What is welding heat input and why does it matter?

Heat input Q = (η × V × I × 60) / (S × 1000) in kJ/mm, where η is thermal efficiency, V is voltage, I is current, and S is travel speed. It controls weld bead size, HAZ width, distortion, residual stress, and mechanical properties. Too high = grain coarsening; too low = lack of fusion.

What are thermal efficiency factors for different welding processes?

Per EN 1011-1: SAW = 1.0, SMAW = 0.8, MIG/MAG = 0.8, FCAW = 0.8, TIG = 0.6. These account for heat lost to radiation and convection rather than entering the workpiece.

What is the cooling time t8/5?

t8/5 is the time (seconds) for the weld HAZ to cool from 800°C to 500°C. This controls the microstructure: short t8/5 → martensite (hard, brittle); long t8/5 → ferrite/pearlite (softer, tougher). Target: 5-25 seconds for most carbon steels.

How is preheat temperature determined?

Preheat depends on carbon equivalent (CE), plate thickness, heat input, and hydrogen content. For CE 0.45: 150-250°C. Per AWS D1.1 Table 3.3 with adjustments for thickness.

ISO 4063 classifies welding processes by reference numbers: 111=SMAW, 131=MIG, 135=MAG, 136=FCAW, 141=TIG, 121=SAW. Used on drawings, WPS documents, and welder qualifications.

Free Engineering Tool

Welding Heat Input Calculator & ISO 4063

Calculate heat input (kJ/mm), cooling time t8/5, preheat recommendation, and carbon equivalent. Includes ISO 4063 process reference.

ISO 4063 EN 1011-1 Heat Input kJ/mm

Welding Process

Arc Voltage (V)

Welding Current (A)

Travel Speed (mm/min)

Plate Thickness (mm)

Carbon Equivalent CE (IIW formula)

Quick presets

Results

Heat Input Q

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Thermal Efficiency η

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Arc Energy (without η)

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Cooling Time t8/5 (est.)

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Carbon Equivalent CE

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Preheat Recommendation

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Weldability Assessment

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ISO 4063 Process

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Heat Input Calculation (EN 1011-1)

Heat input is the energy delivered per unit length of weld:

η — thermal efficiency factor (process-dependent)
V — arc voltage (volts)
I — welding current (amps)
S — travel speed (mm/min)

Thermal Efficiency Factors

ISO 4063 No.	Process	η (EN 1011-1)
121	SAW (submerged arc)	1.0
111	SMAW (stick / MMA)	0.8
131	MIG (inert gas)	0.8
135	MAG (active gas)	0.8
136	FCAW (flux-cored)	0.8
141	TIG / GTAW	0.6

Cooling Time t8/5 Estimation

The time to cool from 800°C to 500°C determines the resulting microstructure. Simplified Rosenthal 2D model for thick plates:

Carbon Equivalent & Preheat

The IIW carbon equivalent formula predicts weldability and preheat needs:

Practical Example

Example — MAG welding 20 mm S355 plate

Given: Process 135 (η=0.8), V=28V, I=250A, S=300 mm/min, CE=0.42

Arc energy = (28 × 250 × 60) / (300 × 1000) = 1.40 kJ/mm

Heat input Q = 0.8 × 1.40 = 1.12 kJ/mm

CE = 0.42 → Fair weldability, preheat 100–150°C recommended

⚠️ Note: t8/5 estimation is simplified. Actual cooling depends on joint geometry, preheat, interpass temperature, and ambient conditions. Consult EN 1011-2 for critical applications.

No.	Process	Description	η
111	SMAW	Shielded metal arc (stick/MMA)	0.8
114	FCAW-S	Self-shielded flux-cored	0.8
121	SAW	Submerged arc welding	1.0
131	MIG	Metal inert gas (Ar) — aluminium, SS	0.8
135	MAG	Metal active gas (CO₂/Ar+CO₂) — carbon steel	0.8
136	FCAW-G	Flux-cored + active gas	0.8
137	FCAW-G	Flux-cored + inert gas	0.8
138	MCAW	Metal-cored wire + active gas	0.8
141	TIG	Tungsten inert gas (GTAW)	0.6
15	Plasma	Plasma arc welding	0.6
311	OAW	Oxy-acetylene welding	0.45
42	Friction	Friction welding	—
52	Laser	Laser beam welding	0.9

Welding Heat Input Calculator & ISO 4063 Reference

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026