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Heat Treating ASTM A36 Structural Steel: Post-Weld Stress Relief and Normalizing

ASTM A36 is the dominant carbon-manganese structural steel grade in North American fabrication — the default specification for wide-flange beams, channels, angles, plates, and structural weldment bases across buildings, bridges, equipment frames, transportation, and heavy industrial machinery. 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. A36 is specified primarily by mechanical properties (36 ksi minimum yield, 58–80 ksi tensile) with a chemistry envelope that tolerates up to 0.26% carbon and 1.2% manganese, giving it excellent weldability but limited heat-treatment response. The two heat treatments that matter for A36 are post-weld stress relief of fabricated weldments (the single highest-volume carbon-steel heat treatment application) and normalizing of thick-section plate when improved fracture toughness or refined grain structure is needed. This article covers thermal cycle parameters, code-driven specifications per AWS D1.1 and ASME Section VIII, and practical considerations for sourcing PWHT on A36 fabrications.

What are the composition and key properties of ASTM A36 structural steel?​

ASTM A36 is specified per ASTM A36/A36M: Standard Specification for Carbon Structural Steel primarily by mechanical properties rather than tight chemistry. The chemistry envelope allows carbon up to 0.26% (up to 0.25% for plates under 3/4 inch and 0.27–0.29% for plates over 2 inches), manganese 0.80–1.20% (for structural shapes and thick plates), phosphorus 0.04% maximum, sulfur 0.05% maximum, silicon 0.40% maximum, and copper 0.20% minimum when copper-bearing steel is specified. The mechanical property requirements are 36 ksi minimum yield strength, 58–80 ksi tensile strength, and 20% minimum elongation in 8 inches. The wide chemistry envelope means A36 heats vary in carbon equivalent (CE) from approximately 0.35 to 0.55 depending on the specific chemistry; this variation affects weldability assessment, preheat requirements, and heat-affected-zone hardness in welding. A36 is a carbon-manganese steel — not a "carbon steel" in the plain-carbon sense (1018, 1020) because of its manganese content — and its weldability is generally excellent for CE values below approximately 0.45, with preheat recommended above that level per AWS D1.1. A36 is not intended for quench-and-temper hardening; its low carbon content and absence of hardenability-enhancing alloys (Cr, Ni, Mo) mean that heat treatment is limited to stress relief, normalizing, and process annealing. The specification dominates North American structural fabrication because it is low-cost, widely stocked in every rolled shape, readily weldable, and backed by more than 60 years of field experience (ASTM A36; AWS D1.1; ASM Handbook, Vol. 1, ASM International, 1990).

When is post-weld heat treatment (PWHT) required for A36 weldments?​

PWHT of A36 weldments is driven by one of three requirement paths: a welding code (AWS D1.1 for structural, AWS D1.5 for bridges), a pressure vessel code (ASME Section VIII Division 1 for vessels, API 510 for in-service vessels), or a customer specification driven by dimensional stability, fatigue resistance, or fracture toughness needs. For AWS D1.1 structural welding, PWHT is addressed in Clause 7 (Stress Relief Heat Treatment) of the 2020 edition; PWHT is not universally required but may be specified by the engineer for welded structures subject to cyclic loading, stress corrosion environments, or dimensional stability requirements. The temperature range per AWS D1.1 Clause 7 is 1,100–1,200 °F (595–650 °C) for carbon steel weldments including A36, with soak time of one hour per inch of thickness (minimum 30 minutes) and heating/cooling rates not exceeding 400 °F per hour above 600 °F for welds over 2 inches thick. For ASME Section VIII Div 1 pressure vessels, PWHT requirements for carbon steel (P-No. 1 materials, which includes A36 when used as pressure-retaining material) are tabulated in Table UCS-56 and referenced by paragraph UW-40. The thresholds are thickness-dependent: generally, PWHT is required for nominal weld thicknesses above 1 1/2 inches for P-No. 1 Group 1 materials, with lower thresholds if welds are performed without preheat. The holding temperature in Table UCS-56 is 1,100 °F minimum with soak time based on the governing thickness. For A36 weldments not subject to code — machine frames, support structures, general industrial fabrications — PWHT is specified when the fabricator or customer determines that dimensional stability after machining, reduced in-service distortion, or improved fatigue life justifies the cost. PWHT of A36 is the single highest-volume external-customer heat treatment service at UTEC Industrial; the car-bottom furnace's 6' × 10' × 17' envelope accepts structural weldments that exceed the working envelope of most commercial heat treating furnaces, and the programmable ramp-and-soak control produces the temperature chart that documents code compliance (ASME Section VIII Div 1, UW-40 and Table UCS-56; AWS D1.1 Clause 7).

What are the thermal cycle parameters for A36 stress relief?​

The standard thermal stress relief cycle for A36 follows the sub-critical heat treatment approach common to all carbon and low-alloy steels: heat to 1,100–1,150 °F (595–620 °C), soak one hour per inch of the thickest governing section (minimum one hour for small work, minimum 30 minutes per AWS D1.1 Clause 7), cool at a controlled rate not exceeding 400 °F per hour down to 600 °F, then allow to cool in still air. The soak temperature is chosen below the A1 lower critical temperature (approximately 1,335 °F for low-carbon steel) so the microstructure and hardness remain essentially unchanged; the process relieves residual stress through creep-driven micro-yielding, typically reducing the initial stress magnitude by 70–85% at 1,150 °F over a one-hour-per-inch soak. For code-governed work, the ramp rate control above 600 °F matters because faster heating produces thermal gradients across the load that can re-introduce stress in thick sections; the 400 °F per hour limit is a conservative value applicable to most fabrications. Below 600 °F during the cool-down, still-air cooling is acceptable because the thermal gradient no longer drives significant creep deformation or stress generation. For a typical 1 1/2-inch-thick A36 weldment specified for PWHT per ASME Section VIII: ramp to 1,150 °F at 400 °F per hour maximum, soak 1.5 hours (one hour per inch of governing thickness), furnace-cool at 400 °F per hour maximum to below 600 °F, then allow natural cooling to ambient. The furnace chart from the cycle becomes the documentation of code compliance. Hardness of the weld metal and heat-affected-zone typically drops slightly after PWHT — typical as-welded HAZ hardness of 200–280 HV in carbon-manganese steel reduces to 180–230 HV after stress relief, which is both a goal (reduced HAZ hardness improves toughness and stress-corrosion resistance) and a side effect to plan for (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1, UCS-56; AWS D1.1 Clause 7).

When should A36 be normalized rather than stress-relieved?​

Normalizing and stress relief are different thermal treatments with different goals, and confusing the two is a common specification error. Stress relief is sub-critical (below A1, no transformation) and reduces residual stress without changing microstructure. Normalizing is super-critical (above A3, full austenitization followed by still-air cooling) and produces a refined, uniform grain structure. For A36, normalizing is specified when the goal is improved fracture toughness on thick sections, grain refinement after hot-working operations that coarsened the structure, or a defined starting microstructure before additional thermal processing. The cycle: ramp to 1,600–1,650 °F (870–900 °C), soak one hour per inch of section, remove from the furnace and cool in still air. The resulting microstructure is fine-grained pro-eutectoid ferrite plus pearlite with ASTM grain size typically 6–8; hardness falls in 131–170 HB range. Normalizing of A36 is commonly specified for thick plate used in low-temperature service (where Charpy impact toughness at low temperature is a qualification requirement), for weld repair of heavy plates where HAZ grain growth has occurred, and for ingot-origin plate that shows banding or directional structure. For a 3-inch-thick A36 plate weldment where the engineer has specified normalizing followed by stress relief, the sequence is: normalize first (1,625 °F, three-hour soak, air cool), then stress relieve (1,150 °F, three-hour soak, controlled cool). For most A36 fabrications, however, stress relief alone is the specified treatment — normalizing is specified only when the code, drawing, or engineer calls for it based on a specific toughness or microstructure requirement. Confusing the two cycles — applying a normalize to a weldment where only stress relief was specified — can produce dimensional changes and distortion that the customer did not expect, and substantially higher cost (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM A36; Machinery's Handbook, 31st ed., Industrial Press, 2020).

How does weldment size and shape affect PWHT furnace selection?​

Weldment size and shape drive furnace selection more than any other factor in PWHT sourcing. The key constraint is that the entire weldment must fit inside the furnace working zone, with adequate clearance for thermocouple placement and for uniform gas circulation (in gas-fired furnaces) or radiation (in electric furnaces) around all surfaces. For small weldments — below approximately 4 feet in any dimension — commercial box furnaces and muffle furnaces are widely available. For medium weldments — up to 8 feet in length — car-bottom or bell furnaces in the 4–6-foot working envelope are common at regional heat treaters. For large weldments — above 8 feet in any dimension or exceeding 5–6 tons load weight — the number of commercial heat treaters able to accept the work drops sharply; these fabrications typically require a large car-bottom furnace or a specialty shop with a pit furnace. Large structural weldments frequently exceed the working envelope of smaller commercial furnaces; a 6' × 10' × 17' car-bottom furnace with 50-ton load capacity accepts weldments that would otherwise need to be subdivided, shipped to a distant heat treater, or have PWHT performed by local electrical-resistance-heating blankets — a field technique that can produce compliant results but is more labor-intensive and produces less uniform documentation than furnace PWHT. For irregular-shaped weldments (tall structural assemblies, machine frames with projecting members, long conveyor frames), the 17-foot furnace length and 10-foot height make a significant difference in what fits without disassembly. Fabricators sourcing PWHT should confirm furnace working dimensions, load capacity, and programmable ramp-and-soak capability early in the project; for code-governed work, the heat treater should also be able to provide temperature uniformity survey results per the applicable pyrometry standard when the end-user's specification requires it (ASM Handbook, Vol. 4B, ASM International, 2014; AWS D1.1 Clause 7).

What documentation ships with an A36 PWHT job?​

The documentation package for a PWHT job on A36 typically includes: the heat treatment record (cycle name, specified temperature and soak time, actual recorded temperature from furnace thermocouples, ramp and cool rates achieved, total cycle time, operator identification, equipment identification), the temperature-time chart from the furnace recorder showing the complete cycle, the thermocouple placement record if thermocouples were attached to the work rather than only monitoring furnace air temperature, any hardness readings taken before and after the cycle if specified, and a statement of conformance to the applicable code paragraph (AWS D1.1 Clause 7.2.2, ASME Section VIII UW-40, or customer specification). For code-governed work under ASME Section VIII Div 1, the customer typically requires the documentation to be traceable to the Manufacturer's Data Report (Form U-1 or similar) for the pressure vessel; the heat treater's record is referenced or attached. For AWS D1.1 structural work, documentation is typically required by the Engineer of Record and becomes part of the fabricator's welding procedure and quality documentation. For private-specification PWHT (machine frames, industrial equipment), the documentation is typically a simple heat treatment certificate with the cycle parameters, actual chart, and statement of completion. The temperature uniformity of the furnace during the cycle is a separate qualification: commercial furnaces meeting AMS 2750 pyrometry for Class 2, 3, or 4 service have documented temperature uniformity across the working zone; for structural PWHT outside of aerospace, the typical industrial expectation is a programmable furnace with reliable process records, not a Nadcap-accredited aerospace facility. The documentation should allow the customer to show compliance during a third-party inspection without requiring additional information from the heat treater (ASME Section VIII Div 1, UCS-56; AMS 2750 for aerospace work; AWS D1.1 Clause 7).

What specification errors are common on A36 heat treatment?​

Specification errors that cause problems in A36 heat treatment work, and how to resolve them: Specifying "heat treat A36" without naming the cycle — leaves the heat treater without guidance. The correct specification states the cycle (PWHT, stress relief, normalize), the temperature range, the soak time basis, and the governing section thickness. Specifying PWHT temperature above 1,200 °F for A36 — approaches the A1 lower critical temperature and risks partial austenitization in thick sections with nonuniform heating; stay in the 1,100–1,200 °F window per AWS D1.1 Clause 7 or Table UCS-56 as applicable. Specifying PWHT without clarifying whether air cooling is acceptable after soak or furnace cooling below 600 °F is required — for code work, the cool rate matters; for general industrial work, practice varies. AWS D1.1 Clause 7 requires controlled cooling to 600 °F at 400 °F per hour maximum. Specifying "normalize and stress relieve" without sequence — the correct sequence is normalize first (above A3, air cool), then stress relieve (below A1, controlled cool); stating the sequence explicitly avoids ambiguity. Specifying PWHT on weldments containing dissimilar metals without considering the thermal expansion mismatch — A36 stress relief at 1,150 °F is fine for A36 alone, but if the weldment incorporates stainless steel or aluminum components, the combined thermal cycle may damage the non-A36 components. Specifying PWHT on weldments with pre-machined precision surfaces or installed bearings — the thermal cycle at 1,100 °F-plus will distort ground surfaces and damage bearings; either remove them before PWHT or specify vibratory stress relief (VSR) as an alternative for assemblies that cannot tolerate furnace temperatures. Specifying a hardness target on A36 after PWHT — hardness after PWHT is in the 110–170 HB range (normalized or annealed equivalent) and is not typically specified on structural A36; hardness targets are a medium-carbon and alloy-steel consideration. Raising these issues at order entry, rather than discovering them after the cycle begins, is where competent intake review pays for itself (ASM Handbook, Vol. 4A, ASM International, 2013; AWS D1.1 Clause 7; ASME Section VIII Div 1, UCS-56).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (2014). ASM Handbook, Volume 4B: Steel Heat Treating Technologies. ASM International.
  • ASM International. (1990). ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys. ASM International.
  • Machinery's Handbook (31st ed.). (2020). Industrial Press.
  • ASTM A36: Standard Specification for Carbon Structural Steel. ASTM International.
  • AWS D1.1: Structural Welding Code — Steel. American Welding Society.
  • ASME Boiler and Pressure Vessel Code, Section VIII Division 1: Paragraphs UW-40 and UCS-56 (Post-Weld Heat Treatment). American Society of Mechanical Engineers.
  • AMS 2750: Pyrometry. SAE Aerospace.

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