THE KINETIC OF PRECIPITATION REACTIONS IN A RAPIDLY AGED HIGH ALLOY HIGH STRENGTH STEEL

This study was launched to investigate the effects ofheating rate and aging parameterson the kinetic ofprecipitation reactions in a high alloy high strength steel having ni, co, mo andti. for this purpose, as quenched specimens were subjected to three types ofaging methods withdifferent heating rates. these methods consisted of aging in pb bath, salt bath, and furnace atdifferent aging cycles. the kinetic of precipitation in each method was studied by hardnessmeasurements and was described adequately by the johnson-mehl-avrami equation. remarkableincrease in hardness and its rate is observed when the rate ofheating increases. the substantialincrease in hardness ofthe specimens aged rapidly in salt & pb baths. compared with those agednormally in furnace, seemed to be due to the formation of thermo elastic stresses during suddenexpansion of the substance subjected to rapid heating. according to the results obtained in thisresearch, increase in the avrami constants, n & k, and decrease in the start time oftransformation. ts, are associated with heating rate increasing. analysis of the observed andcalculated data for hardness using arrhenius equation, shows that for the same amount ofvolumefraction ofprecipitates, the activation energy ofprecipitates decreased for f=25 and 50 %, whileatf=90 % it increased by increasing heating rate.