PWHT in the Welding Workflow: Sequence, Preheat, and Interpass Temperature
Post-weld heat treatment does not happen in isolation — it is one step in a fabrication workflow that begins with material selection and preheat specification, runs through welding and interpass temperature control, and ends with PWHT and final inspection. 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 sequencing of these steps, and the interaction between them, determines whether PWHT achieves its intended purpose. A weldment that enters PWHT with inadequate preheat, uncontrolled interpass temperatures, or pre-existing hydrogen cold cracks will not be restored to spec by the thermal cycle. This article covers the complete welding-to-PWHT workflow for heavy steel fabrications — preheat and interpass requirements, hydrogen management, PWHT timing, sequencing with machining and NDE, and the common workflow errors that defeat PWHT.
How does preheat fit into the welding-and-PWHT workflow?
Preheat is applied to the base metal before welding begins to slow the cooling rate of the weld and heat-affected zone (HAZ), reducing the formation of hard martensite and the buildup of hydrogen-assisted cracking risk. Preheat and PWHT address the same underlying problem — HAZ hardness and residual stress — but at different points in the process and by different mechanisms. Preheat acts during welding to prevent the hardest, most brittle HAZ microstructures from forming in the first place; PWHT acts after welding to temper whatever HAZ microstructure did form and to reduce residual stress. When preheat is inadequate for the steel grade and thickness, the HAZ hardens to a brittle martensitic condition that is susceptible to hydrogen-assisted cold cracking — cracking that typically occurs within hours or days of welding, before PWHT is applied. PWHT performed after the part has already cold-cracked cannot repair the crack; it can only soften the crack flanks and reduce the stress that might extend the crack further. The practical implication: preheat is not optional when the code or metallurgical calculations require it, even if the part will receive PWHT — PWHT and preheat are complementary steps, not alternatives. AWS D1.1 Table 5.8 and ASME Section IX specify minimum preheat temperatures by carbon equivalent and thickness; these are the starting points for preheat specification, with the design authority's procedure qualification records establishing the actual required preheat for the production weld (AWS D1.1, Clause 5.7; ASME Section IX, QW-406; Linnert, G.E., Welding Metallurgy, 4th ed., AWS, 1994).
What is interpass temperature and how does it interact with PWHT requirements?
Interpass temperature is the temperature of the previously deposited weld bead (and surrounding base metal) at the moment the next weld pass is started. It has a minimum and a maximum limit, both important. The minimum interpass temperature is typically the same as the specified preheat temperature — the part must stay at or above preheat between passes to maintain the slow cooling rate that prevents HAZ martensite. If the fabricator allows the weldment to cool below preheat between passes (a common shortcut on long or complex joints), the cooling-rate protection is lost and the HAZ hardens on each thermal cycle from each pass. The maximum interpass temperature is often 550–650 °F for carbon and low-alloy steels (per AWS D1.1 and most fabrication specifications) — exceeding this limit causes excessive heat input to accumulate in the weld zone, resulting in grain coarsening in the HAZ and over-softening of any previously quench-and-tempered base metal adjacent to the joint. For weldments on previously quench-and-tempered base metal (a common situation in industrial fabrication where structural components are welded to hardened shafts, frames, or plates), the maximum interpass temperature must be controlled carefully: if interpass temperatures approach the original tempering temperature of the Q&T base metal, the welding process effectively re-tempers the base metal adjacent to the weld, reducing hardness and strength in those areas even before PWHT is applied. Interpass temperature is measured by contact pyrometer or thermocouple at specified intervals during welding — the records are part of the weld procedure qualification documentation (AWS D1.1, Clause 5.7; ASME Section IX, QW-406).
What is hydrogen bake-out and when is it required before PWHT?
Hydrogen bake-out (also called a post-weld hydrogen bake or low-temperature PWHT) is a thermal hold at 400–600 °F applied immediately after welding — before the weldment cools to ambient — specifically to allow dissolved hydrogen to diffuse out of the weld metal and HAZ. It is distinct from PWHT: bake-out is performed at a temperature too low to produce significant stress relief or HAZ tempering, but high enough to dramatically accelerate hydrogen diffusion. The mechanism: hydrogen enters the weld metal from the arc atmosphere (from moisture in flux coatings, from contaminated base metal, or from humid conditions) during welding. At ambient temperature, hydrogen diffuses very slowly through the steel lattice — the dissolved hydrogen that causes cold cracking may remain in the joint for hours or days. At 400–600 °F, hydrogen diffusivity increases by several orders of magnitude, allowing most of the dissolved hydrogen to diffuse to the surface and escape within 1–4 hours. Bake-out is required when: the weld is performed with high-hydrogen-potential consumables (cellulosic electrodes, unconditioned low-hydrogen electrodes); the joint geometry is highly restrained (preventing natural joint movement that would relieve hydrogen stress); the steel carbon equivalent is high (above ~0.45%, where HAZ martensite is hard and brittle enough for hydrogen cracking to initiate at low stress); or service in hydrogen-containing environments (sour gas, petrochemical) makes any residual hydrogen a long-term embrittlement risk. The distinction between bake-out and PWHT must be clear on the quality plan: bake-out addresses hydrogen only, PWHT addresses stress and HAZ hardness — both may be required in sequence for high-restraint, high-carbon-equivalent weldments (Linnert, G.E., Welding Metallurgy, 4th ed., AWS, 1994; AWS D1.1, Annex I).
What is the correct sequence of PWHT relative to other fabrication operations?
The correct sequence for a typical heavy steel weldment requiring PWHT, NDE, and machining is: (1) preheat and weld with required interpass temperature control; (2) hydrogen bake-out at 400–600 °F immediately after welding if required by material or specification; (3) allow the weldment to cool to ambient (after bake-out, or directly after welding if bake-out is not required); (4) perform all weld completion NDE that is required before PWHT — visual, MT, and PT can be performed before or after PWHT; UT and RT may be performed before or after depending on the code and weld type; (5) PWHT in the furnace, with the weldment in its final assembly configuration (or as close to it as practical — PWHT after final assembly is preferred because subsequent welding after PWHT requires re-PWHT of the new welds); (6) perform final NDE after PWHT — certain codes require NDE after PWHT to detect any PWHT-induced cracking or to confirm the PWHT did not open pre-existing planar defects; (7) machine PWHT'd surfaces to final dimensions (PWHT before final machining is standard for precision-machined weldments — the stress relief makes the machined surfaces dimensionally stable); (8) final dimensional inspection and shipping. Departures from this sequence — machining before PWHT, or performing PWHT before all welding is complete — are the most common workflow errors in fabrication shops dealing with complex assemblies (ASME Section VIII Div 1, UW-40; AWS D1.1, Clause 5.8; Weman, K., Welding Processes Handbook, 2nd ed., Woodhead Publishing, 2011).
What happens if welding is performed after PWHT has been completed?
Welding on a weldment after PWHT has been completed introduces new weld-induced residual stresses and new HAZ microstructures that were not present when the PWHT cycle was run — the original PWHT no longer covers the new welds. Under ASME code requirements, any welding performed after PWHT requires re-PWHT of the affected joint (or, in some limited cases, a localized PWHT of the new weld area per a qualified procedure). Under AWS D1.1, a similar requirement applies when engineer-specified PWHT was part of the original design. In practice, post-PWHT welding is one of the most common non-conformances in fabrication shops — an attachment weld, a fitment correction, or a repair weld is added after PWHT without recognizing that it invalidates the prior PWHT record. The quality control response: any post-PWHT welding, regardless of how minor it appears, must trigger a review of whether re-PWHT is required. For assemblies where post-PWHT welding is anticipated (e.g., customer-added attachments, field modifications), the design should document this expectation and either specify a re-PWHT requirement for the modification or define the conditions under which post-PWHT welding without re-PWHT is acceptable (weld location remote from pressure boundary, low-heat-input process, engineer sign-off). UTEC's quality process flags any post-PWHT modification requests for engineering review before proceeding (ASME Section VIII Div 1, UW-40; AWS D1.1, Clause 5.8).
How does PWHT interact with previously quench-and-tempered base metal?
Welding on previously quench-and-tempered steel — a common situation when structural components are welded to hardened shafts, crane frames, or machinery housings — creates a conflict: the Q&T base metal was heat-treated to a specific hardness for service, but PWHT at 1,100–1,200 °F will re-temper the base metal adjacent to the weld and reduce its hardness. The magnitude of softening depends on the relationship between the PWHT temperature and the original tempering temperature of the Q&T base metal. If the Q&T steel was tempered at 1,150 °F and PWHT is specified at 1,100 °F, the PWHT stays below the original temper temperature — softening in the base metal is minimal (2–5 HB reduction in the HAZ, negligible in remote base metal). If the Q&T steel was tempered at 900 °F and PWHT is specified at 1,100 °F, the PWHT exceeds the original temper temperature — the base metal in the HAZ and the nearby heat-affected base metal will be re-tempered, reducing hardness below the original Q&T specification. For fabrications combining welding and previously hardened base metal, the PWHT temperature must be specified below the original tempering temperature of the hardened base metal — or the hardened region must be protected from furnace heating (local PWHT of the weld zone only, using local resistance heating elements, may be feasible if the hardened region is remote from the joint). The design authority must review this interaction before specifying PWHT for any assembly containing previously hardened steel (ASM Handbook, Vol. 4A, ASM International, 2013; ASME Section VIII Div 1, UW-40).
What are the most common workflow errors that defeat PWHT?
The most frequent workflow failures that undermine PWHT effectiveness: (1) Performing PWHT before all welding is complete, then adding welds afterward — creates unprocessed HAZs and residual stresses not covered by the original PWHT cycle. (2) Allowing the weldment to cool to ambient before bake-out when bake-out is required — hydrogen cold cracks may form in the hours between weld completion and cool-down; once cold cracks exist, PWHT cannot repair them. (3) Machining the weldment to final dimensions before PWHT — stress relief during PWHT then causes dimensional movement in the already-finished surfaces, requiring additional stock removal that may not have been left in the machining plan. (4) Specifying PWHT temperature above the tempering temperature of the base metal in Q&T condition — softens the base metal below service hardness requirements. (5) Performing NDE (particularly MT or PT) before PWHT without also performing NDE after PWHT — the thermal cycle itself occasionally opens pre-existing defects that were below detectability in the stressed condition; post-PWHT NDE is a code requirement in some cases. (6) Inadequate preheat before welding, then relying on PWHT to "fix" the resulting brittle HAZ — if cold cracks formed before PWHT due to inadequate preheat, they cannot be healed by the stress relief cycle. Each of these errors is preventable by treating the preheat–weld–bake-out–NDE–PWHT–machine sequence as an integrated workflow with documented hold points, rather than as independent operations that happen to occur in sequence (ASME Section VIII Div 1; AWS D1.1; ASM Handbook, Vol. 4A, ASM International, 2013).
- Post-Weld Heat Treatment (PWHT): Process Fundamentals and When It Is Required — the PWHT process itself, codes, and documentation
- Thermal Stress Relief: Temperature Ranges, Soak Times, and Applicable Parts — the underlying thermal process
- Car-Bottom Furnace: Equipment, Capacity, and Applicable Heat Treatment Processes — the equipment used for PWHT of large weldments
- Through-Hardening and Quench-and-Temper: Process Fundamentals — Q&T interaction with subsequent welding and PWHT
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; Section IX (current editions). American Society of Mechanical Engineers.
- AWS D1.1: Structural Welding Code — Steel (current edition). American Welding Society. Clauses 5.7, 5.8, Annex I.
- Linnert, G.E. (1994). Welding Metallurgy: Carbon and Alloy Steels (4th ed.). American Welding Society.
- Weman, K. (2011). Welding Processes Handbook (2nd ed.). Woodhead Publishing.
- Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
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