A516 Gr.65/70 Preheat for FCAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A516 Gr.65/70 welded with FCAW at 3/4" to 1-1/2" is 50°F (10°C), Category B. Preheat below this raises hydrogen-cracking risk in the heat-affected zone; the same temperature is the minimum interpass limit maintained through the weld.
Built on AWS D1.1:2025 Table 5.11 — every value traced to the clause.
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FCAW (Flux Cored Arc Welding)
FCAW uses tubular flux-cored wire, available gas-shielded (E71T-1) or self-shielded (E71T-8) for field work. Category B in Table 5.11.
For vessel fabrication, FCAW offers a productivity advantage over SMAW on long circumferential and longitudinal seams. Gas-shielded E71T-1 provides consistent bead profile for radiographic quality. Self-shielded wire is generally not approved for ASME pressure service due to higher inclusion content in the weld deposit.
FCAW Tips for Pressure Vessel and Low-Temperature Steels
For A516 Grades 65/70 pressure vessel plate (35–38 ksi yield, Category B only), FCAW with E71T-1M at 250–280 A delivers consistent multi-pass fill on thick-wall vessel shells; E71T-1M is classified with 75–80% argon, balance CO2, so select the actual gas as the electrode manufacturer recommends and the qualified WPS specifies. On thick A516 Gr.70 plate (common on high-pressure reactors and columns), Category B minimum preheat is 150°F [65°C] over 1-1/2 thru 2-1/2 in and 225°F [110°C] over 2-1/2 in. Use stick-out of 3/4"–1" for E71T-1M to allow adequate resistive preheating of the flux core.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why FCAW for A516 Gr.65/70 at 3/4" to 1-1/2"
Why FCAW for A516 Gr.65/70 at 3/4" to 1-1/2"? FCAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a516-6570/saw/3-4-to-1-1-2-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions. Suitability: field and shop.
A516 Gr.65/70
ASTM A516 Grades 65 and 70 are the higher-strength pressure vessel plates in this specification, with 65 and 70 ksi minimum tensile strength and 35/38 ksi minimum yield respectively. Used in higher-pressure vessels, distillation columns, and reactor shells, they require low-hydrogen welding processes (Category B only in Table 5.11) due to their higher carbon equivalent compared to Grades 55/60. Carbon limits are 0.28% max for Gr.65 and 0.31% max for Gr.70 on thicker plate, pushing the CE-IIW into the 0.43-0.48 range where non-low-hydrogen SMAW is no longer prequalified. A516 Gr.70 is the single most specified plate grade for ASME Section VIII Division 1 pressure vessels, accounting for an estimated 40%+ of all vessel plate orders in North America.
Why This Preheat for A516 Gr.65/70 with FCAW
Higher-strength pressure vessel plate requiring low-hydrogen processes only. This steel is prequalified only with low-hydrogen processes under Table 5.11. With FCAW, flux-cored wire in FCAW provides a combination of deoxidizers and low-moisture flux formulations that control hydrogen. The 50°F minimum preheat balances the steel’s strength level and carbon equivalent against the hydrogen control provided by FCAW. Non-low-hydrogen SMAW is not an option for this grade under D1.1 prequalified WPS.
Typical Applications for A516 Gr.65/70
Used in higher-pressure process vessels, distillation columns, reactor shells, thick-wall accumulators, high-pressure heat exchangers, and flare knockout drums. A516 Gr.70 plate is common in ASME VIII Division 1 vessels above 250 psi design pressure and in columns operating under hydroprocessing conditions. Longitudinal seams on heavy-wall vessels and nozzle-to-shell set-on welds are critical joint configurations requiring strict preheat compliance. Wall thicknesses in high-pressure service can exceed 3" on large-diameter vessels, with shell diameters from 24" to over 15 feet. Post-weld heat treatment per ASME Code is frequently required for A516 Gr.70 above 1-1/4" wall thickness, adding PWHT soak temperature (typically 1100-1200°F) and hold time (1 hour per inch of thickness) to the fabrication sequence. PWHT stress-relieves the weld and HAZ but does not change the preheat requirements during the initial welding operation.
Why Preheat Matters at 3/4" to 1-1/2"
Preheat climbs at this range as thicker material slows heat dissipation, trapping hydrogen at crack-susceptible grain boundaries.
Other Steels with FCAW at 3/4" to 1-1/2"
| Steel | Category | Preheat |
|---|---|---|
| A537 Cl.1/2 | B | 50°F (10°C) |
| A633 Gr.E | C | 150°F (65°C) |
| A709 HPS70W | C | 150°F (65°C) |
| A710 Gr.A | C | 150°F (65°C) |
A516 Gr.65/70 with FCAW
Try Different Combinations
Use the interactive preheat calculator to look up any steel, process, and thickness combination from D1.1:2025 Table 5.11.
A516 Gr.65/70 Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A516 Grade 65/70 plate in the 3/4 to 1-1/2 inch range with FCAW shows up in heavier pressure-vessel-adjacent structural fabrication built from higher-strength PV plate — vessel skid frames carrying large vessels, support saddles for high-volume tanks, and structural connections to higher-strength PV plate where the welding falls under D1.1 (structural) rather than ASME Section IX (pressure boundary). The higher deposition rate of cored wire suits production fillet runs.
Pre-weld notes
Same D1.1 vs ASME Section IX scope question as the 55/60 grades. For D1.1 structural work at this thickness, three constraints layer in. First, the 50°F preheat floor needs active verification per Clause 7.6 — the heated zone must extend at least twice the base-metal thickness from the welding point. Second, FCAW classification on the spool against the WPS-cited variant — self-shielded vs gas-shielded is an essential variable per Table 6.6. Third, matching-strength filler from Table 5.7 Group II FCAW row.
What a CWI verifies
A CWI on A516 Gr.65/70 FCAW mid-thickness structural work first verifies the WPS scope — D1.1 structural vs ASME Section IX pressure-boundary. For D1.1 structural work, the inspection covers (1) preheat through-thickness verification with a contact pyrometer after the first pass group, (2) the FCAW classification on the spool against the WPS-cited variant, (3) the matching-strength filler classification against Table 5.7 Group II (not Group I), and (4) the prequalified FCAW limits in Table 5.4. Pressure-boundary welds get bumped to ASME Section IX-qualified procedures.