Interrelationships Between Thermal History and Mechanical Properties of a Secondary Hardening Steel.

摘要

Commercial secondary hardening steel, VASCO MA,has been subjected to modified heat treatments to introduce mixed microstructures of martensite and lower bainite. Dilatometry,tensile testing,hardness measurements and slow-bend testing have been carried out and the mechanical properties obtained have been correlated to microstructure using scanning electron microscopy and metallography. Duplex microstructures containing small amounts of lower bainite are found to show an increased toughness (at similar strength levels) on tempering in the secondary hardening range,as compared to initially fully martensitic microstructures. As the percentage of lower bainite in the duplex microstructures is increased,the secondary hardening peak is found to attenuate. The effect of different austenitization treatments on the secondary hardening behavior,has also been studied. Thermal cycling (for grain refinement) at lower austenitization temperatures is found to reprecipitate out carbon,which has been taken into solution in earlier high temperature austenitization,thereby leading to a significant depletion in the strength of this secondary hardening steel. Alloy carbides,in the secondary hardening stage,have also been analyzed using energy dispersive analysis of x-rays. (ERA citation 02:030228)