In this work , structural low – alloyed steel (25 Cr Mo4 ) was melted in medium
frequency coreless induction furnace of (60) kgs . capacity , lined with high
alumina 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 manufacture
dilatometric 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 and
microstructures 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 the
structure was accomplished with increasment of Vickers hardness values from (175
) HV with ( 0.1˚C /min ) cooling rate to ( 579 ) HV with water guenching . Those
results enable the choosing of suitable cooling rate ( cooling medium) to obtain the
required hardness and microstructure of this steel