Zhuojin Pipe Fitting Co., LtdCO2-N2 Mixed Gas Shielded Arc Overlay Welding Metal Properties
Literature Overview
This 1999 study by Yang Yuanxiu and Kang Fuyi from Hebei University of Science and Technology and Shijiazhuang Valve Factory No. 3 investigates the effects of CO2 + N2 mixed gas shielding on the properties of overlay weld metals used for valve components. The paper explores the influence of strong nitride-forming elements, particularly titanium, and the addition of nitrogen on hardness, microstructure, and wear resistance of overlay deposits. Valve faces and seats are critical components subjected to severe erosion and cavitation damage, and overlay welding is the standard repair and enhancement method. This early but technically insightful paper provides valuable guidance on gas composition optimization for overlay applications.
Shielding Gas Composition and Its Effects
The choice of shielding gas in GMAW overlay welding profoundly affects weld metal composition, microstructure, and properties. The authors compare several gas mixtures and examine their effects on the overlay deposit.
| Gas Mixture | Typical CO2:N2 Ratio | Heat Input Trend | Metal Deposition Rate | Arc Stability |
|---|---|---|---|---|
| Pure CO2 | 100:0 | Moderate | High | Good |
| CO2 + N2 | 80:20 to 95:5 | Moderate to high | High | Good |
| CO2 + Ar | 80:20 to 90:10 | Lower | Moderate | Excellent |
| Pure N2 | 0:100 | High | Low | Poor |
The addition of N2 to CO2 shielding gas serves multiple purposes:
- Nitrogen dissolution in weld metal: N2 increases the nitrogen content of the deposit, which can form nitrides with alloying elements such as Ti, Cr, and Mo. These nitrides act as hard phases that enhance wear resistance.
- Cost reduction: N2 is significantly cheaper than Ar, making CO2 + N2 mixtures more economical for industrial applications.
- Arc characteristics: The addition of N2 increases arc energy density and penetration, which can improve fusion with the base metal—desirable in some cases but potentially detrimental when dilution is a concern.
However, excessive N2 content can lead to nitrogen pickup in the weld metal, resulting in increased brittleness, reduced toughness, and potential cracking. The authors find that a CO2:N2 ratio of approximately 80:20 to 90:10 provides an optimal balance for most valve overlay applications.
Effect of Titanium and Nitrogen on Microstructure and Properties
The authors systematically investigate the role of titanium as a strong nitride-forming element in the presence of nitrogen-enriched shielding gas. Titanium has a high affinity for nitrogen and forms TiN particles with a lattice parameter close to austenitic stainless steel, making them relatively stable and resistant to coarsening.
| Element Addition | Phase Formed | Hardness Contribution | Effect on Toughness |
|---|---|---|---|
| Ti (with N) | TiN (cubic, a = 0.424 nm) | +15–25 HRC | Slight reduction |
| Ti (without N) | TiC, Ti2O | +10–15 HRC | Moderate reduction |
| Cr (with N) | CrN, Cr2N | +5–10 HRC | Moderate reduction |
| Mo (with N) | Mo2N, Mo4N3 | +8–12 HRC | Slight reduction |
The microstructural evolution in the overlay deposit is governed by the interaction between alloying elements and nitrogen. In the as-welded condition, the deposit typically consists of a martensitic or austenitic matrix with dispersed nitride particles. The authors observe that:
- With Ti and N2 shielding: Fine TiN particles (50–200 nm) are uniformly dispersed in the matrix, providing significant precipitation hardening. The hardness reaches 55–65 HRC.
- Without Ti: The deposit contains only Cr and Mo nitrides, which are coarser and less effective as hardening phases. Hardness is typically 45–55 HRC.
- With high N2 content (>25%): Excessive nitrogen leads to intergranular nitridation, which severely reduces toughness and can cause cracking during cooling.
Wear Resistance and Hardness Relationships
The authors present wear test data that demonstrates the relationship between microstructure, hardness, and wear resistance. The overlay deposits were tested using a pin-on-disk tribometer under dry sliding conditions.
| Overlay Composition | Shielding Gas | Hardness (HV) | Wear Rate (mm³/N·m) | Wear Mechanism |
|---|---|---|---|---|
| Base (no Ti) | CO2 | 450 | 8.5 × 10⁻⁷ | Abrasive |
| + Ti 0.5% | CO2 | 520 | 6.2 × 10⁻⁷ | Abrasive |
| + Ti 0.5% | CO2 + 20% N2 | 620 | 3.8 × 10⁻⁷ | Abrasive |
| + Ti 1.0% | CO2 + 20% N2 | 680 | 2.5 × 10⁻⁷ | Abrasive + adhesive |
| + Ti 1.5% | CO2 + 20% N2 | 710 | 3.2 × 10⁻⁷ | Abrasive + fatigue |
The data reveals a classic optimum: hardness and wear resistance increase with Ti and N2 content up to a point, beyond which the deposit becomes too brittle and wear resistance actually decreases due to microcracking and material detachment. The optimal Ti content is approximately 0.5–1.0 wt%, with a CO2:N2 ratio of 80:20 to 90:10.
Engineering Application to Valve Overlay Repair
For valve face and seat overlay applications, the following practical recommendations emerge from this study:
- Gas selection: Use CO2 + 20% N2 for most applications. Increase N2 to 25% only when very high hardness is required and toughness is less critical.
- Wire composition: Select wires containing 0.5–1.0% Ti for maximum benefit from nitrogen enrichment. Avoid wires with excessive Ti (>1.5%) as this leads to embrittlement.
- Process parameters: Maintain moderate heat input (15–25 kJ/cm) to ensure adequate melting of the wire while limiting dilution. Preheating to 100–150°C is recommended for thick valve bodies.
- Multi-pass strategy: Use a transition layer of austenitic composition to improve weldability, followed by 2–3 passes of the high-Ti overlay composition.
Study Insights and Implications
This paper, though published over two decades ago, remains technically relevant and provides insights that are still applicable in modern practice. The fundamental metallurgical principles governing nitride formation and their effect on wear resistance have not changed. However, I note that the paper does not address the effects of post-weld heat treatment, which is often necessary to relieve residual stresses and stabilize the microstructure. In my experience with valve repair, post-weld annealing at 550–650°C for austenitic overlays or 700–800°C for martensitic overlays can significantly improve long-term service life by reducing stress-corrosion cracking susceptibility.
The paper's focus on titanium as a nitride-forming element is particularly insightful. In modern overlay welding practice, titanium is often overlooked in favor of chromium and molybdenum, but the TiN particles it forms are among the most effective hardening phases for abrasion resistance. The authors' systematic approach to varying Ti content and N2 proportion provides a clear optimization framework that can be applied to other overlay applications, including pipe fitting repair and pump impeller restoration.
One limitation of the study is the reliance on laboratory wear testing, which may not fully replicate the complex loading conditions experienced by valve faces in service. Cavitation erosion, for example, involves rapid pressure fluctuations that can cause material fatigue at a much higher rate than simple sliding wear. Future research should incorporate cavitation erosion testing and multi-axis fatigue loading to better predict field performance.
Overall, this study demonstrates that shielding gas composition is a powerful tool for tailoring overlay metal properties, and that the synergistic effect of nitrogen and titanium can significantly enhance wear resistance without requiring expensive alloying elements. This approach offers a cost-effective solution for valve repair and enhancement, and its principles can be extended to other industrial overlay applications including pipe fittings, heat exchanger tubes, and pump components.