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Optimization of Alloying And Heat Treatment Process Parameters Of Austempered Ductile Iron (ADI) Castings

Diğer Başlık: Östemperlenmiş Küresel Grafitli Dökme Demirlerde Alaşımlama Ve Isıl İşlem Parametrelerinin Optimizasyonu

Oluşturulma Tarihi: 06-10-2020

Niteleme Bilgileri

Tür: Tez

Alt Tür: Yüksek Lisans Tezi

Yayınlanma Durumu: Yayınlanmamış

Dosya Biçimi: PDF

Dil: İngilizce

Konu(lar): TEKNOLOJİ,

Yazar(lar): Yalçın, Mustafa Alp (Yazar),

Emeği Geçen(ler): Davut , Kemal (Tez Danışmanı),


Yayın Tarihi: 11-10-2019


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Anahtar Kelimeler

Austempered ductile iron (ADI), nodularity, ausferrite, retained austenite, microstructure, mechanical properties, correlation 


Özet

Austempered ductile cast iron (ADI) has been extensively used in engineering designs since it offers a good combination of high tensile and fatigue strength, good ductility, toughness, wear resistance and damping characteristics, with lightweighting and low cost. This excellent combination of properties is due to the specific microstructure of ADI; which is composed of spheroidal graphite particles on an ausferritic matrix. The ausferrite consists of carbon free acicular ferrite and carbon enriched retained austenite; which is produced via austempering heat treatment after casting. Problems such as low nodularity level or lower stability of austenite due to inadequate acicular ferrite transformation certainly deteriorate this specific microstructure and hence degrade the mechanical properties of the final product. In the present study the effect of the alloying additions of Cu and Cu + Mo + Ni, heat treatment parameters and sizes and distributions nodular graphite particles on mechanical properties and microstructure of ADI was studied. For that purpose, Y-block specimens having a lean composition and Cu, Cu + high Mo + low Ni and Cu + low Mo + high Ni alloying additions were cast. After austempering treatment, mechanical tests, fractographic and metallographic examinations were performed. The results show that the alloying additions of Cu or Cu + Mo + Ni increased carbon content and stability of austenite in the final microstructure, which means completely homogeneous ausferritic structures can be produced on larger cross-sections without the presence of pearlite or martensite. The Cu + low Mo + high Ni alloyed specimen proves that since it shows the lowest formation of martensite and has higher strength and elongation than other castings. The lean alloy on the other hand, has the highest nodularity and matrix hardness but the lowest strength and ductility because of the inadequate alloying additions, austempering time and austempering temperature. Lastly, the relation between microstructural parameters and mechanical properties were studied using the Pearson Correlation Coefficient. The results indicate a Petchlike relation between grain size of austenite and yield strength, tensile strength, ductility. EBSd studies also revealed two different austenite: i) film type between acicular ferrite platelets and ii) block type around prior austenite grain (boundaries) that are not transformed during austempering. The grain size of acicular ferrite does not correlate well with the mechanical properties; since its variation among the studied samples is smaller than 0.7 m. For the studied set of samples the nodularity and size of graphite nodules are highly correlated; small differences in nodularity doesn‘t have significant effect on the mechanical properties. On the other hand smaller nodule size improves both yield and tensile strength values. In addition a Hall-Petch like relation is found between grain size of retained austenite and strength, ductility. Those results should provide a useful basis for further development and improvement of ―austempered ductile irons".



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