Heat Input in Welding:
Importance, Calculation & Arc Energy
A comprehensive technical guide covering arc energy vs. heat input, formulas per ASME IX & EN ISO 1011-1, thermal efficiency factors, and an interactive calculator.
1. What Is Heat Input in Welding?
In arc welding, energy is transferred from the welding electrode to the base metal through an electric arc. When the welder strikes an arc, both the electrode and the base metal melt, forming a weld pool. The energy that sustains this process is supplied by the power source. The quantity of energy delivered to the weld per unit length of travel is what engineers measure and control.
Arc Energy is the total electrical energy generated by the welding arc per unit length of weld, before accounting for how efficiently that energy reaches the workpiece.
Heat Input is the actual energy that reaches the workpiece. It is calculated by multiplying arc energy by the thermal efficiency of the specific welding process in use.
In American codes such as ASME Section IX and AWS D1.1, the term “heat input” is used to describe what European standards call “arc energy.” This important distinction is addressed in Section 10 — Applicable Codes & Standards.
2. Arc Energy vs. Heat Input — Key Differences
The terms arc energy and heat input are frequently used interchangeably in industry, but they represent distinct concepts under European standards. Understanding the distinction is critical when working across different code systems or comparing welding processes.
| Property | Arc Energy (AE) | Heat Input (HI) |
|---|---|---|
| Definition | Electrical energy generated by the arc per unit weld length | Energy actually transferred to the workpiece per unit weld length |
| Formula | AE = (V × I) / v | HI = AE × η |
| Thermal efficiency | Not included | Included (η factor) |
| European standard (EN ISO 1011-1) | “Arc Energy” | “Heat Input” |
| American standards (ASME IX / AWS D1.1) | Called “Heat Input” (η not applied) | Same formula as AE under this system |
| Units | kJ/mm or kJ/inch | kJ/mm or kJ/inch |
3. Why Heat Input Matters: Effects on Weld Quality
Heat input is arguably the most influential process parameter in arc welding from a metallurgical standpoint. Its primary effect is on the cooling rate of the weld and the adjacent Heat-Affected Zone (HAZ). This in turn governs microstructural development and, ultimately, the mechanical properties of the completed joint.
Effect on Cooling Rate and Microstructure
A higher heat input leads to a slower cooling rate. This is analogous to the effect of preheat — both reduce the thermal gradient between the weld and the surrounding base metal. Slower cooling causes austenite grains in the HAZ to grow larger (coarsening). Coarser grains mean fewer grain boundaries, which translates directly to reduced toughness and impact resistance, particularly at low temperatures.
Risks of Excessive Heat Input
- Coarse-grain microstructure in the HAZ — reduced toughness and Charpy impact values.
- Broad, diluted weld bead with increased risk of solidification cracking (especially in high-carbon or alloy steels).
- Loss of strength in precipitation-hardened or quenched-and-tempered steels.
- Distortion and residual stress in thin or restrained structures.
Risks of Insufficient Heat Input
- Excessively fast cooling rates promote martensite formation — a hard, brittle microstructure.
- Increased risk of hydrogen-induced cold cracking (HIC) in susceptible steels.
- Lack of fusion defects due to insufficient weld pool fluidity.
4. Heat Input & Arc Energy Formulas
There are three principal methods for calculating heat input in arc welding, each applicable to different welding conditions. The appropriate method depends on whether the power source is a conventional DC machine or a waveform-controlled unit.
Method 1 — Standard DC Welding (ASME IX / AWS Approach)
This is the most widely used formula, applicable to conventional DC processes including SMAW, GTAW, GMAW (short-arc), FCAW, and SAW.
I = Welding Current (Amps)
TS = Travel Speed (mm/min or in/min)
AE = Arc Energy / Heat Input (kJ/mm or kJ/inch)
60 = conversion factor (sec → min)
1000 = conversion factor (J → kJ)
Method 2 — Heat Input per EN ISO 1011-1 (European Standard)
The European approach multiplies arc energy by the thermal efficiency factor (η) of the process to yield true heat input at the workpiece. This is described in Section 5.
All other variables as per Formula 1
Travel Speed Calculation
When travel speed is not directly known, calculate it from the weld length and time taken:
t = Time taken to complete the weld (minutes)
TS = Travel Speed (mm/min or in/min)
5. Thermal Efficiency Factors by Process
Not all welding processes transfer the same proportion of their electrical energy into the workpiece. Losses occur through radiation, convection, heating of non-consumable electrodes (e.g., the tungsten in GTAW), and the shielding gas column. The thermal efficiency factor (η) quantifies this transfer ratio.
| Welding Process | Abbreviation | Thermal Efficiency (η) | Notes |
|---|---|---|---|
| Submerged Arc Welding | SAW | 0.95 – 1.00 | Insulating flux blanket minimises losses |
| Shielded Metal Arc Welding | SMAW | 0.75 – 0.85 | Commonly taken as 0.80 |
| Gas Metal Arc Welding (MIG/MAG) | GMAW | 0.75 – 0.85 | Commonly taken as 0.80 |
| Flux-Cored Arc Welding | FCAW | 0.75 – 0.85 | Similar to GMAW |
| Gas Tungsten Arc Welding (TIG) | GTAW | 0.55 – 0.65 | Commonly taken as 0.60; energy lost to tungsten and gas |
| Plasma Arc Welding | PAW | 0.55 – 0.65 | Similar to GTAW |
| Electroslag Welding | ESW | ~1.00 | Very high efficiency due to molten slag blanket |
6. Waveform-Controlled Welding & Instantaneous Energy
The standard arc energy formula (V × I × 60 / TS × 1000) is suitable for conventional DC welding where current and voltage are relatively stable and can be meaningfully averaged. However, it is not valid for waveform-controlled processes such as pulsed GMAW (GMAW-P), advanced inverter-based processes, or other sources where the output changes rapidly.
In waveform-controlled welding, the instantaneous current and voltage undergo rapid phase shifts and synergic changes driven by the power source’s internal software. The meter readings displayed on the machine no longer represent the true energy delivered to the weld. In these cases, the heat input must be determined using one of two instantaneous methods, as defined in ASME Section IX and PD ISO/TR 18491.
Method A — Heat Input from Instantaneous Energy (Joules)
L = Weld bead length (mm or inches)
HI = Heat Input (J/mm → divide by 1000 for kJ/mm)
Method B — Heat Input from Instantaneous Power (Watts)
v = Travel speed (mm/s)
HI = Heat Input (J/mm → divide by 1000 for kJ/mm)
Measurement equipment for waveform-controlled welding must sample at a frequency of at least 10× the waveform frequency to capture accurate instantaneous values. Many modern inverter-based power sources display total energy or instantaneous power natively. For older machines without this capability, an external data-acquisition device may be required.
7. Volume of Weld Deposit Method
ASME Section IX provides an alternative to calculating heat input directly from electrical parameters. Instead, the volume of weld metal deposited per unit length of weld can be used to assess and control the energy delivered. This is particularly practical for processes where instantaneous power measurement is not readily available.
The deposited volume may be determined by either of two methods:
- Bead geometry measurement: Measuring the cross-sectional width and thickness of the weld bead (Volume ≈ Width × Thickness per unit length).
- Run-out length method: Measuring the length of weld produced per unit length of electrode consumed.
This method is most applicable to SMAW, where electrode length can be straightforwardly correlated with deposited metal, and to semi-automatic processes where wire feed speed is constant.
8. Worked Examples
Example 1 — Conventional DC (GTAW, Standard Formula)
A welder uses GTAW with a conventional power source. The machine displays 125 A and 13 V. A 250 mm weld is completed in 4 minutes. Calculate the arc energy and heat input.
Step 1 — Calculate travel speed:
TS = L / t = 250 mm / 4 min = 62.5 mm/min
Step 2 — Calculate Arc Energy (ASME formula):
AE = (V × I × 60) / (TS × 1000)
AE = (13 × 125 × 60) / (62.5 × 1000)
AE = 97,500 / 62,500
AE = 1.56 kJ/mm
Step 3 — Calculate Heat Input (EN ISO 1011-1, η = 0.60 for GTAW):
HI = AE × η = 1.56 × 0.60
HI = 0.936 kJ/mm Example 2 — Waveform-Controlled (Instantaneous Energy Method)
A waveform-controlled power source with GTAW process displays a total energy of 560 kJ for a completed weld bead of 500 mm. Calculate the heat input.
Step 1 — Apply the instantaneous energy formula:
HI = E_total / L
HI = 560 kJ / 500 mm
HI = 1.12 kJ/mm Example 3 — Range Calculation for WPS Qualification
A welding procedure requires qualification across a parameter range: current 140–190 A, voltage 16–18 V, travel speed 80–110 mm/min. Calculate the minimum and maximum heat input values.
Minimum HI (low current & voltage, high travel speed):
HI_min = (140 × 16 × 60) / (110 × 1000)
HI_min = 134,400 / 110,000
HI_min = 1.22 kJ/mm
Maximum HI (high current & voltage, low travel speed):
HI_max = (190 × 18 × 60) / (80 × 1000)
HI_max = 205,200 / 80,000
HI_max = 2.57 kJ/mm
∴ Qualified heat input range: 1.22 – 2.57 kJ/mm 9. Interactive Heat Input & Arc Energy Calculator
Use the calculator below to compute arc energy and heat input for any combination of welding parameters. Select the method that matches your situation.
How to Use the Calculator
For conventional DC welding, enter arc voltage, welding current, travel speed, and select the appropriate welding process. The calculator will display both arc energy (without efficiency) and true heat input (with efficiency factor applied), in both metric (kJ/mm) and imperial (kJ/inch) units. For waveform-controlled welding where your power source displays total energy, switch to the “Instantaneous Energy” tab.
10. Applicable Codes & Standards
Heat input control requirements are embedded in several international fabrication codes. Understanding which code governs a given project is essential before selecting a calculation method.
| Standard | Region | Term Used | Thermal Efficiency Applied? |
|---|---|---|---|
| ASME Section IX (QW-409.1) | Americas / International | “Heat Input” | No (η = 1.0 implicitly) |
| AWS D1.1 (Clause 6.8.5) | Americas | “Heat Input” | No |
| EN ISO 1011-1 | Europe | “Heat Input” = AE × η | Yes |
| PD ISO/TR 18491 | International | Both AE and HI defined | Yes (for HI) |
| BS 5135 (superseded) | UK (historical) | “Arc Energy” (without η) | No |
When working to ASME Section IX, heat input is treated as an essential variable for procedures requiring notch toughness (CVN) testing. An increase in heat input beyond the qualified value requires re-qualification of the welding procedure specification (WPS). The applicable clause is QW-409.1, covering SMAW, GTAW, GMAW, SAW, FCAW, PAW, and EGW processes.
11. Summary
Heat input is a fundamental parameter in arc welding that governs the thermal cycle experienced by the weld metal and the Heat-Affected Zone (HAZ). Controlling it within the ranges qualified in the WPS ensures repeatable microstructure and reliable mechanical properties throughout production welding.
The key points to retain are: arc energy is the raw electrical power divided by travel speed; true heat input adds a thermal efficiency factor (η) that varies by process; conventional DC welding can use the standard formula, while waveform-controlled processes require instantaneous energy measurement. American codes (ASME/AWS) use arc energy as their “heat input,” while European codes (EN ISO 1011-1) define heat input as arc energy multiplied by thermal efficiency.
For any project where toughness, hydrogen cracking susceptibility, or microstructural integrity is a concern, calculating and recording heat input per weld pass is not optional — it is a critical quality control obligation.