11.4 Preheating, Post-Weld Heat Treatment (PWHT) & Weld Flaw Remediation
Key Takeaways
- Preheating reduces the cooling rate of the weld and heat-affected zone (HAZ), facilitating atomic hydrogen diffusion to eliminate delayed cold cracking and controlling peak HAZ hardness.
- Post-Weld Heat Treatment (PWHT) per API 650 Section 5.7.4 and API 653 Section 11.3 relieves residual welding stresses and tempers hard martensite, typically requiring soaking at 1100°F to 1200°F (595°C to 650°C) for 1 hour per inch of thickness.
- API 653 Section 11.3.2 authorizes Controlled Deposition / Temper Bead Welding as an engineered alternative to field PWHT, utilizing multi-pass thermal tempering to refine coarse-grained HAZ microstructures.
- Weld flaw remediation requires precise mechanical excavation via grinding or air carbon arc gouging, followed by grinding at least 1/16 in. (1.6 mm) of metal to remove the carburized recast layer before re-welding.
- Excavated cavities must be verified defect-free via MT or PT prior to welding, welded with a qualified WPS, and re-examined using volumetric (RT/UT) and surface NDE methods conforming to original construction standards.
11.4 Preheating, Post-Weld Heat Treatment (PWHT) & Weld Flaw Remediation
Thermal and Structural Integrity Mandate: In storage tank repair and reconstruction, welding introduces extreme localized thermal gradients, complex metallurgical phase transformations, and near-yield residual tensile stresses. Under API Standard 650 Section 7.2.1 / Section 5.7.4 and API Standard 653 Section 11, the rigorous application of Preheating, Post-Weld Heat Treatment (PWHT), and controlled Weld Flaw Remediation is critical to preventing catastrophic brittle fracture, hydrogen-induced cracking, and environmental cracking in operating tanks.
1. Metallurgical Fundamentals of Preheating
Preheating involves heating the base metal surrounding the joint to a specified minimum temperature prior to initiating the welding arc, and maintaining that temperature continuously between passes (interpass temperature).
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| THREE METALLURGICAL ROLES OF PREHEATING |
| |
| 1. RETARDS COOLING RATE (Transforms Austenite to Ductile Phases) |
| - Slows cooling through the critical 800°C to 500°C range |
| - Prevents formation of brittle, crack-sensitive martensite |
| |
| 2. FACILITATES HYDROGEN EFFUSION (Prevents Delayed Cold Cracking) |
| - Maintains atomic mobility of dissolved interstitial hydrogen |
| - Allows hydrogen gas to diffuse out of weld before cooling |
| |
| 3. MINIMIZES THERMAL GRADIENTS (Controls Residual Stresses) |
| - Reduces differential thermal expansion between joint and shell |
| - Lowers peak residual tensile stress locked into weld toes |
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The Carbon Equivalent (CE) and Hardness Thresholds
The susceptibility of carbon and low-alloy tank steels to hardening is governed by chemical composition, expressed through the Carbon Equivalent (CE) formula:
- Steels with a $CE > 0.40%$ exhibit elevated hardenability. Rapid quenching by the surrounding cold steel mass transforms austenite directly into hard, needle-like untempered martensite in the Heat-Affected Zone (HAZ).
- Untempered martensite possesses high hardness and low ductility. In wet $\text{H}_2\text{S}$ (sour crude) service or caustic environments, hard HAZ zones exceed the critical threshold (200 to 248 Vickers HV / 22 HRC), initiating rapid Sulfide Stress Corrosion Cracking (SSCC) or Hydrogen-Induced Cracking (HIC).
- Adequate preheat retards the cooling rate, allowing the microstructure to transform into soft, ductile ferrite and pearlite, thereby keeping peak HAZ hardness safely below cracking thresholds.
Preheating Guidelines (API 650 Section 7.2.1 / API 653 Section 11)
- Baseline Minimum: For basic carbon steels with thickness $\le 1.0\text{ in. (25 mm)}$, preheat must not be less than 32°F (0°C).
- Moderate Thickness (1.0 in. to 1.5 in.): Minimum preheat of 150°F to 200°F (65°C to 93°C) is mandated.
- Heavy Thickness ($> 1.5\text{ in.}$ / 38 mm) & Group IV–VI Steels: Minimum preheat of 200°F to 300°F (93°C to 149°C) must be maintained, extending at least 3 inches (75 mm) or $2 \times \text{plate thickness}$ on either side of the joint.
- Measurement: Preheat and interpass temperatures must be monitored continuously using temperature-indicating crayons (Tempilstiks), contact pyrometers, or infrared sensors.
2. Post-Weld Heat Treatment (PWHT): Standards and Protocols
Post-Weld Heat Treatment (PWHT) is a controlled thermal cycle applied after welding completion. Unlike preheat, which controls transformation upon cooling, PWHT operates in the subcritical temperature regime to achieve solid-state stress relaxation and microstructural tempering.
Mandatory Code Triggers for PWHT
Under API 650 Section 5.7.4 and API 653 Section 11.3, PWHT is mandatory for specific highly restrained or heavy-wall configurations:
- Flush-Type Cleanout Assemblies & Flush Connections: All flush-type cleanout fittings and flush shell connections must be prefabricated into an insert plate and post-weld heat treated as an integrated assembly in a furnace prior to field erection.
- Heavy-Wall Shell Openings: Shell penetrations and insert plates in materials classified as Group I, II, III, or IIIA with shell thickness $> 1.0\text{ in. (25 mm)}$ (unless modified by specific code exemptions).
- High-Strength Steels (Groups IV, IVA, V, VI): Shell penetrations where the shell thickness exceeds 0.500 in. (12 mm), unless reinforced with thickened insert plates meeting strict stress-relief criteria.
The Standard Thermal Cycle Parameters (API 650 Section 5.7.4.5)
Temperature (°F)
1200 +--------------------------+
| | SOAKING PERIOD
| Heating Ramp | - 1100°F to 1200°F
| Max 400°F/hr / (t/in.) | - 1 hr / inch of thickness (1 hr min)
| | - Uniformity within ±25°F
600 +---/ \------------------+
| / Controlled Heating Controlled Cooling \ Air Cool Below 600°F
| / \ (Still Air)
|/ \
70 +-------------------------------------------------+--------> Time (hr)
- Heating Phase: Above 600°F (315°C), the rate of heating must not exceed $400^\circ\text{F/hr}$ divided by the maximum plate thickness in inches (or $220^\circ\text{C/hr} / [t/25\text{ mm}]$), with a ceiling rate of $400^\circ\text{F/hr}$, and never less than $100^\circ\text{F/hr}$.
- Soaking Period: The component is soaked at 1100°F to 1200°F (595°C to 650°C) for carbon steels (or 1000°F minimum for certain quenched and tempered steels per code limits). The soak duration is 1 hour per inch (25 mm) of thickness, with a mandatory minimum soak of 1 hour.
- Temperature Uniformity: Throughout the soak band, the temperature variation between adjacent thermocouples must not exceed ±25°F (±14°C).
- Cooling Phase: Above 600°F (315°C), the cooling rate must be controlled to prevent thermal shock, not exceeding $500^\circ\text{F/hr}$ divided by plate thickness in inches (ceiling of $500^\circ\text{F/hr}$). Below 600°F, the component may be cooled in still air.
3. Controlled Deposition / Temper Bead Welding: Alternatives to PWHT
Applying thermal PWHT to an existing in-service or reconstructed storage tank shell presents severe practical hazards: heating a localized band on a vertical shell course to 1150°F causes extreme thermal expansion, inducing massive out-of-plumb distortion, severe shell buckling, foundation grout damage, and destruction of adjacent internal tank linings.
The API 653 Section 11.3.2 Alternative Protocol
To eliminate the hazards of field thermal stress relief, API Standard 653 Section 11.3.2 authorizes Controlled Deposition Welding (CDW) or Temper Bead Welding as an approved alternative to PWHT, provided the procedure conforms to ASME Section IX QW-290 and satisfies the following stringent prerequisites:
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| TEMPER BEAD WELDING MICROSTRUCTURAL REFINEMENT |
| |
| Layer 2 (Temper Beads): Deposits heat into Layer 1 HAZ |
| ====================[ Pass 4 ]====[ Pass 5 ]====[ Pass 6 ]========= |
| \ | / |
| v v v |
| Layer 1 (Buttering): ===[ Pass 1 ]====[ Pass 2 ]====[ Pass 3 ]===== |
| ------------------------------------------------------------------- |
| Coarse-Grained HAZ -> RE-HEATED BY LAYER 2 -> REFINED TO DUCTILE |
| (Prior Martensite) FINE-GRAINED FERRITE |
| ------------------------------------------------------------------- |
| Parent Base Metal Plate (No Direct Arc Contact from Layer 2) |
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- Engineering Evaluation: A review by a storage tank metallurgical engineer confirming that the tank service does not promote environmental cracking (e.g., anhydrous ammonia stress corrosion or severe caustic service), where full macroscopic stress reduction is non-negotiable.
- Strict Low-Hydrogen Consumables: Welding must deploy ultra-low hydrogen processes (SMAW with moisture-baked H4 electrodes yielding $< 4\text{ mL}$ diffusible hydrogen per $100\text{ g}$ of weld metal, or GTAW/GMAW).
- Elevated Preheat & Interpass Control: Minimum continuous preheat of 300°F (150°C) maintained across the repair area, with maximum interpass temperature capped at 450°F (230°C).
- The Temper Bead Mechanism: Layer 1 (buttering) is deposited against the base metal. Layer 2 is deposited with overlapping beads positioned such that the heat of Layer 2 penetrates exactly deep enough to reheat and temper the coarse-grained HAZ of Layer 1, recrystallizing brittle martensite into fine-grained, ductile ferrite-pearlite without allowing the arc to strike the base metal directly.
- Post-Weld Hydrogen Bake-Out (Post-Heating): Immediately upon completing welding (prior to allowing the joint to cool), the repair zone must be soaked at 450°F to 550°F (230°C to 290°C) for a minimum of 2 to 4 hours to drive all dissolved atomic hydrogen out of the steel.
- Delayed NDE Window: Final magnetic particle (MT) or ultrasonic testing (UT) must be delayed for at least 24 to 48 hours after the weld has cooled to ambient temperature, ensuring that any delayed hydrogen cold cracking is fully detected.
4. Weld Flaw Remediation Protocols
When non-destructive examination reveals rejectable defects—such as cracks, lack of sidewall fusion, incomplete penetration, or excessive linear slag inclusions—remediation must follow strict code procedures under API 653 Section 11.4 and Section 12:
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| WELD FLAW REMEDIATION LIFECYCLE |
| |
| 1. DEFECT LOCALIZATION: Pinpoint via UT shear-wave, RT, or MT |
| | |
| v |
| 2. EXCAVATION: Air carbon-arc gouging or mechanical grinding |
| | |
| v |
| 3. SURFACE DRESSING: Mechanical grinding of >= 1/16 in. recast layer |
| - Produce smooth boat-shaped cavity (min 3:1 or 4:1 taper) |
| | |
| v |
| 4. CAVITY VERIFICATION: 100% MT or PT to confirm complete flaw removal|
| | |
| v |
| 5. RE-WELDING: Execute using qualified WPS, preheat & low-H consumables|
| | |
| v |
| 6. FINAL RE-EXAMINATION: Visual (VT) + MT/PT + Volumetric (RT/UT) |
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Excavation and Preparation Guidelines
- Air Carbon Arc Gouging (CAC-A): Highly efficient for gouging deep flaws. However, the carbon electrode creates an intense localized carbon-rich molten pool. When the arc extinguishes, a high-carbon, extremely brittle carburized recast layer solidifies along the cavity surface.
- Mandatory Precaution: The excavated cavity must be mechanically ground with an abrasive stone to remove at least 1/16 in. (1.6 mm) of base metal, stripping away all carburized recast material. Welding over an unground carbon-arc gouge introduces severe carbon contamination, producing instant hot cracking and brittle martensitic tears.
- Cavity Geometry: The excavation must be prepared as a smooth, rounded, boat-shaped groove with gentle transition slopes (minimum 3:1 to 4:1 taper). Square ends, vertical sidewalls, and sharp V-notches promote lack of sidewall fusion and trap slag.
- Cavity Inspection: Prior to initiating weld repair, the entire excavated cavity must undergo Magnetic Particle (MT) or Liquid Penetrant (PT) examination to verify that the flaw has been 100% removed and that no crack tips propagate into the base metal.
Re-Welding and Re-Examination (API 653 Section 12.1)
- Welding Control: Repair welding must be performed by a qualified welder using an approved WPS with full preheat maintenance.
- Final Quality Verification: Completed weld repairs must be ground flush (if subject to volumetric NDE) and examined using the identical NDE methods that detected the original flaw, plus supplementary testing:
- Visual Examination (VT) for profile and undercut.
- Magnetic Particle (MT) or Liquid Penetrant (PT) on root pass (if accessible) and completed weld cap.
- Radiographic Testing (RT) or Angle-Beam Ultrasonic Testing (UT) for full-thickness volumetric confirmation on shell butt welds.
5. Heat Treatment and Flaw Remediation Comparison Matrix
The following matrix summarizes the application, operational envelope, and quality criteria for thermal conditioning and repair methods in storage tanks:
| Thermal / Repair Protocol | Governing Code Sections | Temperature & Soaking Envelope | Primary Objective | Critical Quality Control Mandate |
|---|---|---|---|---|
| Preheating | API 650 Sec. 7.2.1 / API 653 Sec. 11.3 | $32^\circ\text{F}$ to $300^\circ\text{F}$ ($0^\circ\text{C}$ to $150^\circ\text{C}$) based on thickness and CE | Retards cooling rate; diffuses hydrogen; prevents untempered martensite | Measure continuously with Tempilstiks $\ge 3\text{ in.}$ on either side of joint |
| Post-Weld Heat Treatment (PWHT) | API 650 Sec. 5.7.4 / API 653 Sec. 11.3 | $1100^\circ\text{F}$ to $1200^\circ\text{F}$ ($595^\circ\text{C}$ to $650^\circ\text{C}$); $1\text{ hr/in.}$ (1 hr min) | Relieves residual welding stresses; tempers hard microstructures | Heating/cooling rates controlled above $600^\circ\text{F}$; thermocouple spread $\le \pm 25^\circ\text{F}$ |
| Controlled Deposition / Temper Bead | API 653 Sec. 11.3.2 / ASME IX QW-290 | Preheat $\ge 300^\circ\text{F}$; Interpass $\le 450^\circ\text{F}$; Bake-out $450\text{--}550^\circ\text{F}$ for 2–4 hrs | Refines coarse HAZ grain structure via multi-pass thermal cycles | Use ultra-low hydrogen consumables (H4); delay final MT/UT by 24–48 hours |
| Carbon Arc Gouge Excavation | API 653 Sec. 11.4 / AWS C5.3 | Localized arc gouging + mechanical grinding | Removes internal weld flaws, slag, and cracks rapidly | Must mechanically grind $\ge 1/16\text{ in.}$ to eliminate carburized recast layer |
| Cavity Weld Remediation | API 653 Sec. 11.4 & Sec. 12.1 | Preheat matched to plate thickness / qualified WPS | Restores sound weld metal to designed joint cross-section | 100% MT/PT of cavity before welding; 100% VT, MT/PT, and RT/UT upon completion |
A repair contractor uses air carbon arc gouging (CAC-A) to excavate an 8-inch long linear crack in a 1.25-inch thick storage tank shell plate. Prior to depositing repair weld metal, what mandatory mechanical preparation must be executed on the excavated cavity?
An API 653 tank repair requires welding a replacement insert plate into a 1.5-inch thick Group V shell course. The owner-user determines that localized field post-weld heat treatment (PWHT) would cause severe shell distortion and foundation settlement. Under API 653 Section 11.3.2, what code-approved alternative is permitted?
What primary metallurgical risk occurs when a welding contractor attempts to weld heavy carbon steel tank plates (thickness > 1.25 inches) without applying the code-mandated preheat?