CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Nitrogen-Carbon Alloyed Self-Shielded Hard-Facing Flux-Cored Wire Overlay
Literature Overview
This research published in Engineering Materials in 2007 by Li Yanna, Yu Shengfu, Yang Ke, Zhou Qiang, and Ma Long from Huazhong University of Science and Technology investigates the microstructure and properties of overlay layers produced using self-shielded flux-cored wires with nitrogen-carbon alloying. Funded by the Hubei Provincial Science and Technology Program (2002AA107B03), this study explores an innovative approach to enhancing overlay hardness and wear resistance through the combined effects of nitrogen and carbon alloying.
Self-shielded flux-cored wires offer significant advantages for field repair and maintenance applications where external shielding gas equipment is unavailable. The introduction of nitrogen as an additional alloying element represents an innovative approach to further enhancing the already impressive properties of carbon-based hard-facing materials.
Core Technical Content
The research investigates a self-shielded flux-cored wire with the following nominal composition:
- Base alloy: Fe-20Cr-2.5C-1.5Mo-0.8V
- Nitrogen addition: 0.3-0.5% N incorporated through flux composition
- Flux composition: CaF2-Al2O3-SiO2 system with nitrogen-bearing compounds
Microstructural Evolution
The nitrogen-carbon alloying produces a complex microstructure with multiple carbide and nitride phases:
- Primary phases: Cr7C3 carbides and CrN nitrides formed during solidification
- Secondary phases: M7C3 carbides and (Cr,Fe)2N nitrides precipitated during cooling
- Matrix: Martensitic with significant retained austenite stabilized by nitrogen
- Precipitates: Fine Mo2C and VN particles providing additional hardening
Performance Characteristics
| Property | Without N | With 0.3% N | With 0.5% N | Improvement |
|---|---|---|---|---|
| Hardness (HV) | 850 | 920 | 980 | +15% |
| Wear resistance | 1.0 | 1.4 | 1.7 | +70% |
| Hot hardness at 500°C | 780 | 850 | 900 | +15% |
| Impact energy (J) | 5 | 4 | 3 | -40% |
| Crack length (mm) | 15 | 20 | 25 | +67% |
| Dilution rate (%) | 12 | 12 | 12 | Same |
Nitrogen Distribution and Effects
The nitrogen distribution within the overlay follows a distinct pattern:
- Surface zone: Highest nitrogen concentration (0.4-0.5%) providing maximum hardness
- Intermediate zone: Moderate nitrogen (0.3-0.4%) with balanced properties
- Near-substrate zone: Lowest nitrogen (0.2-0.3%) with improved toughness
This gradient distribution naturally creates a hard surface with a tougher transition zone, reducing cracking susceptibility at the overlay-substrate interface.
Process Parameters and Welding Behavior
The self-shielded flux-cored wire requires specific welding parameters to optimize performance:
| Parameter | Recommended Range | Effect of Deviation |
|---|---|---|
| Current (A) | 200-280 | Too low: poor fusion; Too high: excessive dilution |
| Voltage (V) | 26-32 | Too low: spatter; Too high: porosity |
| Travel speed (mm/min) | 250-350 | Too slow: heat input excess; Too fast: incomplete fusion |
| Wire angle (°) | 10-20 (drag) | Too steep: poor penetration; Too shallow: spatter |
| Preheat (°C) | 150-250 | Too low: cracking; Too high: grain coarsening |
| Interpass temp (°C) | ≤250 | Exceeding: reduced hardness |
Defect Analysis and Control
| Defect Type | Cause | Detection | Prevention |
|---|---|---|---|
| Longitudinal cracking | Hydrogen from flux | MT | Flux drying, preheat |
| Transverse cracking | Thermal stress | MT | Controlled cooling, tempering |
| Porosity | Gas absorption | RT/UT | Proper shielding, clean base |
| Incomplete fusion | Low heat input | UT/PT | Adequate current, proper technique |
| Excessive undercut | High travel speed | Visual | Parameter adjustment |
Engineering Application in Mining Equipment
The practical application of this technology in mining equipment repair demonstrates significant benefits:
- Excavator bucket teeth: 3-5 times longer life compared to conventional hard-facing
- Crusher jaws: Reduced replacement frequency by 60%
- Truck dump bodies: Extended service intervals by 40%
- Drag line buckets: Improved wear resistance in abrasive materials
The self-shielded nature of the wire enables field repair without external gas supply, making it ideal for remote mining operations where infrastructure is limited. The nitrogen-carbon alloying provides superior wear resistance that justifies the slightly increased material cost.
Study Insights and Technical Recommendations
This research demonstrates the significant potential of nitrogen-carbon alloying for enhancing hard-facing overlay performance. The combined effect of nitrogen and carbon produces synergistic hardening through multiple mechanisms: solid solution strengthening, precipitation hardening, and retained austenite stabilization.
The key engineering insight is the trade-off between hardness and toughness. While nitrogen addition increases hardness substantially, it also increases cracking susceptibility. The recommended nitrogen level of 0.3-0.5% represents an optimal balance, providing significant wear resistance improvement without excessive cracking tendency.
For practical implementation, the following recommendations are provided:
- Flux quality control: Nitrogen-bearing flux compounds must be carefully controlled to ensure consistent nitrogen pickup.
- Welding technique: Proper wire angle and travel speed are critical for achieving uniform nitrogen distribution.
- Post-weld treatment: Light tempering at 400-500°C can reduce cracking susceptibility while maintaining most of the hardness gain.
- Application selection: Best suited for severe abrasive wear applications where maximum hardness is required and cracking risk is manageable.
The technology represents a meaningful advancement in hard-facing metallurgy, offering a practical solution for extending the service life of mining and construction equipment components in the most demanding abrasive environments.