6.1 Welding Procedures (WPS/PQR) & Essential Variables
Key Takeaways
- A Welding Procedure Specification (WPS) is a qualified written instruction providing direction to the welder for making production welds; a Procedure Qualification Record (PQR) is a factual record documenting actual variables used during welding of a test coupon and the certified mechanical test results.
- ASME Section IX classifies welding variables into Essential Variables (changes that alter mechanical properties and require a new PQR), Supplementary Essential Variables (notch-toughness variables required only when Charpy impact testing is specified), and Nonessential Variables (procedural adjustments editable on the WPS without requalification).
- Base metals are assigned P-Numbers based on weldability and chemical/mechanical properties (P-1 Carbon Steel, P-3 Low Alloy C-Mo, P-4 Cr-Mo 1-1.5Cr, P-5A/B/C Cr-Mo 2.25-9Cr, P-8 Austenitic Stainless Steel); filler metals are grouped by usability into F-Numbers (F-1 heavy rutile, F-2 rutile, F-3 cellulosic, F-4 low hydrogen, F-5 stainless, F-6 wire/rods); weld metal chemistry is classified by A-Numbers.
- Per ASME Section IX Table QW-451.1, test coupon thickness T qualifies base metal thickness ranges from 1/16 in. (1.5 mm) to 2T for coupons under 3/8 in., and 3/16 in. (5 mm) to 2T for coupons 3/8 in. to under 3/4 in.; deposited weld metal thickness t qualifies up to 2t for each welding process.
- PQR mechanical testing mandates a minimum of two tension test specimens (must meet or exceed base metal specified minimum tensile strength, or within 5% if failure occurs outside the weld/fusion zone) and four guided-bend test specimens (root/face for T < 3/4 in., side bends for T >= 3/4 in.) with a maximum allowable open defect of 1/8 in. (3 mm).
Welding Procedures (WPS/PQR) & Essential Variables
In pressure vessel manufacturing, alteration, and repair governed by API 510 and ASME Section VIII, welding is the primary method used to join pressure-retaining components. To ensure that every production weld exhibits metallurgical integrity, sound mechanical properties, and sufficient resistance to operational degradation, the welding process must follow proven, qualified procedures.
ASME Boiler and Pressure Vessel Code (BPVC) Section IX (Welding, Brazing, and Fusing Qualifications) establishes the mandatory rules for qualifying welding procedures, welders, and welding operators. API Recommended Practice 577 (Welding Processes, Inspection, and Metallurgy) provides practical guidance for refining and petrochemical equipment inspectors. An Authorized API 510 Inspector must possess an exhaustive understanding of how to audit, review, and verify Welding Procedure Specifications (WPS) and Procedure Qualification Records (PQR).
1. The Tripartite Framework: WPS, PQR, and WPQ Defined
ASME Section IX divides welding qualification into two distinct areas: qualifying the procedure (proving that the proposed combination of materials, processes, and parameters yields acceptable mechanical properties) and qualifying the performance of the welder (proving the individual's manual ability to deposit sound weld metal).
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| ASME SECTION IX QUALIFICATION TRIAD |
| |
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| | PROCEDURE QUALIFICATION RECORD (PQR) | |
| | - Factual historical record of welding a test coupon | |
| | - Records actual variables used (volts, amps, travel speed, etc.) | |
| | - Records certified destructive mechanical test results | |
| | - Never altered or edited (except for verified clerical errors) | |
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| | |
| v Serves as the foundation for |
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| | WELDING PROCEDURE SPECIFICATION (WPS) | |
| | - Written instruction document provided to the welder/shop | |
| | - Defines qualified ranges for all Essential & Nonessential vars | |
| | - Can be revised, edited, and amended within PQR limits | |
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| | |
| v Executed in production by |
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| | WELDER PERFORMANCE QUALIFICATION (WPQ) | |
| | - Validates individual welder dexterity and manual skill | |
| | - Demonstrates ability to deposit sound, defect-free weld metal | |
| | - Follows qualified WPS parameters during performance testing | |
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Detailed Roles & Document Functions
| Document Type | ASME Code Section | Primary Purpose | Modifiability | Key Data Recorded |
|---|---|---|---|---|
| Welding Procedure Specification (WPS) | QW-200.1 | Written operational instruction sheet for welders in the shop and field. | Living Document: May be edited/revised for nonessential variables without new mechanical testing. | Qualified ranges of thickness, base metals (P-Nos), filler metals (F/A-Nos), preheat, PWHT, volts, amps, joint design. |
| Procedure Qualification Record (PQR) | QW-200.2 | Certified record of variables used during the welding of a test coupon and resultant destructive test values. | Permanent Record: Locked historical document; cannot be changed except for demonstrated clerical corrections. | Actual coupon thickness, actual base metal heat/spec, actual heat input, tension test results, bend test results, impact test values. |
| Welder Performance Qualification (WPQ) | QW-300 | Validates that an individual welder or welding operator can make sound welds using a specific process. | Individual Record: Maintained via 6-month continuity logs; renewed if process lapses. | Welder ID, process, position tested, backing, pipe diameter, F-number, test results (bend tests or volumetric NDE). |
2. ASME Section IX Variable Taxonomy
Every welding process (SMAW, GTAW, GMAW, FCAW, SAW) has a dedicated variable table in ASME Section IX (such as Table QW-253 for SMAW and Table QW-256 for GTAW). Section IX categorizes all welding variables into three distinct classes:
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| WELDING VARIABLE CLASSIFICATION HIERARCHY |
| |
| [ESSENTIAL VARIABLES] ------------------------------------------------+ |
| - Directly affect mechanical properties (tensile, yield, ductility) | |
| - Any change beyond qualified range DEMANDS A NEW PQR | |
| - Examples: Welding process, P-Number change, F-Number change, | |
| PWHT addition/deletion, base metal thickness beyond QW-451 limits | |
| | |
| [SUPPLEMENTARY ESSENTIAL VARIABLES] ----------------------------------+ |
| - Directly affect notch toughness (Charpy V-notch impact properties) | |
| - Required ONLY when construction code mandates impact testing | |
| - Treated as Nonessential when impact testing is NOT specified | |
| - Examples: Change in Group Number, heat input increase, | |
| decrease in preheat > 100°F, change in welding position (uphill) | |
| | |
| [NONESSENTIAL VARIABLES] ---------------------------------------------+ |
| - Affect procedural operation and arc stability, NOT mechanics | |
| - May be modified on the WPS without requalifying or writing a new PQR| |
| - Examples: Joint groove design, backing material, cleaning method, | |
| tungsten electrode size, shielding gas flow rate minor adjustments | |
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Comparative Analysis of Variable Types
| Variable Category | Code Clause | Effect of Variable Change | Required Action on Change | Governing Context |
|---|---|---|---|---|
| Essential Variable | QW-200.1(b) / QW-250 series | Alters the tensile strength, yield strength, or ductility of the completed weldment. | Mandatory Requalification: Weld a new test coupon, execute destructive tests, and certify a new PQR. | Applies to all procedure qualifications regardless of service temperature. |
| Supplementary Essential Variable | QW-200.1(c) / QW-401.3 | Alters the low-temperature notch toughness or Charpy impact transition temperature. | Requalification with Impact Testing: Requires a new PQR including Charpy V-notch impact tests. | Applies only when referencing code (e.g., ASME Section VIII Div 1 UCS-66 / UG-84) mandates impact tests. |
| Nonessential Variable | QW-200.1(d) / QW-250 series | Influences operating technique, bead shape, or deposition rate without degrading mechanical strength. | WPS Revision Only: Update the parameters on the face of the WPS; no coupon welding or testing required. | Applies to all procedures; allows operational flexibility in fabrication shops. |
3. Material & Consumable Grouping Architecture
To avoid the crippling economic burden of qualifying a separate welding procedure for every commercial alloy grade and electrode brand, ASME Section IX establishes standardized alphanumeric grouping systems for base metals, filler metals, and weld deposit chemistries.
P-Numbers (Base Metal Grouping — QW/QB-422)
P-Numbers group base metals according to their weldability, chemical composition, and mechanical properties. Qualifying a procedure on one alloy within a P-Number qualifies the procedure for all other base metals within that same P-Number (subject to supplementary essential variable restrictions).
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| ASME P-NUMBER CLASSIFICATION SYSTEM |
| |
| [P-1] ---> Carbon-Manganese Steels (SA-516 Gr 70, SA-106 Gr B, SA-53 Gr B) |
| [P-3] ---> Low Alloy Steels (0.5% Mo, C-Mo, Mn-Mo, e.g., SA-204) |
| [P-4] ---> Cr-Mo Low Alloy Steels (1.0% to 1.5% Cr, 0.5% Mo, e.g., SA-387)|
| [P-5A] ---> Cr-Mo Alloys (2.25% Cr - 1.0% Mo, e.g., SA-387 Gr 22) |
| [P-5B] ---> Cr-Mo Alloys (5.0% to 9.0% Cr, e.g., 5Cr-0.5Mo, 9Cr-1Mo-V P91)|
| [P-5C] ---> Cr-Mo-V Martensitic Alloy Steels |
| [P-8] ---> Austenitic Stainless Steels (AISI 304, 304L, 316, 316L, 321, |
| 347, e.g., SA-240 Type 304/316) |
| [P-9A/B]-> 2.5% to 3.5% Nickel Alloy Steels (Low Temperature Service) |
| [P-10H] -> Duplex and Super Duplex Stainless Steels (2205, 2507) |
| [P-4X] ---> Nickel-Base Alloys (Inconel 600, 625, Hastelloy, Monel) |
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Group Numbers within P-Numbers:
Base metals within P-1, P-3, P-4, P-5, and P-8 are further subdivided into Group Numbers (e.g., P-1 Group 1, P-1 Group 2).
- When notch toughness / impact testing is NOT specified, a change in Group Number is nonessential.
- When impact testing is mandated by the construction code (e.g., ASME Section VIII Div 1 UCS-66), a change in Group Number is a Supplementary Essential Variable requiring procedure requalification.
F-Numbers (Filler Metal Grouping — QW-432)
F-Numbers group filler metals and covered electrodes based on their usability characteristics, which fundamentally determine the skill and technique required by the welder.
| F-Number | AWS Classification Examples | Electrode Characteristics & Primary Use |
|---|---|---|
| F-1 | EXX20, EXX24, EXX27, EXX28 | Heavy rutile or iron powder coatings; high deposition rates; limited to flat and horizontal fillet positions. |
| F-2 | EXX12, EXX13, EXX14 | Rutile-coated general fabrication electrodes; shallow penetration; medium slag. |
| F-3 | EXX10, EXX11 (e.g., E6010, E7010-P1) | Cellulosic flux coating; deep-penetrating, forceful arc; rapid solidification; standard for open-root pipeline and pipe welds. |
| F-4 | EXX15, EXX16, EXX18 (e.g., E7018, E8018-B2) | Low-hydrogen flux coatings with iron powder; resistant to hydrogen-assisted cracking; standard for structural and pressure vessel welds. |
| F-5 | E308-16, E309-16, E316-16, E347-15 | Austenitic and duplex stainless steel covered electrodes. |
| F-6 | ER70S-6, ER80S-D2, E71T-1, E71T-8 | Bare solid wire, composite, and flux-cored electrodes for GTAW, GMAW, SAW, and FCAW processes (carbon and low alloy). |
A-Numbers (Weld Deposit Chemistry — QW-442)
A-Numbers classify the chemical analysis of the as-deposited ferrous weld metal. Unlike P-Numbers (which categorize base metal) and F-Numbers (which categorize electrode usability), A-Numbers verify that the completed weld deposit matches the required metallurgical composition.
- A-1: Plain Carbon Steel weld deposit
- A-2: Carbon-Molybdenum weld deposit ($0.40% \text{ to } 0.65%\text{ Mo}$)
- A-3: Chromium-Molybdenum ($0.40% \text{ to } 2.0%\text{ Cr}, 0.40% \text{ to } 0.65%\text{ Mo}$)
- A-4: Chromium-Molybdenum ($2.0% \text{ to } 4.0%\text{ Cr}, 0.40% \text{ to } 1.5%\text{ Mo}$)
- A-8: Austenitic Stainless Steel ($18% \text{ to } 30%\text{ Cr}, 8% \text{ to } 15%\text{ Ni}$)
4. Base Metal & Weld Metal Thickness Qualification Ranges (ASME Table QW-451.1)
One of the most heavily tested topics on the API 510 examination is determining the qualified thickness limits on a WPS based on the thickness of the test coupon recorded on the PQR.
ASME Section IX Table QW-451.1 establishes the allowable ranges for Base Metal Thickness ($T$) and Deposited Weld Metal Thickness ($t$):
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| TABLE QW-451.1: PQR THICKNESS QUALIFICATION MATRIX |
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| Test Coupon Thickness (T) Qualified Base Metal Thickness Range |
| ------------------------- ------------------------------------ |
| T < 1/16 in. (1.5 mm) T to 2T |
| 1/16 in. <= T < 3/8 in. 1/16 in. (1.5 mm) to 2T |
| 3/8 in. <= T < 3/4 in. 3/16 in. (5.0 mm) to 2T |
| 3/4 in. <= T < 1.5 in. 3/16 in. (5.0 mm) to 2T |
| T >= 1.5 in. (38 mm) 3/16 in. (5.0 mm) to 2T (or 8 in. max) |
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Comprehensive Table QW-451.1 Thickness & Test Specimen Rules
| Coupon Thickness $T$ Tested on PQR | Qualified Base Metal Range ($T_{\text{min}}$ to $T_{\text{max}}$) | Max Deposited Weld Metal ($t$) Qualified | Mandatory Tension Specimens | Mandatory Guided Bend Specimens |
|---|---|---|---|---|
| $T < 1/16\text{ in.} (1.5\text{ mm})$ | $T$ to $2T$ | $2t$ | 2 Tension (QW-150) | 2 Root Bend + 2 Face Bend |
| $1/16\text{ in.} \le T < 3/8\text{ in.} (10\text{ mm})$ | $1/16\text{ in.} (1.5\text{ mm})$ to $2T$ | $2t$ | 2 Tension (QW-150) | 2 Root Bend + 2 Face Bend |
| $3/8\text{ in.} \le T < 3/4\text{ in.} (19\text{ mm})$ | $3/16\text{ in.} (5.0\text{ mm})$ to $2T$ | $2t$ | 2 Tension (QW-150) | 2 Root + 2 Face (or 4 Side Bends) |
| $3/4\text{ in.} \le T < 1.5\text{ in.} (38\text{ mm})$ | $3/16\text{ in.} (5.0\text{ mm})$ to $2T$ | $2t$ | 2 Tension (QW-150) | 4 Side Bends (Mandatory) |
| $T \ge 1.5\text{ in.} (38\text{ mm})$ | $3/16\text{ in.} (5.0\text{ mm})$ to $2T$ (Max $8\text{ in.}$ if pass $t \le 1/2\text{ in.}$) | $2t$ (or $8\text{ in.}$ if pass $t \le 1/2\text{ in.}$) | 2 Tension (QW-150) | 4 Side Bends (Mandatory) |
[!IMPORTANT] Deposited Weld Metal Thickness ($t$) Rule: The maximum thickness of deposited weld metal qualified is $2t$, where $t$ is the thickness of weld metal deposited by a specific process in the test coupon. If a PQR combines GTAW ($t = 0.125\text{ in.}$) for the root pass and SMAW ($t = 0.375\text{ in.}$) for the fill/cap on a $0.500\text{ in.}$ coupon:
- GTAW is qualified for weld metal thickness up to $2 \times 0.125\text{ in.} = \mathbf{0.250\text{ in.}}$
- SMAW is qualified for weld metal thickness up to $2 \times 0.375\text{ in.} = \mathbf{0.750\text{ in.}}$
- The total base metal qualified is $3/16\text{ in.} \text{ to } 2 \times 0.500\text{ in.} = \mathbf{0.1875\text{ in. to } 1.000\text{ in.}}$
5. Mechanical Testing Protocols & Acceptance Criteria
To qualify a PQR, the welded test coupon must undergo destructive mechanical testing in accordance with ASME Section IX Articles I and IV.
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| PQR MECHANICAL TESTING REQUIREMENTS |
| |
| [TENSION TESTS (QW-150)] [GUIDED BEND TESTS (QW-160)] |
| - Minimum 2 specimens required - Minimum 4 specimens req'd |
| - Evaluates ultimate tensile strength - Evaluates sound ductility |
| - Pull until mechanical fracture occurs - Bent around wraparound die |
| |
| Acceptance Criteria (QW-153): Acceptance Criteria (QW-163):|
| 1. Tensile >= Base metal specified min 1. No open defect > 1/8 in. |
| 2. If break occurs in base metal outside (3.2 mm) in any direction |
| weld/fusion line, acceptable if 2. Corner cracks ignored |
| stress >= 95% of specified min SMTS unless from slag/LOF |
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Tension Testing Standards (QW-150 / QW-153)
- Specimen Count: A minimum of two tension test specimens must be pulled to rupture.
- Acceptance Threshold (QW-153.1):
- The specimen must exhibit an ultimate tensile strength equal to or greater than the Specified Minimum Tensile Strength (SMTS) of the base metal (or the weaker base metal if joining dissimilar alloys).
- The 5% Base Metal Break Rule (QW-153.1(d)): If the specimen breaks in the base metal outside the weld and fusion line, the test is fully acceptable provided the ultimate tensile strength is not more than 5% below the specified minimum tensile strength of the base metal (i.e., at least 95% of SMTS).
Guided Bend Testing Standards (QW-160 / QW-163)
- Specimen Count & Orientation: A minimum of four guided-bend specimens are required:
- For coupon thickness $T < 3/4\text{ in.} (19\text{ mm})$: Two transverse root bends and two transverse face bends (or four side bends if $T \ge 3/8\text{ in.}$). Transverse root bends stretch the root pass; face bends stretch the weld crown.
- For coupon thickness $T \ge 3/4\text{ in.} (19\text{ mm})$: Four transverse side bends are mandatory.
- Acceptance Threshold (QW-163):
- Following bending through 180° in a standard guided bend jig, the convex surface of the specimen must contain no open discontinuity in the weld or heat-affected zone (HAZ) exceeding $1/8\text{ in.} (3.2\text{ mm})$ measured in any direction.
- Open discontinuities originating at the specimen corners are ignored during evaluation unless the inspector determines they originate from lack of fusion, slag inclusions, or internal planar flaws.
6. Common Exam Traps in WPS & PQR Evaluation
[!WARNING] Trap 1: PQR vs. WPS Thickness Range Confusion: Exam questions often ask: "A test coupon of 0.500 in. was welded on a PQR. What is the qualified base metal thickness on the WPS?" The PQR records the single exact value tested ($0.500\text{ in.}$). The WPS lists the qualified range per Table QW-451.1: $3/16\text{ in.} \text{ to } 1.000\text{ in.}$ ($5\text{ mm to } 25\text{ mm}$). Do not confuse the PQR test value with the WPS operational range!
[!IMPORTANT] Trap 2: The 5% Tensile Under-Strength Rule: If SA-516 Grade 70 (SMTS = $70,000\text{ psi}$) breaks in the weld metal at $68,500\text{ psi}$, it is a REJECT because all weld breaks must equal or exceed $70,000\text{ psi}$. However, if it breaks in the base metal outside the weld at $68,500\text{ psi}$, it is an ACCEPTABLE PASS because $68,500 \ge 0.95 \times 70,000 = 66,500\text{ psi}$.
[!TIP] Trap 3: Supplementary Essential Variables and Impact Testing: Changing from vertical downhill to vertical uphill, or increasing heat input, is a Nonessential Variable for standard procedure qualification under ASME Section IX. However, the instant impact testing is mandated by ASME Section VIII (UCS-66/UG-84), these changes become Supplementary Essential Variables requiring complete procedure requalification.
An API 510 inspector reviews a PQR coupon welded on SA-516 Grade 70 carbon steel plate (specified minimum tensile strength of 70,000 psi). Two tension test specimens are pulled. Specimen 1 breaks in the base metal at 67,200 psi. Specimen 2 breaks in the base metal at 68,500 psi. What is the status of this tension test?
A welding procedure is qualified using a single SMAW test coupon with a thickness (T) of 0.500 in. (12.7 mm). According to ASME Section IX Table QW-451.1, what is the qualified base metal thickness range that may be listed on the resulting WPS?
Which of the following changes represents an ESSENTIAL variable for a Shielded Metal Arc Welding (SMAW) procedure qualification under ASME Section IX Table QW-253?
When guided-bend test specimens are evaluated for PQR qualification under ASME Section IX QW-163, what is the maximum allowable dimension for an open discontinuity in the weld or HAZ following bending?