EFFECT OF QUENCH RATE ON MICROSTRUCTURE AND TENSILE PROPERTIES OF AISI 4320 AND 4340 STEELS.

The steels were quenched from various temperatures from 1323 to 1473 K at two different quench rates using iced brine (fast quench treatment) and oil held at 373 K (slow quench treatment). Tensile properties after double-tempering at 473 K with intermediate quenching and refrigeration were determined at ambient temperature (293 K). The microstructural changes accompanying these quench rates were examined by optical and thin-foil transmission electron microscopic techniques. It is suggested that the beneficial effect on strength of the slowly-quenched steels is caused by a dispersion-hardening effect due to carbon segregation or fine carbide precipitation in the martensite during the quench (i. e. , autotempering).