A516 Gr.65/70 Preheat for FCAW — over 2-1/2"

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 over 2-1/2"

Why FCAW for A516 Gr.65/70 at over 2-1/2"? FCAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a516-6570/saw/over-2-1-2-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 225°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 over 2-1/2"

The heaviest sections demand the highest preheat in Table 5.11. Multi-pass sequences require maintaining interpass temperature throughout.

Other Steels with FCAW at over 2-1/2"

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