Welding filler metals

Stainless steel is used for many of the welded components in a nuclear facility, including piping and pressure vessels. For operators new to GTAW of stainless alloys, they will comment that the weld pool is difficult to manipulate — “It behaves like chewing gum!” — compared to low alloy or mild steel. The concept of over-alloying may be new as well. For those who ask, “Why is it harder to work with stainless?”, read on.

By Peter Stones, IEng MWeldI IWE/EWE

A: The addition of chromium and nickel to stainless steel welding filler metals has a significant impact on the behaviour of the weld pool, often resulting in a sluggish or slow-moving weld pool. This phenomenon occurs due to several key factors related to the chemical and metallurgical properties of these alloying elements. Chromium is one of the primary alloying elements in stainless steel, known for its excellent corrosion resistance and high-temperature strength.

When chromium is present in the welding filler metal, it forms a stable oxide layer on the surface of the weld pool, commonly referred to as the passive layer. This passive layer acts as a protective barrier, preventing the base metal and filler metal from being exposed to corrosive environments. While this oxide layer is beneficial in terms of corrosion resistance, it also contributes to the sluggishness of the weld pool.

The passive layer formed by chromium oxide has a higher melting temperature compared to the surrounding weld pool. As a result, the weld pool containing chromium-rich filler metal requires more heat input to reach the desired molten state. This higher melting temperature increases the energy required to maintain a fluid weld pool, making it more difficult for the weld pool to flow and merge smoothly with the base metal.

Similarly, the addition of nickel to stainless steel filler metals also plays a role in making the weld pool less fluid. Nickel enhances the mechanical properties and toughness of stainless steel, making it suitable for a wide range of applications. However, nickel has a lower thermal conductivity compared to other elements in stainless steel, such as iron and chromium.

This lower thermal conductivity causes the weld pool to retain heat for a longer duration, resulting in slower cooling rates and reduced fluidity. Moreover, nickel has a higher solidification temperature, which further contributes to the sluggishness of the weld pool. As the weld pool solidifies, the nickel-rich regions tend to solidify last, forming dendritic structures that impede the fluid flow of the weld pool. These dendrites act as barriers, inhibiting the movement of liquid metal and making the weld pool sluggish.

In addition to the chemical and metallurgical factors, the weld pool’s sluggishness can also be influenced by welding parameters such as heat input, travel speed, torch angle and shielding gas composition. Adjusting these parameters can help mitigate some of the sluggishness associated with chromium and nickel-rich filler metals, but it is important to keep within the heat input and interpass temperature required in the weld qualification.

For some grades of stainless steel such as 308L, 309L and 316L, there are modified grades that have a slightly higher silicon content and are referenced as 308LSi, 309LSi and 316LSi. Whilst all of the corrosion resistive and mechanical properties are the same as the ‘L’ grade, the added silicon has the effect of making the weld pool more fluid and making it easier to weld. There is even one grade of duplex stainless steel (2209) that has added silicon for exactly the same reason, 22.8.3.LSi.

Over-alloying the weld filler metal grade for stainless steel can provide several benefits in certain welding applications. This process involves increasing the concentration of alloying elements, such as chromium and nickel, beyond the levels present in the base metal. Here are some advantages of over-alloying the weld filler metal:

1. Enhanced corrosion resistance: Stainless steel is known for its corrosion-resistant properties, and over-alloying the filler metal can further enhance this characteristic. By increasing the chromium content, the passive oxide layer formed on the weld surface becomes thicker and more protective. This offers superior resistance to various corrosive environments, including high-temperature and highly corrosive applications.
2. Improved mechanical properties: Over-alloying the weld filler metal can also lead to improved mechanical properties of the weld joint. By increasing the concentration of nickel and other alloying elements, the weld can exhibit higher tensile strength, toughness, and resistance to deformation. This is particularly beneficial in structural applications where strength and durability are crucial.
3. Matching or exceeding base metal properties: In some cases, the base metal used in a welding application may have higher alloying element content than the standard filler metal grade. In such situations, over-alloying the weld filler metal ensures that the weld joint matches or exceeds the properties of the base metal. This helps maintain consistency and integrity throughout the welded structure.
4. Compensation for dilution: During the welding process, the base metal and filler metal mix together, leading to dilution of the alloying elements. Over-alloying the filler metal compensates for this dilution, ensuring that the resulting weld joint maintains the desired composition and properties.

It is worth noting that over-alloying the weld filler metal should be done with careful consideration of the specific welding application and the requirements of the final product. Balancing the alloying element concentrations is crucial to avoid potential issues such as hot cracking, reduced weld ductility, or increased sensitivity to intergranular corrosion.

It cannot be emphasised enough that following the correct procedure with the correct filler metals and shielding gas as described in the weld qualification is important. Further, operators need to train as if they were welding on the job, working with the exact base materials, filler metals, equipment and weld procedure specification (asking your operators to train on anything less is like expecting an F1 driver to train on a lorry).

Operators have to develop welding techniques that produce successful results, practice them until they become second nature and then trust in those techniques in the field — which we’ll discuss in the next article!