A Post Weld Heat Treatment (PWHT) certificate is the documented record that a welded assembly was heat treated at the required temperature for the required duration, in accordance with the applicable code and procedure. It is a mandatory quality document for most carbon and low-alloy steel pressure equipment above defined thickness thresholds.
Quick Answer
Quick Answer
A PWHT certificate includes the time-temperature chart recorded by calibrated thermocouples during the heat treatment cycle, the procedure reference, the identification of items treated, and a statement that the treatment met code requirements. ASME VIII-1 and EN 13445 define the required soak temperatures and holding times by material P-Number and wall thickness.
Why PWHT Is Required
Welding introduces three types of problematic residual effects that PWHT addresses:
- Residual stresses — weld shrinkage and thermal gradients during cooling create tensile residual stresses that can promote stress corrosion cracking and fatigue crack growth
- Hard microstructures — rapid cooling from welding temperatures creates hard, brittle phases (martensite, bainite) in the heat-affected zone (HAZ) of carbon and alloy steels
- Hydrogen entrapment — dissolved hydrogen from the welding process can cause delayed hydrogen-assisted cracking if not diffused out
PWHT (also called stress relief heat treatment) heats the component to a sub-critical temperature — below the lower transformation temperature (Ac₁) — holds it there long enough to homogenize temperature and relax stresses, then cools it slowly enough to avoid re-introducing thermal stresses.
When PWHT Is Mandatory
ASME BPVC Section VIII Division 1 (UCS-56 for carbon and low-alloy steels)
PWHT is mandatory when nominal weld thickness exceeds:
| P-Number | Mandatory PWHT thickness |
|---|---|
| P-1 (carbon steel, e.g., SA-516) | > 38 mm (1.5 in) nominal |
| P-1 with specific Cr-Mo content | > 19 mm (0.75 in) |
| P-4 (1Cr-½Mo, e.g., SA-387 Gr. 11) | All thicknesses |
| P-5A (2¼Cr-1Mo, e.g., SA-387 Gr. 22) | All thicknesses |
| P-15E (9Cr-1Mo-V, Grade 91) | All thicknesses |
Additional requirements apply for lethal service, low-temperature service, and when impact testing is required.
ASME B31.3 Process Piping (331.1)
Similar P-Number and thickness requirements, with explicit soak temperature and cooling rate specifications.
EN 13445-4 (Unfired Pressure Vessels)
Requirements based on material group (CR ISO 15608), thickness, and preheat temperature used during welding.
PWHT Parameters
Soak Temperature Range
| Material (ASME P-Number) | Soak temperature range |
|---|---|
| P-1 (carbon steel) | 595–650°C (1100–1200°F) |
| P-4 (1.25Cr-0.5Mo) | 620–650°C (1150–1200°F) |
| P-5A (2.25Cr-1Mo) | 675–705°C (1250–1300°F) |
| P-15E (9Cr-1Mo-V, Gr.91) | 730–775°C (1350–1430°F) |
| P-8 (austenitic SS — solution anneal) | 1040–1120°C (1900–2050°F) |
Minimum Holding Time (UCS-56)
Minimum 1 hour per 25 mm (1 in) of thickness, with a minimum of 1 hour for all thicknesses. Longer hold times are required for P-5 and P-15E materials.
Heating and Cooling Rates
Controlled heating and cooling rates prevent re-introduction of thermal gradients:
- Heating rate above 315°C (600°F): ≤ 220°C/hour per 25 mm thickness (max 330°C/hour)
- Cooling rate to 315°C (600°F): ≤ 275°C/hour per 25 mm thickness
Thermocouple Requirements
Thermocouples are attached directly to the component to record actual temperature during PWHT, not furnace air temperature. Requirements:
- Number and placement: At least one thermocouple per 5 m of component length in furnace; additional TCs at high thermal mass locations (nozzle necks, thick sections, head-to-shell junctions)
- Calibration: Thermocouples and recording equipment must be calibrated per ISA 70.01 or equivalent; calibration certificate must be current
- Attachment method: Welded, attached with clamps, or thermally conductive cement — not bare wire laid against the surface
- Recording: Continuous time-temperature record on a chart recorder or data logger; digital records acceptable if tamper-evident
Required Fields on a PWHT Certificate
- Item identification — vessel or spool number, drawing reference, weld joint numbers included
- Material P-Numbers — of base metals in the assembly
- Applicable code and clause — e.g., "ASME VIII-1 UCS-56"
- Procedure reference — PWHT procedure number and revision
- Furnace or heating equipment identification — furnace ID, calibration reference, fuel type (gas, electric, resistance, induction)
- Thermocouple identification — TC numbers, type (Type K, Type N), calibration certificate reference
- Thermocouple attachment locations — drawing showing TC placement on the component
- Recorded soak temperature — actual minimum and maximum temperature recorded during hold
- Soak time — actual hold time at soak temperature
- Heating and cooling rate — maximum rates achieved
- Time-temperature chart — attached to the certificate as the primary evidence record
- Result — Acceptable / Non-conforming
- Heat treater's authorized signature and fabricator QC sign-off
Post-PWHT Hardness Verification
After PWHT, hardness testing is frequently required to confirm that the treatment was effective in tempering the HAZ. ASME VIII-1 UHA-51 requires hardness testing for P-4 and P-5 materials. Results are reported on a separate hardness test certificate, cross-referenced to the PWHT certificate by the item identifier.
See detailed guide: Hardness Test Certificate
What happens if the PWHT temperature drops below the minimum soak temperature during the hold?
If the temperature drops below the minimum specified soak temperature during the holding period, the clock must be restarted from when the temperature returns to within the acceptable range. If the temperature excursion was significant or prolonged, an engineering review is required to determine if additional holding time compensates, or if re-PWHT is required. The time-temperature chart must record the actual excursion, and the non-conformance must be documented.
Can local PWHT be used instead of furnace PWHT?
Yes. ASME VIII-1 UW-40 and ASME B31.3 331.1 permit local PWHT using resistance heating or induction heating in specific circumstances — typically for field welds that cannot be furnished to the furnace. Local PWHT requires a more detailed procedure specifying the heated band width (minimum 75 mm from the weld centerline in each direction), additional thermocouples, and insulation requirements. The certificate must document all these parameters.
Is PWHT required for austenitic stainless steel?
Not for stress relief in the same sense as carbon steel. Austenitic stainless steels do not harden by PWHT in the sub-critical range. However, solution annealing (heating to 1040–1120°C and rapid quench) may be required after welding of certain grades to dissolve sensitization (chromium carbide precipitation). This is a different heat treatment than PWHT and requires its own documentation with different temperature requirements.
Does PWHT reset the impact test qualification?
PWHT is a supplementary essential variable under ASME IX QW-407.1. If the PWHT condition changes for a WPS that requires impact testing (e.g., adding PWHT when the PQR was qualified without it, or changing the PWHT temperature range), a new qualification test coupon must be PWHT'ed under the same conditions and impact tested. The PQR must reflect the PWHT condition used.
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