Weld Heat Input
Calculate the arc energy and heat input of a weld from arc voltage, welding current and travel speed. Pick a process - the thermal efficiency η is set as a DIN EN 1011-1 reference value (or entered freely) - the calculator returns gross arc energy and net heat input live with every input.
Calculation
η reference values and cooling time t8/5The thermal efficiency η depends on the process and, via the heat input, governs the cooling time t8/5 of the heat-affected zone.Show details
η reference values (DIN EN 1011-1)
Relative thermal efficiency per process: SAW 1.0; MAG/MIG 0.8; MMA 0.8; TIG 0.6. Published standard reference values; the formula E = η·U·I/v is textbook-based.
Note t8/5
The heat input governs the cooling time t8/5 (800 to 500 degrees Celsius) and thus microstructure and hardness of the heat-affected zone: higher E lengthens t8/5. The permissible range depends on material and thickness per DIN EN 1011-2.
Model: arc energy E_s = U·I/v and heat input E = η·U·I/v from the electrical process parameters. Formula textbook-based, η values as DIN EN 1011-1 reference values. No t8/5 verification; plate thickness, joint geometry and preheating are not included (use DIN EN 1011-2 for that).
Results
Calculating …
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Formulas and fundamentals
Arc energy
The arc energy E_s is the electrical energy supplied per millimetre of weld length:
With the arc voltage U in volts, the welding current I in amperes and the travel speed v in mm/s. The product U·I is the electrical power in the welding circuit in watts; divided by the travel speed it gives the energy per millimetre of weld directly in J/mm.
Heat input and thermal efficiency η
The heat input E additionally accounts for the process thermal efficiency η, i.e. the ability to actually transfer the supplied energy as heat into the workpiece:
The efficiency depends on the process - submerged arc welding transfers almost all of the energy (η = 1.0), whereas in the TIG process a considerable share is radiated by the open arc (η = 0.6). The formula is textbook-based, the η reference values are published standard values per DIN EN 1011-1.
Significance for the cooling time t8/5
The heat input largely governs the cooling rate of the weld and thus the cooling time t8/5, the time to cool from 800 to 500 degrees Celsius. A high heat input lengthens t8/5, produces a softer microstructure and less hardening, but also a coarser heat-affected zone; a low heat input shortens t8/5 with the risk of hard, crack-prone microstructures. The permissible limits depend on material and plate thickness (DIN EN 1011-2).
Worked example
A MAG weld is made with U = 25 V, I = 250 A and a travel speed of v = 5 mm/s. The thermal efficiency of metal active gas welding is η = 0.8 per DIN EN 1011-1.
The electrical power is P = U·I = 25·250 = 6250 W. This gives the gross arc energy E_s = U·I/v = 6250/5 = 1250 J/mm.
Applying the efficiency yields the heat input E = η·U·I/v = 0.8·1250 = 1000 J/mm = 1.0 kJ/mm. This value is the governing quantity for estimating t8/5 and the hardening of the heat-affected zone.
Frequently asked questions
What is the difference between arc energy and heat input?
The arc energy E_s = U·I/v is the gross value of the electrical energy supplied per millimetre of weld. The heat input E = η·U·I/v is the net value, capturing only the share actually delivered to the workpiece via the thermal efficiency η. Heat input governs the assessment of microstructure and cooling time.
Which η values apply per process?
Per DIN EN 1011-1 the reference values of the relative thermal efficiency are: submerged arc welding (SAW) 1.0; metal active/inert gas welding (MAG/MIG) 0.8; manual metal arc welding (MMA) 0.8; tungsten inert gas welding (TIG) 0.6. These are standard reference values; the formula itself is textbook-based.
In which unit must the travel speed be entered?
The calculator expects v in mm/s, so the result comes out directly in J/mm. To convert: 1 mm/s equals 6 cm/min. A frequently quoted speed of 30 cm/min is therefore 5 mm/s.
Why does heat input matter for the t8/5 time?
The cooling time t8/5 from 800 to 500 degrees Celsius controls microstructure formation in the heat-affected zone. A higher heat input lengthens t8/5 and avoids hard, crack-prone hardening; too high a heat input, however, degrades toughness through grain coarsening. The permissible range depends on material and thickness per DIN EN 1011-2.
Does the calculator account for joint preparation and preheating?
No. The calculator provides arc energy and heat input from the electrical process parameters. Plate thickness, joint geometry, preheating and interpass temperature and the actual t8/5 verification are not included and must be assessed separately per DIN EN 1011-2.
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