Arc Energy - The Electrical Parameter
Arc energy is calculated as: Arc Energy (kJ/mm) = (V × I × 60) / (1000 × S)
Where:
- V = Voltage
- I = Current
- S = Travel speed
This assumes 100% transfer of electrical energy to the joint. It is simple, measurable, and used within ASME Section IX qualification. However, it does not represent actual heat absorbed into the steel.
Heat Input - The Metallurgical Parameter
True heat input accounts for arc efficiency: Heat Input = Arc Energy × η Where η = Arc efficiency factor (process dependent). Typical values from published welding metallurgy literature:
- SAW: 0.95-1.0
- GMAW: 0.8-0.9
- FCAW: 0.8-0.9
- SMAW: 0.7-0.8
- GTAW: 0.6-0.7
The difference between arc energy and true heat input may be 15-35%. Metallurgically, that difference is significant.
Where BS EN 1011-2 Becomes Critical
Unlike qualification codes, BS EN 1011-2 is a fabrication guidance standard. It addresses hydrogen cracking avoidance, preheat determination, cooling time (t8/5) control, HAZ hardness risk, and carbon equivalent influence. Relevant clauses include:
- Clause 5 - Avoidance of hydrogen cracking
- Clause 6 - Preheating and interpass temperature
- Clause 7 - Control of heat input and cooling rate
- Annex C - Calculation of cooling time (t8/5)
Cooling time relationships depend on effective heat input, not raw arc energy.
Why ASME Section IX Does Not Address Efficiency
ASME Section IX is a qualification code. It defines essential variables and uses arc energy methodology without applying an efficiency factor. This simplifies qualification. However, ASME Section IX does not provide fabrication hydrogen control methodology equivalent to EN 1011-2. The two standards serve different purposes.
Engineering Example
Assume:
28 V
220 A
300 mm/min
Arc Energy = 1.23 kJ/mm
GMAW (η = 0.85) → Heat Input = 1.05 kJ/mm
GTAW (η = 0.65) → Heat Input = 0.80 kJ/mm
Under EN 1011-2 Annex C, cooling time is directly proportional to heat input. Incorrect heat input assumptions may lead to incorrect cooling time prediction, incorrect preheat determination, and increased cracking risk.
Practical Implications for UK & European Fabricators
When working to ISO 15614, ISO 9606, EN 1090, or rail/offshore specifications, qualification acceptance does not equal metallurgical optimisation. Fabrication guidance under EN 1011-2 requires understanding effective heat input to manage:
- High carbon equivalent steels
- S690 / Q&T materials
- Thick section structural steel
- Offshore grades
This is particularly relevant where impact testing is required, HAZ hardness limits apply, and low hydrogen consumables are mandated.
Final Technical Summary
- Arc Energy - Electrical measurement.
- Heat Input - Thermal energy absorbed by the joint.
- Arc Efficiency - Bridges the gap between the two.
- BS EN 1011-2 - Provides fabrication guidance based on real thermal behaviour.
Understanding this hierarchy ensures valid WPQR control, correct preheat selection, reliable cooling time estimation, reduced hydrogen cracking risk, and audit defensibility.
When what’s built must not fail, thermal control is not optional. Contact NECIT Services for expert welding inspection services for the UK engineering sector and ensure full safety and compliance within your welding processes.
