Nitriding and Flame Hardening

Nitriding:
In nitriding process, steel is heated in a furnace to a temperature of 4900-5300C ammonia is circulated. Ammonia decomposes to give active nitrogen which diffuses into surface of the steel part to form nitrides. The iron nitride is very brittle, but the nitrides of aluminum, chromium and molybdenum are very hard. For nitriding, ordinary steels cannot be used; special nitriding steels containing aluminum, chromium and molybdenum are used. Nitriding is a very slow process taking anywhere between 50 to 100 hours.
Advantages of nitriding of steels.
1. It gives very hard surface.
2. There is minimum distortion and it gives surfaces free from quenching cracks.
3. Nitrided steel has higher resistance to fatigue.

Flame Hardening:
In flame hardening, the part is heated to a temperature above the upper critical point bye a flame ( usually oxy-acetylene torch) followed by quenching with a jet of water. Rotary parts to be flame-hardened are rotated in front of a flame at slow speed until the entire surface reaches uniform suitable hardening temperature. Then the flame is removed and the part is quenched by a jet of water spray or dropping the part in a quenching bath. If the part is large. It may be rotated very slowly under closely mounted torches so that the part reaches the hardening temperature. This is followed by quenching with water spray. In this case, the part is flame-hardened in one rotation of the part. This process is applied to gears, spindles, cams, etc.