2007 Volume 47 Issue 12 Pages 1776-1780
By a process of combinations of microalloying with Ni, Mo and other elements, cast chilled by metal-mold and special heat-treatment, A martensite/bainite (M/B) dual-phase grey cast iron was fabricated. Quenching into oil after austenization at 900°C for 2 h, the hardness of the sample was above HRC 48. The microstructures of all samples were characterized by optical metallography or X-ray diffraction. The final microstructure was composed of M/B dual-phase and retained austenite. There were some “white-bright zone” composed of few high-carbon martensite plates and retained austenite, full of Mn and Mo elements, concentrating on the boundaries of eutectic cells. The nucleation site and mechanism of bainite were studied by short oil-quenching for 5 s after austenization at 860°C for 2 h. The result showed bainite first nucleated at interface between flake graphite and austenite, and austenite along the direction of flake graphite sharp corner was also tend to transform into bainite. The second site easy for bainite nucleation was the boundary of austenite, also the boundary of “white-bright zone”. Strengthening mechanism of M/B dual-phase to this grey cast iron was studied by using pressing-in method to press round head into the surface of corrosive M/B dual-phase grey cast iron. The initiation and extension of cracks were observed through SEM. The result showed that M/B dual-phase could effectively strengthen graphite–matrix interface and restrain cracks extension, tips of high-carbon martensites in “white-bright zone” were another site prone to initiate cracks under pressure. For lake of phase–phase interfaces, the cracks would extend straightly until to the whole interface of “white-bright zone” and matrix.