7.2 Shielded Metal Arc Welding (SMAW) Procedure Variables

7.2 Shielded Metal Arc Welding (SMAW) Procedure Variables

Core Principle: Shielded Metal Arc Welding (SMAW)—often termed manual metal arc or stick welding—relies on a flux-coated consumable electrode to establish the arc, deposit filler metal, and generate a protective gaseous and slag shield. Under Table QW-253, procedure variables are rigorously categorized to guarantee that changes in chemical composition, cooling rate, and thermal history do not degrade tensile strength, ductility, or notch toughness.

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1. Table QW-253 Architecture & Overview

Table QW-253 is the authoritative variable checklist for SMAW procedure qualification under ASME Section IX. When preparing or auditing an SMAW Welding Procedure Specification (WPS Form QW-482) or Procedure Qualification Record (PQR Form QW-483), every variable paragraph listed in Table QW-253 must be evaluated.

The table spans nine distinct technical categories:

1. QW-402: Joints (Groove design, backing, root spacing, retainers)
2. QW-403: Base Metals (Group numbers, thickness limits, P-numbers, heavy passes)
3. QW-404: Filler Metals (F-numbers, A-numbers, diameter, classification, deposit thickness)
4. QW-405: Positions (Addition of position, vertical progression)
5. QW-406: Preheat (Decrease > 100°F, preheat maintenance, interpass increase > 100°F)
6. QW-407: Postweld Heat Treatment (PWHT) (PWHT condition, time/temp range)
7. QW-408: Gas (Not Applicable to standard SMAW)
8. QW-409: Electrical Characteristics (Heat input, current and polarity, amperage/voltage)
9. QW-410: Technique (Stringer vs weave, cleaning, backgouging, manual/automatic)

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2. Paragraph-by-Paragraph Analysis of Table QW-253

QW-402: Joints

• QW-402.1 (Groove Design - Nonessential): A change in the type of groove (V-groove, U-groove, bevel, J-groove), bevel angle, root opening, or root face. Because joint geometry primarily impacts welder access and deposit efficiency rather than bulk metallurgical properties, these parameters may be altered on the WPS without requalifying the PQR.
• QW-402.4 (Backing Deletion - Essential): The deletion of backing is an essential variable. If a PQR coupon is welded with backing (such as a steel backing strip, ceramic tape, or double-welded from both sides with backgouging), the WPS cannot be written for open-root welding without a backing strip. Conversely, qualifying without backing qualifies for production welding both with and without backing.
• QW-402.10 & .11 (Root Spacing & Retainers - Nonessential): Adjusting root fit-up spacing or altering nonmetallic/nonfusing retainers represents a nonessential change.

QW-403: Base Metals

• QW-403.5 (Group Number - Supplementary Essential): For base metals assigned P-Numbers with Group Numbers (e.g., carbon steels in P-No. 1 Group 1 vs Group 2), changing from one Group Number to another within the same P-Number is a supplementary essential variable. When impact testing is mandated by the construction code (e.g., ASME Section VIII Div 1 UCS-66 or ASME B31.3 Table 323.2.2), this change requires PQR requalification.
• QW-403.6 (Minimum Base Metal Thickness for Toughness - Supplementary Essential): When notch toughness is specified, the minimum base metal thickness qualified is the thickness of the test coupon $T$ or 5/8 in. (16 mm), whichever is less. However, where $T$ is less than 1/4 in. (6 mm), the minimum qualified thickness is $T$.
• QW-403.8 (P-Number Change - Essential): A change from one base metal P-Number to any other P-Number, or to a base metal not listed in Table QW/QB-422. Cross-qualification rules are strictly governed by Table QW-424.1 (e.g., qualifying on P-1 to P-1 qualifies P-1 to P-1 only).
• QW-403.9 (Heavy Pass Rule: Pass Thickness > 1/2 in. - Essential): If any single pass of SMAW weld metal exceeds 1/2 inch (13 mm) in thickness during coupon fabrication, the maximum qualified base metal thickness is limited to $1.1T$ rather than the standard $2T$ permitted by Table QW-451.1! Heavy single passes generate massive thermal heat inputs, resulting in coarse, unrefined grain structures that degrade tensile ductility and notch toughness.
• QW-403.11 (Base Metal Postweld Heat Treated - Essential): For procedure qualification, base metals are categorized by whether they receive PWHT.

QW-404: Filler Metals

• QW-404.4 (F-Number Change - Essential): A change from one filler metal F-Number (Table QW-432) to any other F-Number. In SMAW:
  • F-No. 1: Heavy rutile / iron powder electrodes (EXX20, EXX24, EXX27, EXX28)
  • F-No. 2: Rutile electrodes (EXX12, EXX13, EXX14)
  • F-No. 3: Cellulosic high-penetration electrodes (E6010, E7010, E8010)
  • F-No. 4: Low-hydrogen basic electrodes (E7015, E7016, E7018, E8018, E9018)
  • F-No. 5: Austenitic stainless steel electrodes (E308, E309, E316)
  • Crucial Distinction: Welder performance allows F-4 to qualify F-1 through F-4 under QW-433, but for procedure qualification, switching from E6010 (F-3) to E7018 (F-4) requires welding a new PQR coupon!
• QW-404.5 (A-Number Change - Essential): A change in the chemical composition of the weld deposit from one A-Number (Table QW-442) to another (e.g., A-1 carbon steel to A-2 carbon-molybdenum steel).
• QW-404.6 (Electrode Diameter - Nonessential): Changing electrode diameter (e.g., from 3/32 in. to 1/8 in. or 5/32 in.) is a nonessential variable for standard SMAW procedures. The manufacturer can alter electrode diameters on the WPS to optimize deposition rates.
• QW-404.12 (AWS Classification - Supplementary Essential): A change in the AWS classification of the filler metal within an SFA specification (e.g., changing from E7018 to E7018-1 with improved low-temperature Charpy values) is supplementary essential.
• QW-404.30 (Deposited Thickness $t$ - Essential): A change in the qualified deposited weld metal thickness beyond the limits of Table QW-451.1.

QW-405: Positions

• QW-405.1 (Addition of Position - Nonessential): In SMAW, procedure qualification on a flat plate (1G) qualifies the WPS for all positions (horizontal 2G, vertical 3G, overhead 4G, pipe 5G/6G), provided notch-toughness testing is not required.
• QW-405.2 (Change in Progression - Supplementary Essential): When impact testing applies, changing the vertical progression from uphill to downhill, or from downhill to uphill, is a supplementary essential variable. Uphill progression uses weave beads with high heat input, whereas downhill progression utilizes rapid stringer beads with low heat input.
• QW-405.3 (Change in Progression - Nonessential): When impact testing is not required, changing vertical progression is nonessential.

QW-406: Preheat

• QW-406.1 (Decrease > 100°F / 55°C - Essential): A decrease in preheat temperature of more than 100°F (55°C) below the minimum preheat temperature recorded on the PQR is an essential variable. Lowering preheat increases the cooling rate, promoting the formation of brittle martensite and increasing the risk of hydrogen-induced cold cracking (HICC). Note: Increasing preheat is nonessential.
• QW-406.3 (Interpass Temperature Increase > 100°F / 55°C - Supplementary Essential): An increase in the maximum interpass temperature of more than 100°F (55°C) above that recorded on the PQR is a supplementary essential variable. Excessive interpass temperatures retard cooling, coarsening grains and drastically reducing Charpy impact toughness.

QW-407: Postweld Heat Treatment (PWHT)

• QW-407.1 (PWHT Condition - Essential): A change in any of the following PWHT conditions is an essential variable:
  1. None (As-welded).
  2. PWHT below the lower transformation temperature (e.g., subcritical stress relief at 1100°F–1200°F for carbon steel).
  3. PWHT above the upper transformation temperature (normalizing).
  4. PWHT between upper and lower transformation temperatures (intercritical).
  5. PWHT above upper transformation followed by heat treatment below lower transformation (quenching and tempering).
• QW-407.2 (PWHT Time & Temperature Range - Supplementary Essential): A change in the PWHT holding temperature and time range is supplementary essential because prolonged thermal soaking reduces yield and impact properties.

QW-409: Electrical Characteristics

• QW-409.1 (Heat Input Increase - Supplementary Essential): An increase in heat input beyond that qualified on the PQR is supplementary essential. Calculated as: $\text{Heat Input (J/in)} = \frac{\text{Voltage} \times \text{Amperage} \times 60}{\text{Travel Speed (in/min)}}$
• QW-409.4 (Current and Polarity - Nonessential): In Table QW-253 for SMAW, changing current from AC to DC, or changing polarity from DCEP (reverse) to DCEN (straight), is a Nonessential Variable! (Exam alert: In GTAW Table QW-256, polarity change is an Essential Variable).

QW-410: Technique

• QW-410.1 (Stringer vs Weave - Nonessential): Switching between stringer and weave beads is nonessential for non-impact procedures.
• QW-410.5 (Cleaning Method - Nonessential): Changing from grinding to wire brushing or chipping is nonessential.
• QW-410.25 (Manual vs Automatic - Essential): Changing from manual welding to semiautomatic, machine, or automatic welding is an essential variable.

3. Deep Dive: The Heavy-Pass Rule (QW-403.9)

One of the most heavily tested numerical provisions on the AWS ASME Endorsement examination is paragraph QW-403.9.

Under standard procedure qualification rules (Table QW-451.1), welding a test coupon of thickness $T$ qualifies the procedure to weld production base metals up to a thickness of $2T$ (for $T \ge 3/16$ in.). For example, a 1.0-inch coupon normally qualifies base metal from 3/16 inch up to 2.0 inches.

The Heavy-Pass Exception: If any individual pass deposited during the welding of the test coupon exceeds 1/2 inch (13 mm) in thickness: $\text{Maximum Qualified Base Metal Thickness } T_{\text{max}} = 1.1 \times T_{\text{coupon}}$

Mathematical Comparison Scenario:

• Coupon A: Welded on 1.0-inch thick plate using 10 passes of SMAW (average pass thickness = 0.100 in.). No single pass exceeds 0.50 in. $\text{Base Metal Range Qualified} = 3/16 \text{ in. to } 2.0 \text{ in. } (2T)$
• Coupon B: Welded on 1.0-inch thick plate using a massive root weave measuring 0.550 in. thick, followed by two heavy fill passes. Because at least one pass exceeds 0.50 in., QW-403.9 is triggered. $\text{Base Metal Range Qualified} = 3/16 \text{ in. to } (1.1 \times 1.0) = 1.1 \text{ inches!}$

EXAM TRAP: Candidates seeing a 1.0-inch test coupon automatically select "2.0 inches" as the maximum qualified thickness. Always check the PQR pass breakdown! If any single pass is greater than 0.50 inches, the answer is 1.1T.