Arc Starting, Puddle Control, and Stringer Bead on Carbon Steel (Flat)

Gas Tungsten Arc Welding — Arc Starting, Puddle Control, and Stringer Bead on Carbon Steel (Flat)

What This Lesson Covers

GTAW (TIG) produces the cleanest welds of any common arc process, but that quality depends on precise hand control, correct equipment setup, and disciplined safety habits. In this lab you will learn to start a stable arc using high-frequency (HF) ignition, read and control the molten puddle, and run straight stringer beads on carbon steel plate in the flat (1G) position.

Equipment and Consumables

Power source. Use a GTAW inverter set to DCEN (direct current electrode negative). On DCEN the tungsten stays cooler and carries the highest amperage capacity; the workpiece receives roughly two-thirds of the arc energy, which melts the base metal efficiently.

Torch and tungsten. An air-cooled torch is adequate for the amperage ranges used on 1/4-in. carbon steel in this lab. Use a 2 % ceriated or 2 % thoriated tungsten (follow your school's tungsten policy — ceriated is preferred where radiation exposure is a concern). Grind the tungsten to a tapered point with the grind lines running lengthwise, not across the tip, so the arc starts and stays centered.

Shielding gas. Argon at 99.995 % purity protects the puddle and tungsten from atmospheric contamination. Set flow rate to approximately 15–20 CFH (cubic feet per hour) for flat position work; confirm the exact range with the procedure sheet your instructor provides.

Filler rod. ER70S-2 is a triple-deoxidized rod suitable for carbon steel. Use 1/16-in. rod for lower amperages and thinner beads; 3/32-in. for higher amperages or larger bead profiles.

High-Frequency Arc Starting

HF start sends a high-voltage, high-frequency spark across the gap to ionize the shielding gas and ignite the arc without touching the tungsten to the base metal. This is critical: if the tungsten touches the work (a "touch start"), tungsten particles embed in the weld — this is called tungsten inclusion, a subsurface defect that causes stress concentrations and fails radiographic inspection.

Procedure for HF start:

1. Position the torch at a 15–20° drag angle (torch tilts slightly away from travel direction), tungsten tip 1/8 in. above the plate.

2. Press the foot pedal slowly to ramp amperage up as the arc initiates.

3. Hold still until a bright, stable puddle forms — typically 2–4 seconds at normal amperage.

4. Begin travel only after the puddle reaches the target width.

Reading and Controlling the Puddle

The molten puddle appears as a bright, mirror-like ellipse. Wider puddle = slower travel speed or higher amperage. Narrower puddle = faster travel or lower amperage. The foot pedal lets you modulate amperage in real time — ease off near the plate edge to prevent crater cracking and burnthrough.

Filler addition: hold the rod at a low angle (approximately 15°) on the leading edge of the puddle, on the opposite side from the torch. Dip the rod in short pulses synchronized with torch advancement — never stir it in the arc or it will contaminate the tungsten.

Target bead width for this exercise: 3/8 in. ± 1/8 in. (i.e., between 1/4 in. and 1/2 in.) over the full 6-inch length.

What Tungsten Contamination Looks Like and How to Fix It

A contaminated tungsten turns dark, balls up at the tip, and the arc wanders or "spits." Stop immediately: do not continue welding with a contaminated tungsten — you will drive inclusions into the weld. Break off or re-grind the tip using a dedicated bench grinder or tungsten grinder, then inspect the weld. If inclusions are visible (gray or black specks in the bead surface or slight sink marks), mark the area in your weld log and notify your instructor.

AWS D1.1 Visual Acceptance — What You Are Aiming For

For the peer inspection activity, you will compare beads against AWS D1.1 visual acceptance criteria (Table 6.1 in the structural code). Key points relevant to this exercise:

• Undercut on primary tension members must not exceed limits specified in the code; for this training exercise apply the most restrictive limit your instructor designates.
• Surface porosity — visible pores on the bead surface are not acceptable.
• Overlap (weld metal flowing past the toe without fusing) is not acceptable.
• Tungsten inclusions visible at the surface are a rejection criterion.

A weld gauge (Bridgeport type or equivalent) lets you measure bead height, undercut depth, and fillet weld leg length quickly and accurately.

Safety Requirements for This Lab

PPE: Auto-darkening helmet set to at least shade 10 for GTAW on carbon steel. Leather welding gloves, leather jacket or FR cotton sleeves, closed-toe leather boots, and hearing protection where ambient noise warrants it.

Shielding gas: Argon is an asphyxiant — it displaces oxygen in confined spaces. Ensure the lab has adequate ventilation. Secure cylinders upright with a chain; never drop or strike them.

Lockout/tagout: Before changing tungsten or adjusting internal torch connections, de-energize the power source and follow your facility's LOTO procedure.

Fume control: GTAW produces lower fume volume than SMAW or GMAW, but carbon steel fumes still require local exhaust ventilation (LEV) or a fume extraction system. Position the LEV hood within 6–8 inches of the arc.

Hot metal: Plate coupons retain heat for several minutes. Use pliers or clamps; mark hot pieces with soapstone ("HOT").

Running the Stringer Bead — Step by Step

1. Clean the plate with a stainless wire brush; remove mill scale, oil, and moisture from the bead path.

2. Set up the power source per the procedure card; verify polarity (DCEN), argon flow, and torch connections.

3. Strike the arc with HF start, ramp up with the foot pedal, and pause until the puddle stabilizes.

4. Begin travel at a steady pace while dipping filler in small, rhythmic pulses.

5. At the end of the bead, reduce amperage with the foot pedal (do not lift abruptly — this creates a crater that can crack).

6. Post-flow: keep the torch over the crater until the argon post-flow cycle ends to protect the cooling weld and tungsten.

7. Log each bead: amperage used, travel speed estimate, any contamination events, and inspection results.