Short answer: there is no universal "code limit" on heat input for SA-516 Grade 70 root passes — the limit comes from the qualified Welding Procedure Specification (WPS) the contractor wrote and tested under ASME Section IX. Common practical caps for root passes on SA-516 Gr 70 plate are: GTAW root: 10–25 kJ/in, SMAW root: 25–45 kJ/in, SAW: typically not used for root. The reason these limits exist is to protect the heat-affected zone (HAZ) toughness — SA-516 Gr 70 is a notch-toughness-critical pressure-vessel steel, and excessive heat input destroys the very property it's spec'd for.

What SA-516 Gr 70 actually is

• Spec: ASME SA-516 / ASTM A516, Grade 70 carbon steel for moderate- and lower-temperature pressure vessel service.
• Tensile: 70–90 ksi (485–620 MPa).
• Yield (min): 38 ksi (260 MPa).
• Charpy impact: typically called for at 40°F or lower — this is why HI matters.
• Service: vessels, headers, exchangers, drums — usually with post-weld heat treatment (PWHT) above ~1.5" thickness per UCS-56.

The heat input formula

Heat input (kJ/in) = (60 × Voltage × Amperage) / (Travel speed × 1000)

Heat input (kJ/mm) = (Voltage × Amperage) / (Travel speed in mm/min)

Convert kJ/in → kJ/mm:  divide by 25.4
Convert kJ/mm → kJ/in:  multiply by 25.4

Travel speed: usually inches per minute (IPM) for US shops.

Worked example: GTAW root pass on 1/2" SA-516 Gr 70

Setup:
  Material:    SA-516 Gr 70, 1/2" plate
  Joint:       single V groove, 60° included, 3/32" gap, 1/16" face
  Process:     GTAW root pass
  Filler:      ER70S-2 or ER70S-6, 3/32" rod
  Gas:         100% argon, 18 CFH
  Position:    1G (flat)

Inputs to the math:
  Voltage:     12 V (typical GTAW arc)
  Amperage:    100 A (DCEN, on 1/2" prep)
  Travel:      4 IPM (slow root, careful fusion)

Calculation:
  HI = 60 × 12 × 100 / (4 × 1000)
     = 72000 / 4000
     = 18.0 kJ/in
     = 0.71 kJ/mm

Verdict:
  18 kJ/in is comfortably inside typical 10–25 kJ/in GTAW root range.
  PQR-qualified band is usually ±10% from baseline, so if PQR ran
  this at 16.4 kJ/in, the qualified range is ~14.7–18.0.

Worked example: SMAW root pass with E7018-A1

Setup:
  Material:    SA-516 Gr 70, 3/4" plate
  Process:     SMAW root pass
  Filler:      E7018-A1 (low-hydrogen, ½% Mo for elevated temp)
  Rod:         3/32" diameter
  Position:    1G

Inputs:
  Voltage:     22 V
  Amperage:    85 A (manufacturer chart for 3/32" 7018-A1, root)
  Travel:      6 IPM (root with appropriate fill)

Calculation:
  HI = 60 × 22 × 85 / (6 × 1000)
     = 112,200 / 6,000
     = 18.7 kJ/in
     = 0.74 kJ/mm

Verdict:
  Within the 12–30 kJ/in band typical for SMAW root on SA-516.
  PQR-qualified range usually expressed as ±10% of HI baseline.

Why heat input matters on SA-516 Gr 70

SA-516 Gr 70 is specified for low-temperature pressure-vessel service because the steel chemistry is tuned for HAZ toughness at cold temperatures — typically a 25 ft-lb minimum at 40°F or lower per the impact-test SA. When you weld it, the HAZ on either side of the bead reaches transformation temperatures and re-cools. The cooling rate of that HAZ determines the resulting microstructure (and toughness):

• Fast cooling — martensite or hard bainite, brittle. Low heat input, thick plate.
• Slow cooling — coarse pearlite, low toughness. High heat input, thick plate.
• Optimal cooling — fine ferrite-pearlite or acicular ferrite, good toughness. HI matched to plate thickness via the WPS.

Excess HI on a root pass on SA-516 widens the HAZ, coarsens the grain, and pushes the impact transition temperature up — the property the steel is being purchased for. That's why every SA-516 WPS specifies a heat-input ceiling.

Typical HI ranges for SA-516 Gr 70 by process

Process    Position    Typical HI range    Why
─────────  ──────────  ──────────────────  ──────────────────────────
GTAW root  1G–2G       10–25 kJ/in         Slow travel, low V × A
GTAW fill  1G–2G       15–35 kJ/in         Faster travel, larger pool
SMAW root  1G–2G       12–35 kJ/in         Open root, controllable
SMAW fill  1G–2G       25–55 kJ/in         Larger rod, faster
GMAW spray 1F–2F       30–60 kJ/in         Spray transfer, hot
GMAW SC    All         15–30 kJ/in         Cooler, multipass
FCAW       All         25–60 kJ/in         Bigger wire, hot
SAW        1G          50–120 kJ/in        Submerged, big wire
                                           (usually NOT for roots)

Multi-pass HI budget for a 1" SA-516 Gr 70 V groove

Joint:        1.0" thick V groove, 60° included, GTAW root + SMAW fill/cap

GTAW root:    100 A × 12 V × 4 IPM   = 18.0 kJ/in
GTAW hot:      90 A × 12 V × 5 IPM   = 13.0 kJ/in
SMAW pass 1:  110 A × 22 V × 6 IPM   = 24.2 kJ/in
SMAW pass 2:  120 A × 22 V × 6 IPM   = 26.4 kJ/in
SMAW pass 3:  120 A × 22 V × 6 IPM   = 26.4 kJ/in
SMAW pass 4:  120 A × 22 V × 6 IPM   = 26.4 kJ/in
SMAW pass 5:  130 A × 23 V × 6 IPM   = 29.9 kJ/in
SMAW cap:     120 A × 22 V × 5 IPM   = 31.7 kJ/in
                                       ─────────────
Average HI per pass:                    24.5 kJ/in

If WPS limits SMAW cap to 30 kJ/in: trim cap A or speed travel:
  130 A × 23 V × 6.0 IPM = 29.9 kJ/in  ✓ marginal
  120 A × 22 V × 5.5 IPM = 28.8 kJ/in  ✓ comfortable

The total joint heat budget is the sum × cross-sectional area —
NOT the per-pass — but per-pass is what gets logged on the PQR.

How HI couples to preheat and interpass temperature

• Preheat: SA-516 Gr 70 typically preheated to 100–200°F (38–93°C), higher for thicker sections per ASME Section VIII Div 1 Table UCS-56.
• Interpass max: usually 400°F (204°C). Higher interpass acts like additional heat input, slowing cooling, coarsening microstructure.
• Interpass min: same as preheat — cooling to ambient between passes risks hydrogen-assisted cold cracking.
• PWHT: required on most welded SA-516 above 1.5" (1140–1185°F per UCS-56) — restores HAZ toughness.

The HI envelope: what changes when you exceed it

What happens at HIGH HI (above PQR-qualified range):
  • HAZ widens, grain coarsens
  • Charpy impact toughness DROPS (transition temperature rises)
  • Slower cooling = pearlite-rich microstructure
  • RISK: brittle fracture in service at low temp

What happens at LOW HI (below PQR-qualified range):
  • Lack of fusion (LOF) defects
  • Fast cooling = martensite or bainite formation
  • Hardness in HAZ rises sharply
  • RISK: hydrogen-assisted cold cracking
  • RISK: post-PWHT cracks at HAZ-weld boundary

The "qualified band" exists for a reason. Stay inside it.

How to find the actual limit for YOUR job

• Read the project's WPS — it cites the qualifying PQR.
• Read the PQR — it lists the baseline HI used in the qualifying coupon test.
• The acceptable range on a follow-up production weld is typically baseline ±10% of HI — check QW-409.1.
• If the project is under ASME Section VIII / B31.1 / B31.3, follow the corresponding code-of-construction limits.
• If no WPS exists yet (R&D / one-off): use the typical-band table above and document the as-welded values for future PQR.

Run it on your phone

The WelderCalc app computes heat input instantly for any V/A/travel-speed combination, with kJ/in and kJ/mm units, plus bead-pass budgets for multi-pass joints. It also holds the typical-HI bands per process for low-alloy structural steel, stainless, and Inconel. 100% offline. Free on the App Store and Google Play.

Related

• MIG amperage for 1/4" mild steel with .035 ER70S-6
• TIG tungsten size for 180A on stainless
• MIG amperage cheat sheet (carbon steel)
• WelderCalc — welder's calculator on iOS + Android

Note: This article is informational. Pressure-vessel and code welding requires a qualified WPS, qualified welders, and quality control per the applicable code (ASME Section IX, AWS D1.1, API 1104, etc.). Do not use this article in place of a project WPS.