1. Surface Hardening Methods for Q355NH
Since Q355NH is a low-alloy structural steel, traditional bulk hardening (quenching & tempering) is not suitable. Instead, the following surface treatment techniques can be applied:
| Method | Process | Hardness Achievable | Applications |
|---|---|---|---|
| Flame Hardening | Heating the surface with an oxy-acetylene torch followed by rapid quenching | 45–55 HRC | Gears, shafts |
| Induction Hardening | High-frequency induction heating + quenching | 50–60 HRC | Bearing surfaces |
| Laser Hardening | Precision laser heating + self-quenching | 55–65 HRC | High-wear parts |
| Carburizing | Diffusing carbon into the surface at high temps (~900°C) | 58–63 HRC | Gears, pins |
| Nitriding | Nitrogen diffusion at 500–600°C (gas or plasma) | 800–1200 HV | High-wear, corrosion-resistant parts |
| Hardfacing (Welding) | Depositing a wear-resistant alloy (e.g., Stellite, WC) via welding | 50–65 HRC | Mining, crushers |
2. Recommended Process for Q355NH
(1) Induction Hardening (Best Balance of Hardness & Distortion Control)
Process:
Heat the surface to 850–950°C using an induction coil.
Quench with water, polymer, or oil (depends on required hardness).
Temper at 200–300°C to reduce brittleness.
Hardness: 50–60 HRC (case depth: 1–5 mm).
Advantages:
Fast, localized hardening.
Minimal distortion (compared to flame hardening).
(2) Nitriding (For Corrosion + Wear Resistance)
Process:
Heat to 500–600°C in a nitrogen-rich atmosphere (gas or plasma).
Forms a hard nitride layer (Fe₃N, Fe₄N) without quenching.
Hardness: 800–1200 HV (case depth: 0.1–0.5 mm).
Advantages:
No phase change → minimal distortion.
Improves fatigue & corrosion resistance.
(3) Laser Hardening (Precision Applications)
Process:
Laser beam scans the surface, heating it to austenitizing temperature (900–1000°C).
Self-quenching due to rapid heat dissipation.
Hardness: 55–65 HRC (case depth: 0.2–2 mm).
Advantages:
Ultra-precise, no quenching media needed.
Ideal for complex geometries.
3. Challenges & Considerations
(1) Retaining Weathering Properties
Flame/induction hardening may affect the rust patina formation.
Nitriding is preferred if corrosion resistance must be maintained.
(2) Pre- & Post-Treatment
Pre-Cleaning: Remove rust/mill scale before hardening.
Stress Relieving: Recommended after induction/flame hardening to reduce residual stresses.
(3) Weldability Concerns
If the part is welded post-hardening, HAZ softening may occur.
Solution: Perform hardening after welding.
4. Comparison of Methods
| Factor | Induction | Nitriding | Laser | Flame |
|---|---|---|---|---|
| Hardness (HRC) | 50–60 | 60–70 (HV) | 55–65 | 45–55 |
| Case Depth (mm) | 1–5 | 0.1–0.5 | 0.2–2 | 1–6 |
| Distortion | Low | Very Low | Minimal | Moderate |
| Cost | Medium | High | High | Low |
