Choosing the Right Welding Wire: ER70S-2, ER70S-3, or ER70S-6

When selecting the right filler wire for your welding project, understanding the differences between the commonly used ER70S-2, ER70S-3, and ER70S-6 is essential. These three types of welding wires are often used for the gas tungsten arc welding (GTAW) process, each offering unique properties that make them more suitable for specific applications. The key to making the best choice lies in understanding their composition, performance under different conditions, and how they interact with base metals, particularly with respect to oxidation levels.

Key Differences Between ER70S-2, ER70S-3, and ER70S-6

All three filler metals—ER70S-2, ER70S-3, and ER70S-6—offer similar mechanical properties in terms of yield strength, tensile strength, and elongation when used in GTAW. However, the subtle differences in their chemical composition make each one more suited to particular welding environments.

• ER70S-2: This wire has the highest deoxidant content, including elements like Zr, Al, Ti, and Mn, which help it perform well on surfaces that are rusty or have slight contamination. It provides excellent weldability, especially for single-pass welding of killed, semi-killed, or rimmed steels. However, its strong deoxidation properties can lead to slag formation in certain applications.
• ER70S-3: With a lower deoxidant content than ER70S-2, ER70S-3 is better suited for welding cleaner, non-rusty surfaces. While it performs adequately in bright, polished conditions, it shows more slag formation and porosity when used on rusted or dirty base metals. Its lower deoxidant content results in a higher oxygen concentration in the weld pool, which can affect penetration and bead width.
• ER70S-6: This filler metal lies between ER70S-2 and ER70S-3 in terms of deoxidant content. It works well on moderately contaminated surfaces and provides improved weldability compared to ER70S-3 in rusty conditions. However, like ER70S-2, it may still require purging gas for groove welds to prevent oxidation and maintain weld quality.

The Role of Deoxidants and the Marangoni Effect

One of the most critical factors influencing weld quality is the behavior of the weld pool, which is directly affected by the presence of deoxidants. Deoxidants such as Zr, Al, Ti, and Mn are added to reduce the oxygen content in the molten weld pool, preventing defects like porosity and slag entrapment. This is where the Marangoni effect plays a significant role.

The Marangoni effect describes the flow of liquid in response to a gradient in surface tension, which is influenced by temperature and the concentration of deoxidants. For example, when the area under the arc is heated, the surface tension decreases at the center of the weld pool, causing fluid to flow towards the edges. A higher concentration of deoxidants in the weld pool can strengthen this effect, potentially causing the slag to be trapped, especially when welding without purging gas.

In the case of ER70S-2, the high level of deoxidants leads to a more stable weld pool with low oxygen content, producing wider beads with less penetration. Conversely, ER70S-3, with fewer deoxidants, allows more oxygen into the weld pool, which can increase penetration and produce narrower beads. ER70S-6, offering a balance between the two, provides moderate penetration and bead width.

How to Choose the Right Filler Metal for Your Welding Project

The choice between ER70S-2, ER70S-3, and ER70S-6 depends primarily on the condition of the base metal and the type of weld being performed:

1. For clean, polished base metals: ER70S-3 or ER70S-6 are good choices, as they offer excellent weldability and are less likely to cause excessive slag formation.
2. For rusted or dirty base metals: ER70S-2 is the best option due to its higher deoxidant content, which helps to minimize the effects of oxidation. However, care must be taken with the possibility of slag formation.
3. For groove welds: If welding without purging gas, ER70S-3 is the better choice, as it shows acceptable weldability even without the extra protection of purging gas. However, for both ER70S-2 and ER70S-6, purging gas is essential for maintaining weld quality.

Conclusion

While ER70S-2, ER70S-3, and ER70S-6 all provide similar mechanical properties, their chemical compositions and responses to oxidation make them suitable for different applications. The key factors to consider when selecting a filler metal include the cleanliness of the base material, the necessity for purging gas, and the desired weld pool behavior. By understanding these nuances and the Marangoni effect, you can select the most appropriate filler metal to achieve optimal weld quality and performance.