4.2 Impact Testing Requirements & Toughness Thickness Qualification Ranges

4.2 Impact Testing Requirements & Toughness Thickness Qualification Ranges

Core Principle: ASME Section IX is a testing standard, not a construction code. It does not establish design service temperatures or dictate when Charpy V-notch impact testing is required. However, when a referencing construction code (such as ASME Section VIII Div 1, ASME B31.1, or ASME B31.3) mandates toughness testing, all supplementary essential variables become legally binding essential variables. Under QW-403.6, standard minimum thickness rules are superseded by strict lower-bound limits designed to preserve heat-affected zone (HAZ) toughness.

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1. The Referencing Code Trigger & Supplementary Essential Variables

Paragraph QW-401.3 establishes that supplementary essential variables become active only when notch-toughness testing is specified by the referencing code or engineering design document.

• Without Impact Testing: The procedure qualification requires only transverse tension tests (QW-150) and guided-bend tests (QW-160) to evaluate ultimate tensile strength and gross ductility.
• With Impact Testing: The procedure must also evaluate dynamic fracture toughness at a designated design minimum temperature using Charpy V-notch specimens (QW-170). In addition, variables that govern weld thermal cycles—specifically heat input, bead width, preheat decrease, interpass increase, and postweld heat treatment time—shift from nonessential status to legally binding essential variables.

+-----------------------------------------------------------------------------------------+
|                        THE TOUGHNESS QUALIFICATION ARCHITECTURE                         |
+-----------------------------------------------------------------------------------------+
|  REFERENCING CODE (B31.1 / B31.3 / SEC VIII)                                            |
|   - Sets Minimum Design Metal Temperature (MDMT)                                        |
|   - Mandates Charpy V-notch testing on Weld Metal and HAZ                               |
|   - Establishes minimum energy acceptance values (e.g., 15 ft-lbs / 20 Joules)          |
|                               |                                                         |
|                               V                                                         |
|  ASME SECTION IX (ARTICLE II & IV)                                                      |
|   - Activates Supplementary Essential Variables in QW-250 tables                        |
|   - Enforces QW-403.6 (Restricted minimum base metal thickness)                         |
|   - Enforces QW-409.1 (Maximum heat input and weld bead volume limits)                  |
|   - Enforces QW-407.2 (80% PWHT aggregate time-at-temperature rule)                    |
+-----------------------------------------------------------------------------------------+

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2. Minimum Base Metal Thickness Qualified: Paragraph QW-403.6

In non-toughness procedures, Table QW-451.1 allows very broad base metal qualification ranges. For example, a coupon welded on $1.0\text{ in.}$ plate qualifies base metal down to $3/16\text{ in.}$ ($0.1875\text{ in.}$). However, when notch-toughness testing is invoked, this broad range is legally prohibited.

The Governing Language of QW-403.6

Paragraph QW-403.6 states:

"The minimum base metal thickness qualified is the thickness of the test coupon $T$ or $5/8\text{ in.}$ ($16\text{ mm}$), whichever is less. However, where $T$ is less than $1/4\text{ in.}$ ($6\text{ mm}$), the minimum thickness qualified is $1/2T$."

This creates a three-tier rule for minimum base metal thickness:

1. Coupons Thicker Than $5/8\text{ in.}$ ($T \ge 0.625\text{ in.}$):
   • Qualified minimum thickness is $5/8\text{ in.}$ ($16\text{ mm}$).
   • Example: A $1.5\text{ in.}$ coupon qualifies from $5/8\text{ in.}$ ($0.625\text{ in.}$) to $8.0\text{ in.}$, NOT from $3/16\text{ in.}$!
2. Coupons from $1/4\text{ in.}$ to $5/8\text{ in.}$ ($0.250\text{ in.} \le T < 0.625\text{ in.}$):
   • Qualified minimum thickness is $T$ (the full coupon thickness).
   • Example: A $1/2\text{ in.}$ ($0.500\text{ in.}$) coupon qualifies from $0.500\text{ in.}$ to $1.000\text{ in.}$, NOT from $3/16\text{ in.}$!
3. Coupons Thinner Than $1/4\text{ in.}$ ($T < 0.250\text{ in.}$):
   • Qualified minimum thickness is $0.5T$ (half the coupon thickness).
   • Example: A $0.200\text{ in.}$ coupon qualifies from $0.100\text{ in.}$ to $0.400\text{ in.}$, NOT from $1/16\text{ in.}$!

Metallurgical Rationale: The Quench-and-Temper Cooling Effect

Why does Section IX restrict the minimum thickness when impact testing is required? In thick plates, the vast volume of cold steel acts as a massive heat sink, rapidly quenching the weld and HAZ. When welding thin sheet or plate ($< 5/8\text{ in.}$), the heat cannot dissipate into the body of the material as rapidly, resulting in a significantly slower cooling rate ($t_{8/5}$) or, conversely, if welded with very low heat, an extreme cooling rate that promotes brittle martensite formation. More critically, high heat input per unit thickness in thin sections leads to substantial thermal saturation and severe grain coarsening in the HAZ. Coarse grain microstructures exhibit dramatically higher ductile-to-brittle transition temperatures (DBTT), causing catastrophic brittle failure under impact loading.

3. Heat Input Determination & Cooling Rate Controls: Paragraph QW-409.1

When notch toughness is required, heat input becomes a strictly monitored supplementary essential variable under paragraph QW-409.1:

• The Rule: An increase in heat input, or an increase in the volume of weld metal deposited per unit length of weld bead, over that qualified on the PQR is prohibited without requalification.

The Mathematical Formula for Heat Input

ASME Section IX defines heat input mathematically as: $\text{Heat Input } (J/\text{in.}) = \frac{\text{Voltage } (V) \times \text{Amperage } (A) \times 60}{\text{Travel Speed } (S, \text{ inches per minute})}$

To express heat input in kilojoules per inch ($\text{kJ/in.}$): $\text{Heat Input } (\text{kJ/in.}) = \frac{V \times A \times 60}{S \times 1000} = \frac{V \times A \times 0.06}{S}$

In metric units ($\text{kJ/mm}$ with travel speed in $\text{mm/min}$): $\text{Heat Input } (\text{kJ/mm}) = \frac{V \times A \times 0.06}{S (\text{mm/min})}$

Volume of Weld Metal Deposited per Unit Length

For manual processes (such as SMAW or GTAW) where travel speed may vary considerably, QW-409.1 provides an alternative method of control: weld bead volume per unit length. This is calculated by measuring the length of weld bead deposited per unit length of consumed electrode: $\text{Run-Out Ratio} = \frac{\text{Length of Weld Bead Produced (inches)}}{\text{Length of Electrode Consumed (inches)}}$ An increase in bead width (e.g., changing from a narrow stringer bead to a wide weave bead) increases the volume of weld metal per unit length, which increases local heat input and slows the cooling rate, degrading HAZ toughness.

Advanced Waveform Control Power Sources

Modern pulsed GMAW and computerized power sources modulate current and voltage thousands of times per second. Paragraph QW-409.1 explicitly addresses these power supplies by requiring the use of Instantaneous Energy ($J$) or Instantaneous Power ($W$) measured by specialized internal meters: $\text{Energy per Unit Length } (J/\text{in.}) = \frac{\text{Total Instantaneous Energy } (J)}{\text{Bead Length } (\text{in.})}$ Conventional average meters ($V_{\text{avg}} \times A_{\text{avg}}$) are mathematically inaccurate for pulsed waveforms and cannot be used for code compliance when waveform-controlled machines are specified.

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4. Postweld Heat Treatment (PWHT) Holding Time Limits: Paragraph QW-407.2

In non-toughness procedures under QW-407.1, PWHT is categorized broadly into six basic conditions (e.g., none, below lower transformation temperature, above upper transformation, etc.). However, when impact testing is required, time-at-temperature becomes critical.

The 80% Rule (QW-407.2)

Paragraph QW-407.2 mandates:

• A change in postweld heat treatment temperature range requires requalification.
• For test coupons subjected to PWHT below the lower transformation temperature, the total holding time at temperature recorded on the PQR must represent at least 80% of the total aggregate time to which the production material will be subjected during fabrication, intermediate heat treatments, and anticipated weld repairs.

$\text{PQR PWHT Time} \ge 0.80 \times \text{Total Planned Production PWHT Time}$ $\text{Maximum Qualified Production PWHT Time} = \frac{\text{PQR PWHT Time}}{0.80} = 1.25 \times \text{PQR PWHT Time}$

Metallurgical Rationale: Tempering and Carbide Coarsening

When welded steel is held at PWHT temperatures (typically $1100^\circ\text{F}$ to $1350^\circ\text{F}$ / $595^\circ\text{C}$ to $730^\circ\text{C}$) for extended periods, several metallurgical degradation mechanisms occur:

1. Dislocation Annihilation: The dislocation density decreases, softening the matrix and reducing yield and tensile strength.
2. Carbide Coarsening (Ostwald Ripening): Fine, finely dispersed iron and alloy carbides dissolve and reprecipitate as massive, coarse carbides along grain boundaries.
3. Intergranular Embrittlement: Massive grain boundary carbides act as severe stress raisers and cleavage initiation sites, drastically lowering absorbed Charpy energy. If a PQR coupon is heat treated for only 1 hour, but production vessels undergo multiple PWHT cycles totaling 4 hours (initial PWHT, intermediate cycles, plus post-repair PWHT), the production weldment will suffer catastrophic toughness loss. The 80% rule guarantees that test coupons reflect the real thermal history of the finished component.

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5. Construction Code Interfacing: ASME B31.1 vs. ASME B31.3 Toughness Rules

ASME B31.3 Specimen Location Rules

Under ASME B31.3 paragraph 323.3.4, when procedure qualification impact testing is required:

• Weld Metal Specimens: Three Charpy V-notch specimens extracted transversely from the weld deposit, with the notch oriented perpendicular to the surface and located entirely within the weld centerline.
• HAZ Specimens: Three Charpy V-notch specimens extracted transversely, with the notch located in the heat-affected zone such that the notch intersects the fusion line and HAZ microstructure as required by the code.

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6. Worked Inspection Scenarios & Audit Calculations

Scenario 1: Auditing an Impact-Tested WPS Thickness Range

A quality inspector reviews a WPS for an ASME B31.3 low-temperature ethylene piping system operating at $-50^\circ\text{F}$ ($-45^\circ\text{C}$). The supporting PQR was welded on ASTM A333 Grade 6 seamless pipe with a nominal wall thickness of $T = 0.500\text{ in.}$ ($1/2\text{ in.}$). The PQR passed Charpy impact testing at $-50^\circ\text{F}$. The WPS lists the qualified base metal thickness range as $3/16\text{ in.}$ to $1.0\text{ in.}$

• Audit Evaluation: Nonconformance! The welding engineer incorrectly copied the non-toughness range from Table QW-451.1 ($3/16\text{ in.}$ to $2T$). Because impact testing is required by ASME B31.3, paragraph QW-403.6 governs the minimum thickness. For $T = 0.500\text{ in.}$, $T$ is between $1/4\text{ in.}$ and $5/8\text{ in.}$, so the qualified minimum is equal to $T$ ($0.500\text{ in.}$). The legal WPS range is $0.500\text{ in.}$ to $1.000\text{ in.}$ The WPS cannot be used to weld pipe thinner than $0.500\text{ in.}$

Scenario 2: Heat Input Verification on Shop Traveler

A piping fabricator welds an ASME B31.3 impact-tested joint using SMAW with E7018-1. The certified PQR documents the following parameters:

• Voltage: 24 V, Amperage: 160 A, Travel Speed: 5.5 inches per minute (ipm).
• PQR Qualified Heat Input: $H_{\text{PQR}} = \frac{24 \times 160 \times 60}{5.5 \times 1000} = \frac{230,400}{5,500} = 41.89\text{ kJ/in.}$ During a shop floor audit, the CWI observes a welder running a wide weave cap pass with the following monitored parameters: Voltage: 26 V, Amperage: 175 A, Travel Speed: 4.2 ipm.
• Actual Production Heat Input: $H_{\text{actual}} = \frac{26 \times 175 \times 60}{4.2 \times 1000} = \frac{273,000}{4,200} = 65.00\text{ kJ/in.}$
• Compliance Determination: Critical Violation of QW-409.1. The production heat input ($65.00\text{ kJ/in.}$) exceeds the qualified maximum ($41.89\text{ kJ/in.}$) by 55%. The excessive heat input slows the cooling rate, promoting grain coarsening and severe embrittlement. The weld must be rejected and the joint cut out or requalified.

Scenario 3: PWHT Aggregate Time Verification

A pressure vessel component requires 2 hours of postweld heat treatment during initial shop fabrication, and the engineering specification anticipates a potential 1.5-hour post-repair PWHT cycle during vessel installation, for a total planned aggregate PWHT time of $3.5\text{ hours}$. What is the minimum PWHT soak time that must be applied to the PQR qualification test coupon?

• Calculation under QW-407.2: $\text{Minimum PQR Time} = 0.80 \times 3.5\text{ hours} = 2.80\text{ hours} = 2\text{ hours, } 48\text{ minutes}$
• If the PQR coupon was heat treated for only 2.0 hours, it qualifies a maximum production PWHT time of $2.0 / 0.80 = 2.5\text{ hours}$, which is insufficient to cover the required 3.5-hour aggregate envelope.