Simulation of Solidification Behavior of Nihard-4 White Cast Iron and Its Metallurgical Caracterization

Abstract

NiHard-4 white cast iron is a material mostly used in workpieces subject to load, solid particle erosion and wear in the agricultural and mining industries. Metallurgical definition of the solidification structure, monitoring microsegregation in solidification and identification of critical temperatures for all phase transformations contribute to the determination of heat treatment conditions, which have an important role in optimizing toughness and wear resistance of cast iron. In this study, a thermodynamic modeling is carried out using the CALPHAD methodology on the composition of NiHard-4 white cast iron. Thus, solidification sequence, phase transformations and elemental microsegregations in cast iron are calculated. Calculations reveal that (i) austenite and Cr-rich M7C3 eutectic carbides are present in solidification structure, (ii) solidification begins with the crystallization of the austenite phase from the liquid and is completed after eutectic solidification, and (iii) C, Cr, Mn and Fe elements tend to have positive segregation, whereas Si and Ni elements tend to have negative segregation. Microstructural and thermal characterization studies on cast iron produced as Y block in a sand mold confirm the solidification behavior of the alloy, critical phase transformation temperatures and segregation tendency of the elements according to the model.

Keywords

• NiHard-4 white cast iron
• CALPHAD
• Thermo-Calc
• solidification
• characterization.

NiHARD-4 BEYAZ DÖKME DEMİRİN KATILAŞMA DAVRANIŞININ SİMÜLASYONU VE METALURJİK KARAKTERİZASYONU

Abstract

NiHard-4 beyaz dökme demir tarım ve madencilik endüstrilerinde yüke, katı partikül erozyonuna ve aşınmaya maruz kalan iş parçalarında çoğunlukla kullanılan bir malzemedir. Katılaşma yapısının metalurjik olarak tanımlanması, katılaşmada mikrosegregasyonun izlenmesi ve tüm faz dönüşümlerine ait kritik sıcaklıkların tanımlanması dökme demirin tokluğunun ve aşınma direncinin optimizasyonunda önemli bir rolü olan ısıl işlem koşullarının belirlenmesine katkı sağlar. Bu çalışmada, NiHard-4 beyaz dökme demir kompozisyonu üzerine Thermo-Calc hesaplamaları kullanılarak termodinamik bir modelleme yapılmıştır. Böylece dökme demirde katılaşma sıralaması, faz dönüşümleri ve elementel mikrosegregasyonlar hesaplanmıştır. Hesaplamalar; (i) dökme demirin katılaşma yapısında östenit ve Cr-ca zengin M7C3 ötektik karbürlerin var olduğunu, (ii) katılaşmanın sıvıdan östenit fazının kristallenmesi ile başladığını ve ötektik katılaşma sonrası tamamlandığını ve (iii) C, Cr, Mn ve Fe elementlerinin pozitif segregasyona karşılık Si ve Ni elementlerinin negatif segregasyona eğilimi olduğunu göstermiştir. Y blok olarak kum kalıba dökümü gerçekleştirilen dökme demir üzerine yapılan mikroyapısal ve termal karakterizasyon çalışmaları alaşımın katılaşma davranışını, kritik faz dönüşüm sıcaklıklarını ve elementlerin segregasyon eğilimini modele göre doğrulamıştır.

Keywords

• NiHard-4 beyaz dökme demir
• CALPHAD
• Thermo-Calc
• katılaşma
• karakterizasyon.

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