A516 Gr.65/70 Preheat for SMAW (low-hydrogen) — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A516 Gr.65/70 welded with SMAW (low-hydrogen) 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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SMAW (Low-Hydrogen)
Low-hydrogen SMAW (E7018/E7016) uses basic-coated electrodes requiring rod oven storage, assigned to Category B in Table 5.11.
On pressure vessel plate, E7018 is the standard manual electrode for nozzle welds, manhole reinforcements, and repair welding. ASME Section IX procedure qualifications typically require all-weld-metal tensile and guided bend tests. Rod moisture control is critical in vessel fabrication because vessel codes impose stricter hydrogen limits than structural codes.
SMAW-LH Tips for Pressure Vessel and Low-Temperature Steels
For A516 Grades 65/70 (35–38 ksi yield, Category B only — LH required), E7018 at 130–170 A handles nozzle welds, reinforcement pads, and thick-plate groove welds. On A516 Gr.70 plate over 1-1/2" (common on high-pressure reactors), preheat to 225°F minimum and maintain throughout the entire multi-pass sequence. Verify E7018 meets A516 Gr.70 tensile matching — E7018 at 70 ksi tensile.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why SMAW (low-hydrogen) for A516 Gr.65/70 at 3/4" to 1-1/2"
Why SMAW (low-hydrogen) for A516 Gr.65/70 at 3/4" to 1-1/2"? SMAW (low-hydrogen) delivers 3-5 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 SMAW-LH
Higher-strength pressure vessel plate requiring low-hydrogen processes only. This steel is prequalified only with low-hydrogen processes under Table 5.11. With SMAW-LH, E7018 low-hydrogen electrodes produce typically 4-8 mL/100g diffusible hydrogen under proper rod oven conditions. The 50°F minimum preheat balances the steel’s strength level and carbon equivalent against the hydrogen control provided by SMAW-LH. 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 SMAW (low-hydrogen) at 3/4" to 1-1/2"
| Steel | Category | Preheat |
|---|---|---|
| A36 | 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 SMAW (low-hydrogen)
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 SMAW low-hydrogen shows up in heavier pressure-vessel-adjacent structural fabrication — 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) and the 50°F preheat floor is binding on the section thickness.
Pre-weld notes
Same D1.1 vs ASME Section IX scope question as the 55/60 grades, with three constraints layering in for D1.1 structural work. 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, the LH electrode storage and atmospheric-exposure discipline of Clause 7.3.2.1 and Clause 7.3.2.2 holds. Third, matching-strength filler from Table 5.7 Group II — A516 Grade 65 sits in Group II (35 ksi yield, 65–85 ksi tensile range), Grade 70 sits in Group II (38 ksi yield, 70–90 ksi tensile range). Note that for these grades only low-hydrogen processes are prequalified per Table 5.11 — non-low-hydrogen SMAW is not a prequalified path.
What a CWI verifies
A CWI on A516 Gr.65/70 SMAW-LH 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 LH electrode storage and atmospheric-exposure discipline, (3) the matching-strength filler classification against Table 5.7 Group II (not Group I), and (4) the prequalified SMAW-LH limits in Table 5.1. Pressure-boundary welds get bumped to ASME Section IX-qualified procedures.