Welding Carbon Equivalent, Preheat & Heat Input Calculator

Assess steel weldability with CE(IIW), Pcm, CEN, preheat temperature, and heat input calculations.

Last reviewed: June 2026
Built and maintained by · Last updated June 2026

Steel Chemistry (% by weight)

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Welding Parameters

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V
A
mm/min

Conditions

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Results

CE (IIW)
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Pcm
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CEN
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Heat Input
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Min. Preheat Temperature
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Interpass Maximum
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Risk Assessment
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Disclaimer: This calculator is an engineering aid for educational and planning purposes. It does not replace qualified welding procedure specifications (WPS), professional engineering judgment, or applicable code requirements (AWS D1.1, ASME, etc.).
Important Disclaimer: This calculator is a weldability screening and education aid. It does not qualify a welding procedure, approve a production weld, choose filler metal, set code-required preheat, or replace AWS, ASME, API, EN, project specification, WPS/PQR, inspection, or welding-engineer requirements.

How the Welding Carbon Equivalent Calculator works

This tool evaluates steel weldability from chemistry and welding parameters, not project cost, material quantity, or job time. Enter the steel composition as percent by weight, or choose one of the included steel presets, then enter thickness, welding process, voltage, amperage, travel speed, ambient temperature, and diffusible-hydrogen category. The calculator reports CE(IIW), Pcm, CEN, heat input, a simplified minimum preheat estimate, an interpass maximum, and a qualitative cracking-risk message.

CE(IIW) is calculated as C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. It is a traditional carbon-equivalent index for carbon and low-alloy steels. Higher values generally mean higher hardenability and a greater chance that the heat-affected zone can form hard, crack-sensitive microstructures.

Pcm is calculated as C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B. It is often more useful for modern lower-carbon steels because it gives carbon and small alloy additions a different weighting. CEN uses a carbon-dependent factor, A(C), so its behavior can track low-carbon and higher-carbon steels in one model.

Heat input and preheat logic

Heat input is calculated as (efficiency x voltage x amperage x 60) / (travel speed x 1000), with travel speed in mm/min. The process efficiency defaults in this implementation are 0.80 for SMAW, 0.85 for GMAW and FCAW, 0.95 for SAW, and 0.70 for GTAW. Slower travel speed, higher voltage, or higher amperage raises the kJ/mm value.

The preheat estimate is deliberately simple. The calculator starts at 0 C, 50 C, 100 C, or 150 C based on CE(IIW) and material thickness thresholds, then adds 10 C for medium hydrogen or 25 C for high hydrogen. If the ambient temperature is already higher than the computed preheat, the displayed minimum becomes none required. Real preheat tables also consider restraint, joint detail, consumable classification, heat input range, repair welding, tack welding, material specification, and code rules, so treat this as a first-pass screen.

Worked examples

Default custom steel: C 0.15%, Mn 0.80%, Si 0.25%, 25 mm thickness, GMAW, 25 V, 200 A, and 200 mm/min travel speed gives CE(IIW) about 0.283, Pcm about 0.198, CEN about 0.277, and heat input about 1.28 kJ/mm. CE and Pcm are low, but the 25 mm thickness still triggers the simplified 50 C preheat estimate.

ASTM A36 preset: choosing A36 fills in a higher carbon and copper-bearing composition in this implementation. With the same welding parameters, CE(IIW) is near 0.407 and Pcm is near 0.323, so the page shows more caution even before thickness, hydrogen, and restraint are considered.

High-hydrogen scenario: leaving the default chemistry but changing hydrogen level from low to high adds 25 C to the computed preheat and appends a warning to the risk detail. That reflects the central cold-cracking problem: hydrogen, a hard microstructure, and tensile stress acting together.

Common mistakes

  • Using mill certificates loosely: enter actual chemistry when possible. Presets are approximations and may not match a specific heat of steel.
  • Reading CE as a pass/fail code result: CE is one signal. Procedure qualification, code tables, impact requirements, and inspection acceptance criteria are separate.
  • Ignoring thickness and restraint: even a moderate CE can need preheat when the joint is thick, restrained, or difficult to access.
  • Using heat input without units: the calculator reports kJ/mm. Do not compare it directly with kJ/in limits without conversion.
  • Letting low-hydrogen practice drift: electrode baking, flux handling, clean joint faces, dry consumables, and exposure time can matter as much as the calculated number.

What this calculator does not do

It does not estimate how much metal you need, welding labor cost, weld length, weld size, filler-metal strength, distortion, residual stress, post-weld heat treatment, impact toughness, code acceptance, or repair procedure requirements. It also does not replace hardness testing, procedure qualification, macroetch, bend tests, Charpy testing, NDE, or engineering review.

Sources

This page is aligned with TWI guidance on carbon equivalent formulae and hydrogen cracking, TWI guidance on preheat, interpass control, and hydrogen-crack prevention, and SSAB guidance on welding heat input from voltage, current, travel speed, and thermal efficiency.

Frequently Asked Questions

It calculates CE(IIW), Pcm, CEN, heat input in kJ/mm, a simplified minimum preheat temperature, an interpass maximum, and a qualitative cracking-risk message from the steel chemistry and welding parameters entered on the page.
CE(IIW) is a traditional hardenability index often used for carbon and low-alloy structural steels. Pcm gives more emphasis to carbon and is often used for lower-carbon modern steels. CEN includes a carbon-dependent factor so it can track both lower-carbon and higher-carbon behavior more smoothly.
The calculator uses a simplified screening heuristic based on CE(IIW), material thickness, hydrogen level, and ambient temperature. It is not a code table, WPS requirement, or procedure qualification result.
Heat input is calculated as process efficiency times voltage times amperage times 60, divided by travel speed in mm/min times 1000. The result is shown in kJ/mm.
No. Use it as a planning and education aid only. Production welding should follow the applicable code, material specification, qualified WPS/PQR, consumable requirements, inspection plan, and welding engineer guidance.