456A TIG (GTAW - Gas Tungsten Arc Welding) 3 — Questions and Answers
Question 1: What is the 'keyhole technique' in GTAW root pass welding and when is it used?
- A small hole is melted through the root opening, with filler metal closing the hole as the torch advances, ensuring full penetration (Correct answer)
- The root is welded without filler metal to maximize fusion
- A backing bar is used with a groove to concentrate the root bead
- The root is preheated to create a molten zone before arc is started
Correct answer: A small hole is melted through the root opening, with filler metal closing the hole as the torch advances, ensuring full penetration
The keyhole technique involves melting a small through-hole at the leading edge of the weld pool. As the torch advances, filler metal is added to close the trailing edge of the hole. This technique guarantees complete root penetration and is commonly used for open-butt pipe roots in stainless steel, titanium, and precision piping applications.
Question 2: What is the maximum contamination colour acceptable on a completed GTAW weld in Grade 2 titanium, per industry standards?
- Bright silver or light straw/gold — blue, purple, or grey indicates contamination and requires rejection (Correct answer)
- Any colour up to and including blue is acceptable
- Grey is acceptable if the weld passes tensile testing
- Colour is not an acceptance criterion for titanium welds
Correct answer: Bright silver or light straw/gold — blue, purple, or grey indicates contamination and requires rejection
Titanium weld colour is a direct indicator of oxidation level. Silver is ideal; light straw/gold indicates minor oxidation that is usually acceptable. Blue indicates significant oxygen/nitrogen pickup; purple and grey indicate severe contamination. Contaminated welds have reduced ductility, toughness, and corrosion resistance and are typically rejected in aerospace and critical piping applications.
Question 3: What is the hot-wire GTAW process and what is its advantage?
- Filler wire is resistance-heated before entering the arc, increasing deposition rate without adding arc heat to the base metal (Correct answer)
- A second GTAW arc is used to preheat the joint ahead of the main arc
- The tungsten electrode is operated at elevated temperature for deeper penetration
- The shielding gas is preheated to improve coverage in cold weather
Correct answer: Filler wire is resistance-heated before entering the arc, increasing deposition rate without adding arc heat to the base metal
Hot-wire GTAW uses AC resistance heating to preheat the filler wire to near its melting point before it enters the GTAW arc. This dramatically increases deposition rate (up to 4× over cold wire GTAW) without proportionally increasing heat input to the base metal, making it suitable for cladding and overlay applications on large components.
Question 4: What is 'cold cracking' (hydrogen-induced cracking) in GTAW welds on high-strength low-alloy (HSLA) steel and how is it prevented?
- Cracking that occurs after the weld cools, caused by hydrogen diffusion; prevented by preheat, low-hydrogen filler, clean base metal, and PWHT (Correct answer)
- Cracking during solidification from high restraint; prevented by reducing travel speed
- Surface cracking from thermal shock during rapid cooling; prevented by post-weld water cooling
- Intergranular cracking from sensitization; prevented by using L-grade filler
Correct answer: Cracking that occurs after the weld cools, caused by hydrogen diffusion; prevented by preheat, low-hydrogen filler, clean base metal, and PWHT
Cold cracking (HIC) in GTAW occurs when hydrogen absorbed during welding diffuses to high-stress areas (typically the HAZ) as the weld cools. Prevention includes: preheating to slow cooling and allow hydrogen diffusion, using clean (dry, oxide-free) base metal, minimizing moisture in the shielding gas, and PWHT for thick/restrained joints. GTAW inherently has lower hydrogen than SMAW but is not immune.
Question 5: What is the correct method for adding filler metal to the GTAW weld pool?
- Feed the rod at a low angle (15–20°) into the leading edge of the pool, withdrawing it slightly on each addition without touching the tungsten (Correct answer)
- Plunge the rod directly into the arc to melt it off
- Hold the rod steady and let the pool advance over it
- Melt the rod against the tungsten tip to create droplets
Correct answer: Feed the rod at a low angle (15–20°) into the leading edge of the pool, withdrawing it slightly on each addition without touching the tungsten
Filler metal is fed at a shallow angle (15–20°) into the front edge of the molten pool using a dipping or flowing motion. The rod must not touch the tungsten (which would contaminate the electrode) or be placed directly in the arc (which would cause spattering). The filler melts by contact with the pool, not by the arc directly.
Question 6: What is the purpose of the 'down slope' (current decay) feature on a GTAW power source?
- Gradually reduces current at weld termination to fill the crater and prevent crater cracking (Correct answer)
- Reduces current during root passes to prevent burn-through
- Automatically adjusts current based on arc voltage feedback
- Provides a slow start to prevent cold arc initiation
Correct answer: Gradually reduces current at weld termination to fill the crater and prevent crater cracking
The down slope function gradually reduces welding current over a set time period when the weld is terminated. This allows the welder to fill the weld crater as the current decays, preventing the crater crack that would occur if the arc were suddenly extinguished at full current in a recessed, stress-concentrated pool.
456A TIG (GTAW - Gas Tungsten Arc Welding) 3
What is the 'keyhole technique' in GTAW root pass welding and when is it used?