Zhuojin Pipe Fitting Co., LtdCurrent Status of Nitrogen Alloying Wear-Resistant Surfacing Research
Literature Overview
The review paper by Liu Yue, Zhang Guoshang, and Wei Shizhong (Welding Technology, Vol. 41, No. 12, 2012, pp. 1–4) provides a comprehensive overview of nitrogen alloying in wear-resistant surfacing materials. Funded by the Henan Provincial Science and Technology Program (Project No. 112102213117), this work synthesizes research from multiple institutions and identifies key trends in nitrogen-containing overlay weld metals. The paper covers nitrogen content evolution during welding, carbonitride precipitation behavior, strengthening mechanisms, and the effects of nitrogen on hardness, high-temperature wear resistance, corrosion resistance, and tensile strength.
Nitrogen Behavior During Surfacing
Nitrogen is a highly reactive element that exhibits complex behavior during the welding process:
| Process Stage | Nitrogen Behavior | Impact on Final Composition |
|---|---|---|
| Arc formation | N₂ dissociation, N atom injection into molten pool | Initial nitrogen pickup |
| Molten pool | N dissolution in liquid metal, reaction with C, Cr, Mo | Nitrogen content fluctuation |
| Solidification | N rejection to interdendritic regions | Segregation at grain boundaries |
| Post-solidification | Carbonitride precipitation (Cr₇C₃, Mo₂C, CrN, Mo₅C₃) | Microstructure refinement |
The nitrogen content in the final overlay deposit is typically 0.3–1.5 wt%, depending on the shielding atmosphere, wire composition, and process parameters. Under open-air or CO₂-shielded conditions, significant nitrogen pickup occurs (0.5–1.0 wt%), while argon-shielded processes with nitrogen-containing wire provide controlled nitrogen levels.
Strengthening Mechanisms
The paper identifies four primary strengthening contributions from nitrogen alloying:
- Solid solution strengthening — Nitrogen atoms in solution in the austenite or ferrite matrix create lattice distortion that impedes dislocation motion. The strengthening increment follows the relationship Δσ ∝ C_N^(2/3), where C_N is the nitrogen concentration in at%.
- Carbonitride precipitation strengthening — Nitrogen promotes the formation of fine, coherent carbonitride particles (Cr₇(C,N)₃, Mo₂(C,N), Cr₅(C,N)₃) that are finer and more uniformly distributed than pure carbides. These particles provide Orowan strengthening and may inhibit dislocation cross-slip in austenitic matrices.
- Grain refinement — Nitrogen increases the nucleation rate during solidification by reducing the critical nucleation radius, resulting in finer grain structures with higher Hall-Petch strengthening contribution.
- Phase transformation control — In martensitic systems, nitrogen suppresses the formation of retained austenite and promotes tempered martensite, providing a more stable and wear-resistant microstructure.
Property Relationships
| Property | Effect of Nitrogen Addition | Optimal Range |
|---|---|---|
| Hardness (HV) | Increases significantly | 0.5–1.2 wt% N |
| High-temperature wear resistance | Improves at 400–600 °C | 0.3–0.8 wt% N |
| Corrosion resistance (general) | May improve or degrade depending on matrix | < 0.5 wt% N preferred |
| Tensile strength | Modest increase | 0.3–1.0 wt% N |
| Ductility | Decreases with increasing N | < 0.8 wt% N |
| Hot cracking susceptibility | Increases with N content | Must be managed |
Engineering Applications and Outlook
Nitrogen alloying is particularly valuable in applications where conventional carbide-based surfacing materials reach their performance limits:
- High-temperature wear environments (400–800 °C) where carbonitrides maintain stability better than pure carbides.
- Combined wear-corrosion environments where the nitrogen-strengthened matrix provides better crack resistance than brittle carbide networks.
- Thermal spray and HVOF coatings where nitrogen is used as a process gas and simultaneously alloying element.
The review identifies several open challenges: precise control of nitrogen content in arc welding processes, understanding of nitrogen-induced hot cracking in thick overlay sections, and long-term thermal stability of carbonitride microstructures under cyclic loading. The authors correctly identify nitrogen alloying as a high-priority research direction with broad industrial potential, particularly for power generation, mining, and oil/gas equipment where overlay welds must withstand severe combined damage mechanisms.