Heat Input Considerations in Welding
Technically speaking, heat input is the amount of electrical energy supplied by a welding arc to a workpiece. In reality, heat input is actually the ratio between the electrical energy supplied by a welding arc to an electrode, and the travel speed of the arc.
Heat input is important because it affects weld cooling rates, which in turn affects the heat affected zone and the microstructure of welded materials. A change in the microstructure of welded materials can have an immediate, direct effect on the quality and mechanical properties of a weld.
For the vast majority of welding processes, heat input is not a huge concern. However, when welding materials that are susceptible to cracking, adequate heat input is crucial given the potential impact it can have on cooling rates. Cooling rates that are too fast can cause embrittlement in the heat affected zone.
As such, when welding materials such as stainless steel and duplex, there is an unwritten rule that the heat input should not rise above 1.5KJ per millimeter. With stainless steel, a heat input above 1.5KJ per millimeter can cause carbide precipitation. With duplex, a heat input that is too low can cause excessive ferrite, while a heat input that is too high can cause precipitation of intermetallic phases.
How to Calculate Welding Heat Input
The following formula can be used as a general guide to calculate welding heat input:
60 x volts x amps
travel speed (mm) x 1000 = KJ per mm
Clearly, travel speed is a major factor in determining heat input.
For instance, while you might run 440amps when welding 6mm thick material, you’ll also need to travel at 550mm per minute. This means that heat input is actually reasonably low, despite the heat of the arc going in.
Factors Not Considered by the Formula
It is important to remember that the formula above is purely a guide. There are many factors that the formula does not consider. For example:
- Gas mixtures: if hydrogen is added to the welding gas, then the arc heat can be increased. The standard formula outlined above does not account for the addition of hydrogen.
- Process efficiency: when using keyhole welding processes, escaped heat is often an unquantifiable - because of the jet at the bottom of the keyhole, heat can escape before it reaches the plate. Again, the standard formula outlined above does not account for this escaped heat.
How to Ensure You Have the Correct Heat Input
While it is prudent for welders and welding workshop supervisors to be concerned about heat input in the welding process, this particular welding parameter is not something to be afraid of.
- If you want to ensure that you’re using the correct heat input, industry guidelines are a good place to start. However, the best way to ensure that your heat input is set at the most appropriate level is to perform a test weld, and evaluate the results.
Heat Input When Using K-TIG
When it comes to K-TIG, many people believe that the keyhole TIG process would necessitate high heat inputs due to the high amperage.
Although currents are higher with K-TIG welding, the very high speed of the process combined with its deeper penetration that allows for fewer passes results in modest heat inputs which are well within normal ranges.
For example, the heat input for a K-TIG weld on 10mm (13/32 inch) thick, 304 stainless steel is approximately 1.4kJ/mm gross.