15.6 D1.1 Nondestructive Examination Protocols, PWHT Scheduling & Visual Audit Practice

Non-Destructive Examination (NDE) Code Protocols (Clause 8 Part C)

Visual testing is complemented by surface and volumetric NDE specified in contract documents:

• Magnetic Particle Testing (MT): Governed by ASTM E709. Mandated on cut edges after discontinuity removal and on heavy weld passes to detect surface/subsurface cracks.
• Ultrasonic Testing (UT): Governed by AWS D1.1 Clause 8 Part F. Utilizes straight-beam and angle-beam transducers (45°, 60°, 70°) calibrated against an IIW reference block. Indications are evaluated based on decibel attenuation ratings: d = a - b - c where a is the test indication level (dB), b is the reference level (dB), and c is the sound path attenuation factor (c = (path - 1 in) x 2). Severity levels determine acceptance (Class A through D per Table 8.2 and Table 8.3).
• Delayed Inspection Rule (Clause 8.15): For high-strength steels with Fy >= 70 ksi (Group IV or Q&T steels), final VT and NDE must be delayed for at least 48 hours after weld completion to allow any latent hydrogen-induced cold cracking to initiate and propagate to detectable size.

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Comprehensive Worked Engineering Example: PWHT Schedule Calculation & CWI Visual Audit

Problem Statement

A heavy bridge crane runway box girder fabricated from ASTM A572 Grade 50 steel has a maximum flange thickness of t = 3.5 in (88.9 mm) and is welded to a 1.25 in (31.8 mm) web plate using FCAW-G. The project specification mandates a furnace Post-Weld Heat Treatment per AWS D1.1 Clause 7.8, followed by a Certified Welding Inspector (CWI) visual audit per Table 8.1 (Cyclically Loaded).

Part A: PWHT Schedule Calculation Calculate:

1. The minimum required soak holding time at 1150°F (620°C).
2. The maximum allowable heating rate above 600°F (315°C).
3. The maximum allowable cooling rate above 600°F (315°C).
4. The total minimum time elapsed during the controlled thermal cycle above 600°F.

Part B: CWI Visual Audit Evaluation During the post-PWHT visual audit, the inspector records three specific observations:

• Observation 1: On the tension flange transverse CJP butt splice, the toe undercut depth is measured at 0.018 in (0.46 mm).
• Observation 2: On a longitudinal web-to-flange fillet weld, two surface pores of diameter 1/16 in (1.6 mm) are found spaced 6 inches (150 mm) apart.
• Observation 3: A continuous 12 mm specified fillet weld has a measured leg size of 10.5 mm (1.5 mm ~ 1/16 in undersize) along a 150 mm stretch of an 1800 mm continuous joint, located in the middle third of the girder span.

Determine whether each observation is Acceptable or Rejectable under Table 8.1 cyclically loaded criteria.

                              PWHT TEMPERATURE PROFILE CALCULATION

        1150°F + - - - - - - - - - - +-----------------------+ - - - - - - - - - - - -
               |                    /   Soak Time = 2.375 hr  |   Cooling Rate
               |    Heating Rate   /    (2 hr 23 min)         |   R_C = 142.9°F/hr
               |   R_H = 114.3°F  /                           |  (t_cool = 3.85 hr)
               | (t_heat = 4.81h)/                            |
         600°F +----------------+                             +-------------------
               |<-------------- Total Cycle Above 600°F = 11.035 hr -------------->|

Step-by-Step Engineering Solution

Part A: PWHT Schedule Calculations

Step 1: Soak Holding Time

• Thickness of thickest section joined: t = 3.5 in (88.9 mm).
• Per AWS D1.1 Clause 7.8.1, for thickness exceeding 2.0 in: t_soak = 2.0 hr + 0.25 hr x (t - 2.0 in) t_soak = 2.0 hr + 0.25 hr x (3.5 - 2.0) = 2.0 + 0.375 = 2.375 hr t_soak = 2 hours and 22.5 minutes (~ 2 hr 23 min)

Step 2: Maximum Heating Rate Above 600°F

• Per Clause 7.8.1: R_H = (400°F/hr) / t = 400 / 3.5 = 114.29°F/hr
• Code bounds check: 100°F/hr <= R_H <= 400°F/hr. Since 114.29°F/hr > 100°F/hr, the maximum heating rate is 114.3°F/hr (63.5°C/hr).

Step 3: Maximum Cooling Rate Above 600°F

• Per Clause 7.8.1: R_C = (500°F/hr) / t = 500 / 3.5 = 142.86°F/hr
• Code bounds check: 100°F/hr <= R_C <= 500°F/hr. The maximum allowable cooling rate is 142.9°F/hr (79.4°C/hr).

Step 4: Total Minimum Thermal Cycle Duration Above 600°F

• Heating temperature rise from 600°F to 1150°F: Delta T = 1150°F - 600°F = 550°F
• Minimum heating time: t_heat = 550°F / (114.29°F/hr) = 4.8125 hr (4 hr 49 min)
• Minimum cooling time: t_cool = 550°F / (142.86°F/hr) = 3.8500 hr (3 hr 51 min)
• Total cycle time above 600°F: t_total = t_heat + t_soak + t_cool = 4.8125 + 2.3750 + 3.8500 = 11.0375 hr Total Duration Above 600°F = 11 hours and 2.25 minutes (~ 11 hr 2 min)

Part B: CWI Visual Audit Disposition

• Observation 1 (Transverse CJP Butt Undercut = 0.018 in):

  • Joint is in a crane runway girder subjected to cyclic fatigue, and the butt weld is transverse to tension.
  • Per Table 8.1 (Cyclically Loaded Nontubular), undercut transverse to tensile stress must not exceed 0.01 in (0.25 mm).
  • Measured undercut is 0.018 in > 0.01 in.
  • Disposition: REJECTABLE. The undercut notch must be excavated by light grinding or filled with a qualified TIG/SMAW repair pass and ground smooth.

• Observation 2 (Fillet Porosity: two 1/16 in pores spaced 6 in apart):

  • Joint is a longitudinal fillet weld in a cyclically loaded member.
  • Per Table 8.1, maximum allowable diameter is 3/32 in (2.4 mm), and frequency must not exceed one pore in 4 inches (100 mm).
  • Pore diameter (1/16 in = 0.0625 in) is less than 3/32 in (0.0938 in). Pore spacing (6 in) exceeds the 4 in minimum separation.
  • Disposition: ACCEPTABLE.

• Observation 3 (Fillet Undersize: 1.5 mm on 150 mm of 1800 mm weld):

  • Measured undersize: 12 mm - 10.5 mm = 1.5 mm (< 1/16 in / 1.6 mm). Complies with undersize threshold.
  • Length percentage: 150 mm / 1800 mm = 8.33% <= 10%. Complies with cumulative length limit.
  • Location check: The defect is in the middle third of the girder span, not at girder ends (no restriction regarding the 2 x bf end zone applies).
  • Disposition: ACCEPTABLE.

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Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Field Disaster Scenario

A heavy open-pit mining dragline boom fabricated from quenched and tempered ASTM A514 Grade B steel (Fy = 100 ksi) underwent major field splice modifications. The repair contractor's quality manager, accustomed to conventional carbon steel piping codes (ASME B31.3), instituted a Post-Weld Heat Treatment at 1250°F (675°C) for two hours to "relieve residual stresses." During the subsequent shift under full digging payload, the entire upper boom structure buckled catastrophically. The metallurgical failure investigation revealed that because the 1250°F PWHT temperature far exceeded the mill tempering temperature (1150°F), the quenched martensite had completely over-tempered, coalescing carbides and dropping the yield strength from 100 ksi to 54 ksi (46% loss of capacity). The contractor was held liable for $12 million in equipment replacement and lost mine production.

Certified Welding Engineer Exam Pitfalls

Exam Trap 1: The Soak Time Over 2 Inches Formula When calculating PWHT soak times for plates thicker than 2 in, candidates often mistakenly apply 1 hr/inch across the entire thickness (e.g., calculating 4 hours for a 4 in plate). Per AWS D1.1 Clause 7.8.1, the rule changes after 2 inches: it is 2 hours for the first 2 inches, plus only 15 minutes for each additional inch! For a 4 in plate: t = 2.0 + (2 x 0.25) = 2.5 hours. Calculating 4 hours is a classic exam failure.

Exam Trap 2: The Undercut Threshold Trap Exam questions routinely test whether candidates know the difference between static and cyclic undercut limits. For a static building frame, an undercut of 0.03 in (1/32 in) is completely acceptable. For a cyclically loaded bridge or crane girder tension member, that exact same 0.03 in undercut is a severe violation, because the code enforces an absolute cap of 0.01 in (0.25 mm).

Exam Trap 3: Cooling Below 600°F An exam question will ask for the required furnace cooling rate between 600°F and room temperature. The answer is: None. Below 600°F (315°C), the furnace may be opened, and the assembly cooled in still air.