Development and characterization of a wear resistant bainite/martensite ductile iron by combination of alloying and a controlled cooling heat-treatment

In this paper, a bainite/martensite (B/M) dual-phase ductile iron was fabricated by combining alloying and a controlled cooling heat-treatment. The microstructure, the mechanical properties and the wear performance were investigated and discussed. The ductile iron containing 3.2–3.8 wt.% carbon was alloyed with 2.5–3.0 wt.% manganese and 2.5–3.0 wt.% silicon. In general, manganese is no more than 0.7 wt.% and silicon <2.5 wt.% in commercial grade lower-bainite ductile irons. So, manganese contained in the ductile iron in this work is several times higher, and silicon slightly higher. In order to control the phase transition in the ductile iron during the heat-treatment, its continuous cooling transformation (CCT) curve was determined. The controlled cooling heat-treatment process was determined according to the CCT curve, which included three stages. The first stage was water quenching of the sample rapidly from the austenization temperature to a temperature below 350°C in a few minutes. The second stage was heat preservation of the sample from the spraying end temperature to 200°C in 2 h. The last stage was air cooling of the sample from 200°C to RT. According to the analysis using the scanning electron microscope (SEM) and the X-ray diffraction (XRD), the matrix of the ductile iron had a microstructure of bainite, martensite and a little retained austenite. The hardness and impact toughness of the heat-treated ductile iron were HRC 51.5 and 21.7 J/cm2, respectively. The high values of the hardness and toughness were attributed to (1) the refined structure, (2) the presence of B/M dual-phase and (3) the presence of retained austenite. The impact abrasive wear resistance of the B/M ductile iron was observed to be comparable with that of a high chrome cast iron, and twice that of Mn13 steel.