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

Abstract

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)3C is of the same structure as that of Fe3C 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.

Influence of alloy element distributions on austempered ductile irons

“…Taking into account the mentioned relation between f αp and f γf , the volume fractions of ferrite platelets (f αp ) and austenite films (f γf ) are computed as f αp = 0.89286f sh (12) f γf = 0.10714f sh (13) Carbon distribution: After the growth of a carbonsupersaturated ferrite platelet within a sheaf, part of the ferrite carbon is incorporated in the austenite films (placed within the sheaf) and the remaining part is instantaneously rejected to the austenite blocks (placed next to the sheaf). Then, the received carbon is diffused in the austenite blocks.…”

“…Moreover, the increment of graphite nodule count increases the interface area between nodules and matrix, which increases the transformation rate [3]. Other technique that has been employed to reduce the microsegregations of Mo and Mn, in parts with thick sections, is the partial melting homogenisation [12, 13].…”