What is Heat Input in Welding?

Heat Input is a measure of how much energy has been supplied to the work-piece to form a weld. It is measured in units of energy per unit length. In Europe, this tends to be in kJ/mm, whereas in America it tends to be kJ/in.

When welding on certain materials, take into consideration that cooling rate can significantly affect its micro-structure. High cooling rates and heat transfer can cause embrittlement in the heat affected zone. This is likely to happen when welding material that is susceptible to hydrogen-induced cracking.

Maintaining the heat input in the range provided by WPS will control the cooling rate thus avoiding unwanted changes in its micro-structure.

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How Do You Calculate Heat Input?

Heat input is the energy supplied by the welding arc to the work-piece. It is expressed in terms of:

Heat input ⇒ Arc energy x Thermal efficiency factor

Where thermal efficiency factor is the ratio of heat energy into the welding arc to the electrical energy consumed by the arc.

Thermal efficiency factor ⇒ Heat energy into the welding arc ÷ Electrical energy consumed by the arc

Thermal efficiency factors for the most common welding process are presented below:

• SAW (wire electrode) 1.0
• MMA (covered electrode) 0.8
• MIG/MAG 0.8
• FCAW (with or without gas shield) 0.8
• TIG 0.6
• Plasma 0.6

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The different between Heat Input and Arc Energy

Arc energy (AE) is the energy supplied by the welding arc to the work piece before the efficiency of the process is considered. It is the amount of heat generated in the welding arc per unit length of the weld and is usually expressed in kilojoules per millimeter length of the weld (kJ/mm).

A number of American codes also use the term Heat Input when referencing arc energy.

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Heat input is mainly influenced by the travel speed, while travel speed itself is strongly influenced by the welding position and the process used.

For manual and semi-automatic welding, the following are general principles:

Position:

• Vertical-up progression – Highest heat input due to the necessity to weave a suitable profile and the relatively slow forward travel speed.
• Vertical-down – Lowest heat input because of the high travel speed.
• Horizontal-vertical – Relatively low heat input welding position because welder can’t weave in this position.
• Overhead welding – Low heat input since it is necessary to use low current and relatively fast travel speed.
• Flat position (down hand) – Can be a low or high heat input position since the welder has more flexibility handling the travel speed.

Process:

In arc welding processes, SAW has the potential to give the highest heat input and deposition rates while TIG and MIG / MAG produce very low heat input.

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Example on determining heat input;

Example 1:

You are welding using MMA process at 500 mm per minute and 29 volts. Your weld is 2500mm long and it takes the welder 2 minutes to weld it. While he is welding, you notice the machine is displaying an amperage of 325. What is your heat input? Before calculating heat input you need to determine your travel speed.

Travel Speed = Length of Weld / Time to weld

= 2500 mm / 2 minutes = 1250 mm per minute

Arc Energy   = [(60 sec/min) x (325 amps) x (29 volts)] / [(1,000 joules/kilojoule) x (1250 mm/minute)]

= 0.4524 KJ/mm

Heat input   =   Arc energy x Thermal efficiency factor

= 0.4524 KJ/mm x 0.8 = 0.3619 KJ/mm

Example 2:

Your heat input at 0.3619 KJ/mm is too high, but you also notice your weld is over-sized. Instead of lowering your amperage and voltage you decide to travel faster. You now complete the same weld in 90 seconds at the exact same parameters. What is your heat input now?

Travel Speed = Length of Weld / Time to weld

= 2500 mm / 1.5 minutes

= 1666 mm per minute

Arc Energy   = [(60 sec/min) x (325 amps) x (29 volts)] / [(1,000 joules/kilojoule) x (1666 mm/minute)]

= 0.3394 KJ/mm

Heat input = Arc energy x Thermal efficiency factor

= 0.3394 KJ/mm x 0.8 = 0.2715 KJ/mm

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As you can see from those two examples, the heat input dropped by almost 25% simply by traveling faster. Therefore, before reducing your amperage and voltage to get a lower heat input, try to increase the travel speed and deposit a smaller weld. This will overcome over-welding.

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Source;

1. Weldinganswers.com
2. CSWIP 3.1 Course note.
3. How does heat input affect weld strength?

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