Development of high Mo carbide free thick section nodular cast iron by partial melting homogenisation

• Published in: Materials science and technology Vol. 29; no. 11; pp. 1346 - 1353
• Main Authors: Rahgozar, N, Nili-Ahmadabadi, M, Forghani, F
• Format: Journal Article
• Language: English
• Published: London, England Taylor & Francis 01.11.2013; SAGE Publications; Taylor & Francis Ltd
• Subjects: Carbide characterisation; Carbides; Diffraction; Ductile iron; Hardenability; Heat treating; Heat treatment; High molybdenum ductile iron; Homogenizing; Iron; Materials science; Molybdenum; Nodular iron; Partial melting homogenisation; Precipitation hardening; Scanning electron microscopy; High molybdenum ductile iron; Partial melting homogenisation; Carbide characterisation
• ISSN: 0267-0836; 1743-2847
• DOI: 10.1179/1743284713Y.0000000279

Sufficient hardenability is required for thick section ductile irons which are the subject of heat treatments. Molybdenum is known as one the best hardenabilty promoter elements; however, it is not common to use it in an extensive content because it segregates severely and promotes the carbide precipitation in the intercellular regions. These carbides have a detrimental effect on the mechanical properties of casting such that keeping the Mo content to a minimum as much as possible is recommended. In this study, a 1·24 pct molybdenum ductile iron which provides sufficient hardenability for about 75 mm thick section iron was used to investigate the precipitated carbides in the intercellular regions and their annihilation by partial melting homogenisation (PMH) process to get a homogenous and carbide free structure. Heating for 9 h at 1090°C was the most successful procedure to annihilate all carbides. Carbides were then extracted and subjected to scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis, with the results showing that chemical composition of (Fe, Mo, Si) 3 C is of the same structure as that of Fe 3 C in which some iron atoms are replaced by Mo and Si. The optimum PMH treated ductile iron was austempered at 370°C for different periods after austenitising at 900°C for 2 h. The hardness tests and microstructural study by optical, scanning electron microscopy and EDS analysis revealed a carbide free and fully homogenised austempered ductile iron.