Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Nitrogen-Carbon Alloyed Self-Shielded Hardfacing Flux-Cored Wire Overlay
Literature Overview
This paper by Li Yanna and colleagues from Huazhong University of Science and Technology (published in Materials in Mechanical Engineering, Vol. 31, No. 7, 2007, pp. 38–41) reports on the development and characterization of a nitrogen-carbon alloyed self-shielded hardfacing flux-cored wire. The research investigates the effect of tempering temperature on overlay hardness and wear resistance, and elucidates the wear mechanisms of the resulting overlay metal.
Core Technical Findings
The developed wire produces overlay layers with hardness ranging from 36.5 to 45.5 HRC, depending on tempering condition. The microstructure consists of lenticular (plate-like) martensite combined with chromium, titanium, vanadium, and niobium carbonitrides. A critical finding is that these carbonitride particles exhibit exceptional thermal stability and do not decompose readily during tempering, providing excellent high-temperature temper resistance.
Tempering Temperature Effects
| Tempering Temperature (°C) | Hardness (HRC) | Notes |
|---|---|---|
| As-welded (no tempering) | Baseline | Maximum carbonitride stability |
| 480–520 | Optimal range | Best wear resistance |
| 550 | Maximum hardness | Hardness peak |
| >550 | Stable, no further change | Carbonitrides resist decomposition |
Microstructural Features
- Lenticular martensite provides the base hardness and retains good toughness.
- Cr, Ti, V, and Nb carbonitrides (M23C6, TiC, VC, NbC) are thermodynamically stable and resist coarsening.
- The multi-element carbonitride system creates a synergistic effect: Ti and Nb stabilize the carbonitrides at high temperatures, while Cr and V contribute to solid solution strengthening.
Wear Mechanism Analysis
The wear behavior was characterized as a combination of abrasive cutting followed by plastic deformation marks and block detachment of carbonitride particles. This mixed wear mechanism indicates that the hard carbonitride particles resist cutting, but under severe sliding conditions, they may fracture and detach as blocks. The optimal tempering temperature range of 480–520°C represents a balance between matrix toughness (which prevents crack propagation) and carbonitride stability (which resists wear).
Process and Standards Considerations
For hardfacing applications in the piping and equipment industry — such as valve seats, pump impellers, and erosion-resistant overlays on pipe components — this type of self-shielded flux-cored wire offers practical advantages:
- Self-shielding eliminates the need for external shielding gas, making field application feasible.
- The carbonitride stability means the overlay maintains its properties even after subsequent heat treatment or service at elevated temperatures.
- The hardness range of 36.5–45.5 HRC is appropriate for moderate to severe abrasion without being so hard as to be brittle.
Comparison with Conventional Hardfacing Consumables
| Feature | N-C Alloyed Wire | Conventional Cr-C Hardfacing |
|---|---|---|
| Shielding | Self-shielded | Requires external gas |
| Hardness range | 36.5–45.5 HRC | Typically 50–60 HRC |
| High-temperature stability | Excellent (carbonitrides stable) | Moderate (carbides decompose) |
| Toughness | Good (martensite + stable particles) | Poor (very hard, brittle) |
| Field applicability | High | Limited |
Engineering Practice Integration
In my experience with hardfacing applications on piping components, the challenge is often not achieving high hardness but maintaining it under thermal cycling. The nitrogen-carbon alloyed approach addresses this directly. For applications such as:
- Hardfacing of ball valve seats where thermal cycling from process temperature changes occurs.
- Overlay repair of pump impellers that may be exposed to elevated temperatures.
- Surface protection of pipe components in hot oil or hot gas service.
The stability of the carbonitride phase above 550°C is particularly valuable. In contrast, conventional high-carbon Cr hardfacing alloys often suffer from significant hardness loss after tempering above 500°C due to carbide decomposition and matrix softening.
Study Insights and Implications
The research demonstrates a rational approach to hardfacing consumable development: rather than maximizing hardness through high carbon and chromium content, the strategy of incorporating nitrogen along with multiple carbide-forming elements (Ti, V, Nb) creates a more thermally stable and balanced overlay. The optimal tempering window of 480–520°C provides a practical post-weld treatment guideline for field applications.
One reflection that emerges from this study is the importance of matching the tempering condition to the service environment. If the component will operate at temperatures approaching 500°C, tempering at 550°C ensures the overlay has already been stabilized. If the service temperature is lower, tempering at 480–520°C provides the best combination of hardness and wear resistance. This type of service-condition-aware post-weld treatment is often overlooked in practice but can significantly extend component life.