8.2 Temper-Bead Deposition Techniques & Sequence
Key Takeaways
- Temper-bead welding deposits each subsequent pass so its thermal envelope reheats and tempers the hard, brittle HAZ left by the previous pass — no PWHT possible on a live line.
- Annex B distinguishes heat-input control procedures (high enough heat input to overcome the contents' heat removal) from temper-bead procedures (deposition sequence engineered for tempering); the WPS specifies the heat-input range for each layer and the bead sequence with spacing tolerances.
- The tempering layer needs enough heat input to reach the underlying HAZ but not so much that it burns through the first layer and creates a new untempered HAZ.
- Stringer beads with controlled overlap temper the HAZ continuously; weaving is avoided because it makes heat input and HAZ depth inconsistent.
- Consumable discipline is strict: low-hydrogen electrodes (H4/H8 class diffusible hydrogen), sealed containers, holding ovens at 250°F-300°F (120°C-150°C), and rebake or discard after excess atmospheric exposure.
8.2 Temper-Bead Deposition Techniques & Sequence
Post-weld heat treatment is impossible on an operating hydrocarbon pipeline, so API 1104:2021 Annex B achieves HAZ softening in the weld sequence itself. Temper-bead welding positions and sizes each later pass so its heat reheats — and tempers — the hard HAZ created by the earlier pass. Figure B.1 of the standard shows typical temper-bead deposition sequences.
1. The Metallurgy
A first bead on a cold, flow-cooled pipe wall produces a coarse-grained HAZ (CGHAZ) of untempered martensite — hard and crack-susceptible. A correctly placed second bead pushes a controlled thermal pulse into that CGHAZ. If the CGHAZ reheats into the subcritical tempering range (below the Ac1 transformation temperature):
- carbon precipitates out of the supersaturated martensite as fine carbides,
- hardness drops and toughness recovers,
- and no new hard HAZ forms, because the second bead's own HAZ lands mostly in the first bead's weld metal.
The window is narrow: too little second-layer heat and the CGHAZ never tempers; too much and the arc penetrates the first layer, re-austenitizing fresh pipe steel and creating a NEW untempered HAZ deeper in the wall. That is why Annex B treats the weld deposition sequence and the heat input range for each layer as specified WPS content (B.2.2.1.3/B.2.2.1.4), with bead-spacing tolerances, and makes sequence changes an essential variable (Table B.1; also 5.4.2.11 for temper-bead techniques generally).
2. Heat-Input Control vs. Temper-Bead Procedures
Annex B recognizes two strategies for overcoming the heat-sink effect of flowing contents:
| Strategy | How it works | WPS content |
|---|---|---|
| Heat-input control procedure | Heat input high enough to slow cooling despite the flowing contents | Required heat-input range |
| Temper-bead procedure | Deposition sequence tempers each prior HAZ | Heat-input range for each layer plus the deposition sequence with bead-spacing tolerances |
Preheat, where practicable, supplements either strategy — though the contents often make effective preheat difficult to hold.
3. Execution Discipline
- Stringer beads only. Weaving makes heat input and HAZ depth irregular; temper-bead work uses tight stringers with precise toe placement.
- Overlap control. Adjacent beads overlap by a controlled fraction of bead width (about half) so each new bead tempers its neighbor's HAZ across the whole band.
- Layering. The first (buttering) layer uses small electrodes and modest heat input to avoid burning through; the second (tempering) layer runs hotter to push the tempering pulse through the first layer into the pipe HAZ; later fill layers build to size.
- Fit-up first. Sleeve/saddle gaps are minimized (weld-metal buildup on the carrier pipe is permitted) to cut restraint stress (B.4.2).
4. Consumable Discipline
Temper-bead control fixes the microstructure leg; hydrogen control fixes the second leg of the triad:
- Low-hydrogen consumables — SMAW electrodes to AWS A5.1/A5.5 such as E7018-H4R, or controlled processes — with diffusible hydrogen designators H4 (≤ 4 mL/100 g) or H8 (≤ 8 mL/100 g).
- Storage: factory-sealed containers until use; opened electrodes into holding ovens at 250°F-300°F (120°C-150°C); exposure beyond the manufacturer limit (commonly 4 hours, less for high-strength grades in humidity) → rebake per manufacturer (typically 500°F-800°F / 260°C-430°C for 1-2 hours) or discard.
- Cellulosic electrodes (E6010/E7010/E8010) produce high diffusible hydrogen and high HAZ hardness — Annex B's Table B.4 hardness limits effectively drive in-service welding to low-hydrogen practice.
5. Where Temper-Bead Fits in Annex B Geometry
Figure B.1's sequences apply to the recurring in-service geometries: full-encirclement sleeves, reinforcing pads and saddles, encirclement tees, and weld-deposition repairs (Figures B.7-B.12 illustrate the fitting configurations). In each case the fillet or groove weld onto the carrier pipe gets the temper-bead treatment: a buttering layer on the pipe surface, a tempering layer over it, then structural fills.
The technique's acceptance is proven in qualification, not assumed: the procedure test assembly (Section 8.3 of this guide) must show the hardness results of Table B.4 across the macrosection. A deposition sequence change after qualification is an essential variable (Table B.1) — the sequence IS the heat treatment, so improvising bead order in the field voids the procedure.
6. Layering Summary
| Layer | Typical electrode | Heat input | Function |
|---|---|---|---|
| Buttering (first) | Small-diameter low-hydrogen (3/32-1/8 in.) | Lower | Bond to pipe with minimum penetration — burn-through control |
| Tempering (second) | Larger diameter (1/8-5/32 in.) | Higher | Thermal pulse tempers the buttering layer's CGHAZ |
| Fill/cap | Per WPS | Per WPS | Build to size; each pass continues the tempering chain |
The exact electrode sizes, heat inputs, and bead placements are properties of the qualified WPS — the table shows the logic, not universal values.
What is the purpose of the temper-bead deposition sequence in API 1104:2021 Annex B in-service welding?
In a temper-bead procedure under Annex B, what happens if the second (tempering) layer heat input is too HIGH?
What is the maximum allowable ambient atmospheric exposure time commonly applied to opened E7018-H4R low-hydrogen electrodes before rebaking or discard?