Carbon Equivalent & Preheat Calculator

A36 plate. AZoM’s typical A36 analysis gives 0.25–0.29% carbon, 1.03% manganese and 0.20% copper. At the top of the carbon range, CE = 0.29 + 1.03/6 + 0.20/15 = 0.29 + 0.1717 + 0.0133 = 0.475. That is in TWI’s 0.4–0.5 group: thin sections without preheat, thicker sections with low preheat and low-hydrogen electrodes. At 0.25% carbon the same steel is 0.435.

4140 alloy steel. In the middle of AZoM’s ranges (0.405% C, 0.875% Mn, 0.95% Cr, 0.20% Mo), CE = 0.405 + 0.875/6 + (0.95 + 0.20)/5 = 0.405 + 0.1458 + 0.23 = 0.781, so preheat, low-hydrogen consumables, post-weld heating and slow cooling are all called for.

The common mistake: dropping the small numbers on the certificate. Leave chromium and molybdenum out of the 4140 sum and you get 0.405 + 0.1458 = 0.551, which still says “preheat” but understates the hardenability badly; the right figure is 0.781. Every element in the formula counts, divided by its own number.

The heat-affected zone next to a weld cools fast enough to harden, and hard steel plus hydrogen from the welding consumable plus stress is the recipe for hydrogen (cold) cracking, which can appear hours after welding. Carbon raises hardenability most, but manganese, chromium, molybdenum and the rest add to it too. A carbon equivalent rolls them into one number.

TWI traces the idea to 1940, when Dearden and O’Neill proposed a formula to predict strength, hardenability and HAZ hardness. In 1967 the International Institute of Welding adopted a simplified version, the CE (IIW) used here, which went into EN 1011-2 (replacing BS 5135) and, with silicon added, AWS D1.1. Ito and Bessyo published Pcm in 1968 from a wider range of Japanese steels, and Yurioka’s CEN covers both ends: it tracks CE (IIW) above about 0.15% carbon and gives the alloying elements less weight as carbon falls. The Japan Welding Engineering Society notes that CEN is written into ASTM A1005 and ASME B16.49.

Hydrogen control matters as much as the steel. EN 1011-2 grades consumables by diffusible hydrogen in ml per 100 g of weld metal: scale A above 15 (high), B 10–15, C 5–10, D 3–5 and E 3 or less (ultra-low), as TWI lists them. For crack-sensitive work TWI mentions preheat up to 200 °C, and 250–300 °C held for three to four hours in the worst cases.

This calculator gives CE (IIW), Pcm and CEN for any composition, places CE (IIW) in TWI’s three weldability groups with their preheat and hydrogen guidance, and shows how much each term adds. It does not estimate a preheat temperature: that depends on thickness, restraint, heat input and hydrogen level, and comes from a method such as EN 1011-2 or the alternative methods in AWS D1.1. The presets are labelled compositions from published data sheets (AZoM for A36 and 4140, Alro’s maxima for A572 Grade 50); use your own mill certificate for real work.

The guidance is a starting point; the welding procedure specification and the code for the job decide the preheat. Everything runs in your browser; nothing you enter is sent anywhere.

Related tools: Weld Heat Input Calculator, Filler Rod / Electrode Selection Guide, and Fillet Weld Size & Strength Calculator.