Post-weld heat treatment (PWHT) is a controlled thermal cycle applied after welding to reduce residual stresses, temper hard microstructures, improve toughness, and promote hydrogen diffusion. The process involves slowly heating the weldment to a specific temperature, holding at that temperature for a calculated duration, and slowly cooling back to ambient temperature.
PWHT is mandatory under ASME codes for thick carbon steel, all chrome-moly alloys, and sour service applications. AWS D1.1 rarely requires PWHT for structural steel. The decision to apply PWHT is driven by the construction code, material type, thickness, and service conditions.
Why PWHT Is Performed
PWHT accomplishes four things:
Residual stress reduction. Welding introduces residual stresses from thermal contraction. These stresses can approach the yield strength of the material. PWHT at elevated temperature allows the metal to partially relax (creep), reducing residual stresses to 10-20% of their as-welded levels.
HAZ tempering. The heat-affected zone develops hard, potentially brittle microstructures during welding. PWHT tempers these structures, improving ductility and toughness. For carbon and low-alloy steels, this means converting untempered martensite to tempered martensite or bainite.
Hydrogen removal. Hydrogen trapped in the weld and HAZ can cause delayed cracking. Holding at PWHT temperature (or even at lower intermediate temperatures) accelerates hydrogen diffusion out of the metal.
Metallurgical stability. For some alloys (notably P91/T91 chrome-moly), PWHT is essential to develop the correct microstructure for high-temperature service.
When PWHT Is Required
ASME Section VIII (Pressure Vessels)
ASME Section VIII Division 1, UCS-56 specifies mandatory PWHT for:
| P-Number | Material Type | PWHT Required When |
|---|---|---|
| P-1 (Carbon steel) | A516, A106, A105 | Thickness exceeds 3/4 in (some exceptions for preheated joints) |
| P-3 (1/2 Mo) | A335 P1, A204 | All thicknesses |
| P-4 (1-1/4 Cr-1/2 Mo) | A335 P11, A387 Gr 11 | All thicknesses |
| P-5A (2-1/4 Cr-1 Mo) | A335 P22, A387 Gr 22 | All thicknesses |
| P-5B (5 Cr, 9 Cr) | A335 P5, P9, P91 | All thicknesses |
| P-8 (Austenitic stainless) | 304, 316, 321 | Generally not required (PWHT can be harmful) |
ASME B31.3 (Process Piping)
B31.3 Table 331.1.1 specifies PWHT based on P-number and thickness. Requirements generally align with Section VIII but may have additional conditions for specific service classifications (e.g., severe cyclic service requires PWHT at lower thickness thresholds).
NACE MR0175/ISO 15156 (Sour Service)
For equipment exposed to hydrogen sulfide (H2S) in oil and gas service, PWHT is mandatory regardless of thickness for most carbon and low-alloy steels. The purpose is to ensure HAZ hardness doesn’t exceed 22 HRC (Rockwell C), which is the threshold above which sulfide stress cracking can occur.
AWS D1.1 (Structural Steel)
AWS D1.1 generally doesn’t require PWHT for structural steel. The engineer can specify it for special situations (highly restrained joints, thick members, or fatigue-sensitive applications), but it’s not a default requirement.
PWHT Temperatures and Hold Times
Temperature Chart by Material
| P-Number | Material | PWHT Temperature Range | Minimum Hold Time |
|---|---|---|---|
| P-1 | Carbon steel | 1100-1200F (593-649C) | 1 hr/in thickness (1 hr minimum) |
| P-3 | 1/2 Mo steel | 1100-1200F | 1 hr/in (1 hr min) |
| P-4 | 1-1/4 Cr-1/2 Mo | 1250-1300F (677-704C) | 1 hr/in (1 hr min) |
| P-5A | 2-1/4 Cr-1 Mo | 1300-1350F (704-732C) | 1 hr/in (1 hr min) |
| P-5B (Gr 91) | 9 Cr-1 Mo-V | 1350-1425F (732-774C) | Per code (typically 2 hr min) |
| P-9A | 2.5% Nickel steel | 1050-1100F (566-593C) | 1 hr/in (1 hr min) |
Hold time calculation: For material 2 inches thick at 1 hour per inch, the hold time is 2 hours at the specified temperature. The clock starts when all thermocouples read within the specified temperature band and stops when controlled cooling begins.
Heating and Cooling Rates
Controlled Heating Rate
The heating rate must be slow enough to prevent thermal gradients that could cause distortion or cracking in the already-welded joint.
ASME standard heating rate: Maximum 400F per hour divided by the maximum metal thickness in inches, with a maximum of 400F/hr. For a 2-inch thick vessel, the maximum heating rate is 200F/hr.
In practice, heating rates of 100-300F/hr are common. Slower rates are used for heavy-wall, highly restrained joints.
Soaking (Hold) Phase
During the hold phase, all thermocouples must read within the specified temperature band. The maximum temperature must not exceed the upper limit, and the minimum must stay above the lower limit.
Temperature uniformity: The difference between the hottest and coldest thermocouple should not exceed 150F (some codes specify tighter ranges, like 50F or 100F, depending on the application).
Controlled Cooling Rate
Cooling must be controlled to prevent thermal shock and the formation of unwanted microstructures.
ASME standard cooling rate: Maximum 400F per hour divided by thickness (same formula as heating), with a maximum of 400F/hr. Below 800F, cooling can be accelerated by removing insulation and allowing natural cooling, provided the rate doesn’t cause distortion.
In practice: Controlled cooling to 600-800F (in the furnace or under insulation), then natural cooling to ambient temperature.
PWHT Methods
Furnace PWHT
The entire weldment is placed in a furnace designed for heat treatment. The furnace provides uniform temperature across the entire piece.
Advantages:
- Best temperature uniformity
- Entire piece is stress-relieved simultaneously
- Automatic temperature control and recording
- Handles large assemblies
Limitations:
- Requires access to a furnace large enough for the piece
- Transportation to and from the furnace
- The entire piece is heated, including areas that don’t need treatment
- Cost of furnace time
Local PWHT
When furnace treatment isn’t practical (field installations, very large assemblies, repairs on existing equipment), PWHT can be applied locally using:
Electric resistance heaters: Ceramic pad heaters strapped to the pipe or vessel around the weld. Powered by a programmable controller that manages heating rate, hold temperature, and cooling rate.
Induction heating: Electromagnetic coils provide rapid, uniform heating. More expensive equipment but faster and more uniform than resistance heaters.
Local PWHT Procedure
Local PWHT requires careful planning to ensure adequate coverage:
Heated band width: The area heated to the full PWHT temperature must extend at least 3 times the material thickness on each side of the widest point of the weld. For a 1-inch thick pipe weld, heat at least 3 inches on each side of the weld (6 inches total minimum heated band, plus the weld width).
Gradient control band: The temperature gradient outside the heated band must be controlled. Insulation extends well beyond the heated band to prevent steep gradients.
Insulation: Ceramic fiber blankets (at least 2 inches thick) are wrapped around the heated band and extending at least 12 inches beyond on each side. This minimizes heat loss and maintains temperature uniformity.
Thermocouple Placement
Thermocouples are the temperature measurement devices attached directly to the metal surface. Their placement determines if the reading reflects the actual temperature at the weld zone.
Placement Rules
| Location | Purpose |
|---|---|
| On the weld (crown) | Verify the weld reaches PWHT temperature |
| On the HAZ (both sides of weld) | Confirm the HAZ is treated |
| At the edge of the heated band | Monitor temperature gradient |
| On the opposite side (180 degrees on pipe) | Verify uniform heating around the circumference |
| Outside the insulated zone | Monitor that the temperature drops off appropriately |
Attachment Methods
- Capacitor discharge welding: Small thermocouple wire spot-welded directly to the metal surface. The most reliable attachment method. The small weld mark is easily ground off after PWHT
- Mechanical clips: Spring-loaded clips hold the thermocouple against the surface. Less reliable because the clip can shift during heating
- High-temperature tape: Holds the thermocouple in place but may fail at high temperatures
Number of Thermocouples
Minimum thermocouple count depends on the component size and code requirements:
- Pipe joints (local PWHT): Minimum 2 thermocouples per joint (12 o’clock and 6 o’clock). For larger diameters, add thermocouples at 3 o’clock and 9 o’clock
- Vessel shell seams: One thermocouple every 10-15 feet of weld length, plus one at each end
- Furnace PWHT: Thermocouples on the component surface at the thickest and thinnest sections, plus furnace air temperature sensors
Documentation
PWHT documentation must include:
- PWHT procedure: Written procedure specifying temperature, hold time, heating rate, cooling rate, and thermocouple placement
- Chart recorder output: Time-temperature recording from every thermocouple during the entire cycle (heating, hold, cooling)
- Thermocouple calibration records: Verification that thermocouples are accurate
- Visual inspection report: Post-PWHT inspection for distortion, discoloration, or damage
The time-temperature chart is the permanent record that proves PWHT was performed correctly. It must show that all thermocouples stayed within the specified temperature band for the full hold time, and that heating and cooling rates didn’t exceed limits. This chart becomes part of the project quality documentation and is typically required for code compliance.
Common PWHT Mistakes
Insufficient insulation. Without adequate insulation, the heat source struggles to maintain temperature, energy costs skyrocket, and temperature uniformity suffers. Use at least 2 inches of ceramic fiber blanket, more for high-temperature PWHT.
Heating too fast. Rapid heating creates thermal gradients that distort or crack the weldment. Follow the code-specified maximum heating rate.
Cooling too fast. Removing insulation at high temperature and letting the piece air-cool creates thermal shock risk and may produce undesirable microstructures. Control cooling per the code until below 800F.
Wrong temperature for the material. P-5A (2-1/4 Cr-1 Mo) at 1100F instead of 1300F won’t achieve the desired metallurgical transformation. Verify the correct temperature for the specific P-number.
Measuring the heater instead of the metal. Thermocouples must be on the metal surface, not on the heating element. The heater surface temperature may be 100-200F higher than the metal surface during heating.
PWHT is a critical quality step that, when done correctly, prevents in-service failures in pressure equipment, piping, and other critical applications. The investment in time, equipment, and documentation is significant, but the alternative is operating equipment with unrelieved welding stresses and potentially hard, brittle microstructures that can fail catastrophically. Follow the code, document the process, and verify the results.