Carbonitriding is a thermochemical heat treatment process in which carbon (C) and nitrogen (N) are simultaneously diffused into the surface of steel while it is in the austenitic state.
It is a modified form of carburizing (carbon diffusion), with the addition of a nitrogen source (commonly from ammonia gas – NH₃) within the treatment atmosphere.

➡️ Purpose: to form a thin, hardened surface layer (0.07–1.25 mm) with high hardness and excellent wear resistance, while maintaining a tough and ductile core.

Basic Process

• Treatment temperature: typically in the range of 750–880 °C (lower than Carburizing: 900–950 °C).

• Diffusion atmosphere: a carbon-rich gas mixture (CO, CH₄, or modified natural gas) combined with NH₃ as the nitrogen source.

• Diffusion mechanism:

  • Carbon diffuses into the austenite lattice → surface enrichment with carbon.

  • Nitrogen dissociates from NH₃ → atomic nitrogen diffuses simultaneously, partly dissolving in austenite, and partly forming nitrides/carbonitrides (Fe₃(C,N)).

• Quenching: usually oil, water, or high-pressure gas quenching → transforms the C+N enriched austenite surface into supersaturated hard martensite.

Characteristics & Applications

• Case characteristics:

  • Shallower case depth compared to carburizing (typically 0.1–1.25 mm).

  • High surface hardness (55–62 HRC), with improved tempering resistance.

  • Nitrogen addition enhances wear resistance, fatigue strength, and reduces distortion during quenching.

• Advantages:

  • Shorter processing time compared to carburizing.

  • Minimal dimensional distortion.

  • Effective for low-carbon and low-alloy steels.

  • Lower cost compared to deep carburizing.

• Limitations:

  • Thin case layer, not suitable for components requiring deep wear resistance.

  • More difficult to control case uniformity in complex-shaped parts.

• Typical applications:

  • Small gears, shafts, pins, and bolts subjected to fatigue loading.

  • Automotive and motorcycle components (camshafts, piston pins, gears, shafts).

  • Small mechanical tools requiring wear resistance and high fatigue strength.

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