Nitriding Surface Treatment: Process & Technical Specifications

Nitriding is a thermochemical surface treatment process where nitrogen atoms diffuse into a workpiece’s surface layer under controlled temperature and medium conditions. Primary methods include liquid nitriding, gas nitriding, and plasma (ion) nitriding.

Core Process Fundamentals

1. Gas Nitriding Mechanism
   Workpieces are sealed in a retort with flowing ammonia gas (NH₃). At 500-590°C, ammonia decomposes into active nitrogen atoms that adsorb onto and diffuse into the surface, modifying its chemical composition and microstructure to enhance surface properties.

2. Nitrocarburizing Variant
   When carbon is simultaneously introduced to accelerate nitrogen diffusion, the process becomes nitrocarburizing (also known as ferritic nitrocarburizing).

Pre-Treatment Requirements

1. Heat Treatment Conditioning

• Quenching & Tempering:
  Must achieve tempered sorbite microstructure.
  Critical Parameter: Tempering temperature > Subsequent nitriding temperature.

2. Stress Relief Protocol

• Annealing temperature < Prior tempering temperature

• Extended dwell time (exceeding standard tempering duration)

• Controlled slow cooling to room temperature

• Note:

  • Avoid normalizing for large cross-sections

  • Tool steels require quenching/tempering (annealing prohibited)

Workpiece Preparation Standards

Geometric Limitations

• ✘ Sharp edges/corners (risk of embrittlement)

• ✘ Unmachined surfaces (grinding mandatory)

• ✘ Partial-coverage methods using machining allowances

Process Comparison Table

Technical Footnotes

1. Salt bath nitriding typically uses cyanide-containing compounds (e.g., NaCN) – require eco-friendly disposal

2. Post-nitriding surface porosity may necessitate honing/polishing for precision components

3. Case depth follows Fick’s diffusion law: Depth ∝ √(Time × Temperature)

Compliance Note: AMS 2759/10, ASTM E18, and ISO 9001:2015 standards govern critical aerospace/automotive applications.