Duplex Explained: A Guided Welding Procedure

In the realm of metallurgy and welding, there are materials that stand out due to their unique properties and versatile applications. Among these, duplex and super duplex stainless steels have gained prominence for their remarkable combination of strength, corrosion resistance, and affordability. In this article, we will delve into the world of duplex and super duplex stainless steels, exploring their characteristics, storage, welding procedures, and best practices.

Understanding Duplex and Super Duplex Stainless Steels

Duplex stainless steels are a class of stainless steels that exhibit a two-phase microstructure consisting of approximately equal proportions of austenite and ferrite phases. This unique microstructure grants duplex stainless steels their exceptional mechanical and corrosion-resistant properties. Super duplex stainless steels take this concept a step further, with an even higher content of chromium, nickel, and molybdenum, resulting in enhanced corrosion resistance and strength. These materials find extensive use in various industries, including oil and gas, chemical processing, marine environments, and more.

Storage and Handling

Before delving into the welding process, it's crucial to understand the proper storage and handling of duplex and super duplex stainless steel materials. These materials should be stored in dry and clean environments, away from contaminants that could compromise their integrity. Always handle them with gloves to prevent the transfer of oils and other residues from the skin, which could lead to corrosion.

Purging and Shielding Gas

During the welding process, maintaining the right atmosphere is essential to preserve the material's corrosion resistance properties. To achieve this, it's recommended to use purging and shielding gases. A common choice is 100% Argon (Ar) or a mix of 98% Argon and 2% Nitrogen (N2). The addition of nitrogen helps influence the balance between the austenite and ferrite phases. During purging, gas flow rates should be adjusted according to the pipe diameter to ensure oxygen levels remain below 50 parts per million (ppm) or 0.005%.

Managing Oxygen Levels

Excessive oxygen levels during welding can negatively impact the corrosion properties of duplex and super duplex stainless steels, potentially leading to material loss. If oxygen levels exceed 100 ppm, the welding process should be paused, and adjustments to the purging gas should be made to maintain the recommended oxygen level of 50 ppm.

Preparation and Joint Configuration

Proper preparation of the joint surfaces is crucial for successful welding. Here are some key steps:

1. Clean joint surfaces thoroughly using stainless steel brushes.
2. Remove dirt, oil, and grease using non-chlorinated solvents like acetone.
3. Eliminate oxides, paints, and primers within a minimum of 50mm from the joint edges.
4. Moisture should be removed using methods like blowers or controlled flames, staying below 100°C.
5. Preheating and Interpass Temperature.

Preheating is generally not recommended for duplex and super duplex stainless steels. However, in ambient temperatures below 5°C, warming the field welds up to 100°C can help prevent condensation and ensure joint dryness. When it comes to interpass temperatures:

• For duplex applications, the interpass temperature should not exceed 200°C (300°F).
• For super duplex, the interpass temperature should be limited to 150°C (300°F) for thick applications and 100°C (210°F) for thinner applications.

Welding Heat Input and Technique

Achieving the correct heat input during welding is vital to maintaining the desired microstructure and properties. Here are some recommendations:

• For duplex root passes, a heat input of 1.5 kJ/mm is recommended, along with 1.3 kJ/mm for hot passes.
• The balance filling caps should be between 0.5 and 2.5 kJ/mm for duplex, depending on the thickness.

For super duplex:

• Root passes should have a heat input of 1.2 kJ/mm, and hot passes should use 1 kJ/mm.
• The balance filling caps should range from 0.8 to 1.2 kJ/mm, depending on thickness.
• Welding Technique and Distortion Minimization

To minimize distortion, it's advisable to divide the welding process into segments, especially for long welding seams. Excessive heat input in a single area can lead to distortion and compromise the material's integrity. In conclusion, duplex and super duplex stainless steels are exceptional materials that offer an optimal balance between strength and corrosion resistance. Proper storage, handling, and adherence to welding procedures are paramount to ensuring the longevity and performance of welded structures made from these materials. By following the guidelines mentioned above and implementing best practices, welders can confidently work with duplex and super duplex stainless steels and create structures that stand the test of time.