The Mechanical Properties of a low Alloyed Austempered Ductile Iron in the Upper Ausferrite Region

摘要

Microstructural observation, X-ray diffraction testing and measurements of mechanical properties were used to study the characteristics of austempering and the influence of tempered martensite on mechanical properties as a function of austempering time at 400 deg C after austenitising at 900 deg C for a 0.77/100Cu-0.5/100Ni ductile iron. The austempering times were derived from a resistivity curve measured by a vacuum heat treatment system. The experimental results show that the resistivity change curve could be used effectively for selecting the isothermal holding times of austempering treatment, with all the mechanical properties of ADI austempering at times corresponding to the processing window defined by the electrical resistance measurement satisfying the standard requirement. The formation of martensite in austempering reduce the mechanical properties of ADI but these properties could be increased by a treatment tempering at 200 deg C after cooling and to obtain more ductility and toughness as compared with that undergoing single heat treatment. This increase also extended the effective austempering time interval of ADI over which the ASTM standards were satisfied.