11.3 Essential, Non-Essential & Supplementary Essential Variables

11.3 Essential, Non-Essential & Supplementary Essential Variables

The Engineering Hierarchy of Variables: The core mechanism by which ASME Section IX controls welding quality is the systematic classification of welding parameters into Essential Variables, Non-Essential Variables, and Supplementary Essential Variables. In aboveground storage tank construction and repair, an Authorized Inspector must instantly discern which field alterations require a completely new Procedure Qualification Record (PQR) with destructive mechanical testing, and which may be resolved through an administrative revision to the Welding Procedure Specification (WPS).

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1. Classification Architecture of Welding Variables (QW-250)

Under ASME Section IX Article II, each welding process—Shielded Metal Arc Welding (SMAW, QW-253), Submerged Arc Welding (SAW, QW-254), Gas Metal Arc Welding (GMAW/FCAW, QW-255), and Gas Tungsten Arc Welding (GTAW, QW-256)—has a dedicated variable table. Variables are categorized into three distinct classes:

+-------------------------------------------------------------------------+
|                   ASME SECTION IX VARIABLE TAXONOMY                     |
+-----------------------+-----------------------+-------------------------+
| Variable Category     | Metallurgical Effect  | Action Required on      |
|                       |                       | Parameter Change        |
+-----------------------+-----------------------+-------------------------+
| **Essential**         | Alters mechanical     | **Mandates New PQR**    |
| **Variables**         | tensile, yield, or    | with coupon welding and |
| (QW-200.1(b))         | bend properties       | destructive testing     |
+-----------------------+-----------------------+-------------------------+
| **Non-Essential**     | Affects bead shape,   | **WPS Revision Only**;  |
| **Variables**         | deposition rate, or   | no new coupon or PQR    |
| (QW-200.1(c))         | operational technique | testing required        |
+-----------------------+-----------------------+-------------------------+
| **Supplementary**     | Alters Charpy V-notch | **Mandates New PQR**    |
| **Essential**         | low-temperature       | **ONLY when toughness** |
| (QW-200.1(d))         | impact toughness      | is invoked by code      |
+-----------------------+-----------------------+-------------------------+

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2. Essential Variables: Mechanics and Requalification Triggers

An Essential Variable is defined under QW-200.1(b) as a condition where a change in the variable is considered to affect the mechanical properties (other than notch toughness) of the weldment. If an essential variable is changed beyond the permitted range, the existing WPS is invalidated, and a new test coupon must be welded and mechanically tested to generate a supporting PQR.

Critical Essential Variables Across Storage Tank Welding

1. Base Metal P-Number (QW-403.11 & QW-420):
   • ASME Section IX assigns base metals to P-Numbers based on chemical composition, weldability, and mechanical properties. Carbon steels commonly used in storage tanks (ASTM A36, A283 Grade C, A516 Grade 70, A573 Grade 70) are assigned to P-No. 1.
   • A change from one P-Number to another (e.g., from P-No. 1 carbon steel to P-No. 8 austenitic stainless steel) is an essential variable requiring full requalification. Welding dissimilar P-numbers (e.g., P-No. 1 to P-No. 8) requires a dedicated PQR qualifying that specific bimetallic combination.
2. Base Metal Thickness Range Qualified ($T$) (QW-403.8 & Table QW-451.1):
   • The thickness of the test coupon welded during procedure qualification dictates the minimum and maximum base metal thickness envelope qualified for production:
     • For test coupon thickness $T < 3/8\text{ in. (10 mm)}$: Qualified range is $1/16\text{ in. (1.5 mm)}$ to $2T$.
     • For $3/8\text{ in.} \le T < 3/4\text{ in. (19 mm)}$: Qualified range is $3/16\text{ in. (5 mm)}$ to $2T$.
     • For $T \ge 3/4\text{ in. (19 mm)}$: Qualified range is $3/16\text{ in. (5 mm)}$ to $2T$, except that when $T \ge 1.5\text{ in. (38 mm)}$, maximum qualified thickness is $8\text{ in. (200 mm)}$.
   • If a contractor qualifies on a 0.500-inch plate, the maximum shell plate thickness they may weld in production is $2 \times 0.500 = 1.000\text{ inch}$. Welding a 1.250-inch shell insert plate requires a new PQR welded on a coupon of sufficient thickness.
3. Welding Process (QW-401.1):
   • A change from SMAW (stick) to GMAW (MIG), FCAW (flux-cored), or SAW (submerged arc) is a fundamental essential variable. In multi-process procedures (e.g., GTAW root followed by SMAW fill), each individual process must be qualified for its deposited thickness ($t$) per QW-404.30.
4. Filler Metal F-Number (QW-404.4):
   • A change in filler metal from one F-Number (Table QW-432) to another (e.g., switching from F-No. 3 cellulosic E6010 to F-No. 4 low-hydrogen E7018) is an essential variable requiring procedure requalification.
5. Filler Metal A-Number (QW-404.5):
   • A-Numbers (Table QW-442) classify the chemical composition of deposited carbon and alloy weld metal. A change from A-No. 1 (plain carbon steel weld metal) to A-No. 2 (carbon-molybdenum) or A-No. 8 (austenitic stainless steel) is an essential variable.
6. Post-Weld Heat Treatment (QW-407.1):
   • A change in PWHT condition—such as qualifying without PWHT and subsequently applying PWHT in production, or vice versa—is an essential variable requiring a new PQR.

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3. Non-Essential Variables: Engineering Flexibility & Administrative Revisions

A Non-Essential Variable is defined under QW-200.1(c) as a condition where a change may be made in the WPS without requalifying the procedure. These variables influence the appearance, bead shape, operating ease, or deposition rate of the weld puddle, but do not alter the tensile strength, ductility, or bend properties of the completed joint.

Common Non-Essential Variables in Storage Tank Procedures

• Joint Design and Geometry (QW-402.1): Modifying groove preparation, such as changing a single-V butt joint to a double-V butt joint, changing the included bevel angle from 60° to 75°, or adjusting root face land and root opening dimensions.
• Backing (QW-402.4): Adding or deleting a permanent steel backing strip, ceramic backing, or changing backing material.
• Electrode Diameter (QW-404.14): Changing the core wire size of an electrode (e.g., from 1/8-in. E7018 to 5/32-in. E7018) within the same AWS classification and F-Number.
• Method of Cleaning and Preparation (QW-410.1): Changing from mechanical wire brushing to disc grinding, chipping, or carbon-arc backgouging.
• Bead Technique (QW-410.5): Switching between a stringer bead technique and a weave bead technique (provided supplementary essential toughness rules do not restrict heat input).
• Peening (QW-410.6): The addition or deletion of mechanical peening on intermediate weld passes.

Administrative Rule: When a non-essential variable is changed, the contractor updates the WPS document, increments the revision number, logs the date, and issues the document for production. No test coupons are welded, and no mechanical testing is conducted.

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4. Supplementary Essential Variables: The Notch Toughness Regime

A Supplementary Essential Variable (QW-200.1(d)) is a specialized variable that is classified as non-essential under standard conditions, but becomes an Essential Variable whenever the referencing code mandates notch toughness (Charpy V-notch / CVN) testing.

When is Toughness Mandated in Storage Tanks?

In storage tank engineering, notch toughness is not universally required for every atmospheric vessel. However, under API Standard 650 Section 4.2.9, Table 4.4, and Section 9.2.2, as well as API 653 Section 11.1.2, toughness testing becomes mandatory under specific operational and material envelopes:

1. Tanks constructed from Group IV, Group IVA, Group V, or Group VI steels (e.g., ASTM A537 Class 1 or 2, normalized A516 Gr. 70, A662, A737).
2. Tanks operating at low Design Metal Temperatures (DMT) falling below the exemption curves in API 650 Figure 4.1.
3. Shell plates exceeding specific thickness thresholds (e.g., plates thicker than 1.5 inches) operating under high hydrostatic tensile membrane stress.
4. Low-temperature or refrigerated product storage.

Critical Supplementary Essential Variables

When notch toughness is specified, the following parameters immediately become essential variables, requiring a new PQR if altered:

1. Increase in Heat Input (QW-409.1): An increase in calculated heat input or volume of weld metal deposited per unit length over that qualified on the PQR.
2. Change in Welding Position (QW-405.2): A change in position (e.g., from flat 1G to vertical 3G) or a change in vertical progression from uphill to downhill.
3. Decrease in Preheat Temperature (QW-406.1): A decrease of more than 100°F (55°C) in the minimum preheat temperature qualified.
4. Increase in Interpass Temperature (QW-406.3): An increase of more than 100°F (55°C) in the maximum interpass temperature recorded on the PQR.
5. Change in Filler Metal Classification (QW-404.12): A change in the specific AWS classification (e.g., from E7018 to E7018-1) or manufacturer trade name where impact properties are guaranteed by classification.
6. Change in PWHT Time and Temperature (QW-407.2): A change in PWHT soaking temperature or a change in soaking time exceeding 25%.

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5. Heat Input Calculation and Microstructural Toughness Mechanics

Heat input ($HI$) is the primary thermodynamic metric governing the cooling rate and final grain morphology of the weld metal and heat-affected zone (HAZ).

The Heat Input Formula (ASME IX QW-409.1)

Heat input is quantified as electrical energy delivered per unit length of weld bead:

$HI = \frac{V \times I \times 60}{S \times 1000}$

Where:

• $HI$ = Heat Input in kilojoules per inch ($\text{kJ/in.}$) or kilojoules per millimeter ($\text{kJ/mm}$)
• $V$ = Arc Voltage (Volts)
• $I$ = Welding Current (Amperes)
• $S$ = Travel Speed of the arc in inches per minute ($\text{in/min}$) or millimeters per minute ($\text{mm/min}$)
• $60$ = Conversion factor (seconds per minute)
• $1000$ = Conversion factor (Joules to kiloJoules)

   LOW HEAT INPUT (Fast Travel Speed)            HIGH HEAT INPUT (Slow Weave / High Amps)
   -----------------------------------            ----------------------------------------
   - Rapid Cooling Rate (T8/5)                    - Slow Cooling Rate (Extended T8/5)
   - Fine-Grained Acicular Ferrite                - Coarse Grain HAZ (Grain Growth)
   - High Tensile Strength                        - Upper Bainite / Coarse Ferrite Networks
   - EXCELLENT CHARPY IMPACT TOUGHNESS            - SEVERE LOSS OF CHARPY IMPACT TOUGHNESS

Why High Heat Input Destroys Impact Toughness

• When heat input is excessively high (e.g., dragging a heavy weave pass at slow travel speed or operating at excessive amperage), the cooling rate through the critical transformation range (800°C down to 500°C, known as $\Delta t_{8/5}$) is severely retarded.
• Slow cooling allows austenite grains in the coarse-grained HAZ (CGHAZ) immediately adjacent to the fusion line to grow excessively large. Coarse grains transform into coarse pearlite, upper bainite, and pro-eutectoid ferrite networks along prior austenite grain boundaries.
• Under dynamic shock loading at low operating temperatures, these coarse microstructures provide low-energy cleavage planes, causing catastrophic brittle fracture at dramatically reduced impact energy levels.
• Consequently, when toughness is specified, ASME Section IX strictly prohibits increasing the production heat input above the maximum heat input validated during PQR coupon fabrication.

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6. Comparative Welding Variables Matrix for Storage Tank Processes

The following comparative table contrasts the classification of critical welding variables across the three primary processes deployed in aboveground storage tank fabrication and repair: