Nitriding:-
It is the process of producing a hard surface layer in alloy steels, which contains strong nitride forming elements. Since Nitriding is carried out in the ferritic region below 590— C, and hence there is no phase transformation. The parts are to be subjected to proper heat treatment prior to Nitriding. All machining and grinding operations are finished before Nitriding. There are different methods of Nitriding.
i) Gas Nitriding:-
In this process anhydrous ammonia gas is passed in to the furnace at about 550— C. The gas dissociates to from nascent nitrogen and hydrogen.
2NH3 →► 2[N] Fe + 3H2
The treatment time varies from 50 to 100 hours depending on the case depth required. The steel part is allowed to cool in the furnace itself after the treatment in the presence of ammonia. The furnace container is made of heat resisting alloy steel. The hard layer achieved by this process imparts good wear resistance, hot hardness and corrosion resistance. During this process a very brittle white layer may be formed on the surface due to high nitrogen potential. Keeping the nitrogen potential at a desired level can eliminate this.
ii) Plasma Nitriding:-
Plasma Nitriding is also known as ion Nitriding. In this process the steel component is kept at 450— C, in vacuum at a negative potential in the order of 1kv with respect to the chamber. Then a mixture of nitrogen and hydrogen is passed at a pressure of
0.2 – 8m bar .As a result plasma formation of these gases take place. Time required for achieving a case depth of 4microns is about 20 hours at 450—C. The formation of the white later can be avoided since a close control of nitrogen is possible.
COMMENTS