10.3 Preheating Standards, Interpass Temperature Control & Post-Weld Heat Treatment (PWHT)

Metallurgical Purpose of Preheating

Preheating involves heating the base metal immediately surrounding a weld joint to a specified minimum temperature prior to initiating an electric arc or thermal cutting operation. In boiler and pressure vessel fabrication, preheat is not merely a convenience—it is a critical metallurgical necessity governed by ASME Section I (PW-38/39), ASME Section VIII (UW-30/UCS-56), and ASME B31.1.

                                  METALLURGICAL BENEFITS OF PREHEAT
                                                  │
       ┌──────────────────────────┬───────────────┴───────────────┬──────────────────────────┐
       ▼                          ▼                               ▼                          ▼
  SLOWER COOLING RATE        PREVENTS MARTENSITE             HYDROGEN EFFUSION          REDUCES RESTRAINT
  Reduces temperature        Suppresses formation of         Gives dissolved H2 time    Minimizes thermal
  gradient between puddle    brittle, untempered             to escape into atmosphere  contraction stress
  and cold base plate        martensite in the HAZ           before cold cracking       across rigid joints

The Four Primary Mechanisms of Preheat

1. Reduces the Cooling Rate (Quench Severity):

   • In heavy-wall pressure vessels (such as steam drums and thick headers), the cold mass of steel acts as a massive thermal heat sink. When liquid weld metal solidifies at ~2,800°F, the adjacent steel rapidly pulls heat away from the fusion zone.
   • Preheating raises the background temperature of the component, reducing the temperature differential (Delta-T) and dramatically slowing the cooling rate through the critical transformation range ($1,333^\circ\text{F}\text{ to }800^\circ\text{F}$).

2. Suppresses Brittle Martensite Formation in the HAZ:

   • When carbon and alloy steels cool slowly from austenitic temperatures, the austenite transforms safely into soft, ductile ferrite and pearlite or tough bainite.
   • If the cooling rate is too rapid (rapid quenching), carbon atoms become trapped within the crystal lattice, forming untempered martensite—a body-centered tetragonal phase that is glass-brittle, highly stressed, and exceptionally prone to cracking under load.

3. Promotes Hydrogen Effusion (Outgassing):

   • Hydrogen atoms absorbed into the liquid weld puddle remain dissolved in the hot metal. At temperatures above 200°F (93°C), hydrogen possesses high mobility and diffuses rapidly through the crystal lattice into the atmosphere.
   • If the weldment cools too quickly to ambient temperature, hydrogen becomes trapped in microscopic voids and grain boundaries. Under residual tensile stress, trapped hydrogen generates immense internal pressure, triggering Hydrogen-Induced Underbead Cracking.

4. Minimizes Thermal Contraction and Residual Stress:

   • Preheating thermally expands the base metal surrounding the joint prior to welding. As the weld puddle solidifies and shrinks, the surrounding preheated base metal contracts concurrently, substantially reducing localized residual shrinkage stresses.

ASME P-Number Groupings and Minimum Preheat Requirements

To standardize welding procedures (WPS/PQR) and heat treatment rules, ASME Section IX assigns base metals specific P-Numbers based on chemical composition, weldability, and mechanical characteristics.

ASME Section I Table PW-39 and ASME Section VIII Division 1 Table UCS-56 establish mandatory minimum preheat temperatures based on the material's P-Number and thickness:

Preheat Heating Zone (Soak Band Width)

Preheat cannot be applied merely as a thin stripe along the bevel edge. ASME Section I mandates that preheat must be uniformly applied through the full material thickness and extend across a heated soak band extending a minimum of 3 inches (75 mm) or 3 times the nominal component thickness (whichever is greater) on each side of the weld centerline.

Interpass Temperature Measurement and Control

Interpass temperature is defined as the temperature of the weldment in the immediate joint area immediately before the subsequent weld pass is deposited (or between passes in multi-pass welding).

                                  INTERPASS TEMPERATURE CONTROL

                      MINIMUM INTERPASS                             MAXIMUM INTERPASS
                      Must be AT OR ABOVE                           Must NOT EXCEED
                      specified preheat temp                        specified upper limit
                                │                                             │
                                ▼                                             ▼
                      Prevents martensite &                         Prevents grain coarsening,
                      hydrogen cracking                             loss of toughness & sensitization

Measurement Tools and Field Techniques

1. Temperature-Indicating Crayons (Tempilstiks):

   • Formulated with calibrated chemical phase-change materials that melt precisely at a rated temperature (+/- 1% accuracy).
   • Proper Technique: Boilermakers must rub the Tempilstik on the base metal surface adjacent to the weld groove (approximately 1 inch back from the bevel).
   • Critical Inspection Rule: NEVER stroke a Tempilstik directly inside the weld groove or onto a molten weld bead. The melting wax and chemical binders introduce organic contaminants that cause immediate porosity, carbon pickup, and cracking.
   • Verification Strategy: Welders utilize two crayons:
     • Minimum Tempilstik (e.g., 300°F): Must melt instantly upon touching the steel, confirming preheat is satisfied.
     • Maximum Tempilstik (e.g., 500°F): Must leave a dry, un-melted chalk mark, confirming the joint has not overheated.

2. Direct-Contact Digital Pyrometers (Thermocouple Probes):

   • Surface contact thermocouple probes provide instantaneous digital readouts. Surface must be cleaned of heavy loose scale for accurate heat conduction.

3. Infrared (IR) Optical Pyrometers (Non-Contact Laser Guns):

   • Fast and convenient, but subject to severe optical errors caused by surface emissivity. Bare, ground, shiny steel reflects infrared energy and produces falsely low temperature readings. Pyrometers must be adjusted for an emissivity of 0.85 to 0.95 on oxidized steel, or targeted on dark, non-reflective surfaces.

Consequences of Exceeding Maximum Interpass Temperature

Allowing a weld joint to exceed its qualified maximum interpass temperature causes severe metallurgical degradation:

• Creep-Strength Enhanced Ferritic Steels (P91 / P92): Exceeding the maximum interpass temperature ($550^\circ\text{F} / 288^\circ\text{C}$) causes extreme austenite grain coarsening and destroys elevated-temperature creep-rupture strength.
• Austenitic Stainless Steels (304/316): Exceeding $350^\circ\text{F} (177^\circ\text{C})$ holds the heat-affected zone in the sensitization window ($800^\circ\text{F}\text{ to }1,500^\circ\text{F}$), causing chromium carbide precipitation along grain boundaries and catastrophic intergranular corrosion.
• High-Strength Quenched & Tempered Steels: Overheating slows the cooling rate, over-tempering the base metal and permanently destroying yield and tensile strength.

Post-Weld Heat Treatment (PWHT) and Stress Relieving

Post-Weld Heat Treatment (PWHT), often termed stress relieving, is a controlled thermal cycle applied to a completed weldment. The assembly is heated uniformly to a sub-critical temperature (below the lower critical transformation temperature $Ac_1$), held at that temperature for a specified soak time, and slowly cooled at a controlled rate.

                                  PWHT THERMAL CYCLE PROFILE

  TEMP (°F)
     ▲
     │                                [ SOAK / HOLD TEMPERATURE ]
     │                            ┌─────────────────────────────────┐
     │                           /  (P1: 1100-1200°F; P4: 1200-1300°F) \   CONTROLLED COOLING RATE
     │                          /   (Hold: 1 Hr per Inch Thickness)     \  (Max 500°F/hr / t)
     │  CONTROLLED HEATING RATE/                                         \ (Down to 600°F-800°F)
     │  (Max 400°F/hr / t)    /                                           \
     │                       /                                             \──────── Air Cool to
 800°│──────────────────────/                                               Ambient (Still Air)
     │                     /
     │  Unrestricted Rate /
     └───────────────────┴─────────────────────────────────────────────────────────────► TIME (HR)

The Metallurgical Objectives of PWHT

1. Thermal Stress Relief (Plastic Relaxation):
   • Solidified weld deposits contain residual tensile stresses approaching or exceeding the yield strength of the steel (up to 50,000–80,000 psi).
   • When heated into the PWHT range ($1,100^\circ\text{F}\text{ to }1,400^\circ\text{F}$), the yield strength of the metal drops drastically to less than 10% of its room-temperature value. Residual stresses exceeding this reduced yield strength are relieved through microscopic plastic creep relaxation.
2. Tempering of Martensite Microstructures:
   • PWHT tempers the hard, brittle martensite formed in the weld metal and HAZ. The trapped carbon precipitates out as microscopic spheroidal carbides within a ductile ferrite matrix, drastically increasing impact toughness and elongation.
3. Dimensional Stability & Hydrogen Extraction:
   • Prevents stress-induced distortion during post-weld machining and drives any remaining trace hydrogen completely out of the component.
4. Resistance to Stress Corrosion Cracking (SCC):
   • Lowering residual tensile stresses below threshold limits prevents catastrophic cracking in boiler environments containing sulfides, chlorides, or caustic boiler water.

PWHT Parameters: Heating Rates, Soak Temperatures, Hold Times & Cooling Rates

ASME Section I (Table PW-39) and Section VIII (Table UCS-56) dictate strict mathematical rules governing every stage of the PWHT cycle:

1. Heating Rate Above 800°F (427°C)

Below 600°F–800°F, heating may proceed at an unrestricted rate. Above 800°F, thermal shock and differential expansion must be prevented by limiting the heating rate according to component thickness ($t$ in inches):

$\text{Maximum Heating Rate (°F/hr)} = \frac{400^\circ\text{F}}{\text{Thickness in inches } (t)}, \quad \text{with a maximum limit of } 400^\circ\text{F}/\text{hr}$

(Note: In no case may the required rate be mandated below 100°F/hr.)

2. Soak Temperatures and Holding Times

The soak temperature must be maintained uniformly across the entire weldment and heated band. The standard holding duration is 1 hour per inch of thickness (with a minimum of 15 to 30 minutes for thin sections):

The Critical Temperature Ceiling for Grade 91 ($Ac_1$ Rule): For CSEF Grade 91 steel, the upper PWHT temperature must never exceed the lower critical transformation temperature ($Ac_1 \approx 1,470^\circ\text{F} / 800^\circ\text{C}$). If heated above $1,470^\circ\text{F}$, the steel re-transforms to un-tempered austenite and permanently dissolves its micro-alloy precipitate structure, destroying the creep strength of the pipe and requiring complete replacement of the component.

3. Cooling Rate Down to 800°F

Upon completion of the soak hold time, the cooling rate must be controlled to prevent thermal stress re-introduction:

$\text{Maximum Cooling Rate (°F/hr)} = \frac{500^\circ\text{F}}{\text{Thickness in inches } (t)}, \quad \text{with a maximum limit of } 500^\circ\text{F}/\text{hr}$

Once the temperature cools below 600°F to 800°F (315°C to 427°C), the insulation blankets may be left intact while the component cools naturally in still ambient air down to room temperature.

Field PWHT Equipment: Resistance Ceramic Mats vs. Induction Heating

In field boiler construction, components cannot be placed in a shop furnace. Heat treatment is applied locally using specialized electric heating methods:

               ELECTRIC CERAMIC RESISTANCE PADS vs. INDUCTION HEATING

       [ CERAMIC RESISTANCE PADS ]                     [ INDUCTION HEATING ]
      High-Purity Ceramic Fiber Blankets             Water-Cooled Copper Induction Coils
            ┌──────────────────┐                            ┌──────────────────┐
       ────►│ Nichrome Wire    │◄────                  ────►│ Electromagnetic  │◄────
            │ Ceramic Beads    │                            │ Eddy Currents    │
            └──────────────────┘                            └──────────────────┘
      Conduction / Radiant surface heating           Direct internal volumetric heating;
      Flexible; low initial equipment cost           fast cycle; superior gradient control

1. Flexible Ceramic Resistance Heating Pads (FCP)

• Construction: Nickel-chromium (Nichrome 80/20) resistance heating wire threaded through interlocking, high-alumina ceramic beads to form flexible heating pads (mats).
• Operation: Powered by 80-volt low-voltage power consoles or 480-volt multi-zone heat treatment units. Heat transfers into the pipe wall through thermal conduction and radiation.
• Installation: Pads are wrapped tightly around the circumference of the pipe/vessel, secured with stainless steel banding wire, and insulated with 2 to 4 inches of high-temperature ceramic fiber blanket (Kaowool / Superwool).

2. Induction Heating

• Construction: Flexible, water-cooled copper cables or induction blankets wrapped around the pipe spool. High-frequency alternating electric current (10 to 25 kHz) passes through the coils.
• Operation: The alternating magnetic field penetrates the ferromagnetic steel, inducing intense eddy currents and magnetic hysteresis within the pipe wall itself.
• Advantages: Heat is generated internally within the metal, rather than conducted from the outside surface. Provides rapid, uniform heating across the entire wall thickness, eliminates burn-out of heating elements, and reduces thermal hold cycle times on heavy steam lines.

3. Thermocouple Placement and Gradient Control

• Type K Thermocouples (Chromel-Alumel): Welded directly to the pipe surface using capacitive discharge (CD) wire welders.
• Placement Strategy: Thermocouples must be placed at the top (12:00), bottom (6:00), and sides of horizontal pipe joints to monitor chimney-effect thermal stratification.
• Heated Band & Gradient Insulation: The soak band must encompass the weld plus $1t$ on either side; gradient insulation must extend a minimum of $2 \times \sqrt{R \cdot t}$ along the pipe to ensure axial thermal stresses do not exceed code allowable limits.

Step-by-Step Worked Calculation: PWHT Cycle for a Steam Drum Seam

To understand the practical execution of ASME Section I / Section VIII heat treatment, consider a field engineering scenario for stress-relieving a boiler pressure boundary:

  ═════════════════════════════════════════════════════════════════════════════════════
  WORKED FIELD CALCULATION: ASME SECTION VIII STEAM DRUM GIRTH SEAM PWHT
  ═════════════════════════════════════════════════════════════════════════════════════
  - Component: Boiler Steam Drum Shell Girth Butt Weld
  - Material: ASME SA-516 Grade 70 (P-No. 1 Carbon Steel)
  - Nominal Shell Thickness (t): 2.50 inches (63.5 mm)
  - Drum Outer Diameter: 60.0 inches (1,524 mm)

  STEP 1: Calculate Maximum Allowable Heating Rate (Above 800°F)
          Max Rate = 400°F / t = 400°F / 2.50 in = 160°F per hour
          Result: The heating rate above 800°F must not exceed 160°F/hr.

  STEP 2: Determine Soak (Holding) Temperature Range
          Per ASME Section VIII Table UCS-56 for P-No. 1:
          Target Soak Range = 1,100°F to 1,200°F (Nominal Target: 1,150°F ± 25°F)

  STEP 3: Calculate Mandatory Minimum Soak (Hold) Duration
          Rule: 1 hour per inch of thickness for P-No. 1
          Soak Time = 2.50 in × 1.0 hr/in = 2.50 hours (2 hours, 30 minutes)
          Result: Hold temperature between 1,100°F and 1,200°F for a minimum of 2.5 hrs.

  STEP 4: Calculate Maximum Allowable Cooling Rate (Down to 800°F)
          Max Rate = 500°F / t = 500°F / 2.50 in = 200°F per hour
          Result: The cooling rate from soak down to 800°F must not exceed 200°F/hr.

  STEP 5: Ambient Air Cooling Phase
          Below 800°F, heating power is turned off; insulation blankets remain in place
          while the drum cools naturally in still air down to ambient room temperature.
  ═════════════════════════════════════════════════════════════════════════════════════