A516 Gr.65/70 Preheat for GMAW — up to 3/4"

GMAW (Gas Metal Arc Welding)

GMAW (MIG) feeds continuous solid wire with shielding gas — an inherently low-hydrogen process assigned to Category B in Table 5.11.

For pressure vessel work, GMAW with ER70S-6 provides excellent deposition rates on long seam welds. Gas backing with argon on back-purged root passes produces a smooth internal weld profile that satisfies radiographic acceptance criteria. Shielding gas purity is critical for vessel code quality; dew point must be below -40°F.

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 GMAW

Higher-strength pressure vessel plate requiring low-hydrogen processes only. This steel is prequalified only with low-hydrogen processes under Table 5.11, which is why it appears in Category B but not Category A. The 32°F minimum preheat with GMAW balances the steel's strength level and carbon equivalent against the controlled hydrogen input from the consumable. 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 up to 3/4"

Thin material sheds heat quickly, allowing hydrogen to escape the HAZ readily — lowest preheat tier in Table 5.11.

Other Steels with GMAW at up to 3/4"

A516 Gr.65/70 with GMAW

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