A weld can look perfect and still let you down if the heat input was wrong. Heat input has a direct effect on the mechanical properties of the joint, which is why welding procedures (WPS) give a range for it on critical work.
How heat input is calculated
For a conventional arc, heat input is worked out from the voltage, current and travel speed:
Heat input (kJ/mm) = k × (V × A × 60) ÷ (1000 × travel speed in mm/min)
Here k is the thermal efficiency of the process: 0.8 for MIG/MAG and MMA, and 0.6 for TIG (as given in BS EN 1011-1). For example, 24 V at 200 A travelling at 300 mm/min with MIG gives 0.8 × (24 × 200 × 60) ÷ (1000 × 300) = 0.77 kJ/mm.
With pulse welding the current and voltage are changing all the time, so a simple meter reading can be misleading. The correct method is to use the arc energy or average power that the machine measures from the actual waveform, which most modern pulse machines can display.
Too much heat
- Lower toughness: in steels, high heat input and slow cooling coarsen the grain in the heat-affected zone (HAZ), which can reduce impact (Charpy) toughness.
- Loss of strength: quenched and tempered steels and heat-treatable aluminium alloys such as 6082 soften in a wider HAZ.
- Stainless steel problems: more risk of heat tint, sensitisation and reduced corrosion resistance.
- More distortion (see Welding thin sheet without distortion).
Too little heat
- Lack of fusion and poor penetration.
- Fast cooling, which can produce hard, brittle zones and raise the risk of hydrogen cracking in thicker or higher-strength steels, which is why preheat is sometimes specified.
Where pulse helps
Pulse welding lets you keep heat input down without giving up fusion: the peak current does the melting and the background keeps the mean low. In our simple example, pulsing between 100 A and 50 A gives a 75 A mean, about 25% less heat than a steady 100 A. Lower, more controlled heat input generally means a narrower HAZ and better mechanical properties, as long as you stay inside the range your procedure allows.
Always follow your qualified WPS on coded work. If you are writing or reviewing procedures, we can help with the equipment and consumables side.
Talk to us
Questions about heat input on a particular job or material? Call 0141 959 5666 or email info@weldsafe.co.uk.
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