Effect of cooling rate of steel ( 25 Cr Mo4 ) on its hardness and microstructure


In this work , structural low – alloyed steel (25 Cr Mo4 ) was melted in mediumfrequency coreless induction furnace of (60) kgs . capacity , lined with highalumina lining powder .Samples of disc shaped (Ø25- Ø30) mm and about (10) mm thick ,for spectro –analyzing and cylindrical parts (Ø10×100) mm, were poured to manufacturedilatometric specimens by machining to the required standard dimensions of (Ø5.5×50) mm. Testing of (12) specimens by electronic dilatometer type ( 402) ,using different cooling rates , enable getting results of hardness andmicrostructures Tests showed that ferritic structure with slow cooling i.e.( 0.1 ,0.2 and 0.5) ˚C /min .was obtained –(hipoeutectoid). While pearlite started with(1.0) ˚C /min .Cooling rate , and full pearlitic appeared by (10.0) ˚c /min .Bainitic structure , started with ( 20.0) ˚C /min .cooling rate together with pearlite ,reaching full bainitic by cooling with (100) ˚C /min.By oil guenching (i.e about 120 ˚C / sec .) and by water guenching ( i.e about 400˚C/ sec.) [ 8] martensitic structure was achieved. Those transformations in thestructure was accomplished with increasment of Vickers hardness values from (175) HV with ( 0.1˚C /min ) cooling rate to ( 579 ) HV with water guenching . Thoseresults enable the choosing of suitable cooling rate ( cooling medium) to obtain therequired hardness and microstructure of this steel