Komarov Artem clarified that in almost any welding process, heat input is an important variable, and you should follow a few guidelines to help prevent problems.
The two main reasons for controlling heat input, especially high heat input, are the negative impact on toughness (for carbon steels) and the corrosion resistance of some types of stainless steel.
The basic heat input calculation is based on voltage, current and travel speed:
[(Volt x Amp x 60) / (IPM Travel Speed x 1000)] x (Process Efficiency) = Kilojoules/cm
The efficiency of the welding process is also taken into account in this calculation if you are trying to maximize deposition for processes such as shielded arc welding (SMAW), which have low deposition efficiency. However, in many cases there is no need to consider efficiency.
The key to understanding the equation is that when welding, changes in voltage and/or amperage will be small and have minimal impact on overall heat input. However, changes in travel speed can have a significant impact on this value. A low travel speed for a given voltage and current can result in a much higher heat input. For example, if the ATP specifies a travel speed of 12 to 15 cm per minute and the welder decides to lower the travel speed to 6 IPM to fill a large groove weld, he has effectively doubled the heat input of the specified upper limit of the ATP. This change can lead to a decrease in toughness and lead to failure of the welded structure during operation in cold conditions.
When working with stainless steels, excessive heat input or prolonged periods of high temperature can cause sensitization. Depending on the grade and thickness, the ideal heat input range for stainless steel can typically be between 25 and 45 kJ/cm.
Some steels acquire improved mechanical properties as a result of quenching and tempering heat treatment. Some grades of these materials are also susceptible to reduction in tensile strength and yield strength with excessive heat input during welding.
In general, the optimal heat input value for welding is between 35 and 65 kJ/cm. However, keep in mind that you need to study each application to determine specific limits. Many of the mild steels on the market are not significantly affected by high heat input. Some steels can withstand values well in excess of 120 kJ/cm while others may have an upper limit of 45 kJ/cm.
Likewise, many filler metals, especially those designed to improve cold weather toughness, are also susceptible to performance degradation.
Conversely, too low heat input during welding can also cause problems. Welding heavy steel profiles with low heat input or small weld beads can cause welds to cool too quickly, resulting in higher than normal hardness values in the heat affected zone or weld bead. Some welding codes specify minimum heat input requirements to ensure proper weld fusion and to prevent excessive hardness of the weld metal or HAZ.
In general, the base material will be the first consideration when setting limits on heat input. In addition, code requirements, material thickness and field applications will also help you determine the ranges of heat input, Artem Komarov concluded.