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PWHT Process Parameters for Welded Fabrications: Temperatures, Soak Times, and Ramp Rates

Post-weld heat treatment of a welded fabrication is a code-governed thermal cycle in which the entire weldment — or a qualified local band around each weld — is heated to a specified holding temperature, held long enough to relieve residual stress and temper the hardened heat-affected zone, then cooled at a controlled rate. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. The temperature, soak time, heating and cooling rates, and thermocouple placement are not matters of shop preference: they are specified by the governing code (ASME Section VIII for pressure vessels, AWS D1.1 or D1.5 for structural welding, API specifications for piping and wellheads) based on the material's P-number classification and section thickness. This article covers the heat-treater-side parameters that define a code-compliant PWHT cycle for welded fabrications — what drives each parameter, how soak time is calculated, what the furnace chart must document, and when exemption from PWHT is available under the governing code.

What does PWHT accomplish metallurgically, and what process parameters control those outcomes?​

PWHT of a welded steel fabrication simultaneously accomplishes two objectives: reduction of residual welding stress through creep-driven micro-yielding, and tempering of the hardened heat-affected zone (HAZ) microstructure produced by the weld thermal cycle. Both objectives require the weldment to be held at a temperature high enough that thermally activated mechanisms — dislocation climb, diffusional recovery of strained lattice, carbide precipitation and coalescence — become significant on a workshop time scale. For carbon and low-alloy steels, this means holding in the sub-critical range of roughly 1,050–1,250 °F (565–675 °C), below the Ac1 transformation temperature but high enough that 70–90% of the original residual stress is relieved within 1–2 hours of holding. The principal parameters that control PWHT outcome are (1) the holding temperature, which sets the rate of stress relief and HAZ tempering; (2) the soak time at holding temperature, which sets how far each mechanism proceeds; (3) the heating ramp rate, which limits the thermal gradient developed inside the part during warm-up and therefore the risk of introducing new distortion; and (4) the cooling rate after holding, which for most code applications must be slow enough to avoid re-introducing residual stress from differential cooling. A PWHT that misses any of these parameters — even one — does not meet the code requirement and cannot be corrected by extending another parameter after the fact (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1, UW-40; AWS D1.1, Clause 5.8).

How does the ASME Section VIII P-number system determine required PWHT temperature and time?​

ASME Section VIII Division 1 classifies base metals into P-numbers (and low-alloy steels into P-number groups) in Section IX, Table QW/QB-422, based on chemical composition and typical metallurgical response to welding and heat treatment. The PWHT requirements in UCS-56 (carbon and low-alloy steels) and UHA-32 (austenitic stainless), along with the companion tables in UHT, UNF, and UHX, are keyed to these P-numbers. P-No. 1 covers carbon steel (A36, A516, A53, A106, A333-1, A350-LF2 and similar); P-No. 3 covers low-alloy steel with limited chromium and molybdenum (0.5 Mo and 0.5 Cr–0.5 Mo grades); P-No. 4 and 5A cover 1.25 Cr–0.5 Mo and 2.25 Cr–1 Mo respectively; P-No. 5B covers higher-chromium grades up to 9 Cr–1 Mo. The minimum PWHT holding temperature rises with P-number — from 1,100 °F (593 °C) for P-No. 1 Group 1 carbon steel, through 1,100–1,200 °F for most P-No. 3 and P-No. 4 materials, to 1,250 °F for P-No. 5A and 1,350–1,400 °F for P-No. 5B. The soak time for all P-numbers is calculated from nominal section thickness at the weld, with most requirements set at 1 hour per inch of thickness, 15 minutes minimum. The holding-temperature range is specified as a minimum, not a target — exceeding the minimum is acceptable up to the material's upper temper limit, but below the minimum is a non-conformance regardless of how long the soak is extended (ASME Section VIII Div 1, UW-40, UCS-56, Tables UCS-56.1 through UCS-56-11; ASME Section IX, QW-420).

What are the typical PWHT temperatures and soak times for the most common weldment materials?​

The table below summarizes PWHT holding-temperature minimums and soak times for the materials UTEC Industrial most frequently processes in its car-bottom furnace. Specific code edition tables govern each actual job — the values below are representative and should be verified against the current ASME, AWS, or API edition cited on the drawing.

Material groupExample gradesP-No.Holding temperature (min)Soak time
Carbon steel, plate/pipeA36, A516 Gr 70, A106 Gr B1 Gr 11,100 °F (593 °C)1 hr/in, 15 min min
Carbon-manganese, higher strengthA516 Gr 65/70, A350-LF21 Gr 21,100 °F (593 °C)1 hr/in, 15 min min
Carbon-molybdenumA335 P1, A2043 Gr 11,150 °F (620 °C)1 hr/in, 15 min min
½ Cr–½ Mo low-alloyA335 P23 Gr 21,150 °F (620 °C)1 hr/in, 15 min min
1¼ Cr–½ Mo low-alloyA335 P11, A387 Gr 1141,200 °F (649 °C)1 hr/in, 15 min min
2¼ Cr–1 Mo low-alloyA335 P22, A387 Gr 225A1,250 °F (677 °C)1 hr/in, 15 min min
5 Cr–½ MoA335 P55B Gr 11,250 °F (677 °C)1 hr/in, 15 min min
9 Cr–1 Mo–V (Grade 91)A335 P9115E1,350–1,425 °F (732–774 °C)1 hr/in, 30 min min

For AWS D1.1 structural steel welding, the PWHT requirement in Clause 5.8 and Table 5.8 specifies 1,100 °F minimum holding temperature for most Group I and II carbon steels (A36, A572 Gr 50, A588), with soak at 1 hour per inch of thickness and minimum 1 hour. AWS D1.5 (bridge welding) uses similar parameters but imposes tighter acceptance criteria on the resulting notch-toughness values of the PWHT'd weld, which can drive the holding temperature down toward 1,100 °F minimum to avoid over-tempering. API specifications for wellheads, piping, and pressure piping use similar tables, with the specific temperature and time drawn from the applicable section (e.g., ASME B31.3 Chapter III for process piping). When multiple codes apply to a single fabrication — a common situation on pressure vessels that carry a nozzle attached by structural welding to a non-pressure support — the most restrictive applicable parameter governs (ASME Section VIII Div 1, UCS-56; AWS D1.1, Table 5.8; API Specification 6A; ASME B31.3).

How is soak time calculated, and how is it adjusted for variable section thickness?​

Soak time for PWHT is calculated from the nominal thickness of the weld being heat-treated — specifically, the thickness of the thicker member at each PWHT'd weld, not the average weldment thickness. For a 1½-inch-thick shell butt-welded to a 2-inch-thick head, the PWHT soak time at the girth weld is calculated from the 2-inch dimension: 2 hours at temperature, regardless of the thinner shell thickness elsewhere on the vessel. For a fabrication containing welds of varying thickness, the controlling thickness for the whole cycle is the thickest weld being processed. This is conservative — the thinner welds are over-processed — but it guarantees that every weld receives at least its code-required soak. When welds in a single assembly differ by more than a 2:1 thickness ratio and the cycle time penalty for the thin sections becomes significant, some specifications allow zone-selective PWHT using a qualified local PWHT procedure, in which thermocouples and heaters localize the cycle to one weld at a time — but this is an instrumented, procedure-qualified alternative, not a routine option. The 15-minute minimum soak (30 minutes for Grade 91) applies regardless of how thin the material is; a very thin weld still requires the minimum hold for the mechanisms of stress relief and HAZ tempering to run to completion at the molecular scale. For repair welds added to a previously PWHT'd vessel, the soak time is calculated from the depth of the repair cavity, not the full wall thickness — in these cases, a qualified local PWHT procedure with calibrated thermocouple spacing around the repair is often used (ASME Section VIII Div 1, UW-40(f); ASM Handbook, Vol. 4A, ASM International, 2013).

What ramp rate and cooling rate limits apply during a code-compliant PWHT cycle?​

The ASME Section VIII Div 1 rules for heating and cooling rate (UW-40(f) and UCS-56) apply only above 800 °F — below 800 °F, heating and cooling rates are uncontrolled. Above 800 °F, the heating rate shall not exceed 400 °F/hour divided by the maximum thickness in inches (and need not be less than 100 °F/hour for any thickness). For a 2-inch weldment, that means a maximum heating rate of 200 °F/hour above 800 °F; for a 4-inch weldment, 100 °F/hour. The cooling rate above 800 °F follows the same calculation in the opposite direction, except that for ASME it is keyed to 800 °F with the same 400 °F/hr-per-inch limit (minimum 100 °F/hour). The cooling rate below 800 °F is uncontrolled by the code, though good practice is to furnace-cool below 800 °F to reduce the risk of re-introducing residual stress from differential cooling. AWS D1.1 Clause 5.8 uses slightly different thresholds — heating and cooling rates are controlled above 600 °F (not 800 °F as in ASME), with heating rate limited to 400 °F/hour per inch and cooling rate limited to 500 °F/hour per inch, minimum 100 °F/hour for both. These rate limits exist to control the thermal gradient between the outer surface of the weldment and its core during the warmup and cooldown legs: if the surface heats or cools much faster than the core, the resulting thermal stress can introduce distortion in a part that is specifically being processed to remove stress. Programmable ramp-and-soak controllers, such as the one on UTEC's car-bottom furnace, are configured with the code-required rate limits before the cycle starts and enforce them automatically through modulated burner firing — a handwritten cycle on a manually tended furnace cannot meet these rate requirements repeatably on a 50-ton load (ASME Section VIII Div 1, UW-40(f), UCS-56; AWS D1.1, Clause 5.8.2).

How are thermocouples placed and validated for a PWHT cycle on a large weldment?​

Thermocouples attached directly to the weldment are the primary evidence that the part reached and held the required PWHT temperature — the furnace-wall thermocouple provides cycle control, but it does not document that the load reached temperature. ASME Section VIII Div 1 and AWS D1.1 require a documented number of part-mounted thermocouples based on weldment size and complexity. Typical practice for a vessel up to 8 feet in length and moderate wall thickness is a minimum of two thermocouples: one on the thickest section and one at the expected coolest location (the location farthest from burner impingement and with the greatest thermal mass around it, often a nozzle intersection or a heavily reinforced seam). Larger or more complex weldments may require 4–8 part-mounted thermocouples, one per PWHT-qualified zone. Each thermocouple is attached by capacitor-discharge tack-welding of the junction directly to the part surface (most common for Type K thermocouples), with the leads routed out of the furnace through a feed-through port to the chart recorder. Thermocouple accuracy must be calibrated and traceable — AMS 2750 provides the detailed pyrometry requirements for aerospace-class work, and most commercial fabrication specifications reference similar accuracy requirements (typically ±10 °F at PWHT temperatures). Local PWHT using resistance heating blankets around a circumferential weld uses the same thermocouple philosophy — a minimum of one thermocouple at each of four equally spaced points around the circumference, at the weld centerline, with additional thermocouples on the heated band edges to verify the soak band extends at least 2× the wall thickness or 4 inches (whichever is greater) on each side of the weld centerline per ASME Section VIII Div 1 Appendix R (ASME Section VIII Div 1, UW-40, Appendix R; AMS 2750; AWS D10.10 for local heating of pipe).

When is PWHT exempt under the code, and when should it be specified even without a code requirement?​

ASME Section VIII Division 1 UCS-56 provides PWHT exemption tables keyed to material P-number, section thickness, and design temperature. For P-No. 1 Group 1 carbon steel pressure vessels, PWHT is commonly exempt when nominal thickness at the weld is less than 1.5 inches (specific exemption limits vary with the current code edition — always verify against the edition cited on the drawing). For thicker sections or higher-alloy materials, exemption is either unavailable or comes with restrictions (no hydro-pressure test below a specified temperature, Charpy impact testing required, etc.). The engineer on record makes the code-exemption decision; the heat treater performs what the drawing specifies. Outside the code requirement, PWHT is often specified for reasons the code does not mandate: thick-section weldments that will be finish-machined to tight tolerances benefit from PWHT to stabilize dimensions against post-service distortion; weldments containing hardened components adjacent to welds benefit from stress relief to avoid fatigue-driven cracking at HAZ boundaries; assemblies that will carry cyclic loads (crane end trucks, machine bases, bridge components) benefit from the fatigue-life improvement that comes from reducing tensile residual stress at weld toes. In these non-code-mandated cases, the specification may call for "stress relief per ASME UW-40" or "thermal stress relief at 1,100 °F for 1 hr/in" — either phrasing invokes the same cycle parameters, and the heat treater processes the job to the same quality standard as a code-required PWHT. UTEC Industrial's 6' × 10' × 17' car-bottom furnace accepts both code-required and specification-driven PWHT work up to 50 tons, with the thermocouple records and furnace chart delivered as part of the standard documentation package on every job (ASME Section VIII Div 1, UCS-56; ASM Handbook, Vol. 4A, ASM International, 2013).

What are the alternative PWHT holding conditions allowed under the code, and when are they useful?​

ASME Section VIII Div 1 UCS-56 permits alternative holding temperatures and times for most P-No. 1 and P-No. 3 materials, trading a lower holding temperature for a longer soak time to achieve equivalent stress relief. Table UCS-56.1 lists alternative conditions such as 1,050 °F for 4 hours per inch, 1,000 °F for 10 hours per inch, or 950 °F for 20 hours per inch — each option achieves the same mechanistic outcome as the 1,100 °F / 1 hour-per-inch baseline, through the temperature–time compensation implicit in the Larson–Miller parameter that governs creep-driven stress relief. The alternative conditions are useful in three situations: (1) the weldment contains base metal in the quench-and-tempered condition that was tempered below 1,100 °F and would be re-tempered by a standard PWHT — a lower-temperature PWHT stays below the base metal's original tempering temperature and preserves its as-heat-treated hardness; (2) the part contains attached components that cannot tolerate 1,100 °F exposure (brazed joints, coatings, pre-machined bearing surfaces that would grow out of tolerance at higher temperature); or (3) the furnace's practical upper limit, or load-specific uniformity constraints, makes the lower temperature easier to achieve across the entire soak band. The trade-off is cycle time — a 4-hour-per-inch soak is four times longer than a 1-hour-per-inch soak — and the engineer on record must approve the alternative condition. Most high-strength-alloy grades (P-No. 5A Class 2, P-No. 15E Grade 91, and similar) do not permit the alternative-condition substitution because the higher holding temperature is necessary to produce the carbide redistribution required to meet notch-toughness and creep-rupture specifications — these materials require the higher-temperature baseline cycle (ASME Section VIII Div 1, UCS-56, Table UCS-56.1; ASM Handbook, Vol. 4A, ASM International, 2013; Totten, Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASME Boiler and Pressure Vessel Code, Section VIII Division 1 (current edition). American Society of Mechanical Engineers. UW-40, UCS-56, Appendix R.
  • ASME Boiler and Pressure Vessel Code, Section IX (current edition). American Society of Mechanical Engineers. Table QW/QB-422.
  • AWS D1.1: Structural Welding Code — Steel (current edition). American Welding Society. Clause 5.8, Table 5.8.
  • AWS D1.5: Bridge Welding Code (current edition). American Welding Society.
  • AWS D10.10: Local Heating of Pipe, Tube, and Fittings. American Welding Society.
  • AMS 2750: Pyrometry. SAE Aerospace.
  • ASME B31.3: Process Piping (current edition). American Society of Mechanical Engineers.
  • API Specification 6A: Specification for Wellhead and Christmas Tree Equipment. American Petroleum Institute.
  • Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.

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