Zhuojin Pipe Fitting Co., LtdCO2-N2 Mixed Gas Shielded Arc Overlay Welding Metal Properties
Literature Overview
The study by Yang Yuanxiu and Kang Fuyi, published in Valve (1999, No. 1, pp. 10-12), investigates the metallurgical and mechanical properties of overlay weld deposits produced using CO2+N2 mixed gas shielding. The research focuses on the influence of titanium (a strong nitride-forming element) and nitrogen content on the hardness, microstructure, and wear resistance of overlay welds applied to valve components.
Core Technical Content
Process Configuration
| Parameter | Specification |
|---|---|
| Shielding gas composition | CO2 + N2 (variable N2 fraction) |
| Welding process | Gas metal arc welding (GMAW) |
| Electrode wire | Low-alloy steel with Ti addition |
| Base material | Carbon steel valve body |
| Welding current | 180-250 A |
| Arc voltage | 22-28 V |
| Travel speed | 300-500 mm/min |
The introduction of nitrogen into the CO2 shielding atmosphere serves a dual purpose: it acts as a diluent to moderate the thermal effect of CO2 and introduces nitrogen atoms into the weld pool, promoting nitride precipitation. The presence of titanium creates strong TiN precipitates with a melting point of 2950°C, providing exceptional hardness and wear resistance.
Microstructural Evolution
The overlay weld microstructure is governed by three competing factors:
- Carbon activity: CO2 dissociation provides carbon atoms that promote cementite (Fe3C) and alloy carbide formation
- Nitrogen solubility: N2 dissolution in the molten pool increases with temperature and decreases with cooling rate
- TiN precipitation: Titanium preferentially combines with nitrogen to form hexagonal TiN particles
The resulting microstructure typically consists of:
- Martensitic matrix (hardened by carbon and nitrogen in solid solution)
- Dispersed TiN particles (200-800 nm diameter)
- Secondary carbides (M7C3, M23C6)
- Possible retained austenite at high carbon levels
Property-Composition Relationships
| N2 Content (vol%) | Hardness (HV) | Wear Rate (mg/1000 r) | TiN Volume Fraction (%) |
|---|---|---|---|
| 0 (pure CO2) | 480-520 | 85-95 | 0 |
| 10 | 580-620 | 55-65 | 8-12 |
| 20 | 650-700 | 35-45 | 15-20 |
| 30 | 680-720 | 28-38 | 18-24 |
| 40 | 660-700 | 32-42 | 15-20 |
The data reveals an optimal N2 content of approximately 20-30 vol%, beyond which excessive nitrogen causes porosity formation and brittleness increase. The hardness plateau at 30% N2 indicates that TiN precipitation reaches saturation, while further N2 addition primarily contributes to gas porosity rather than strengthening.
Process Optimization and Defect Control
Critical Parameters for Quality
- N2 fraction control: The N2/CO2 ratio must be precisely controlled. Excess N2 (>35%) leads to nitrogen porosity, while insufficient N2 (<10%) does not provide adequate TiN precipitation.
- Weld pool stability: N2 addition increases arc force and spatter compared to pure CO2. The arc becomes more constricted, requiring careful travel speed adjustment.
- Heat input sensitivity: The overlay deposit is highly sensitive to cooling rate. Rapid cooling (high travel speed) increases martensite fraction but may cause cracking due to high residual stresses.
Common Defects and Solutions
| Defect | Cause | Solution |
|---|---|---|
| Gas porosity | Excessive N2 dissolution | Reduce N2 fraction; increase wire feed speed |
| Cracking | High carbon + nitrogen brittleness | Reduce carbon content; add Mn, Ni |
| Excessive spatter | High arc energy with N2 | Reduce current; optimize nozzle distance |
| Incomplete fusion | Low heat input | Increase current or reduce travel speed |
Engineering Application Insights
The study demonstrates that nitrogen-enhanced overlay welding offers a viable pathway for valve seat and trim repair, where the combination of hardness and corrosion resistance is critical. The TiN-reinforced overlay deposits achieve hardness values comparable to cast iron while maintaining the toughness of a steel matrix.
From a practical standpoint, the CO2+N2 mixture provides a cost-effective alternative to argon-based shielding gases, which is particularly important for large-scale industrial applications where gas consumption is substantial. The use of commercially available low-alloy wires with titanium addition eliminates the need for specialized electrode development.
The key engineering lesson is that nitrogen, traditionally considered a detrimental element in steel welds, can be deliberately introduced to create beneficial precipitation strengthening when alloyed with appropriate nitride formers. This principle has broader implications for overlay welding applications in mining, power generation, and chemical processing equipment.