3. CARBONITRING
• It is a modified form of gas carburizing.
• It is a case Hardening process which involves the diffusion of both
carbon and nitrogen into the base metal in the temperature
range700-900° C.
• Carbonitriding is carried out at a lower temperature and for a
shorter time than is gas carburizing.
• It produces a shallower case (0.075 to 0.75mm) than carburizing.
• Used for Plain Carbon Steel or Low Alloy Steel.
4. Process
• 2 - 12 % Ammonia is added to the carburizing atmosphere, which provides
a source of nitrogen.
• Process variables –
• Temperature - 700-900° C
• Gas Composition – Methane + Ammonia
• Duration
• Ammonia decomposes at the surface as :
2NH3 ↔ N2 + 3h2
• This molecular Nitrogen can not be Absorbed to the steel surface.
5. • It is only the portion that does not decompose – called residual ammonia –
that is the active component for nitriding expressed by the reaction.
NH3 (residual) → N + 3/2H2
• 0.5 to 0.8% carbon and 0.2-0.4% Nitrogen is added to the Steel surface.
• The carbon provides the base metal with a high carbon
surface, and the nitrogen provides the case with an added
boost of hardenability to insure full case hardness.
6. • Carbonitriding is used primarily to impart a hard, wear-resistant case.
• A carbonitrided case has better hardenability than a carburized case.
• Quenching is done after Heating to get hard martensitic structure.
• Sometimes Tempering is done to ensure ductility by eliminating internal
micro stresses and by somewhat reducing the hardness.
Carbonitriding cycle including cooling(quenching) and
Tempering Step.
7. Objectives Of Carbonitriding
• To obtain higher surface hardness
• To increase wear resistance & Fatigue Strength
• To obtain higher corrosion resistance
• To have good high temperature properties
8. ADVANTAGES
• It has a greater resistance to softening during Tempering.
• Increased Fatigue and impact strength.
• Better case hardenability in lower alloy or plain carbon steels.
• The carbonitrided case has better wear and temper resistance than a straight
carburized case.
• It is carried out at a lower temperature and for a shorter time than is gas
carburizing.
• Reduced distortion due to lower temperature.
9. Disadvantages
• It produces shallower cases.
• It is not possible to obtain higher core hardness and deeper case depths.
• Only useful for Plain carbon steel or Low alloy Steel.
• Ammonia can produce harmful effects.
10. APPLICATIONS
• Carbonitriding is used to treat Plain Carbon Steel or Low Alloy Steel.
• It is successfully applied to generally mass produced components, and
those of smaller dimensions, where great resistance to wear is required
and where the case depth requirements ranges from 0.1 to max 0.75 mm.
• Typical applications include:
• Gears and Shafts
• Pistons
• Rollers and Bearings
• levers in hydraulic, pneumatic and mechanical actuated systems.
• Tools and Dies
