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PWHT for Thick-Section Weldments: Cooling Rate, Soak Time, and Restraint

Thick-section weldments — pressure vessel shells, heavy structural nodes, large forged-and-welded shaft assemblies — concentrate the challenges of post-weld heat treatment. 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 stored elastic strain energy scales with thickness, the thermal gradient during ramp-up and cool-down amplifies as the ratio of surface area to volume drops, and the restraint imposed by the part's own stiffness becomes large enough that a poorly controlled cycle can introduce distortion rather than remove stress. This article covers the PWHT parameters that thick-section weldments require under ASME Section VIII Div 1 UCS-56 and AWS D1.1 Clause 7 — P-number-keyed soak temperatures, the 1-hour-per-inch soak-time rule, ramp and cool-down rate limits above 800 °F, and the restraint considerations that shop planners must address before rolling a heavy assembly into the furnace.

At what thickness does PWHT become mandatory under ASME UCS-56 and AWS D1.1?​

ASME Section VIII Division 1 UCS-56 establishes PWHT thresholds by P-number and nominal weld thickness. For P-No. 1 Group 1 carbon steel (A36, A516, A106 Gr B and similar), PWHT is generally required when the nominal thickness at the weld exceeds 1.5 inches, though exemption tables in UCS-56 allow specific exemptions up to this threshold when minimum preheat and Charpy impact testing requirements are met. For P-No. 3 carbon-molybdenum steels, the mandatory threshold drops to approximately 0.625 inches; for P-No. 4 (1.25 Cr–0.5 Mo) the threshold is typically 0.625 inches; for P-No. 5A (2.25 Cr–1 Mo) PWHT is required regardless of thickness on pressure-boundary welds. AWS D1.1 Clause 7 (the current edition's PWHT section, formerly Clause 5.8 in older editions) does not impose a thickness-driven mandate for structural carbon steel, but PWHT becomes engineer-specified when the structure carries cyclic loads, when the base metal is higher-strength Group II or Group III steel (A572 Gr 50, A588, A709 Gr 50W), or when the weldment will be finish-machined to close tolerances. The controlling thickness at each weld is the thicker member, not the thinner one — a 1-inch shell butt-welded to a 2-inch head is PWHT'd to the 2-inch requirement (ASME Section VIII Div 1, UCS-56; AWS D1.1, Clause 7.8).

What holding temperature and soak time apply to thick-section carbon and low-alloy steel weldments?​

The holding-temperature minimum for P-No. 1 carbon steel under ASME UCS-56 is 1,100 °F (593 °C), with soak time calculated at 1 hour per inch of controlling thickness and a 15-minute minimum for any thickness. For P-No. 3 Group 1 carbon-molybdenum grades the minimum rises to 1,150 °F (620 °C); for P-No. 4 (1.25 Cr–0.5 Mo) to 1,200 °F (649 °C); for P-No. 5A (2.25 Cr–1 Mo) to 1,250 °F (677 °C). The 1-hour-per-inch rule means a 3-inch-thick weldment receives a 3-hour soak at temperature, a 5-inch weldment a 5-hour soak, and an 8-inch weldment an 8-hour soak — plus ramp-up and cool-down time, which for a large, thick load can triple the total cycle duration. The holding-temperature figure is a minimum, not a target: the cycle must achieve at least the minimum at every thermocouple on the part throughout the entire soak period, and the furnace set point is typically 15–25 °F above the minimum to ensure the coolest thermocouple stays on-temperature through load thermal lag. AWS D1.1 Clause 7 specifies 1,100 °F minimum with 1 hour per inch (1-hour minimum) for most Group I and Group II structural steels, with the same controlling-thickness rule (ASME Section VIII Div 1, UCS-56, Tables UCS-56.1 through UCS-56-11; AWS D1.1, Clause 7.8; ASM Handbook, Vol. 4A, ASM International, 2013).

What ramp rate limits apply above 800 °F to thick-section weldments?​

ASME Section VIII Div 1 UW-40(f) imposes rate controls on heating and cooling above 800 °F for thick-section PWHT. The heating rate shall not exceed 400 °F per hour divided by the maximum thickness in inches, with an absolute maximum of 400 °F per hour and no requirement to go below 100 °F per hour regardless of thickness. For a 4-inch-thick weldment, this calculates to 100 °F per hour maximum above 800 °F; for a 2-inch weldment, 200 °F per hour; for a 6-inch weldment, the minimum 100 °F per hour still applies. Below 800 °F, the ramp rate is not code-controlled — programmable furnaces typically warm from ambient to 800 °F at 300–400 °F per hour without issue because thermal gradients at lower temperatures do not yet drive significant thermal stress. The rate limits exist to control the temperature differential between the outer skin and the core of the thick section during ramp-up: if the surface climbs much faster than the core, the outer fibers expand against a cooler, stiffer core, introducing new transient stress that competes with the stress relief the cycle is trying to accomplish. AWS D1.1 Clause 7 uses a similar calculation but begins rate control at 600 °F rather than 800 °F and uses 400 °F per hour per inch with a 100 °F per hour minimum (ASME Section VIII Div 1, UW-40(f); AWS D1.1, Clause 7.8).

What cooling rate limit applies during the controlled cool-down leg?​

The cooling rate limit above 800 °F under ASME UW-40(f) follows the same calculation as the heating leg — 400 °F per hour divided by the maximum thickness in inches, with a minimum of 100 °F per hour. A 4-inch weldment cools no faster than 100 °F per hour from the hold temperature down to 800 °F; a 2-inch weldment no faster than 200 °F per hour. Below 800 °F the code does not impose a rate, but industry standard practice is to furnace-cool below 800 °F — keeping the doors closed and letting the furnace mass carry the part down slowly — until the part reaches roughly 600 °F, at which point it may be removed to still air without risk of re-introducing thermal-gradient stress. Pulling a thick-section weldment from the furnace immediately at the end of soak, while it is still at 1,100 °F, defeats the slow-cool objective: the outer surface chills rapidly in shop air while the core remains near hold temperature, re-introducing differential contraction stress that can equal or exceed the original welding stress. AWS D1.1 Clause 7 specifies 500 °F per hour per inch as the cooling rate limit above 600 °F (100 °F per hour minimum) and requires still-air cool below that threshold. For large loads in a programmable car-bottom furnace, the controlled cool is handled by the ramp-and-soak program itself, which modulates burner firing downward and vents the furnace at a controlled rate (ASME Section VIII Div 1, UW-40(f); AWS D1.1, Clause 7.8; ASM Handbook, Vol. 4A, ASM International, 2013).

Why does the 1-hour-per-inch soak rule apply at every thickness, and when is more time justified?​

The 1-hour-per-inch soak rule emerges from the kinetics of creep-driven stress relaxation and carbide tempering at sub-critical temperature. At 1,100 °F, about 70–90% of residual welding stress relaxes within 1 hour, and HAZ tempering of the hardened martensitic or bainitic structures produced by welding reaches a stable condition on the same time scale. The rule scales linearly with thickness because the slowest-responding region is the weld-metal core, which for a thicker weld has more thermal mass to bring to temperature and more volume of material to relax through dislocation climb. Extending soak time beyond 1 hour per inch yields diminishing returns on stress relief — the relaxation curve flattens — but continues to temper the HAZ further, which can be beneficial when the welding procedure produced a harder-than-expected HAZ or when the base metal is high-strength low-alloy steel that benefits from additional tempering. More time is justified when: (1) the weld geometry produced a highly restrained joint where residual stress estimates are unusually high; (2) NDE after PWHT reveals hardness readings in the HAZ above the specification limit (commonly 248 HV or 22 HRC for sour-service applications), and a re-PWHT with extended soak is used to drive additional tempering; (3) the code alternative-condition table (UCS-56 Table UCS-56.1) is invoked with a lower holding temperature and correspondingly longer soak — 1,050 °F for 4 hr/in, 1,000 °F for 10 hr/in, 950 °F for 20 hr/in — to protect heat-sensitive attached components or previously Q&T base metal (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).

How does restraint during ramp-up and cool-down affect thick-section weldments?​

Restraint is the combined stiffness of a weldment that opposes thermal expansion and contraction during PWHT. A thick-section assembly with welded-in stiffening plates, integral nozzles, or heavy end closures cannot expand freely as it warms — its own geometry imposes internal constraint, and any external fixturing (car blocking, ring supports, hold-down clamps) adds more. When restraint is high, the stress that PWHT is trying to relieve can be partially offset by new transient stress generated during the thermal ramp itself. Two practical consequences follow: (1) the fixturing plan for a thick-section PWHT load should minimize restraint to what is necessary for handling and uniform heating — blocking should support the part evenly without clamping it against expansion; and (2) ramp rate should be set at or below the code maximum rather than pushed to the limit, because a slower ramp reduces the thermal gradient that drives restraint-induced stress. Thick assemblies that contain welded-in mismatched materials — for example, a carbon steel shell with a stainless steel internal liner, or a large casting welded into a forged frame — are particularly prone to restraint-driven distortion because the two materials have different coefficients of thermal expansion, creating a fixed mismatch stress that rises linearly with temperature. In these cases, the design authority and the heat treater review the assembly together before PWHT to determine whether the weldment can safely complete the cycle or whether alternative stress relief (VSR, local PWHT) is required (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1, UW-40).

How does UTEC Industrial's car-bottom furnace configuration support thick-section PWHT?​

UTEC Industrial's 6' × 10' × 17' car-bottom furnace with 50-ton load capacity accommodates thick-section weldments that exceed the working envelope of many commercial heat treatment furnaces — heavy pressure vessel shells, machine base fabrications, large welded shaft assemblies, and structural weldments up to the full car dimensions can roll in on the cart without disassembly or field welding of subsections. The programmable ramp-and-soak controller is configured per job with the code-required heating and cooling rate limits — typically 100–200 °F per hour above 800 °F for thick sections — and enforces those rates automatically through modulated burner firing, which a manually tended furnace cannot reliably maintain on a 50-ton load over a 20-hour cycle. Part-mounted thermocouples tack-welded at the thickest section and the expected coolest location document that the part reached and held the required temperature throughout the soak period; the chart record ships with the heat treatment documentation package. For thick-section weldments up to 50 tons that require PWHT under ASME UCS-56 or AWS D1.1 Clause 7, the combination of envelope, load capacity, and programmable cycle control is what determines whether the job can be processed as a single load — the practical alternative for parts that exceed a smaller furnace is to subdivide the weldment into sections and weld them together after PWHT, which introduces un-stress-relieved welds that can compromise the service application (ASME Section VIII Div 1, UW-40, UCS-56; ASM Handbook, Vol. 4A, ASM International, 2013).

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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, Tables UCS-56.1 through UCS-56-11.
  • ASME Boiler and Pressure Vessel Code, Section IX (current edition). American Society of Mechanical Engineers.
  • AWS D1.1: Structural Welding Code — Steel (current edition). American Welding Society. Clause 7.8.
  • AMS 2750: Pyrometry. SAE Aerospace.
  • Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.

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