THE MICROSTRUCTURE AND PROPERTIES OF DIRECT QUENCHED MARTENSITE SUBJECTED TO BOTH ULTRA SLOW TEMPERING AND CONVENTIONAL TEMPERING CYCLES

摘要

The effect of different tempering methods on the microstructure and mechanical properties of laboratory scale direct quenched and tempered martensitic steels has been studied. Two types of laboratory tempering processes were applied: conventional furnace tempering and ultra slow tempering to investigate the effect of ultra slow and long tempering cycles. The effects of vanadium in the range 0.00-0.08% and boron in the range 0-15 ppm has been evaluated. Microstructural characterization has been performed with light optical microscopy (OM), laser scanning confocal microscopy (LSCM) and field emission scanning electron microscopy (FESEM). It was found that irrespective of the heat treatment details, final hardness was well predicted using a modified Hollomon-Jaffe tempering parameter. Vanadium is effective in retarding softening in long and demanding heat treatments. The absence of boron decreased the hardenability of the experimental steels to the extent that they did not achieve a fully martensitic structure. This led to lower hardness, but higher impact toughness, in both the direct quenched and tempered conditions.