Welding heat input is the amount of thermal energy applied to the material during the welding process per unit length of the weld joint.
The magnitude of heat input is primarily related to current, voltage, welding speed (or travel speed), and the efficiency of the process employed.
Heat input must be controlled because the amount of heat received by the material can influence the characteristics of the weld pool, the heat-affected zone (HAZ), the cooling rate, distortion, and even the material's properties after welding.
Therefore, heat input is not merely a matter of using high or low current. Its value must be viewed as a combination of several welding parameters working simultaneously.
Heat input is the effective thermal energy entering the workpiece during the welding process per unit length of the weld.
In arc welding, energy is generated from a combination of voltage and electric current. However, not all of the energy from the arc is actually absorbed by the workpiece; some energy may be lost to the surroundings or through other mechanisms during the process.
Therefore, in technical calculations, it is necessary to distinguish between arc energy and heat input.
According to TWI, arc energy is the energy delivered by the arc before the process efficiency is taken into account. Meanwhile, heat input factors in process efficiency to estimate the energy actually transferred to the workpiece.
What is the difference between heat input and arc energy?
Both represent the energy per unit length of welding, but there is a difference regarding the efficiency factor.
Simply:
Arc Energy = arc energy per unit length
Whereas:
Heat Input = Arc Energy × process efficiency
It should be noted that in some welding codes or practices, the term heat input is also used to refer to the calculated arc energy without the efficiency factor.
Therefore, the calculation methods used in production work must comply with the WPS, standards, codes, or procedures applicable to that project.
Welding Heat Input Formula
If the welding speed is expressed in mm/min, the arc energy can be calculated using the formula:
Arc Energy (kJ/mm) = (V × I × 60) ÷ (Travel Speed × 1,000)
Information:
| Parameters | Information |
|---|---|
| V | Welding voltage in volts |
| I | Welding current in amperes |
| 60 | Time conversion from minutes to seconds |
| Travel Speed | Welding speed in mm/min |
| 1.000 | Convert joules to kilojoules |
Once the arc energy is known, the heat input can be calculated using:
Heat Input = η × Arc Energy
Where η (eta) is the welding process efficiency factor.
Heat Input Efficiency Factor
Energy transfer efficiency varies across different welding processes.
TWI lists the following efficiency factor values:
| Welding Process | Efficiency Factor |
| SAW | 1,0 |
| SMAW / MMA | 0,8 |
| FCAW | 0,8 |
| MIG/MAG / GMAW | 0,8 |
| TIG / GTAW | 0,6 |
| Plasma Arc Welding | 0,6 |
This value indicates that not all of the arc energy is converted into heat received directly by the workpiece.
For work following a WPS or specific standard, use the factors and calculation methods prescribed by that procedure.
Example of How to Calculate Welding Heat Input
For example, SMAW welding is performed using the following parameters:
- Current: 120 A
- Voltage: 24 V
- Travel speed: 180 mm/min
First, calculate the arc energy:
Arc Energy = (24 × 120 × 60) ÷ (180 × 1.000)
Arc Energy = 0,96 kJ/mm
If an SMAW efficiency factor of 0.8 is used:
Heat Input = 0,8 × 0,96
Heat Input = 0,768 kJ/mm
Based on those parameters, the heat input value—accounting for process efficiency—is approximately:
0,77 kJ/mm
This example demonstrates that changes in current, voltage, or travel speed can directly alter the amount of energy per unit length delivered to the material. An efficiency factor is then used to estimate the effective energy received by the workpiece.
Factors Affecting Welding Heat Input
There are four main factors to pay attention to.
1. Welding Current
With other parameters held constant, an increase in current will increase arc energy and potentially increase heat input.
However, the current should not be analyzed in isolation, as changes in current can also affect arc characteristics, electrode melting rate, penetration, and welding speed.
2. Welding Voltage
Voltage is one of the components in the calculation of arc energy.
If the current and travel speed remain constant, an increase in voltage will increase the energy per unit length.
However, voltage should not be considered the sole parameter determining welding "heat." The interaction of all parameters must still be taken into account.
3. Travel Speed
Travel speed, or welding movement speed, is inversely related to heat input.
With the same current and voltage:
- slower travel speed → energy per unit length increases;
- Higher travel speed → energy per unit length decreases.
That is why two welders using the same current and voltage do not necessarily produce the same heat input if their travel speeds differ.
The relationship between current, voltage, and travel speed is also used to determine heat input when controlling welding parameters.
4. Welding Process Efficiency
Each process has distinct heat transfer characteristics. Therefore, two processes generating the same arc energy do not necessarily transfer the same amount of effective energy to the material.
The efficiency factor helps account for those differences.
What Happens If Heat Input Is Too High?
Excessive heat input means the material receives a large amount of thermal energy per unit length of the weld.
This condition can cause:
- the HAZ area becomes larger;
- cooling proceeds more slowly;
- distortion or deformation increases;
- microstructural changes resulting in a coarser structure;
- Certain mechanical properties can change depending on the type of material.
Excessive heat is also a factor that must be controlled when working with thin materials, as it can increase the risk of the material deforming or even burning through.
In Intan Pertiwi's guide to welding thin iron, controlling heat and travel speed is crucial for preventing heat buildup in a single area.
However, the specific impact of high heat input varies depending on the material type, thickness, joint configuration, and welding procedure.
What Happens If the Heat Input Is Too Low?
Excessively low heat input does not always result in a better weld.
Under certain conditions, excessively low energy can result in insufficient heat to produce the fusion required at the joint.
Possible consequences include:
- inadequate fusion;
- penetration does not meet requirements;
- the bead is too tight;
- the cooling rate becomes faster;
- The metallurgical characteristics of the joint may change.
For certain materials, excessively rapid cooling must also be considered, as it can affect hardness and the tendency for cracking.
In other words, the goal of controlling heat input is not to keep the value as low as possible, but rather to maintain it within a range appropriate for the material, process, joint, and WPS.
Relationship between Heat Input and HAZ
The Heat-Affected Zone (HAZ) is the portion of the base metal that does not melt but undergoes changes due to the heat cycle of welding.
Heat input is one of the factors influencing the heating and cooling cycle in that area.
In general, an increase in heat input can cause heat to affect a larger area of the material and slow down the cooling process. Conversely, lower heat input tends to result in a different thermal cycle and faster cooling.
However, the characteristics of the HAZ are not determined solely by heat input. Material type, thickness, initial temperature, interpass temperature, joint design, and welding conditions also play a role.
Therefore, the heat input value must be considered in conjunction with the welding procedure requirements, rather than as a standalone parameter.
Heat Input Relationship with Welding WPS
In controlled welding operations, parameters such as current, voltage, travel speed, polarity, and heat input can be specified or controlled through Welding Procedure Specification (WPS).
The objective is to ensure that the welding process can be carried out consistently in accordance with the predetermined parameters.
Intan Pertiwi previously discussed that welding parameters are a crucial part of the WPS and can include current, voltage, travel speed, and heat input.
Therefore, when a WPS specifies a particular heat input range, it is not enough for the welder to simply ensure the amperage falls within that range. Voltage and travel speed must also be maintained to ensure the energy per unit length remains in accordance with the procedure.
How to Control Welding Heat Input
Heat input control can be achieved by maintaining a balance among the key parameters.
Some steps that can be implemented include:
- use a current within the recommended range;
- maintain voltage or arc length consistently;
- maintain the travel speed so that it is neither too slow nor too fast;
- use parameters appropriate to the diameter of the electrode or wire;
- follow the parameter ranges specified in the WPS;
- record parameters if the work requires quality control;
- Avoid holding the arc in one area for too long.
In the SMAW process, the stability of the arc length and the welder’s travel speed greatly affect process consistency. The current setting must also be adjusted according to the diameter of the electrode being used.
Welding Heat Input Summary Table
| Parameter Changes | Status Unchanged | Heat Input |
|---|---|---|
| The current is increasing. | Voltage & travel speed | Increase |
| Downstream flow | Voltage & travel speed | Decrease |
| Tensions are rising. | Current & travel speed | Increase |
| Voltage drops | Current & travel speed | Decrease |
| Travel speed increases. | Current & voltage | Decrease |
| Travel speed decreases. | Current & voltage | Increase |
The table illustrates the fundamental mathematical relationship regarding heat input. In practice, a change in one parameter can affect other process characteristics; therefore, welding parameters must still be evaluated as an integrated whole.
Conclusion
Welding heat input is the thermal energy received by the material per unit length of the joint during the welding process.
Its value is primarily influenced by:
- current;
- voltage;
- travel speed;
- welding process efficiency.
Excessively high or low heat input can affect the welding results.
Therefore, the goal is not to generate the highest or lowest possible heat input, but to maintain it within a range appropriate for the material, process, joint design, and the WPS being used.
For welding operations using electrodes or KOBELCO welding elctrode, follow the applicable product parameters and welding procedures to ensure the process is carried out consistently in accordance with application requirements.
