16.6 QW-451 Mechanical Test Protocols & Procedure Qualification Test Evaluation

Mechanical Test Specimen Protocols (QW-202 & Table QW-451.1)

To qualify a groove weld procedure, ASME Section IX QW-202 mandates destructive testing of specimens machined from the welded coupon. For standard plate and pipe coupons, Table QW-451.1 dictates the specimen quantities:

+------------------------------------+-------------------+-------------------+-------------------+
| Coupon Thickness (T)               | Tension Specimens | Transverse Root/  | Transverse Side   |
|                                    | (QW-150)          | Face Bends (QW-160)| Bends (QW-160)   |
+------------------------------------+-------------------+-------------------+-------------------+
| T < 1/16 in (1.5 mm)               | 2                 | 2 Root, 2 Face    | Not Permitted     |
| 1/16 in <= T < 3/8 in (10 mm)      | 2                 | 2 Root, 2 Face    | Permitted (alt)   |
| 3/8 in <= T < 3/4 in (19 mm)       | 2                 | 2 Root, 2 Face    | 4 Side Bends (rec)|
| T >= 3/4 in (19 mm)                | 2                 | Not Permitted     | 4 Side Bends      |
+------------------------------------+-------------------+-------------------+-------------------+

                          MECHANICAL TEST SPECIMEN EXTRACTION LAYOUT

        +-------------------------------------------------------------------+
        |                      Discard (Top 1.0 inch)                       |
        +-------------------------------------------------------------------+
        |  [T-1] Transverse Reduced-Section Tension Specimen (QW-462.1)     |
        +-------------------------------------------------------------------+
        |  [S-1] Guided Transverse Side Bend Specimen (QW-462.2)            |
        +-------------------------------------------------------------------+
        |  [S-2] Guided Transverse Side Bend Specimen (QW-462.2)            |
        +-------------------------------------------------------------------+
        |  [CVN] Charpy V-Notch Impact Set (Weld Centerline & HAZ)          |
        +-------------------------------------------------------------------+
        |  [S-3] Guided Transverse Side Bend Specimen (QW-462.2)            |
        +-------------------------------------------------------------------+
        |  [S-4] Guided Transverse Side Bend Specimen (QW-462.2)            |
        +-------------------------------------------------------------------+
        |  [T-2] Transverse Reduced-Section Tension Specimen (QW-462.1)     |
        +-------------------------------------------------------------------+
        |                     Discard (Bottom 1.0 inch)                     |
        +-------------------------------------------------------------------+

1. Transverse Tension Testing (QW-150)

Reduced-section tension specimens (QW-462.1) evaluate the ultimate tensile strength of the welded joint under uniaxial loading:

• Cross-Sectional Area Calculation: A = W * T_specimen
• Ultimate Tensile Stress: σ_u = P_max / A

Acceptance Criteria (QW-153.1):

1. Tensile Strength >= SMTS: The specimen must display an ultimate tensile strength not less than the Specified Minimum Tensile Strength (SMTS) of the base metal (listed in QW/QB-422).
2. Dissimilar Metals: When joining base metals of different tensile strengths, the specimen must meet or exceed the SMTS of the weaker base metal.
3. The 5% Rule (QW-153.1(d)): If the specimen breaks in the base metal outside of the weld and weld fusion boundary, the test is accepted as meeting the code requirements provided the calculated tensile strength is not more than 5% below the specified minimum tensile strength of the base metal.

Engineering Rationale for the 5% Rule: When a tension specimen necks down and fractures in the parent plate well away from the joint, it physically proves that the weld metal and heat-affected zone overmatched the base metal. Minor strength variations in the parent plate (up to 5% below nominal mill minimum) are accepted because the integrity of the welded joint itself has been proven superior to the adjacent base metal.

2. Guided Bend Testing (QW-160)

Guided bend specimens evaluate the soundness and ductility of the weld metal and fusion boundaries by straining the outer fibers in tension over a standardized plunger jig:

• Specimen Types: Transverse Root Bends (root in tension), Transverse Face Bends (face in tension), Transverse Side Bends (entire cross-section strained across 3/8 in thickness), and Longitudinal Bends (used for dissimilar metals with large yield strength disparities per QW-161.5).
• Jig Geometry (QW-466.1): Standard guided bend jigs utilize a plunger radius of A/2 = 2t (where t is specimen thickness, typically 3/8 in, resulting in plunger radius R_A = 0.75 in and die radius R_B = 1.3125 in). This imposes approximately 20% outer fiber strain.
• Adjusted Bend Radius (QW-466.1 / QW-466.2): For low-ductility base metals having specified elongation E < 20%, the plunger dimension A is adjusted to prevent premature mechanical tearing:

$A = \frac{100 \times t}{E} - t$

Defect Acceptance Criteria (QW-163):

After bending through 180°, the convex surface of the specimen is examined for discontinuities:

• Maximum Defect Size: The weld and HAZ shall have no open discontinuity exceeding 1/8 in (3.2 mm) measured in any direction.
• Corner Cracks Disregarded: Cracks occurring on the corners of the specimen during bending are disregarded unless there is definite evidence that they result from lack of fusion, slag inclusions, or internal planar defects.

3. Charpy V-Notch Impact Testing (QW-170 / ASTM A370)

When notch toughness is mandated, impact testing evaluates resistance to brittle fracture under high-strain dynamic impact:

• Specimen Dimensions: Standard full-size Charpy specimens measure 10 x 10 x 55 mm with a 2 mm deep 45° V-notch (R = 0.25 mm).
• Notch Placement: Specimens must sample both the Weld Metal Centerline (WM) and the Heat-Affected Zone (HAZ). HAZ specimens must be etched to locate the fusion boundary, with the notch precisely positioned at the fusion line (FL), FL + 1 mm, FL + 2 mm, or FL + 5 mm as specified by the construction code.
• Acceptance Metrics: Minimum absorbed energy (ft-lbf or Joules) and lateral expansion (minimum 0.015 in / 0.38 mm for austenitic/nickel steels per Section VIII Division 1 UG-84).

---

Comprehensive Worked Engineering Example: PQR Mechanical Test Evaluation

Problem Statement

A welding engineer evaluates destructive mechanical test data from a procedure qualification coupon. The coupon is a 1.00 in (25.4 mm) thick plate of SA-516 Grade 70 (P-No 1 Group 2). Per ASME Section IX Table QW/QB-422, SA-516 Grade 70 has a Specified Minimum Tensile Strength of 70.0 ksi (485 MPa).

Destructive testing produces the following laboratory data:

1. Tension Test Specimens (Reduced Section):

• Specimen T-1: Width W = 0.752 in, Thickness T = 0.995 in, Maximum Load P1 = 53,200 lbf, Fracture Location: Weld metal (silky cup-and-cone).
• Specimen T-2: Width W = 0.750 in, Thickness T = 1.002 in, Maximum Load P2 = 50,450 lbf, Fracture Location: Base metal, 1.4 in outside the fusion line.

2. Guided Side Bend Specimens (4 Side Bends, t = 0.375 in):

• Specimen S-1: Zero open discontinuities.
• Specimen S-2: One open pore measuring 0.06 in (1.5 mm).
• Specimen S-3: One corner crack measuring 0.15 in (3.8 mm) with clean parent metal shear lip and no inclusion evidence.
• Specimen S-4: One open fissure along the fusion line measuring 0.16 in (4.1 mm).

3. Charpy V-Notch Impact Specimens (Tested at -50°F, Code Mandate: 20 ft-lb avg, 15 ft-lb min):

• Weld Metal: 28, 25, 22 ft-lbf (Average = 25.0 ft-lbf)
• HAZ: 21, 14, 19 ft-lbf (Average = 18.0 ft-lbf)

Evaluate each test result against ASME Section IX and render a final PQR disposition.

                             TENSION TEST SPECIMEN PROFILES & FRACTURE

       Specimen T-1 (Broke in Weld Metal @ 71.1 ksi)          Specimen T-2 (Broke in Base Metal @ 67.1 ksi)
     +----------------------------------------------+       +----------------------------------------------+
     |      Base Metal   |  Weld  |   Base Metal    |       |      Base Metal   |  Weld  |   Base Metal    |
     |                   |  ***   |                 |       |           *       |        |                 |
     +-------------------|--***---|-----------------+       +-----------*-------|--------|-----------------+
                            Fracture                                 Fracture (1.4" from weld)
                           (>= 70 ksi)                                (Within 5% Rule: >= 66.5 ksi)

Step-by-Step Evaluation

Step 1: Tension Specimen T-1 Evaluation

• Cross-sectional area: A1 = W * T = 0.752 * 0.995 = 0.74824 sq in
• Ultimate tensile strength: $\sigma_{u,1} = \frac{53,200\text{ lbf}}{0.74824\text{ sq in}} = 71,101\text{ psi} = 71.1\text{ ksi}$
• Failure Location: Weld metal.
• Code Requirement: Must be >= SMTS = 70.0 ksi.
• Disposition: 71.1 ksi >= 70.0 ksi. PASS.

Step 2: Tension Specimen T-2 Evaluation

• Cross-sectional area: A2 = W * T = 0.750 * 1.002 = 0.75150 sq in
• Ultimate tensile strength: $\sigma_{u,2} = \frac{50,450\text{ lbf}}{0.75150\text{ sq in}} = 67,132\text{ psi} = 67.1\text{ ksi}$
• Failure Location: Base metal, 1.4 inches outside weld.
• Code Requirement per QW-153.1(d) (5% Rule): $\sigma_{\text{allowable}} = 0.95 \times \text{SMTS} = 0.95 \times 70.0\text{ ksi} = 66.5\text{ ksi}$
• Disposition: Because failure occurred in the base metal outside the weld and σ_u,2 (67.1 ksi) >= 66.5 ksi, Specimen T-2 is ACCEPTED per QW-153.1(d).

Step 3: Guided Side Bend Evaluation

• Specimen S-1: Zero defects. PASS.
• Specimen S-2: Open pore = 0.06 in <= 1/8 in (0.125 in). PASS.
• Specimen S-3: Corner crack = 0.15 in. Under QW-163, corner cracks are disregarded unless resulting from internal flaws. Visual examination confirmed clean shear tearing without slag or cold lap. PASS (Disregarded).
• Specimen S-4: Open fusion line fissure = 0.16 in (4.1 mm). The discontinuity exceeds the maximum allowable limit of 1/8 in (0.125 in / 3.2 mm).
• Disposition: Specimen S-4 FAILS per QW-163.

Step 4: Charpy V-Notch Toughness Evaluation

• Weld Metal: Average = 25.0 ft-lbf >= 20.0 ft-lbf; Individual minimum = 22.0 ft-lbf >= 15.0 ft-lbf. PASS.
• HAZ: Average = 18.0 ft-lbf < 20.0 ft-lbf; Individual minimum = 14.0 ft-lbf < 15.0 ft-lbf. Both average and minimum fail code criteria.
• Disposition: HAZ Toughness FAILS.

Overall PQR Disposition

PQR REJECTED. The procedure qualification fails due to lack of sidewall fusion on bend specimen S-4 and insufficient notch toughness in the heat-affected zone. Remedial engineering requires optimizing joint cleaning/groove angle to eliminate sidewall lack of fusion, and reducing arc heat input or lowering interpass temperature to refine the CGHAZ microstructure and restore Charpy impact toughness.

---

Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Field Disaster Scenario: The Cryogenic Tank Failure

A LNG satellite terminal installed a 3.5% nickel steel (P-No 9B) liquid ethylene storage tank. The contractor's welding engineer qualified the PQR on SA-203 Grade E plate. During tension testing, both specimens fractured in the weld metal at 68.2 ksi and 69.1 ksi against a specified minimum tensile strength of 70.0 ksi. The testing lab technician applied the "5% rule" and marked the tension tests as passing.

Two years later, during thermal cooldown, thermal contraction stresses induced joint failure across the under-strength weld seam, releasing 1,500 cubic meters of cryogenic liquid. The regulatory investigation revealed that the 5% reduction allowance applies strictly and exclusively to fractures occurring in the base metal outside the weld. When a specimen breaks in the weld metal, it must meet 100% of the SMTS (70.0 ksi). Applying the 5% rule to weld metal failure was a catastrophic misapplication of QW-153.1(d), which had masked severe weld metal undermatching.

Certified Welding Engineer Exam Pitfalls

Exam Trap 1: Misapplication of the 5% Tensile Rule The CWEng exam frequently presents a tension specimen breaking in the weld metal at 97% of the base metal SMTS (e.g., 68.0 ksi on a 70.0 ksi steel). Candidates frequently apply the 5% rule and mark the test as passing. This is an automatic failure. If the fracture occurs in the weld metal or fusion boundary, it must meet or exceed 100% of the SMTS. The 5% allowance is permitted only when fracture occurs entirely in the base metal outside the weld.

Exam Trap 2: Corner Cracks on Guided Bend Specimens An exam question will describe a guided bend specimen with a 5/32 in (4.0 mm) crack located precisely at the corner edge, with no evidence of internal slag or lack of fusion. Candidates often fail the specimen because 5/32 in exceeds 1/8 in. Per QW-163, corner cracks are explicitly disregarded unless evidence shows they originated from internal defects.

Exam Trap 3: P-No 1 Group Number Interchangeability When an exam question specifies an impact-tested vessel, candidates often assume that qualifying a procedure on SA-516 Grade 70 (Group 2) automatically qualifies SA-516 Grade 60 (Group 1). Under supplementary essential variable QW-403.5, Group Numbers are not interchangeable when impact testing is required.