Purging Gas in Gas Tungsten Arc Welding: Enhancing Weld Quality and Integrity

Introduction:

In the realm of welding, achieving high-quality welds with excellent integrity is paramount. One crucial technique that aids in this endeavor is the use of purging gas. Purging gas plays a vital role in preventing oxidation and ensuring a clean, controlled environment during welding. In this blog, we will explore the significance of purging gas, its purpose, techniques, and benefits in various welding applications. Join us as we delve into the world of purging gas and its impact on weld quality.

Back purging is most important phenomenon in GTAW process because this process is mostly used in Stainless steel. Stainless steel is widely used fabrication of chemical, petrochemical, food etc. plant. All thin section and root welding is performed by GTAW process. GTAW process is also very popular in Aluminum welding.

In all large diameter pipe the root pass welding is done by GTAW process where the back purging is mandatory. Purging gas protect the weld metal from atmospheric contamination & oxidation because due to contamination & oxidation porosity, oxides, cracks like defect generates in welding.

1. Understanding the Purpose of Purging Gas:

1.1 Controlling Oxidation: Purging gas is used to displace oxygen from the vicinity of the weld joint, preventing the formation of oxides that can weaken the weld.

1.2 Protecting Reactive Materials: Certain materials, such as stainless steel and titanium, are highly reactive and prone to oxidation during welding. Purging gas helps shield these materials from oxygen exposure.

2. Techniques for Purging Gas:

2.1 Back Purging: This technique involves introducing the purging gas on the backside of the weld joint, creating a protective environment. We'll explain the process and benefits.

2.2 Flow Rate and Direction: Controlling the flow rate and direction of purging gas is crucial for effective shielding. We'll discuss optimal practices for achieving a consistent purge.

Argon and helium is most common purging gases .The properties of Argon and Helium gases is explained below.

	Argon Gas	Helium Gas
Chemical name	Ar	He
Atomic number	18	2
Atomic mass	39.948 g.mol -1	4.00260 g.mol -1
Density	1.78.10 -3 g.cm -3 at 0 °C	0.178*10 -3 g.cm -3 at 20 °C
Melting point	-189 °C	- 272.2 (26 atm) °C
Boiling point	-185.7 °C	- 268.9 °C
Ionization potential	15.8eV	24.6eV
Arc initation	Good	Poor
Cleaning	Good	Poor

 

These gases are chemically inert, colorless and odorless in liquid and gaseous forms.

Argon gas:-
Pure Argon is generally used in GTAW & GMAW welding but some time mixture of Helium and Argon is also very useful in weld quality point of view. Helium has higher ionization potential and thermal conductivity. So greater heat generates during welding using Helium as purging gas.

During GTAW Argon is preferred gas in AC Current. Because pure argon gives following characteristics in AC Current.

1.Lower cost.
2. Due to its lower ionization potential, Argon produces consistent high frequency arc.
3. Reduced penetration during welding.
4. Lower gas consumption during welding.
5. Greater metal cleaning action on aluminum and magnesium in AC.
6. Better cleaning action in DCEP.

The atomic number of argon is 18. It has 18 electrons. K, L and M shells contain 2,8,8 electrons respectively. The valence shell contains 8 electrons. Thus, argon has completed its octet and is highly stable. Usually, argon does not form compounds. So, it is chemically unreactive inert gas, also called noble gas. Thus there is no reaction between argon and the metal to be welded. Due to low reactivity of argon, during welding, argon can be used to create an inert gas shield.

Due to High heat generation characteristics, Helium/Argon mixture is used for high heat generation during welding. 25% Helium and 75% Argon gas mixture is used in AC-GTAW Process but Helium more than 25% creates instability in AC Current.  

Helium Gas:-

90%Helium and 10% Argon mixture is useful in DCEN-GTAW process. High heat generation in DC Current provide increased welding speed, full penetration  weld. Following characteristics of Helium gas is given below.

1. Lightest Inert Gas.
2. Helium yields a much higher available heat on metal than Argon. Due to this helium is used for welding of heavy section.
3. Higher welding speed with any defect like undercut.
4. Better cleaning action in DCEP.

Mixture of Helium & Argon also used to provide combination of characteristic of each gases. It provides good arc starting with arc stability, deep penetration and welding speed.

1. So High Helium gas are very useful for GMAW on thicker and GTAW welding with DCEN Polarity.

2. Pure Argon is useful for GTAW & GMAW. Pure Argon is very useful in Orbital welding of Bimetallic tube (25Ni:22Cr:2Mo).

3.Helium is expensive than Argon so many times Argon is preferred gas for purging & Shielding gas.

4. Due to higher density of Argon than Helium low flow rate of Argon required  during welding.

3. Benefits of Purging Gas in Welding:

3.1 Minimizing Weld Defects: Purging gas helps reduce the likelihood of weld defects such as porosity, excessive oxidation, and lack of fusion, leading to improved weld quality.

3.2 Enhanced Corrosion Resistance: By minimizing oxidation, purging gas contributes to the formation of corrosion-resistant welds, particularly in materials like stainless steel.

3.3 Increased Strength and Integrity: Purging gas helps maintain the mechanical strength and integrity of the weld joint by minimizing oxygen-induced embrittlement.

4. Applications of Purging Gas:

4.1 Stainless Steel Welding: Purging gas is extensively used in stainless steel welding, where it helps achieve welds with superior corrosion resistance and aesthetic appearance.

4.2 Titanium Welding: Given the reactivity of titanium, purging gas is crucial in titanium welding to prevent contamination and maintain its exceptional properties.

4.3 Pipe Welding: Purging gas finds widespread use in pipe welding, ensuring clean and defect-free welds in critical applications such as oil and gas pipelines.

5. Types of Purging Gases:

5.1 Inert Gases: Inert gases, such as argon and helium, are commonly used as purging gases due to their low reactivity and ability to displace oxygen effectively.

5.2 Gas Mixture Considerations: Depending on the specific welding application, gas mixtures with controlled oxygen content may be employed. We'll discuss the considerations involved in gas selection.

6. Safety Considerations:

6.1 Ventilation: Proper ventilation is essential when working with purging gases to prevent the accumulation of potentially hazardous gases in the welding environment.

6.2 Gas Monitoring: Regular monitoring of gas levels, especially in confined spaces, ensures a safe working environment for welders.

Conclusion:

The use of purging gas in welding is a critical technique that contributes to the production of high-quality welds with superior integrity. By effectively displacing oxygen and preventing oxidation, purging gas minimizes weld defects, enhances corrosion resistance, and maintains the mechanical strength of the weld joint. Whether in stainless steel, titanium, or pipe welding applications, the controlled environment provided by purging gas ensures clean, reliable welds. Understanding the purpose, techniques, and benefits of purging gas allows welders to optimize their welding processes, resulting in welds that meet stringent quality standards.

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