7.3 Gas Tungsten Arc Welding (GTAW) Procedure Variables

7.3 Gas Tungsten Arc Welding (GTAW) Procedure Variables

Core Principle: Gas Tungsten Arc Welding (GTAW / TIG) utilizes a nonconsumable tungsten electrode to establish an electric arc, with an externally supplied inert or reducing gas shielding the molten pool. Because GTAW provides surgical thermal control and exceptionally clean deposition, it is the premier process for high-pressure power piping (ASME B31.1), severe cyclic chemical process piping (ASME B31.3), and nuclear components (ASME Section III). However, because the molten weld pool is entirely dependent on external shielding and precise electrical dynamics, Table QW-256 classifies many variables as Essential that are merely nonessential in SMAW.

---

1. Table QW-256 Architecture & Contrast with SMAW

When evaluating GTAW procedure qualification, candidates must recognize the dramatic shifts in variable criticality between Table QW-253 (SMAW) and Table QW-256 (GTAW):

+-----------------------------------------------------------------------------------------+
|                   CRITICAL VARIABLE DIVERGENCE: SMAW (QW-253) vs GTAW (QW-256)          |
+-----------------------------------------------------------------------------------------+
| VARIABLE DESCRIPTION      | SMAW (Table QW-253)          | GTAW (Table QW-256)          |
+---------------------------+------------------------------+------------------------------|
| Electrical Polarity       | NONESSENTIAL (QW-409.4)      | ESSENTIAL (QW-409.4)         |
| Addition/Deletion Filler  | NOT APPLICABLE               | ESSENTIAL (QW-404.14)        |
| Consumable Inserts        | NOT APPLICABLE               | ESSENTIAL (QW-404.22)        |
| Shielding Gas Composition | NOT APPLICABLE               | ESSENTIAL (QW-408.2)         |
| Backing Gas Deletion      | NOT APPLICABLE               | ESSENTIAL (QW-408.5)         |
| Trailing Shielding Gas    | NOT APPLICABLE               | ESSENTIAL (QW-408.8)         |
+-----------------------------------------------------------------------------------------+

In SMAW, switching from DCEP to DCEN or AC does not fundamentally alter the metallurgical structure of carbon steel welds. In GTAW, however, switching polarity completely alters arc physics:

• DCEN (Direct Current Electrode Negative / Straight Polarity): 70% of heat concentrates at the workpiece (anode), producing deep, narrow penetration and preserving the tungsten tip. Used for 95% of steel and stainless piping.
• DCEP (Direct Current Electrode Positive / Reverse Polarity): 70% of heat concentrates at the tungsten electrode (cathode), causing tungsten melting/balling with shallow workpiece penetration. Unusable for standard GTAW manual joint welding.
• AC (Alternating Current): Rapidly cycles between cathodic cleaning (removing stubborn aluminum/magnesium oxides) and workpiece penetration. Essential for aluminum (P-No. 21–25).

Because changing current type or polarity fundamentally changes penetration, fusion, and dilution, QW-409.4 is an ESSENTIAL variable in Table QW-256.

---

2. Paragraph-by-Paragraph Analysis of Table QW-256

QW-402: Joints

• QW-402.1 (Groove Design - Nonessential): Revisions to root opening, groove angle, bevel, or joint geometry can be made on the WPS without PQR requalification.
• QW-402.4 (Backing Deletion - Essential): Deletion of backing (converting a joint welded with backing or double-welded into an open-root joint) requires requalification.
• QW-402.10 & .11 (Root Spacing & Retainers - Nonessential): Root gap and nonfusing retainer adjustments are nonessential.

QW-403: Base Metals

• QW-403.5 (Group Number - Supplementary Essential): Within a P-Number, changing from one Group Number to another is supplementary essential when notch-toughness testing is mandated by the construction code.
• QW-403.8 (P-Number Change - Essential): Changing base metal P-Number requires PQR requalification per Table QW-424.1.
• QW-403.11 (Base Metal Postweld Heat Treated - Essential): Base metal thermal condition.

QW-404: Filler Metals

• QW-404.4 (F-Number Change - Essential): A change from one F-Number (e.g., F-No. 6 for solid carbon/stainless bare wire under SFA-5.18 / SFA-5.9) to another is an essential variable.
• QW-404.5 (A-Number Change - Essential): A change in the chemical composition of the deposited weld metal from one A-Number to another.
• QW-404.12 (AWS Classification - Supplementary Essential): Changing filler metal classification within an SFA specification is supplementary essential when impact testing is required.
• QW-404.14 (Addition or Deletion of Filler Metal - Essential): Switching from autogenous welding (fusion welding without adding wire) to manual wire feeding, or vice versa, is an essential variable. Autogenous welds exhibit high base-metal dilution and zero deoxidizers, whereas filler-added welds introduce manganese and silicon deoxidizers that dramatically alter mechanical properties.
• QW-404.22 (Consumable Inserts - Essential): Adding or deleting a preformed consumable insert (such as an AWS A5.30 Class 1–5 insert ring), or changing the nominal composition or shape of the insert, is an essential variable.
• QW-404.30 (Deposited Thickness $t$ - Essential): Deposited thickness limits outside Table QW-451.

QW-405: Positions

• QW-405.1 (Addition of Position - Nonessential): A PQR coupon welded in the flat position (1G) qualifies the WPS for all positions (2G, 3G, 4G, 5G, 6G), provided impact testing is not required.
• QW-405.2 (Progression - Supplementary Essential): Changing vertical progression from uphill to downhill or vice versa is supplementary essential when impact testing applies.

QW-406: Preheat

• QW-406.1 (Decrease > 100°F / 55°C - Essential): Decreasing preheat by more than 100°F below that recorded on the PQR requires requalification.
• QW-406.3 (Interpass Increase > 100°F / 55°C - Supplementary Essential): Increasing maximum interpass temperature by more than 100°F above that recorded on the PQR requires requalification if notch-toughness testing is mandated.

QW-407: Postweld Heat Treatment (PWHT)

• QW-407.1 (PWHT Condition - Essential): Adding, deleting, or altering the PWHT thermal regime requires requalification.
• QW-407.2 (PWHT Time & Temperature - Supplementary Essential): PWHT time/temp range changes require requalification when impact testing is specified.

QW-408: Gas (The Defining GTAW Domain)

Paragraph QW-408 contains the most critical process-specific rules for GTAW:

• QW-408.1 (Addition/Deletion of Trailing/Backing Gas - Essential): The addition or deletion of trailing shielding gas or backing gas is an essential variable.
• QW-408.2 (Shielding Gas Mixture & Percent - Essential): A change from a single shielding gas to another shielding gas (e.g., 100% Argon to 100% Helium), or a change in the nominal composition of a gas mixture (e.g., 75% Ar / 25% He to 90% Ar / 10% He), or a change of more than 5% in the specified gas mixture ratio, is an essential variable.
• QW-408.3 (Shielding Gas Flow Rate - Nonessential): An increase or decrease in the shielding gas flow rate (e.g., adjusting from 20 CFH to 30 CFH) is a nonessential variable.
• QW-408.5 (Backing Gas Addition, Deletion, or Composition - Essential): The addition, deletion, or change in composition of backing gas (internal purge) is an essential variable.
  • Metallurgical Context: When welding stainless steel (P-No. 8), duplex stainless, or nickel alloys, atmospheric oxygen on the unshielded root side causes severe oxidation known as "sugaring." This granulated oxide destroys corrosion resistance and creates severe stress risers. Deleting an Argon backing purge from an active P-8 WPS without qualifying an open-root PQR with backing gas deletion is a severe code violation!
• QW-408.8 (Deletion of Trailing Shielding Gas - Essential): Trailing gas shields hot, solidified weld metal as it cools. Deleting trailing gas is an essential variable, especially on reactive materials (titanium P-51, zirconium P-61) where atmospheric exposure above 800°F causes catastrophic embrittlement.
• QW-408.9 (Backing Gas Flow Rate - Nonessential): Adjusting backing purge flow rate is nonessential.
• QW-408.10 (Shielding Gas Environment - Essential): Deleting or adding a complete environmental chamber (glove box) is an essential variable.

QW-409: Electrical Characteristics

• QW-409.1 (Heat Input Increase - Supplementary Essential): Heat input increases beyond the PQR are supplementary essential when impact tested.
• QW-409.4 (Current and Polarity - Essential): Changing from DCEN to DCEP, or to AC, or vice versa, is an essential variable.
• QW-409.8 (Tungsten Electrode Size - Nonessential): Changing electrode diameter (e.g., 3/32 in. to 1/8 in.) is nonessential.
• QW-409.12 (Tungsten Electrode Classification - Nonessential): Changing tungsten electrode classification under AWS SFA-5.12 (e.g., from 2% Thoriated EWTh-2 to 2% Ceriated EWCe-2 or 1.5% Lanthanated EWLa-1.5) is a nonessential variable in Table QW-256.

QW-410: Technique

• QW-410.1 (Stringer vs Weave - Nonessential): Nonessential unless impact testing applies.
• QW-410.3 (Orifice / Cup Size - Nonessential): Changing the ceramic gas nozzle diameter (e.g., #6 cup [3/8 in.] to #8 cup [1/2 in.]) is nonessential.
• QW-410.5 (Method of Cleaning - Nonessential): Wire brushing, chemical etching, or solvent wiping changes are nonessential.
• QW-410.9 (Single vs Multiple Passes per Side - Supplementary Essential): When impact tested, changing from multipass to single-pass per side is supplementary essential.
• QW-410.25 (Manual vs Automatic - Essential): Changing between manual and automated equipment is essential.

3. Practical Piping Inspection Scenarios & Audit Findings

Scenario 1: Shielding Gas Composition Substitution

A mechanical contractor qualifies a GTAW procedure on 2-inch NPS Schedule 80 carbon steel pipe using 100% Argon shielding gas. For a heavy-wall alloy project, the welding engineer revises the WPS to specify 75% Argon / 25% Helium to increase heat input and travel speed. The engineer argues: "Both Argon and Helium are inert gases under SFA-5.32, and Helium increases penetration, so no requalification is needed."

• Audit Finding: Critical Violation of QW-408.2. In Table QW-256, changing from a single shielding gas (100% Ar) to a mixture (75% Ar / 25% He), or altering the mixture ratio by more than 5%, is an Essential Variable. Helium significantly raises the arc voltage and thermal conductivity compared to Argon, altering weld penetration, puddle fluidity, and base-metal dilution. The contractor must qualify a new PQR with the 75/25 Ar/He mixture.

Scenario 2: Deleting Purge Gas on Stainless Steel Piping

An ASME B31.3 process piping contractor is welding 3-inch NPS Schedule 40S Type 304L stainless steel (P-No. 8) pipe. The supporting PQR was welded with 100% Argon torch shielding and 100% Argon backing purge gas. To accelerate field production on tie-in welds where internal damming is difficult, the supervisor instructs welders to weld the root pass open-root without backing gas.

• Audit Finding: Severe Violation of QW-408.5. In Table QW-256, the deletion of backing gas is an Essential Variable. Welding austenitic stainless steel without an internal inert gas purge exposes the molten root bead to atmospheric oxygen and nitrogen, producing severe oxidation and porosity ("sugaring"). This destroys corrosion resistance and creates severe notch defects. The contractor cannot delete backing gas without qualifying a dedicated open-root PQR without backing gas.

Scenario 3: Switching Tungsten Electrode Classification

A shop switches its GTAW torch setups from 2% Thoriated tungsten (AWS EWTh-2, red band) to 2% Ceriated tungsten (AWS EWCe-2, orange band) to eliminate airborne radioactive thorium dust during tip grinding. The Authorized Inspector reviews the updated WPS.

• Audit Finding: Fully Compliant. Under Table QW-256 paragraph QW-409.12, tungsten electrode classification is a Nonessential Variable. The contractor is permitted to update the tungsten type on the WPS document without retesting or welding a new PQR coupon.