takes place with other plates and rejection of carbon into austenite re duced the driving forces for further growth.structure occurs. This is the structure which is desired for optimum properties. The second stage (stage II) is the time period which slow fer rite plate thickening occurs together with the nucleation and growth of the iron carbide at the expense of the austenite. Transformation to lower bainite is similar to that of upper bainite, except that iron carbides form with the ferrite in stage I. In an actual casting, an overlap between stage I and stage II may be unavoidable, however if stage I is not completed there will be unreacted retained auste nite in the matrix. This austenite will most likely transform to martensite when the part is cooled to room temperature. Relatively small amounts of martensite may cause seve embrittlement in the casting. Because of this, stage I should be completed. Casting conditions can cause si gni fici ant segrega tion in the structure. Segregation influences the local rate of reaction and austenite decomposition occurs i nhomogenousl y. The aim of the present work is to research the potential changes of the particle size and distribution of spheroidal graphite in ductile iron during the whole austempering process < austeni ti si ng + austempering > releated to the heat treatment process variables and matrix structure. For this purpose, two different classes of ductile iron castings have been used. Castings have been prepared in form of the Keel blocks Compositions and grades of the castings are listed on the next page. IX GGG-40 : Fully Ferritic matrix GGG-80 : Fully Pearlitic Matrix Keel blocks were cut into pieces and these pieces machined to get experimental samples. After that, two different heat treatment route was applied to the samples for differentiating between the effects of austenitising and aus tempering. In former path, these samples were held different combinations of temperature <800°C - 850°C) and time <30 min. - 90 min. > ranges following rapid quenching into water. Latter case involved to austenitising of samples at a constant temperature and time <850°C - 90 min. > level and than quenching them into a molten salt bath maintained for different isothermal transformation temperature (250°C - 450°O and time C 15 min. - 120 min. > ranges. Applied experimental procedure was to measure the effect of the heat treatment process variables onto the predetermined particle size and distribution of sphero idal graphite, in terms of nodule count and nodule size. For this purpose, a spatial particle size and distribution method was used through a computerized image analyzer. Since the limitations of the particle sizing of the method and image analysing technique resulted in considerably high error in critisizing of the experia- mental data, just qualitative approch could be applied to the results. It was the way that taken account to discuss the results. It is well known that aus tempering is a two stage heat treatment process and austempering follows auste nitising. Therefore, a started size increasing process of the nodules in austenitising stage is expected to continue during the austempering stage related to the predetermined nodule size and a further increase latterly. However, experimental values have given controversial results depending upon error that was considered before. According to the results, increasing the heat treatment temperature has given increased nodule size and reduced nodule count (X). Castings that have ferritic matrix structure at the beginning of the heat treatment process have shown higher rate of change than pearl i tic castings, in terms of nodule count and nodule size. However, there is no information in the literaure confirming this result.