WELDING OF HEAD HARDENED RAILS
Head Hardened Rails have a chemical composition similar to that of 900A grade rails (Tensile strength 90Kg/mm2). The head of the rails is heat treated to a finely lamellar pearlite structure resulting in high hardness (340-360 HB) and strength.
The Thermit portion we use for welding these head hardened rails is similar to the one used for the welding of 900A grade rails. This welding follows the same sequence used in the normal SKV technique up to trimming of the extra metal. Immediately thereafter, the rail head and heat affected zones are air cooled by passing air from the compressed air petrol system through a specially designed cooling head. Doing this for 8 minutes brings down the temperature of the rail head from over 1000°C to about 400°C.
This method of controlled cooling results in the hardness of the weld metal matching favorably with that of the parent rail.