The first stage consists of drastic water cooling treatment to the bars as they leave the last finished stands. The efficiency to the water cooler equipment used at this stage has to be as high as to produce a very hard cooling.
Tempering is the second stage of the bar leaves the water quenching line and is exposed to air. The heat flux form the still hot core reheats the quenched by the conduction and the martensite formed in the first stage is thus, subjected to self-tempering giving structure called "Tempered martensite" which is strong and tough. The Core is still authentic at the stage.
The Third stage of "Atmospheric Cooling" occurs on the cooling bed, where the austenitic core is transformed to a ductile ferrite pearlite core. Thus the final structure consists of the combination of strong outer layer of tempered martensite and a ductile core of ferrite-pearlite