Weld Heat Input Calculator

Why Is Weld Heat Input Important?

Heat input is the energy delivered per unit length of weld and directly controls the cooling rate, microstructure, and mechanical properties of the weld and heat-affected zone (HAZ). Too much heat input causes grain coarsening, reduced toughness, and excessive distortion. Too little causes incomplete fusion and high hardness in the HAZ.

Carbon equivalent (CE) predicts the hardenability and cracking susceptibility of the base metal. The IIW formula (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) is used for steels with C > 0.18%. The Pcm formula is used for low-carbon steels (C < 0.18%). CE > 0.45 generally requires preheat to prevent hydrogen-induced cracking.

Preheat temperature is determined by the combined effects of CE, material thickness, hydrogen level, and heat input. EN 1011-2 provides calculation methods. Preheat slows the cooling rate, allowing hydrogen to diffuse out before the weld cools below 150°C (where hydrogen becomes trapped and causes cold cracking).

Formula: Heat Input (kJ/mm) = (η × V × I × 60) / (Travel Speed × 1000) CE(IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

Example Calculation

GMAW welding at 28V, 250A, travel speed 300 mm/min, arc efficiency 0.85: Heat input = (0.85 × 28 × 250 × 60)/(300 × 1000) = 1.19 kJ/mm. For a steel with C=0.18%, Mn=1.2%: CE(IIW) = 0.18 + 1.2/6 = 0.38. Preheat not required (CE < 0.45).

When to Use This Calculator

• Developing a Welding Procedure Specification (WPS) and needing to determine preheat temperature based on carbon equivalent and material thickness
• Reviewing weld quality issues — checking whether the heat input was within the acceptable range to prevent HAZ property degradation
• Qualifying a new base material for welding — calculating CE to assess cracking susceptibility before procedure trials
• Auditing welder compliance with code requirements — verifying that recorded parameters produce heat input within the WPS-qualified range

Common Mistakes to Avoid

• Using the wrong CE formula — IIW formula is for steels with C>0.18%, Pcm is for low-carbon steels; using the wrong formula gives incorrect preheat recommendations
• Forgetting arc efficiency — GMAW is ~0.85, SMAW ~0.80, GTAW ~0.65; omitting efficiency overstates the actual heat delivered to the weld
• Applying a single-pass heat input calculation to a multi-pass weld without considering interpass temperature — accumulated heat changes cooling rate and HAZ properties
• Ignoring hydrogen control — even with correct preheat, using non-low-hydrogen consumables or moist electrodes can still cause cold cracking

How to Interpret Results

• CE (IIW) < 0.40: Low hardenability — preheat generally not required unless thickness exceeds 25 mm or ambient temperature is below 5°C
• CE (IIW) 0.40-0.60: Moderate hardenability — preheat typically required, temperature depends on thickness and hydrogen level
• CE (IIW) > 0.60: High hardenability — mandatory preheat (often 150-250°C), strict hydrogen control, and post-weld heat treatment likely required
• Cracking risk 'High' or 'Very High' — stop and review the procedure; additional precautions (higher preheat, lower hydrogen consumables, PWHT) are mandatory before welding

Related Standards & References

• EN 1011-2 — Welding — Recommendations for welding of metallic materials — Arc welding of ferritic steels
• AWS D1.1 — Structural welding code — Steel (Section 5: Prequalification and preheat requirements)
• ISO/TR 17671-2 — Guidance on the application of EN 1011-2
• ASME BPVC Section IX — Welding, brazing, and fusing qualifications

Frequently Asked Questions

What are the consequences of exceeding the maximum heat input?

Excessive heat input causes: (1) grain coarsening in the HAZ, reducing toughness (Charpy impact values drop significantly), (2) increased distortion and residual stress, (3) wider HAZ with degraded properties, (4) risk of hot cracking in susceptible alloys, (5) potential violation of code requirements (AWS D1.1 limits heat input for prequalified WPSs). For quenched and tempered steels, excessive heat can soften the HAZ below minimum specified hardness.

When is preheat required and how is the temperature determined?

Preheat is required when CE > 0.45 (IIW formula), material thickness > 25 mm, or the ambient temperature is below 5°C. Minimum preheat temperature is calculated using EN 1011-2 or AWS D1.1 guidelines based on CE, thickness, hydrogen level, and heat input. Typical values: 75°C for CE=0.45 in 25 mm plate, up to 200°C for high-CE steels. Preheat must be applied uniformly over at least 75 mm on each side of the joint.

What is the difference between t8/5 cooling time and its practical significance?

The t8/5 (time to cool from 800°C to 500°C) determines the microstructure of the HAZ. Fast cooling (short t8/5 < 5s) produces martensite — hard, brittle, and crack-susceptible. Slow cooling (long t8/5 > 25s) produces ferrite/pearlite — soft but with reduced toughness due to grain coarsening. The optimal range depends on the steel grade, typically 8-15s for structural steels.