Carbon Equivalent Calculator

Carbon Equivalent Calculator: Check How Weldable Your Steel Is

Not all steel welds the same way. Some grades weld easily on a cold day with no special care. Others crack next to the weld a few hours after you finish, even when the weld looks perfect. The carbon equivalent, often written as CE or CEV, is a single number that tells you which kind of steel you have.

This carbon equivalent calculator takes the chemical analysis of your steel and works out the value with six common formulas: IIW, AWS D1.1, Pcm, CET, CEN and JIS. It rates the weldability, checks the number against your own limit, and gives you an estimated preheat temperature using the EN 1011-2 method. You can compare several steels side by side and download the result.

It is made for welders, welding inspectors, fabricators, engineers and students. You do not need to know the maths. Type in the numbers from the mill certificate and read the answer.

What Is Carbon Equivalent?

When steel is welded, the metal right next to the weld gets very hot and then cools fast. This strip is called the heat affected zone, or HAZ. If the steel can harden when it cools quickly, the HAZ turns hard and brittle, and a crack can start there. This is called hydrogen cracking, cold cracking or delayed cracking.

Carbon is the main element that makes steel harden. But carbon is not the only one. Manganese, chromium, molybdenum, vanadium, nickel and copper also make steel harder to weld, each to a different degree. The carbon equivalent adds them all up and turns them into one number, as if all of that hardening effect came from carbon alone.

A higher number means the steel hardens more and is harder to weld. A lower number means it is easier. That is the whole idea.

The Carbon Equivalent Formulas

There is no single official formula. Different groups made different ones for different kinds of steel. All element amounts are in weight percent. These are the six used in this calculator.

• IIW formula: CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. The most popular one. It was adopted by the International Institute of Welding and suits plain carbon and carbon-manganese steels.
• AWS D1.1 formula: CE = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. It is the IIW formula with a silicon term added, and it is used for structural steel.
• Pcm formula: Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B. Made in Japan for low carbon, high strength steels such as modern pipeline grades.
• CET formula: CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40. It is part of the European standard EN 1011-2 and is used to work out preheat.
• CEN formula: CEN = C + A(C) x [Si/24 + Mn/6 + Cu/15 + Ni/20 + (Cr + Mo + Nb + V)/5 + 5B]. Made by Yurioka. The A(C) part changes with the carbon level, so it works across a wide range of steels.
• JIS formula: CE = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14. Common on Japanese mill certificates.

How to Calculate Carbon Equivalent Step by Step

Take the chemical analysis from the mill test certificate. Say a steel has 0.18% carbon and 1.40% manganese, which is close to an S355 structural plate. With the IIW formula you only need the carbon and the manganese, because the other elements are close to zero.

Divide the manganese by 6. That is 1.40 divided by 6, which is 0.233. Add the carbon, 0.18, and you get a carbon equivalent of 0.413. On the usual scale that sits in the good range, so the steel welds well but deserves a little care on thick plate.

Now try a quenched and tempered grade like 4140. It has about 0.40% carbon, 0.85% manganese, 0.95% chromium and 0.20% molybdenum. Manganese gives 0.142. Chromium and molybdenum together give (0.95 + 0.20) divided by 5, which is 0.230. Add the carbon and you get 0.772. That is far above the safe range, which is why 4140 needs a strict welding procedure.

The calculator does exactly this, but for all six formulas at once and with every element you enter.

Carbon Equivalent and Weldability: What the Numbers Mean

The IIW value is usually read with a simple scale. It is a rule of thumb, not a law. Thickness, hydrogen and heat input matter just as much, as you will see in the next sections.

• Up to 0.35: excellent weldability. Preheat is rarely needed.
• 0.36 to 0.40: very good weldability.
• 0.41 to 0.45: good weldability. Take care on thick sections.
• 0.46 to 0.50: fair weldability. Preheat and low-hydrogen welding are wise.
• Above 0.50: poor weldability. Expect to need preheat.
• Above 0.60: a strict procedure is needed, with preheat, controlled cooling and often post-weld heat treatment.

How to Use This Carbon Equivalent Calculator

You can start with a preset steel or type your own numbers. Everything updates as you type.

• Pick a steel such as A36, S355J2, A516 Gr 70, API 5L X65 or 4140. The numbers are typical, so replace them with your own mill certificate values for real work.
• Fill in the elements. Leave an element empty if the certificate does not list it. Boron is very small, so type it like 0.0005.
• Read the six carbon equivalent values. The tool also shows which formula suits your steel best and why.
• Set your limit. If your specification says CE must not be above 0.43, type that in and choose the formula. The tool tells you if the steel passes.
• Open the preheat section. Enter the plate thickness, the hydrogen level and the heat input. You can type the heat input directly or work it out from volts, amps and travel speed.
• Save the steel to the comparison table, change the composition, and save another one to compare heats or grades.
• Copy the result, download the CSV file or print the page.

Which Carbon Equivalent Formula Should You Use?

This is the question that trips people up, because the formulas can give quite different numbers for the same steel. A modern pipeline steel with 0.09% carbon, 1.55% manganese and a little niobium and vanadium shows it well. Its IIW value is about 0.40, which looks borderline. Its Pcm is about 0.20, which looks very safe. The truth is closer to the Pcm side, because IIW was never meant for low carbon micro-alloyed steel and it over-counts manganese.

A good rule is to match the formula to the steel. Use IIW or AWS D1.1 for plain carbon and carbon-manganese steels. Use Pcm for low carbon steels, around 0.12% carbon or lower. Use CET when you work to European standards or want a preheat estimate. Use CEN when you are not sure, because it adjusts itself to the carbon level.

Most of all, use the formula your specification asks for. If the contract says CE by IIW, then IIW is the number that counts, even if another formula looks better for your steel.

Carbon Equivalent and Preheat Temperature

Preheating means warming the steel before you weld. It slows the cooling, which lets hydrogen escape and keeps the HAZ from getting too hard. The higher the carbon equivalent, the more preheat you usually need.

Carbon equivalent is only one input. Four things decide the preheat. The first is the chemistry, shown by the CET value. The second is the plate thickness, because thick plate pulls heat away faster. The third is the amount of hydrogen in the weld, which depends on your consumables. The fourth is the heat input, because a hotter, slower weld cools more gently.

This calculator uses the preheat method from EN 1011-2, known as Method B. For an S355J2 type steel with a CET of about 0.32, a heat input of 1.5 kJ/mm and low hydrogen of 5 ml per 100 g, the estimate is about 65 degrees C for 20 mm plate and about 115 degrees C for 40 mm plate. Raise the hydrogen to 10 and the 40 mm plate goes up to about 145 degrees C.

That example shows two things. Thicker plate needs more preheat, and cleaner, drier consumables can save you a lot of heating. The method is valid for a CET from 0.20 to 0.50, thickness from 10 to 90 mm, hydrogen from 1 to 20 ml per 100 g and heat input from 0.5 to 4.0 kJ/mm. The calculator warns you when you step outside those limits.

Typical Carbon Equivalent of Common Steels

Here are the IIW values for some steels people weld every day. They are worked out from typical chemistry, so your own heat can be a little higher or lower. Always check the certificate that came with your steel.

• S235JR, about 0.28: excellent weldability.
• A36, about 0.33: excellent weldability.
• A516 Grade 70, about 0.39: very good weldability.
• A572 Grade 50, about 0.41: good weldability.
• S355J2, about 0.41: good weldability.
• A588 weathering steel, about 0.49: fair weldability.
• 1045 carbon steel, about 0.58: poor weldability, preheat needed.
• 4130, about 0.61: very poor weldability, strict procedure needed.
• 4140, about 0.77: very poor weldability, strict procedure needed.

Hydrogen and Heat Input in Simple Words

Hydrogen is the hidden cause of most weld cracks. It comes from moisture in electrode coatings, flux, rust and the air. Hydrogen creeps into the hard HAZ and builds pressure inside it, and that pressure opens a crack, sometimes a day or two after welding. Low-hydrogen electrodes and dry, clean joints keep the number down. The calculator uses hydrogen in millilitres per 100 grams of weld metal, from very low at 3 to very high at 20.

Heat input is the energy you put into each millimetre of weld. A higher heat input warms a wider area and makes the steel cool more slowly, which helps. But too much heat input can soften the steel and spoil its toughness, so the answer is balance, not the highest number you can reach.

How to Lower the Risk of Cracking

If your carbon equivalent is high, you still have plenty of ways to weld the steel safely. These are the steps that make the biggest difference.

• Use low-hydrogen consumables and keep them dry. Bake and store stick electrodes as the maker says.
• Clean the joint. Rust, oil, paint and moisture all add hydrogen.
• Preheat to the temperature you calculated and keep it through the weld. Check it with a temperature stick or a contact thermometer.
• Use a heat input that is not too low. Very small, fast passes cool too quickly.
• Control the interpass temperature so the steel does not get too hot between passes.
• Let the weld cool slowly. Wrap thick parts in a blanket if needed.
• Reduce restraint where you can. Heavily clamped joints crack more easily.
• For very high carbon equivalent steels, add post-weld heat treatment if the procedure calls for it.

Common Mistakes With Carbon Equivalent

Most mistakes come from treating the number as a final answer. Here are the ones we see most often.

• Comparing numbers from different formulas. A Pcm of 0.25 and an IIW of 0.25 are not the same thing.
• Using the typical values from a table instead of the real mill certificate. Two heats of the same grade can differ a lot.
• Forgetting small elements. Copper, nickel and vanadium add up on weathering and micro-alloyed steels.
• Ignoring thickness. A steel that welds fine at 10 mm may need preheat at 50 mm.
• Using the IIW formula on low carbon micro-alloyed steels, where it can give a false alarm.
• Assuming a low CE means you can skip good practice. Wet electrodes can crack even a low CE steel.

Important Notes Before You Weld

This calculator is a planning and learning tool. It gives an estimate, not a welding procedure. A real procedure must be qualified under your code, such as AWS D1.1, EN ISO 15614 or ASME IX, and it must follow the rules in your contract.

The weldability bands and the preheat formula are well known guides, but they are built on averages. If a joint is critical, such as a pressure vessel, a bridge or a pipeline, ask a qualified welding engineer to check the numbers before work starts.