Skip to main content

PWHT Code Requirements: ASME Section VIII and AWS D1.1

Post-weld heat treatment is mandated by code for a wide range of welded fabrications, and the code requirements — not heat-treater preference — determine what temperature the cycle holds at, how long the soak runs, how fast the part can be heated or cooled, and what documentation must ship with the finished assembly. 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 two dominant codes for heavy industrial work in the United States are ASME Boiler and Pressure Vessel Code Section VIII Division 1 (pressure vessels) and AWS D1.1 Structural Welding Code — Steel (structural steel fabrications). They take similar approaches but diverge in the temperature floor, the ramp and cool rate formulas, and the thickness thresholds that trigger mandatory PWHT. This article summarizes what each code requires, how the P-number classification system drives cycle selection, when PWHT is exempt, and how related codes (AWS D1.5 for bridges, API 510 for in-service vessel repairs) interact with the Section VIII and D1.1 framework.

What do ASME Section VIII Division 1 and AWS D1.1 require for PWHT?​

ASME Section VIII Division 1 requires post-weld heat treatment for most pressure-retaining weldments, with the specific cycle parameters set by the base metal's P-number grouping in Table UCS-56. The baseline requirement for P-No. 1 carbon steels is a minimum holding temperature of 1,100 °F with a hold time of 1 hour per inch of the governing thickness (or fraction thereof), but not less than 15 minutes. For low-alloy chromium-molybdenum steels classified under P-No. 3, 4, 5A, and 5B, the minimum holding temperature rises to 1,250 °F or higher depending on the specific P-number and Group. The requirement applies to the vessel's pressure boundary welds and to non-pressure attachments welded to the pressure boundary above certain size thresholds. AWS D1.1 takes a narrower approach — the code covers structural steel fabrications for buildings, industrial structures, and tubular construction, and it does not mandate PWHT for most structural applications. Where D1.1 PWHT is required (typically for tubular connections in specific service conditions or when project specifications call it out), the cycle parameters sit in Clause 5.8: minimum 1,150 °F holding temperature, 1 hour per inch of thickness, with heating and cooling rate limits that depend on section thickness (ASME Section VIII Div 1, UCS-56; AWS D1.1, Clause 5.8).

How do P-numbers classify base metals for PWHT purposes?​

The P-number system, defined in ASME Section IX for welding procedure qualification, groups base metals by their metallurgical similarity so that one qualified welding procedure can cover multiple related grades. Section VIII Division 1 uses the same P-numbers to index PWHT requirements. P-No. 1 covers plain carbon and carbon-manganese steels (A36, A516, A515, A106, A53 Grade B, SA-350 LF2, and similar) — the largest group by volume and the grades UTEC Industrial processes most often in the car-bottom furnace for structural and pressure-vessel weldments. P-No. 3 covers low-alloy steels with up to 0.75% molybdenum or up to 1.25% nickel (A302, A335 P11 partial, certain A387 grades); P-No. 4 covers 1.25% chromium-0.5% molybdenum alloys (A335 P11, A387 Grade 11); P-No. 5A covers 2.25% Cr-1% Mo alloys (A335 P22, A387 Grade 22); P-No. 5B covers higher-chromium Cr-Mo grades (A335 P5, P9, P91). Each P-number — and for some groups each Group number within the P-number — maps to a specific minimum PWHT temperature in Table UCS-56. Knowing the base metal's P-number is a prerequisite to any PWHT specification; the welding procedure qualification record (PQR) identifies it, and the heat-treating operator verifies it against the job packet before cycle selection (ASME Section IX, QW-422; ASME Section VIII Div 1, UCS-56).

What are the temperature and time requirements for PWHT per code?​

Table UCS-56 in Section VIII Division 1 sets the minimum holding temperature and holding time for each P-number group. For P-No. 1 Group 1 and 2 carbon steels, the minimum is 1,100 °F; for P-No. 3 (Cr-Mo-V and Mn-Mo steels), the minimum rises to 1,100 °F for Group 1 and 1,100–1,200 °F for higher groups; for P-No. 4 (1-1/4 Cr-1/2 Mo), the minimum is 1,200 °F; for P-No. 5A (2-1/4 Cr-1 Mo), the minimum is 1,250 °F; for P-No. 5B and P-No. 6 higher-alloy grades, temperatures rise further. Hold time is 1 hour per inch of the governing thickness for thicknesses up to 2 inches; for thicknesses between 2 and 5 inches, hold time is 2 hours plus 15 minutes per inch over 2 inches; minimum hold time is 15 minutes regardless of thickness. Governing thickness is defined by the weld configuration — for a butt weld it is the thinner of the two plates; for a fillet weld attaching a non-pressure component, it is the throat of the fillet. AWS D1.1 Clause 5.8 uses a simpler formula where PWHT is required: 1,150 °F minimum and 1 hour per inch, but applied more narrowly because D1.1 rarely mandates PWHT in the first place (ASME Section VIII Div 1, UCS-56 and Table UCS-56; AWS D1.1, Clause 5.8).

What ramp rate and cooling rate limits does each code impose?​

Heating and cooling rates are limited above a threshold temperature to prevent thermal gradients from inducing stress that would negate the stress-relief benefit or cause cracking. Section VIII Division 1 limits heating above 800 °F to 400 °F per hour divided by the maximum metal thickness in inches, with a maximum heating rate not exceeding 400 °F per hour regardless of thickness, and a minimum rate not less than 100 °F per hour. For a 2-inch-thick vessel shell, that formula yields a maximum heating rate of 200 °F per hour above 800 °F; for a 4-inch section, 100 °F per hour; below 800 °F, no code-imposed rate limit applies and typical industrial practice heats at 300–400 °F per hour. Cooling above 800 °F is limited to 500 °F per hour divided by the maximum metal thickness in inches, also capped at 500 °F per hour maximum and 100 °F per hour minimum. Below 800 °F, cooling is typically in the closed furnace until below 600 °F, then the part may be removed to still air. AWS D1.1 Clause 5.8 uses a similar but simpler pair of limits — 400 °F per hour heating and 500 °F per hour cooling above 600 °F — applied across the full relevant temperature range without the thickness-scaled formula. UTEC Industrial's car-bottom furnace programmable ramp-and-soak controller holds actual heating and cooling rates to a commanded profile and records the result on the cycle chart for documentation (ASME Section VIII Div 1, UCS-56(d); AWS D1.1, Clause 5.8.3).

When is PWHT exempted under Section VIII Division 1?​

Not every pressure vessel requires PWHT. Section VIII Division 1 provides exemptions in Table UCS-56 and in the notes to that table, primarily based on thickness and preheat. The most commonly invoked exemption covers P-No. 1 Group 1 and 2 carbon steels at thicknesses up to 1-1/2 inches (38 mm) for joints not subject to the specific cases requiring PWHT — but exceptions apply, including service in lethal substances (UW-2(a)), service at low temperatures where the material's impact properties must be preserved, and fabrications with joint geometries that concentrate residual stress. For P-No. 3 steels, an exemption applies up to 5/8 inch thickness with 200 °F preheat maintained. Similar thickness-plus-preheat exemptions appear for P-No. 4 (up to 1/2 inch with 300 °F preheat) and for higher-alloy P-numbers at progressively smaller thicknesses. Preheat maintained at or above the specified temperature during and after welding acts as a slow-cool substitute that allows hydrogen diffusion and reduces the martensite formation that makes PWHT beneficial. Even when an exemption is available, many owners and specification writers require PWHT anyway as a service-condition hedge — PWHT on a borderline-thickness vessel handling cyclic loads is conservative engineering (ASME Section VIII Div 1, UCS-56, Table UCS-56; UW-2(a)).

How does AWS D1.1 PWHT differ from ASME Section VIII?​

AWS D1.1 and ASME Section VIII Division 1 address different product categories with different failure-mode priorities, and the PWHT requirements reflect that. Section VIII covers pressure-containing vessels where through-wall leak, fatigue cracking, and hydrogen-induced cracking in welded zones are the dominant concerns — PWHT is the default, exemption is the exception. D1.1 covers structural steel for buildings, mill structures, offshore topsides, and similar load-bearing non-pressure fabrications where residual stress is a design input but not typically a service-life-limiting factor for most member sizes — PWHT is rare, preheat is the primary weld-quality control. Where D1.1 does require PWHT (specific tubular connection details in Section 9, certain project-specification overlays), the temperature is 1,150 °F minimum versus Section VIII's 1,100 °F for the same P-No. 1 base metal. The higher D1.1 number reflects the code's concern with residual-stress-driven fatigue in high-cycle structural applications. Both codes require 1 hour per inch soak, but D1.1's rate limits are uniform (not thickness-scaled) and its cooling-method allowances are slightly more permissive. A fabrication drawing that calls for PWHT under D1.1 produces a cycle that meets Section VIII requirements if the P-number and thickness fall within Section VIII's coverage, but the reverse is not always true — a Section VIII cycle at 1,100 °F does not meet D1.1's 1,150 °F floor (AWS D1.1, Clause 5.8; ASME Section VIII Div 1, UCS-56).

What thermocouple placement and documentation does each code require?​

Both codes require that actual metal temperature — not just furnace air temperature — be monitored during the cycle, and both require that the temperature record be preserved as part of the fabrication quality documentation. Section VIII Division 1 requires thermocouples attached directly to the part (capacitor-discharge weld attachment is standard) at locations that represent the thickest section, the thinnest section, and locations where temperature might lag due to shielding or thermal mass — the specific number and placement is determined by the fabricator's PWHT procedure qualified under Section IX. For pressure vessels with through-wall thermocouples, at minimum one thermocouple is placed on each major component (shell, heads, nozzles). AWS D1.1 requires temperature monitoring of the weldment surface and the area immediately adjacent to the weld, with thermocouple spacing not greater than 5 feet for large weldments. Both codes require that the chart or electronic record of the cycle be retained as a quality record tied to the specific fabrication — UTEC Industrial's programmable controller produces a time-stamped temperature trace for each load, and the chart ships with the cycle documentation as part of the heat treatment record. The PWHT record, together with the welding procedure qualification record and welder performance records, forms the documentation package that demonstrates code compliance to the Authorized Inspector or project owner (ASME Section VIII Div 1, UW-51 and UCS-56; AWS D1.1, Clause 5.8.4).

How do API 510 and AWS D1.5 extend the PWHT requirement picture?​

Two additional codes routinely invoke PWHT requirements that fabricators and heat treaters encounter in industrial work. API 510 (Pressure Vessel Inspection Code) governs in-service inspection and repair of pressure vessels originally constructed to ASME Section VIII. When a repair weld is made under API 510, the PWHT requirement of the original construction code applies — a repair weld on a Section VIII vessel at a location where the original construction required PWHT also requires PWHT after repair, unless a controlled-deposition welding technique qualified as a PWHT alternative is used. AWS D1.5 (Bridge Welding Code) covers welded bridge structural steel and is more stringent than D1.1 on thick-section PWHT requirements, reflecting the fatigue-driven service life of bridge welded joints under repeated truck loading. D1.5 Clause 4.3 addresses heat treatment and typically requires PWHT for certain weld joint categories above specified thickness thresholds at 1,100 °F minimum for 1 hour per inch. In both cases, the cycle parameters the heat treater executes are dictated by the construction code, not by the inspection code or the welding code alone — the job packet that accompanies the fabrication to the furnace must identify which code and which section govern, so that cycle selection matches the requirement. Ambiguity in the code reference is resolved at intake review, before the part enters the furnace (API 510, Section 8; AWS D1.5, Clause 4.3; ASME Section VIII Div 1, UW-40).

Related Articles

References​

  • ASME. (2023). ASME Boiler and Pressure Vessel Code, Section VIII Division 1: Rules for Construction of Pressure Vessels, Paragraph UW-40, UCS-56, and Table UCS-56. American Society of Mechanical Engineers.
  • ASME. (2023). ASME Boiler and Pressure Vessel Code, Section IX: Welding, Brazing, and Fusing Qualifications, QW-422. American Society of Mechanical Engineers.
  • American Welding Society. (2020). AWS D1.1/D1.1M: Structural Welding Code — Steel, Clause 5.8. American Welding Society.
  • American Welding Society. (2020). AWS D1.5/D1.5M: Bridge Welding Code, Clause 4.3. American Welding Society / AASHTO.
  • American Petroleum Institute. (2022). API 510: Pressure Vessel Inspection Code — In-service Inspection, Rating, Repair, and Alteration (11th ed.). American Petroleum Institute.
  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.

Need In-House Heat Treating for Heavy Industrial Parts?​

UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co

Last updated on