Gases for moulding/root protection gases: selection and application

Forming gases, also known as root protection gases, are crucial to the welding process as they protect the weld root from oxygen. These gases create an inert atmosphere to prevent oxidation of the weld and ensure the formation of a clean, high quality weld. The selection of the right forming gas has a significant influence on the welding quality, as different materials have different requirements.

Types of forming gases

Inert gases are the most commonly used forming gases:

• Argon (Ar) is the most common forming gas, as it is inert, colourless and universally applicable. It is ideal for a wide range of materials, including unalloyed and alloyed steels, stainless steel and aluminium.
• Helium (He) offers high thermal conductivity and good penetration, which makes it useful for special applications, especially when processing thicker and thinner materials. However, it is more expensive than argon and is often used in combination with other gases.

Gas mixtures offer additional advantages:

• Argon-hydrogen mixtures (Ar/H₂) are often used for austenitic chromium-nickel steels, as the hydrogen content provides additional protection against oxidation.
• Nitrogen (N₂) and nitrogen-hydrogen mixtures are used especially for the forming of austenitic CrNi steels and duplex steels, as they create a suitable protective climate and preserve the mechanical properties of the material.
  
  

Active gases such as oxygen (O₂) and carbon dioxide (CO₂) are not forming gases and should not be confused here. They are shielding gases that are used for welding but are not suitable for root protection.

Selection of the forming gas: influencing factors

The choice of forming gas is determined by various factors:

• MaterialDifferent materials require different shielding gases, e.g. stainless steel often requires a combination of argon and nitrogen.
• Welding processWelding methods such as MIG, TIG or electric welding have specific requirements for the composition of the gas.
• Material thicknessThin-walled materials often benefit from helium or helium mixtures to increase penetration, while argon mixtures are more commonly used for thicker materials.
• Desired welding propertiesRequirements for the weld seam such as surface quality, penetration and mechanical properties influence the choice of gas.

Recommendations for gas selection by material

Depending on the material, there are different recommendations for the most suitable forming gas:

• Unalloyed and alloyed steelsArgon or argon-hydrogen mixtures are usually suitable as they effectively displace oxygen.
• Stainless steelPure argon or argon-hydrogen mixtures are standard to prevent corrosion and to obtain a high-quality weld seam.
• AluminiumArgon is the first choice for aluminium due to its inert properties. Helium can also be used for improved penetration.
• Thin materialsHelium or argon-helium mixtures ensure deep, clean penetration of the weld seam.
• Thick materialsArgon-helium mixtures are suitable for thicker workpieces in order to improve heat distribution and protect the weld seam.

Application notes for forming gases

Certain aspects must be taken into account when using forming gases:

• Purity of the gasHigh purity gas (99.99 % or higher) is important to avoid impurities that could affect the welding quality.
• Gas flow rateThe flow rate must be set so that the oxygen is quickly and efficiently displaced from the moulding chamber without creating turbulence.
• Forming timeThe duration of purging with forming gas should be long enough to ensure that the oxygen is completely displaced.
• Preheating the workpiecePreheating can help to minimise the oxygen absorption of the material and thus improve the weld seam quality.

 

Conclusion: The importance of choosing the right throttle when moulding

The selection of the right forming gas is a decisive factor for the quality of the weld seam. The wrong choice of gas can lead to oxidation, poor gas coverage or pore formation, which can impair the corrosion resistance, strength and service life of the welded joint. Particularly with demanding materials such as stainless steel or high-alloy steels, this can result in additional costs due to reworking and reduced component longevity. It is therefore essential to carefully select the gas according to the material and process requirements in order to achieve optimum results.