15.5 PWHT Fundamentals, D1.1 Thermal Cycles & Visual Acceptance Criteria

15.3 Post-Weld Heat Treatment (PWHT) & Visual/NDE Acceptance Criteria

Quick Answer: AWS D1.1 Clause 7.8 governs Post-Weld Heat Treatment (PWHT) for carbon steels, specifying a holding range of 1100°F - 1200°F (590°C - 650°C) to relax residual stresses through high-temperature creep and temper hard HAZ microstructures. Above 600°F (315°C), heating rates are capped at 400°F/hr divided by thickness in inches (maximum 400°F/hr, minimum 100°F/hr) and cooling rates at 500°F/hr divided by thickness, with soak times of 1 hr/in up to 2 in plus 15 min per additional inch. Visual inspection per Table 8.1 strictly prohibits cracks and lack of fusion across all structures, but establishes vital operational distinctions: cyclically loaded tension welds cap undercut at 0.01 in (0.25 mm) compared to 1/32 in (1 mm) for static structures, while cyclic porosity is restricted to one pore per 4 inches of weld (d <= 3/32 in).

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Post-Weld Heat Treatment (PWHT) Engineering Fundamentals (Clause 7.8)

During arc welding, steep thermal gradients and localized weld metal solidification induce severe tensile residual stresses that reach or exceed the room-temperature yield strength (Fy) of the base metal. In heavy structural weldments subjected to dynamic fatigue, stress corrosion cracking (SCC), or low-temperature impact service, Post-Weld Heat Treatment (PWHT)—also designated thermal stress relief—is mandated by design specifications to restore mechanical integrity.

Metallurgical Mechanisms of Thermal Stress Relief

Thermal stress relief does not alter the fundamental macro-geometry of the weldment; rather, it operates via two distinct metallurgical phenomena:

1. Yield Strength Reduction & Creep Relaxation: As structural steel is heated into the subcritical range (590°C - 650°C / 1100°F - 1200°F), its yield strength plummets to roughly 10% to 20% of its ambient room-temperature value. Residual elastic strain (e_e) converts into plastic strain (e_p) and time-dependent creep strain (e_c): e_total = e_e + e_p + e_c Because the material can no longer support high elastic stresses at elevated temperatures, peak residual stresses relax down to the elevated-temperature yield strength (< 5-8 ksi / 35-55 MPa).
2. HAZ Tempering & Hydrogen Effusion: Holding at 1100°F - 1200°F tempers hard, brittle martensite and bainite in the Coarse-Grained Heat-Affected Zone (CGHAZ). High-carbon martensitic laths decompose into soft, ductile ferrite and coarsened cementite (carbide spherodization), reducing peak HAZ hardness below 22 HRC (250 HV). Simultaneously, high thermal kinetic energy accelerates the effusion of any residual diffusible hydrogen from the crystal lattice.

                 RESIDUAL STRESS RELAXATION DURING PWHT CYCLE

     Stress Level (ksi)
      ^
   50 +---------------------------+   <-- Ambient Room-Temp Yield (~50 ksi)
      |   Peak As-Welded          |
   40 |   Residual Stress          v
      |                             .
   30 |                              .
      |                               .
   20 |                                .
      |                                 +-------------------------+
   10 |                                 | Stress Relaxed to       |
      |                                 | Elevated-Temp Yield     |
    0 +---------------------------------+-------------------------> Temperature
     Ambient (70°F)                   600°F                   1150°F (PWHT)

The Quenched and Tempered (Q&T) Steel Danger (ASTM A514 / A517)

Critical Caution: PWHT on High-Strength Q&T Steels High-strength quenched and tempered steels (e.g., ASTM A514, with Fy = 100 ksi) derive their strength from a carefully tempered martensitic microstructure achieved at mill tempering temperatures of 1150°F - 1250°F. Subjecting A514 weldments to indiscriminate PWHT can be catastrophic. If the PWHT holding temperature exceeds the original tempering temperature, the steel over-tempers, causing an irreversible 30% to 50% drop in tensile and yield strength. Furthermore, PWHT of high-alloy Q&T steels induces reheat cracking (stress-relief cracking) along CGHAZ prior-austenite grain boundaries. AWS D1.1 Clause 7.8 strictly mandates that PWHT of A514/A517 steels shall not exceed 1150°F (620°C) and requires engineering concurrence.

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AWS D1.1 Thermal Cycle Specifications & Governing Equations

AWS D1.1 Clause 7.8 establishes rigorous limits on holding temperatures, heating rates, soak durations, cooling rates, and furnace temperature uniformity to prevent thermal shock and severe thermal gradient-induced distortion.

                         AWS D1.1 PWHT THERMAL CYCLE PROFILE

    Temperature (°F)
          ^
          |                         Soak Period (t_soak)
   1150°F |                     +-------------------------+
          |                    /  1100°F - 1200°F Holding  |
          |   Controlled      /    (1 hr/in <=2", +15m)     |  Controlled Cooling
          |   Heating Rate   /                              |  (R_C <= 500/t)
    600°F + - - - - - - - - + - - - - - - - - - - - - - - - + - - - - - - - -
          |  Unrestricted  /                                |  Air Cool
          |  Heating (<600)                                 | (<600°F)
     70°F +---------------+---------------------------------+-------------> Time

1. Holding Temperature Ranges

• Carbon Steels (Groups I, II, III): 1100°F to 1200°F (590°C to 650°C).
• Low-Alloy Steels (Group IV): 1100°F to 1250°F (590°C to 675°C).

2. Heating Rate Restrictions (T > 600°F / 315°C)

Below 600°F, heating rate is unrestricted. Above 600°F, the rate of heating (R_H) must not exceed:

R_H = (400°F/hr) / t (in) = (222°C/hr) / (t / 25.4 mm)

• Maximum Ceiling: In no case shall R_H exceed 400°F/hr (222°C/hr).
• Minimum Floor: R_H need not be less than 100°F/hr (56°C/hr), regardless of thickness.

3. Holding Time (Soak Duration)

The holding time at temperature (t_soak) is dictated by the thickest section joined:

• Thickness t <= 2.0 in (50 mm): 1 hour per inch (2.36 min/mm) of thickness, with a minimum holding time of 15 minutes for sections under 1/4 in.
• Thickness t > 2.0 in (50 mm): 2 hours plus 15 minutes for each additional inch (0.59 min/mm) of thickness over 2.0 in:

t_soak = 2.0 hr + 0.25 hr x (t - 2.0 in)

4. Cooling Rate Restrictions (T > 600°F / 315°C)

Above 600°F, cooling must proceed in a closed furnace or chamber at a rate (R_C) not exceeding:

R_C = (500°F/hr) / t (in) = (278°C/hr) / (t / 25.4 mm)

• Maximum Ceiling: In no case shall R_C exceed 500°F/hr (278°C/hr).
• Minimum Floor: R_C need not be less than 100°F/hr (56°C/hr).
• Below 600°F (315°C): The weldment may be removed from the furnace and cooled in still air.

5. Temperature Uniformity in Furnace

During heating and cooling, the variation in temperature throughout the component must not exceed 250°F (140°C) within any 15 ft (4.6 m) interval. During the holding soak period, the temperature across the entire component must remain within a 50°F (28°C) band.

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Visual Inspection Acceptance Criteria: AWS D1.1 Table 8.1

Visual Testing (VT) is the foundational Quality Assurance method in AWS D1.1 Clause 8. Every weld must be visually inspected along its entire length. AWS D1.1 Table 8.1 (Visual Inspection Acceptance Criteria) establishes fundamentally different criteria depending on whether a structure is statically loaded (buildings) or cyclically loaded (bridges, crane girders).

+---------------------------------------------------------------------------------------------------------+
|                    AWS D1.1 TABLE 8.1 VISUAL INSPECTION ACCEPTANCE CRITERIA MATRIX                      |
+----------------------------+------------------------------------+---------------------------------------+
| Discontinuity Category     | Statically Loaded Nontubular       | Cyclically Loaded Nontubular          |
+----------------------------+------------------------------------+---------------------------------------+
| 1. Crack Prohibition       | NO CRACKS PERMITTED                | NO CRACKS PERMITTED                   |
|                            | (Zero tolerance, any size)         | (Zero tolerance, any size)            |
+----------------------------+------------------------------------+---------------------------------------+
| 2. Weld/Base Metal Fusion  | Complete fusion required between   | Complete fusion required between      |
|                            | passes and weld to base metal      | passes and weld to base metal         |
+----------------------------+------------------------------------+---------------------------------------+
| 3. Crater Cross-Section    | All craters filled to full weld    | All craters filled to full weld       |
|                            | cross-section (except ends)        | cross-section (except ends)           |
+----------------------------+------------------------------------+---------------------------------------+
| 4. Fillet Weld Undersize   | Leg undersize <= 1/16" (1.6 mm) on | Leg undersize <= 1/16" (1.6 mm) on    |
|                            | max 10% of length; ends restricted | max 10% of length; ends restricted    |
+----------------------------+------------------------------------+---------------------------------------+
| 5. Maximum Undercut        | <= 1/32" (1 mm) general;           | Transverse to tension: <= 0.01"       |
|                            | <= 1/16" (1.6 mm) on accum 2" in 12" | (0.25 mm) STRICT MAXIMUM;          |
|                            |                                    | All other cases: <= 1/32" (1 mm)      |
+----------------------------+------------------------------------+---------------------------------------+
| 6. Porosity Limits         | CJP Butt: NO piping porosity       | CJP Butt: NO piping porosity          |
|                            | Other: Sum of dia >= 1/32" shall   | Other: Frequency <= 1 pore in 4"      |
|                            | not exceed 3/8" in 1" & 3/4" in 12" | (100 mm); Max dia <= 3/32" (2.5 mm)   |
+----------------------------+------------------------------------+---------------------------------------+

                CRITICAL UNDERCUT THRESHOLDS: STATIC VS CYCLIC

         STATIC CONNECTIONS                           CYCLIC TENSION CONNECTIONS
         ==================                           ==========================
         Max Undercut = 1/32" (1.0 mm)                Max Undercut = 0.01" (0.25 mm)

          +---------------------------+                +---------------------------+
          |       Weld Crown          |                |       Weld Crown          |
      ----+                           +----        ----+                           +----
          |                           |                |                           |
          v                           v                v                           v
         h <= 1/32" (1.0 mm)                          h <= 0.01" (0.25 mm)
         (Permits up to 1/16" on accum 2" in 12")     (STRICT ZERO-TOLERANCE TO DEEP NOTCHES)

In-Depth Analysis of Table 8.1 Discontinuity Rules

1. Crack Prohibition

Under no circumstances does AWS D1.1 permit cracks, microcracks, or crater cracks. Any crack detected visually, via Liquid Penetrant (PT), Magnetic Particle (MT), or Ultrasonic Testing (UT) constitutes immediate rejection and requires complete excavation and repair.

2. Undercut Limits: The 0.01 in Cyclic Rule

• In statically loaded structures, undercut is permitted up to 1/32 in (1.0 mm) along the entire length, and up to 1/16 in (1.6 mm) for an accumulated length of 2 inches (50 mm) in any 12 inches (300 mm) of weld.
• In cyclically loaded structures subjected to calculated tensile stress normal to the weld axis, undercut must not exceed 0.01 in (0.25 mm). This microscopic threshold (250 micro-m) exists because toe undercut in cyclic tension creates an acute stress concentration notch that drastically slashes fatigue initiation life.

3. Fillet Weld Undersize Allowance

Table 8.1 permits a fillet weld leg size to be undersized by up to 1/16 in (1.6 mm) without correction, provided that:

1. The undersized portion does not exceed 10% of the continuous weld length.
2. On girder web-to-flange welds, no undersize is permitted at the ends for a distance equal to twice the flange width (2 x bf), where end shears and principal stress rotations peak.

4. Convexity and Weld Reinforcement Limits

For fillet welds, face convexity (C) must not exceed the limits shown below, based on weld face width (W):

• W <= 5/16 in (8 mm): C_max = 1/16 in (1.6 mm)
• 5/16 in < W < 1 in (25 mm): C_max = 1/8 in (3.2 mm)
• W >= 1 in (25 mm): C_max = 3/16 in (4.8 mm)
• Formula: C <= 0.1 W + 0.03 in (1 mm).
• Groove weld face reinforcement must not exceed 1/8 in (3.2 mm) and must blend smoothly with the parent plate without sharp weld toe re-entrant corners.

5. Porosity Limitations

• Complete Joint Penetration (CJP) Butt Welds Transverse to Tension: In both static and cyclic structures, zero visible piping porosity is permitted. Volumetric sound weld metal is non-negotiable.
• Other Groove & Fillet Welds (Static): The sum of visible piping porosity 1/32 in (1 mm) or larger in diameter must not exceed 3/8 in (10 mm) in any linear inch of weld, and must not exceed 3/4 in (20 mm) in any 12 in (300 mm) length.
• Other Groove & Fillet Welds (Cyclic): The frequency of piping porosity must not exceed one pore or cluster in 4 inches (100 mm) of weld length, and the maximum diameter must not exceed 3/32 in (2.5 mm).

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