The Effect of Niobium Addition in Welding

1. Niobium Serves as an Alloying Element to Improve Welding Performance in Stainless Steel.

--Let’s learn about the Effect.

In certain austenitic stainless steels, such as 347 stainless steel, niobium is added as an alloying element to enhance resistance to intergranular corrosion. This is particularly important during welding.

This is because niobium forms niobium carbide (NbC) when it interacts with carbon, preventing the carbon from sequestering chromium and forming chromium carbides (Cr23C6). Chromium carbides are prone to causing intergranular corrosion, which can severely compromise the material's integrity.

By forming niobium carbide, niobium effectively prevents this process, thus improving the corrosion resistance of the weld and the heat-affected zone (HAZ).

Thus, niobium-alloyed stainless steels ideal for use in high-performance environments, such as heat exchangers, pressure vessels, and components exposed to high temperatures and aggressive chemical conditions.

--Such effect has been proven by several experiments.

Several studies have confirmed the beneficial effects of niobium in welding applications. For instance, the role of niobium in the heat-affected zone (HAZ) has been analyzed using Electron Backscatter Diffraction (EBSD) to study the grain structure in welded joints. These studies show that niobium addition leads to grain refinement in the HAZ, reducing the extent of the coarse-grained heat-affected zone (CGHAZ). Finer grains in the HAZ result in improved microstructure and toughness, making the welded material more resistant to failure under stress.

Fig. 1 Comparing the Weld of Stainless Steel with Different Niobium Addition (The niobium addition of the upper one is lower than the other.)

Source:
“The State of the Art of Long Distance Gas Pipeline in China.” Chengjia Shang - IGRC - Rio 2017

Also, research data from projects like the CBMM collaboration in Moscow indicate that steels with higher niobium content exhibit greater tolerance to increased heat input during welding. This flexibility enhances the material's ability to withstand welding thermal cycles and ensures the reliability of the welded joint.

2. Niobium Alloy and Stainless Steel Dissimilar Metal Welding Technology

--Importance and Challenges of Dissimilar Metal Welding

Niobium alloys and stainless steel are often welded together in industries where both high-performance properties are required. The combination of niobium's exceptional high-temperature performance, corrosion resistance, and superconductivity, with stainless steel’s strength and cost-effectiveness, makes this dissimilar metal welding particularly attractive. However, welding these two materials presents several challenges due to their differences in physical, chemical, and metallurgical properties.

• Thermal Property Differences: The coefficient of thermal expansion for niobium alloys (7.3×10⁻⁶/°C) differs significantly from that of stainless steel (17.3×10⁻⁶/°C), leading to residual stresses.
• Metallurgical Incompatibility: There is a risk of forming brittle intermetallic compounds, such as Fe₂Nb and Cr₂Nb.
• Oxidation Sensitivity: Niobium is highly susceptible to oxidation at elevated temperatures, requiring strict protective measures.
• Weld Dilution Control: Controlling the composition in the molten pool is challenging.

--Welding Methods Comparison and Selection

--Traditional Welding Methods

When welding niobium alloys to stainless steel, the selection of the appropriate welding method is crucial. Here is a list of traditional welding methods.

--Advanced Welding Technologies

Advanced welding techniques, such as Friction Welding (FW), Diffusion Welding (DFW), and Explosive Welding (EXW), can also be used to address the unique challenges of welding niobium alloys to stainless steel.

1. Friction Welding (FW)

• Solid-state joining avoids welding defects.
• Particularly suitable for pipe joints.
• Requires strict control of parameters (speed, pressure, time).

2. Diffusion Welding (DFW)

• Using intermediate layers (Ti, Cu, Ni) to improve bonding.
• Requires strict control of temperature (800-950°C) and pressure.
• Joint strength can reach 90% of base material strength.

3. Explosive Welding (EXW)

• Suitable for large-area plate composites.
• The bonding interface has a wavy shape, mechanically interlocked.
• Requires subsequent heat treatment to relieve residual stress.

--Key Process Control Points

1.  Intermediate Layer Material Selection

The selection of intermediate materials plays a key role in improving the weld quality and mitigating material mismatch between niobium alloys and stainless steel. Intermediate layers such as pure nickel, copper-based composites, and vanadium/titanium transition layers help alleviate residual stresses and prevent the formation of brittle phases.

For instance, a pure nickel intermediate layer effectively suppresses the formation of Fe-Nb brittle phases, while copper-based layers can relieve thermal stresses and provide good electrical conductivity. The use of vanadium or titanium transition layers offers excellent compatibility with both niobium and stainless steel, but requires careful control of diffusion temperatures.

2.  Protective Atmosphere Control

Controlling the welding environment is essential when welding niobium alloys, which are highly sensitive to oxidation at elevated temperatures. The use of inert gases such as argon or helium with a purity of at least 99.999% is recommended. Oxygen content must be carefully controlled to below 10ppm, and dual-gas protection systems should be used to ensure the integrity of the weld. For vacuum welding, pressure must be kept below 5×10⁻³Pa to prevent oxidation.

Conclusion

The welding technology for niobium alloys and stainless steel has made significant progress. The addition of niobium to stainless steel significantly improves welding performance by enhancing corrosion resistance, refining grain structure, and increasing toughness. Hope that you can have a better understanding of the effect of niobium addition in welding. For more information, please check Stanford Advanced Materials (SAM).

Reference:

[1] CHEN Guoqing, GAN Zhanhua, ZHANG Ge, LENG Xuesong (2023). Research Progress of Welding Technology Between Niobium Alloy and Stainless Steel Dissimilar Metal. Aeronautical Manufacturing Technology. https://doi.org/http://www.amte.net.cn/CN/10.16080/j.issn1671-833x.2023.19.093

[2] Xingwen Zhou, Yuhua Chen, Yongde Huang, Yuqing Mao, Yangyang Yu, Effects of niobium addition on the microstructure and mechanical properties of laser-welded joints of NiTiNb and Ti6Al4V alloys, Journal of Alloys and Compounds, Volume 735, 2018, Pages 2616-2624, https://www.sciencedirect.com/science/article/pii/S0925838817340896