Welding Heat Input Calculator — kJ/mm for All Arc Processes

Welding heat input is the electrical energy delivered to the weld per unit length of weld pass, corrected for the thermal efficiency of the welding process. It is the single most important parameter governing the heat-affected zone (HAZ) microstructure: it controls peak temperature, time above transformation temperatures, cooling rate, grain growth, and the risk of hydrogen cracking or grain boundary sensitisation. This calculator computes heat input per ASME Section IX QW-409.1 and ISO 1011-1 using arc energy multiplied by the process thermal efficiency factor η, with full step-by-step formula output, a visual HAZ assessment, and dual unit output in kJ/mm and J/in.

Heat Input Formula — Derivation and Standard References

The fundamental heat input equation relates the electrical power delivered at the arc to the length of weld deposited per unit time. Both ISO 1011-1 and ASME Section IX QW-409.1 use the same underlying expression, with ISO 1011-1 explicitly requiring the thermal efficiency correction that ASME adopted in more recent editions:

Arc energy (kJ/mm) — uncorrected:
  E = (I × U) / v_s                   [where v_s is travel speed in mm/s]

In practical units (v in mm/min):
  E = (I × U × 60) / (v × 1000)      [kJ/mm]

Heat input (kJ/mm) — ISO 1011-1 / ASME IX QW-409.1:
  Q = E × η = (I × U × 60) / (v × 1000) × η

Where:
  I   = welding current (A)            — measured at arc, averaged over pass
  U   = arc voltage (V)                — measured at arc or WFS point
  v   = travel speed (mm/min)         — arc-on time only, measured distance / time
  η   = process thermal efficiency factor (dimensionless, 0 < η ≤ 1.0)
  Q   = corrected heat input (kJ/mm)

Unit conversions:
  Q (kJ/mm) × 25.4 = Q (kJ/in)
  Q (kJ/mm) × 1000 = Q (J/mm)
  Q (J/in)  / 25.4 = Q (J/mm)

Process thermal efficiency factors η (ISO 1011-1:2009 Table A.1):
  SAW  (Submerged Arc Welding)          η = 1.0
  SMAW (Shielded Metal Arc / Stick)     η = 0.8
  GMAW (Gas Metal Arc / MIG/MAG)        η = 0.8
  FCAW (Flux-Cored Arc Welding)         η = 0.8
  GTAW (Gas Tungsten Arc / TIG)         η = 0.6
  PAW  (Plasma Arc Welding)             η = 0.6

Note: Some codes (AWS D1.1 Annex I) permit η = 1.0 for all processes as
a conservative simplification. Always confirm with the applicable code.

Worked Example — SMAW Pass on S355 Steel

Given:
  Process:       SMAW (η = 0.80)
  Current I:     180 A
  Voltage U:     24 V
  Travel speed:  280 mm/min (measured over 420 mm in 90 s arc-on time)

Step 1 — Arc energy:
  E = (180 × 24 × 60) / (280 × 1000)
    = 259,200 / 280,000
    = 0.926 kJ/mm

Step 2 — Heat input:
  Q = E × η = 0.926 × 0.80 = 0.741 kJ/mm

Step 3 — Convert to J/in:
  Q = 0.741 × 25.4 × 1000 = 18,820 J/in

Assessment:
  S355 structural (EN 1011-2): Max HI ~3.5 kJ/mm
  Q = 0.741 kJ/mm → WELL WITHIN LIMIT ✓
  HAZ: Fine-grained bainite/acicular ferrite expected → good CVN toughness

Heat Input Limits by Material and Code

Every structural and pressure welding code specifies permissible heat input ranges — both to protect the HAZ properties of the base material and to control the weld metal microstructure. The values below are typical ranges; always consult the applicable project specification and approved WPS for the definitive limits.

t_8/5 Cooling Time and Heat Input

The t_8/5 cooling time — the time in seconds for the HAZ to cool from 800°C to 500°C — is the metallurgically relevant thermal parameter that determines which microstructural phases form in the HAZ. Heat input and t_8/5 are directly linked through the Rykalin heat flow equations for 2D (thin plate) and 3D (thick plate) geometries:

3D heat flow (thick plate, t > d_cr):
  t8/5 = (6700 − 5×T₀) × Q × [1/(500−T₀)² − 1/(800−T₀)²] × 1/(2π×λ)

2D heat flow (thin plate, t ≤ d_cr):
  t8/5 = (4300 − 4.3×T₀) × 10⁵ × Q²/λ²·c·ρ · [1/(500−T₀)² − 1/(800−T₀)²] / t²

Simplified form (structural steel, T₀ = 20°C preheat, 3D):
  t8/5 ≈ 0.67 × Q    (Q in kJ/mm, t8/5 in seconds, approximate)

Where:
  T₀  = preheat / interpass temperature (°C)
  λ   = thermal conductivity (W/m·K) ≈ 0.04 kJ/mm·s·K for steel
  c·ρ = volumetric heat capacity (J/mm³·K) ≈ 0.005 for steel
  t   = plate thickness (mm)
  Q   = heat input (kJ/mm)

Microstructural interpretation (structural steel, CE ≈ 0.40):
  t8/5 < 5 s   → Martensite / lower bainite dominant → HIC risk if moisture present
  t8/5 5–15 s  → Upper bainite / acicular ferrite → optimal CVN toughness
  t8/5 15–35 s → Polygonal ferrite + pearlite → reduced toughness, lower hardness
  t8/5 > 35 s  → Coarse ferrite, Widmanstätten plates → poor HAZ properties

Measurement of Welding Parameters for Accurate Heat Input

The accuracy of a heat input calculation is only as good as the accuracy of the three measured input parameters. Each introduces potential error in practice:

Current (I)

Welding current at the arc is typically 2–5% lower than the ammeter reading on the power source display due to cable and connection resistance. For PQR qualification, current must be measured with a calibrated clamp meter directly on the welding cable as close to the workpiece as practicable. For SMAW and FCAW with conditioned electrodes, current varies continuously with arc length; the measured value should be a time-averaged reading over the complete run. For SAW and orbital GTAW, machine-logged data from the welding controller provides accurate continuous records.

Voltage (U)

Arc voltage should be measured across the arc where possible — between the electrode or wire contact tube and the workpiece — rather than at the power source terminals. Power source output voltage includes the voltage drop across the welding cables, which can be 1–3 V for long cable runs, introducing significant error in heat input calculation. For GTAW, arc voltage is sensitive to tungsten condition, arc length, and shielding gas composition; a consistently held arc length is essential for reproducible measurements.

Travel Speed (v)

Travel speed is frequently the largest source of error. The correct approach is to mark the start and end of a measured weld length, measure the arc-on time only (excluding electrode changes, repositioning, arc strikes, and restarts), and divide: v = L / t_arc-on. For manual processes, speed should be measured over a minimum 150 mm run length. A stopwatch accurate to 0.1 s and a steel rule are the minimum required instruments. The average of three measured runs at representative welding conditions should be used.

Interpass Temperature and Heat Input Interaction

The maximum interpass temperature is as important as heat input in controlling HAZ properties in multi-pass welds. Interpass temperature affects the effective t_8/5 of each subsequent pass: a high interpass temperature (equivalent to a high preheat T₀) slows the cooling rate of subsequent passes and effectively increases the thermal impact on the HAZ grain structure even at the same nominal heat input. This is why welding codes specify both a maximum heat input and a maximum interpass temperature as independent controls.

For duplex stainless steel, interpass temperature ≤150°C is a firm requirement regardless of heat input: the sigma-phase precipitation nose in the CCT diagram of duplex SS is at approximately 700–900°C, and any significant time in this range — from either high heat input or slow inter-run cooling — causes embrittlement. For HAZ microstructure and transformation diagrams relevant to specific steel grades, see the dedicated guide. The connection between hydrogen-induced cracking and inadequate heat input or preheat control is covered in the HIC guide. For the preheat temperature calculated from the weld parameters and CE, use the preheat temperature calculator.

Frequently Asked Questions

Recommended References

Further Reading