Development and Characterization of a New Type of Ductile Iron with a Novel Multi-phase Microstructure

Abstract

Over the last few years two new types of Ductile Iron (DI) have been developed based on Austempered DI (ADI), called Dual Phase ADI and Carbidic ADI (CADI), respectively. In the light of the experience acquired when studying these DI variants, the authors found motivating the possibility of obtaining a new type of multi-phase DI (MPDI), combining their main microstructural characteristics. A DI melt alloyed with 2%Cr, exhibiting free carbides and a pearlitic matrix in its as-cast condition, was used. The upper (UCT) and lower (LCT) critical temperatures of the Fe-C-Si equilibrium diagram region, where ferrite (α), austenite (γ) and graphite (Gr) coexist (called "intercritical interval"), were determined by heat treatment and microstructural analysis on samples previously annealed. Carbides stability was studied, and its dissolution was found to be negligible. It was possible to obtain free or allotriomorphic ferrite and austenite by isothermal austenitizing at temperatures within the intercritical interval, and then transform the remaining austenite into ausferrite after an austempering step. Therefore, multi-phase microstructures composed of graphite nodules, free carbides, free ferrite and ausferrite were obtained. The possibility of obtaining MPDI directly from the as-cast structure was also analyzed, and it was found that very similar microstructures to those previously annealed can be obtained by austenitizing samples at the same temperature range. These results proved that the pearlite → austenite transformation kinetics is rapid, as it took only one hour for the transformation to occur.

Current Status of Carbidic Austempered Ductile Iron

“…In recent years, on the basis of austempering ductile iron (ADI), by adding a small amount of strong carbide forming elements (such as Cr, Ti, V, Nb, etc.) and developed with carbide (CADI), isothermal quenching nodular cast iron has good casting performance, small density, wear-resisting corrosion resistance and the lower cost of production advantages, used as mine wet grinding medium and small ball mill grinding balls, It shows advantages of less abrasion, low crushing rate, power saving and low noise [2][3][4] .However, compared with forged (rolled) steel balls, CADI grinding balls still have low strength and toughness. When used in large wet grinding ball mills in mines, CADI grinding balls have high crushing rate, resulting in reduced grinding efficiency.…”

Improved corrosive wear resistance of carbidic austempered ductile iron by addition of Cu

“…hardness, impact toughness, tensile strength, and wear resistance) of carbidic austempered ductile iron (CADI) have been significantly improved after several works regarding processing design and composition control [11][12][13][14][15]. CADI is widely applied in the engineering field [16]. After Ti refinement, the maximum diameter of graphite nodules decreased from 58 to 40 μm, and the shape of carbides transformed from linked network to dispersed.…”

The influence of chill thickness and austempering temperature on dry sliding wear behaviour of a Cu-Ni carbidic austempered ductile iron (CADI)

“…A new type of ADI which contains carbides immersed in the typical matrix of ductile iron called carbidic austempered ductile iron (CADI) has been developed in recent years. CADI is produced by austempering heat treatment of ductile iron which has a controlled amount of carbides present in the microstructure for a higher abrasion resistance than that of the base material [18][19][20]. The standard practice in carbide introduction is chill-cast of ductile iron parts.…”