Arc Striking Methods in TIG Welding

Literature Overview

This technical article by Xu Heshui, published in Welding Technology (焊接技术) in 1997, addresses a fundamental yet often neglected aspect of TIG welding: the arc striking method. Although arc striking may seem like a minor operational detail, it has a significant impact on weld quality, particularly in the initiation zone of the weld bead. The article was authored by an engineer from Nanjing Chenguang Machinery Factory, reflecting the practical, hands-on nature of the content.

Core Technical Content

Types of Arc Striking Methods

The article discusses several arc striking methods used in TIG welding, each with distinct characteristics and applications:

High-Frequency Arc Striking: Detailed Analysis

The high-frequency (HF) arc striking method is the most commonly used and recommended method for TIG welding. The principle involves applying a high-frequency (typically 100–400 kHz), high-voltage (5–20 kV) oscillating signal between the tungsten electrode and the workpiece. This high-frequency voltage ionizes the gas in the gap, creating a conductive path that allows the DC welding current to flow and establish the main arc.

The key parameters of the HF arc striking system include:

Impact of Arc Striking Method on Weld Quality

The arc striking method has a direct impact on the quality of the weld initiation zone:

1. Tungsten contamination: The lift arc method carries a significant risk of tungsten contamination, where tungsten particles transfer to the weld pool and create inclusions. Tungsten inclusions in the weld metal can cause:

• Arc instability during subsequent welding
• Cracking susceptibility
• Reduced mechanical properties
• In some cases, the entire weld must be removed and rewelded

1. Porosity: Inadequate arc striking can lead to porosity in the initiation zone due to incomplete shielding gas coverage at the start of the weld. The gas flow pattern is not fully established during the first few milliseconds of arc operation, allowing atmospheric contamination of the molten pool.
2. Undercut: If the arc is not properly established at the correct position, the initial weld bead may exhibit undercut due to insufficient heat input at the joint edges.
3. Weld bead appearance: The initiation zone often has a different appearance from the rest of the weld bead, with a more rounded or irregular profile. This is primarily due to the transient thermal conditions during arc establishment.

Practical Recommendations for Arc Striking

Based on the analysis in this article, the following practical recommendations are provided for TIG welding arc striking:

1. Always use HF arc striking when possible: This eliminates the risk of tungsten contamination and provides a clean, consistent arc start.
2. Use a trailing shield or gas cup: This extends the shielding gas coverage to the end of the weld bead, preventing atmospheric contamination during arc termination.
3. Preheat the tungsten tip: For difficult-to-strike arcs (such as on stainless steel or nickel-based alloys), preheating the tungsten tip with a separate flame or arc can improve arc initiation reliability.
4. Use a proper tungsten electrode: The tungsten electrode should be properly ground to a sharp point (for DC welding) or a dull radius (for AC welding) to facilitate arc initiation and maintain arc stability.
5. Ensure proper gas flow: The shielding gas flow rate should be sufficient to establish full coverage before the arc is struck. A minimum of 15–20 L/min of argon is typically required for most TIG welding applications.
6. Use a purge system for internal welding: For welding of pipes and vessels, a purge system should be used to maintain an inert atmosphere inside the component during arc striking and welding.

Engineering Practice Implications

Application to Cladding and Overlay Welding

In cladding and overlay welding operations, the arc striking method has particular significance because:

• First-pass quality is critical: In overlay welding, the first pass establishes the bond between the overlay layer and the base metal. Any defects in the initiation zone (such as lack of fusion or tungsten contamination) can compromise the entire overlay layer.
• Multiple passes are common: Overlay welding often requires multiple passes to build up the required overlay thickness. Each pass requires a separate arc strike, increasing the number of potential initiation defects.
• Sensitive materials: Many overlay applications involve corrosion-resistant materials (such as stainless steel, nickel-based alloys, or titanium alloys) that are highly sensitive to contamination. Tungsten contamination from lift arc striking can severely degrade the corrosion resistance of the overlay layer.

Quality Control for Arc Initiation Zones

For TIG overlay welds in pressure vessel applications, the following quality control measures should be applied to the arc initiation zones:

• Visual inspection: Examine the initiation zone for tungsten inclusions, porosity, and undercut. Any visible defects should be marked for further evaluation.
• Radiographic testing (RT): Perform RT on the initiation zone to detect internal defects such as tungsten inclusions, porosity, and lack of fusion.
• Ultrasonic testing (UT): Evaluate the bond quality at the overlay-base metal interface in the initiation zone.
• Macrograph examination: If defects are suspected, perform macrograph examination to evaluate the fusion characteristics and microstructural features of the initiation zone.

Connection to Bimetal Pressure Vessel Fabrication

In the fabrication of bimetal pressure vessels, TIG welding is frequently used for:

• Clad plate edge welding: Welding the edges of clad plates before forming or rolling
• Overlay repair: Repairing localized damage on clad surfaces
• Small component welding: Welding small-diameter components (such as heat exchanger tubes) that require precise control

In all these applications, the arc striking method must be carefully controlled to ensure weld quality. The use of HF arc striking is strongly recommended, and the initiation zone should be included in the inspection scope.

Key Questions and Reflections

A fundamental question raised by this article is: why is such a basic aspect of TIG welding (arc striking) often neglected in welding procedure development and qualification? The answer lies in the perception that arc striking is a minor operational detail that does not significantly affect weld quality. However, this perception is incorrect, particularly for critical applications such as pressure vessel fabrication and overlay welding of sensitive materials.

Another important consideration is the training of welders. The arc striking technique requires skill and practice to perform consistently. Welders should be trained to:

• Strike the arc at the correct position and angle
• Maintain stable arc during the first few milliseconds of arc operation
• Avoid tungsten contamination during lift arc striking (if HF arc striking is not available)
• Recognize and correct arc striking problems before they affect weld quality

The article also highlights the importance of equipment maintenance. HF arc striking equipment requires regular maintenance to ensure reliable arc initiation. Faulty HF generators can cause:

• Difficulty in arc initiation, leading welders to resort to lift arc striking
• Arc instability during welding, causing quality problems
• Radio frequency interference with other equipment in the workshop

Study Insights and Implications

Although this 1997 article addresses a seemingly simple topic, it provides valuable insights into the practical aspects of TIG welding that are often overlooked in academic literature and standards. The arc striking method is a fundamental aspect of TIG welding that directly affects weld quality, and its proper implementation is essential for achieving consistent, high-quality welds.

For engineers working in cladding and bimetal product manufacturing, the key takeaway is that even the most basic aspects of the welding process must be carefully controlled and documented. The welding procedure specification (WPS)