Research On The Microstructure And Mechanical Properties Of Austempered High Boron White Cast Iron

White cast iron is a kind of widely used metal wear-resistant materials, and it hasexperienced a process from conventional plain white cast iron, nickel-hard white castiron, high chromium white cast iron. These white cast irons all contain eutectic carbidesas the hard phase, and performance well in abrasive conditions. The austempered highboron white cast iron is a new kind of wear-resistant metal material. It take eutecticborides substitute for eutectic carbides as the hard phase. Its chemical composition isthat of the high silicon cast steel with proper boron element. In the as-cast condition, themicrostructure of austempered high boron white cast iron is comprised of dentriticmatrix and interdentritic eutectic borides of crystal structure of M2B, while M presentsFe、Cr、Mn、Mo. And the matrix is made up of pealite and some ferrite around theeutectic borides in the as-cast state. After the austempering treatment, the matrix istransformed to ausferritic (bainite and retained austenite) structure, and differentausferritic microstructure can be obtained by applying different austemperingtemperatures. The borides changes little during the austempering heat treatment.Because the matrix is supersaturated of boron, secondary particles of boride precipitateduiring the austenitizing course and the austempering course.The hardness of austempered high boron cast iron decrease as the austemperingtempering temperature increases, from HRC55-58of250°C to HRC45-48of400°C.The eutectic borides affect the hardness little for the boron content of the testedspecimen was not much enough. The hardness is decided by the matrix. The impacttoughness is about3.5-6.5J/cm~2, and the impact toughness decrease as the boroncontent increase. That is because the borides are brittle and the continuous eutecticboride deteriorate the continuity of the matrix. The fracture is intergranular fracture,brittle fracture and dimple are observed by scanning electron microscope. The weightloss of high boron white cast iron austempered at250℃is less or close to that of highchromium white cast iron in high stress sliding friction tests, performing a goodwear-resistant ability. The impact toughness can be improved to7-12J/cm~2by hightemperature treatment. The austempered high boron white cast iron is a wear-resistantmaterial with bright application prospect.In order to break the continous network type borides and thus improve the impact toughness,3.42wt.%,6.20wt.%, and8.79wt.%chromium was added to the white castirons. Martensite was obtained when the chromium is8.79%in the as-cast condition.Comparing with the austempering high boron white cast iron with lower chromiumcontent, the matrix of the irons after high content of chromium added is made up ofmartensite, bainite and retained austenite after austempering heat treatment.