2.2 Mechanical Test Types, Specimen Dimensions & Acceptance Criteria

2.2 Mechanical Test Types, Specimen Dimensions & Acceptance Criteria

In ASME Section IX Article I, mechanical testing constitutes the definitive barrier between an unproven welding concept and a qualified, code-compliant fabrication procedure. When qualifying a groove welding procedure, Section IX relies on two primary destructive test types:

1. Tension Tests (QW-150): Designed to measure the ultimate tensile capacity and load-bearing efficiency of the weldment relative to the base metal.
2. Guided-Bend Tests (QW-160): Designed to assess the ductility, soundness, and fusion integrity of both the weld metal and the heat-affected zone (HAZ).

Inspectors taking the AWS Endorsement examination must be fluent in specimen geometry, dimensional tolerances, mathematical formulas, and the precise pass/fail criteria stipulated by paragraphs QW-153 and QW-163.

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1. Tension Tests (QW-150)

Tension test specimens are machined from the welded procedure test coupon after all required non-destructive examinations (and postweld heat treatments, if applicable) are completed. Section IX requires a minimum of two tension test specimens for standard groove weld procedure qualification (QW-451.1).

Specimen Preparation & Geometry (QW-151 & QW-462.1)

Tension specimens may take several standardized forms depending on material thickness, form (plate vs. pipe), and testing machine capacity:

1. Reduced Section — Plate (QW-462.1(a)):
   • Standard specimen width: $0.750\text{ in.}$ (19 mm) for thickness $T \le 1\text{ in.}$, or $1.50\text{ in.}$ (38 mm) for thin sheet/plate.
   • Parallel reduced section length: The weld width plus at least $0.50\text{ in.}$ (13 mm) on each side, with a minimum parallel length of $2.25\text{ in.}$ (57 mm).
   • Transition radius: Minimum $1.0\text{ in.}$ (25 mm) radius to prevent stress concentrations at the grips.
   • Weld reinforcement removal: Under QW-151.1, the weld reinforcement must be machined or ground flush with the surfaces of the base metal.
2. Reduced Section — Pipe (QW-462.1(b)):
   • Used for pipe diameters $\ge 3\text{ in.}$ OD. Longitudinal strip specimens machined with parallel sides and weld reinforcement removed flush.
3. Turned (Round) Specimens (QW-462.1(c)):
   • For thick materials ($T > 1\text{ in.}$ [25 mm]), round turned specimens with gage diameters of $0.505\text{ in.}$, $0.353\text{ in.}$, or $0.252\text{ in.}$ may be extracted. Alternatively, full-thickness slabs can be sliced into multiple longitudinal rectangular slices (multi-specimen sets).
4. Full-Section Pipe (QW-462.1(d)):
   • Used for small diameter pipe (nominal OD $\le 3\text{ in.}$). Snug-fitting metal plugs are inserted into the pipe ends to prevent collapse inside the testing machine grips.

Tensile Strength Calculation

The ultimate tensile strength ($R_m$ or UTS) is calculated by dividing the maximum ultimate load ($P$) sustained by the specimen prior to rupture by the original cross-sectional area ($A$) of the reduced section: $\text{Tensile Strength (psi or MPa)} = \frac{\text{Ultimate Breaking Load } (P)}{\text{Original Cross-Sectional Area } (A)}$ Where cross-sectional area for rectangular specimens is: $A = \text{Width } (w) \times \text{Thickness } (t)$

Note for Inspectors: All cross-sectional dimensions must be measured with calibrated precision micrometers prior to pulling the specimen.

Tensile Acceptance Criteria (QW-153.1)

Paragraph QW-153.1 dictates the four distinct conditions under which a tension specimen is accepted:

                          [Tension Specimen Ruptures]
                                      │
               ┌──────────────────────┴──────────────────────┐
               ▼                                             ▼
     [Break in Weld Metal]                         [Break in Base Metal]
               │                                             │
               ▼                                             ▼
  Tensile Strength MUST be                      Did break occur outside the
   >= Minimum Specified                           weld and fusion line?
  Tensile Strength (MSTS)                                    │
     of Base Metal!                                          ├───────────────┐
  (No 95% reduction allowed!)                                ▼               ▼
                                                           [YES]            [NO]
                                                             │          (At Fusion Line)
                                                             ▼               │
                                                    Tensile Strength         ▼
                                                      MUST be >= 95%     Treated as weld
                                                       of Base Metal     break: MUST be
                                                          MSTS!             >= MSTS!

1. Failure in Weld Metal (QW-153.1(a)): If the specimen ruptures in the weld metal, the calculated tensile strength shall not be less than the Minimum Specified Tensile Strength (MSTS) of the base metal as listed in Section IX Table QW/QB-422.
2. Failure in Base Metal Outside Weld/Fusion Line (QW-153.1(b)): If the specimen ruptures in the base metal outside of the weld and weld interface (fusion line), the test is acceptable provided the tensile strength is not more than 5% below the MSTS of the base metal (i.e., at least 95% of MSTS).
   • Metallurgical Rationale: Heat from welding can produce localized annealing in cold-worked or heat-treated base metals, slightly softening the HAZ or adjacent parent metal. Section IX tolerates up to a 5% drop in base metal strength as an inherent artifact of thermal welding cycles.
3. Dissimilar Base Metal Welds (QW-153.1(c)): If two base metals of different MSTS are joined, the tensile strength shall not be less than the MSTS of the weaker base metal. If the rupture occurs in the weaker base metal outside the fusion line, the strength shall not be less than 95% of the MSTS of that weaker base metal.
4. Base Metals Without Specified Tensile Strength (QW-153.1(d)): For materials not listed in QW/QB-422, acceptance is governed by the referencing construction code.

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2. Guided-Bend Tests (QW-160)

While tension tests establish load capacity, guided-bend tests verify that the weld metal and HAZ can undergo severe plastic deformation without initiating or propagating planar defects.

Specimen Types & Orientations (QW-161)

Bend specimens are categorized based on the surface that is stretched into convex tension:

• Transverse Side Bend (QW-161.1): The specimen is cut transverse to the longitudinal axis of the weld and bent so that one of the side cut surfaces becomes the convex (stretched) surface. Standard side bend thickness is $t = 3/8\text{ in.}$ ($10\text{ mm}$). Side bends evaluate the entire through-thickness of the weldment from face to root simultaneously.
• Transverse Face Bend (QW-161.2): The specimen is cut transverse to the weld axis and bent so that the original weld face becomes the convex outer surface.
• Transverse Root Bend (QW-161.3): The specimen is cut transverse to the weld axis and bent so that the original weld root becomes the convex outer surface.
• Longitudinal Face Bend (QW-161.4) & Longitudinal Root Bend (QW-161.5): The specimen is cut parallel to the weld axis. Used primarily when joining base metals or depositing filler metals that have substantially different mechanical properties (e.g., austenitic stainless steel welded to ferritic carbon steel, or high-nickel alloys), where transverse bending would cause uneven straining across the joint.

Thickness Selection Rules (QW-451.1)

• Coupon Thickness $T < 3/8\text{ in.}$ (10 mm): Transverse Face and Root bends are standard.
• Coupon Thickness $3/8\text{ in.} \le T < 3/4\text{ in.}$ (10 mm to 19 mm): Either 4 side bends OR 2 face bends and 2 root bends are permitted.
• Coupon Thickness $T \ge 3/4\text{ in.}$ (19 mm): Side bends are mandatory (4 side bend specimens required). Face and root bends are not permitted for thick coupons because outer fiber strain cannot effectively interrogate the core of the weldment.

Bend Test Jigs (QW-162 & QW-466)

Section IX specifies three distinct jig configurations in Article IV:

1. Standard Guided-Bend Jig (QW-466.1): Uses a hydraulic or mechanical plunger forcing the specimen into a contoured die. For standard P-No. 1 carbon steels with $t = 3/8\text{ in.}$, the plunger radius is $A/2 = 0.75\text{ in.}$ ($A = 1.50\text{ in.} = 4t$), and die width is $B = 2.0625\text{ in.}$ ($2-1/16\text{ in.}$). This imposes approximately 20% outer fiber elongation.
2. Guided-Bend Roller Jig (QW-466.2): Replaces fixed die shoulders with hardened rotating rollers to reduce sliding friction and prevent galling.
3. Wrap-Around Guided-Bend Jig (QW-466.3): Clamps one end of the specimen securely against a central mandrel while a roller wraps the specimen around the pin. This eliminates compressive buckling and is strongly favored for thin sheets and work-hardening materials.

Guided-Bend Acceptance Criteria (QW-163)

Paragraph QW-163 provides clear, quantitative rejection thresholds. After bending through a full $180^\circ$ arc:

1. The weld metal and HAZ must be completely within the bent portion of the specimen.
2. Discontinuity Limit: There shall be no open discontinuity exceeding 1/8 in. (3 mm) measured in any direction on the convex surface of the specimen in the weld metal or HAZ.
3. Corner Crack Exception: Cracks occurring on the corners of the specimen during bending shall not be considered, provided they do not exceed 1/4 in. (6 mm) and there is no metallurgical evidence that they resulted from lack of fusion, slag inclusions, or other internal weld defects.
   • If a corner crack exhibits slag, lack of penetration, or porosity, it is treated as an internal defect and must not exceed the strict 1/8 in. (3 mm) limit.
4. Corrosion-Resistant Overlay Cladding: Open defects in the cladding layer must not exceed $1/16\text{ in.}$ ($1.5\text{ mm}$), and open defects at the bond line must not exceed $1/8\text{ in.}$ ($3\text{ mm}$).

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3. Practical Calculation & Evaluation Scenarios

Case Study 1: Tensile Evaluation on SA-516 Grade 70

A PQR test coupon welded from $0.500\text{ in.}$ thick SA-516 Grade 70 plate is tested. From ASME Section IX Table QW/QB-422, the Minimum Specified Tensile Strength (MSTS) for SA-516 Gr 70 is 70,000 psi.

Two reduced-section specimens are pulled:

• Specimen T-1:
  • Measured dimensions: Width $w = 0.752\text{ in.}$, Thickness $t = 0.495\text{ in.}$
  • Area $A = 0.752 \times 0.495 = 0.37224\text{ sq in.}$
  • Ultimate Load $P = 25,900\text{ lbf}$
  • Failure location: Weld metal
  • Calculated UTS: $25,900 / 0.37224 = 69,579\text{ psi}$
  • Evaluation: FAIL. The specimen ruptured in the weld metal at $69,579\text{ psi}$, which is below the mandatory MSTS of $70,000\text{ psi}$. Under QW-153.1(a), weld breaks receive no tolerance.
• Specimen T-2:
  • Measured dimensions: Width $w = 0.750\text{ in.}$, Thickness $t = 0.498\text{ in.}$
  • Area $A = 0.750 \times 0.498 = 0.37350\text{ sq in.}$
  • Ultimate Load $P = 25,200\text{ lbf}$
  • Failure location: Base metal, 1.25 inches outside the fusion line
  • Calculated UTS: $25,200 / 0.37350 = 67,470\text{ psi}$
  • Minimum required for base metal failure: $0.95 \times 70,000\text{ psi} = 66,500\text{ psi}$
  • Evaluation: PASS. The specimen ruptured in the base metal outside the weld at $67,470\text{ psi}$, which exceeds the 95% threshold of $66,500\text{ psi}$ per QW-153.1(b).

Case Study 2: Side Bend Evaluation with Discontinuities

A CWI inspects four side bend specimens tested in a guided-bend jig:

• Specimen B-1: Displays an opening in the center of the weld metal measuring $0.100\text{ in.}$ long by $0.040\text{ in.}$ wide. ($0.100\text{ in.} < 1/8\text{ in.}$ [0.125 in.]). -> ACCEPTABLE.
• Specimen B-2: Displays an opening along the fusion line measuring $0.140\text{ in.}$ in length. ($0.140\text{ in.} > 1/8\text{ in.}$). -> REJECT.
• Specimen B-3: Displays a tear on the sharp corner of the specimen measuring $0.1875\text{ in.}$ ($3/16\text{ in.}$) long. Visual examination shows cleanly torn parent grain structure with no slag, inclusions, or unbonded sidewall. -> ACCEPTABLE (corner crack $\le 1/4\text{ in.}$ without underlying weld defects per QW-163).
• Specimen B-4: Displays a corner tear measuring $0.150\text{ in.}$ ($< 1/4\text{ in.}$), but optical magnification reveals trapped black slag particles in the root of the tear. -> REJECT (because slag is present, the $1/8\text{ in.}$ [0.125 in.] threshold applies, and $0.150\text{ in.} > 0.125\text{ in.}$).