Enter the steel chemistry (C, Mn, Cr+Mo+V, Ni+Cu, Si) and the tool computes the international IIW carbon equivalent CE_IIW and the Ito-Bessyo Pcm cracking parameter, then judges weldability, recommended preheat temperature and hydrogen-induced cold-cracking risk. Use it as an input check for welding procedure specifications (WPS) and when screening structural steels for new fabrications.
Parameters
Carbon %C
%
By far the strongest hardener — biggest contributor to CE
Manganese %Mn
%
Main alloy element for strength and hardenability
%Cr + %Mo + %V
%
Hardenability boosters common in HSLA and Cr-Mo steels
%Ni + %Cu
%
Toughness and weathering — weakest weight (1/15)
Silicon %Si
%
Deoxidiser — appears in Pcm only (not in CE_IIW)
Results
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Carbon equivalent CE_IIW
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Cracking parameter Pcm
—
Carbon share of CE (%)
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Weldability rating
—
Recommended preheat (°C)
—
Cold-cracking risk
—
CE_IIW stacked contribution with thresholds
Stacked bar of each alloy group (C / Mn/6 / (Cr+Mo+V)/5 / (Ni+Cu)/15) building up to CE_IIW, with the three standard thresholds (0.40 / 0.50 / 0.60) shown as horizontal lines and the current Pcm drawn as a side bar.
The IIW carbon equivalent CE_IIW and the Ito-Bessyo Pcm cracking parameter. Both combine mass percent concentrations into a dimensionless "effective carbon" number. As rules of thumb, CE_IIW > 0.50 typically requires preheat, and Pcm > 0.30 indicates serious cold-cracking risk.
Carbon equivalent and weldability
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My mill certificate has a line that says "CE = 0.42". What does that number actually mean, and why does it matter if it is high?
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Good question. CE stands for "carbon equivalent" and it boils a steel's tendency to crack during welding down to a single number. When you weld a steel, the heat-affected zone (HAZ) right next to the bead is heated above 1300 degC for a moment and then quenched back to room temperature in a few seconds. If that quench produces martensite — a hard, brittle microstructure — and you combine it with the diffusible hydrogen and tensile residual stress that come with welding, the joint can crack hours or even days after the welder has finished. That delayed failure is hydrogen-induced cold cracking, and it's the most feared welding defect.
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It can crack days later? That sounds impossible to inspect. Why do we predict it from the chemistry alone?
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Exactly — the part looks perfect when it leaves the shop and only fails in the customer's hands. The thing that decides cracking is "hardenability", i.e. how hard the steel gets when it quenches. Carbon is by far the strongest hardener, but Mn, Cr, Mo, V, Ni, Cu and a few others also push hardenability up to varying degrees. So the idea behind carbon equivalent is "convert every alloy element into an equivalent amount of carbon, and add them up". The most widely used formula is the IIW one: CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. So Mn counts at one sixth, the Cr+Mo+V group at one fifth and Ni+Cu at one fifteenth.
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So the coefficients are the "potency" of each element. Does CE_IIW directly tell us if we need to preheat?
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Yes — the rough ladder goes like this. CE_IIW < 0.40 % is readily weldable, no special procedure. 0.40-0.50 % calls for a little care — low-hydrogen electrodes, slower cooling, decent housekeeping. 0.50-0.60 % means you have to preheat the parent metal to 100-200 degC before striking the arc. Above 0.60 % you need preheat of 200-300 degC plus low-hydrogen consumables and often post-weld heat treatment (PWHT). In bridges, pressure vessels, shipyards, nuclear piping — every WPS in the world starts by reading the CE off the mill certificate and choosing preheat from there.
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Then what is Pcm? The simulator shows both numbers side by side.
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Pcm is the "Cracking Parameter for Material", a formula developed in Japan by Ito and Bessyo specifically for low-carbon high-strength steels like HT780. The expression is Pcm = C + Si/30 + Mn/20 + (Cr+Mo+V)/20 + (Ni+Cu)/60, with much smaller coefficients than CE_IIW. For steels with only 0.10-0.16 % carbon, CE_IIW becomes insensitive to the real cracking tendency, whereas Pcm tracks it well. Pcm > 0.30 % is the accepted threshold for serious cold-cracking risk. A common rule is "use CE_IIW for ordinary structural steel, Pcm for HSLA and quenched-and-tempered grades". If both stay green, you are safe; if either turns red, that weld procedure needs extra controls.
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Got it. Besides preheat, what other levers do we have to stop cold cracking?
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Cold cracking needs the three legs of the stool — hard HAZ, diffusible hydrogen and tensile residual stress — so removing any one of them prevents the failure. Preheat (and post-heat) slows the cooling rate and bakes hydrogen out. Low-hydrogen consumables (well-dried E7016 or E7018 stick electrodes, properly stored basic flux) cut hydrogen at source. Higher heat input also slows cooling but trades against toughness. Reducing joint restraint with smaller groove angles or a sensible welding sequence eases residual stress. CE_IIW and Pcm tell you "how much intervention is needed"; the engineering choice of which lever to pull is the welder's craft.
Frequently Asked Questions
The International Institute of Welding (IIW) formula is CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Carbon counts at full weight; manganese contributes one sixth, the Cr+Mo+V group one fifth and the Ni+Cu group one fifteenth, giving a single "effective carbon" number. As a rule of thumb, CE_IIW < 0.40 % means the steel is readily weldable with no special precautions; 0.40-0.50 % calls for some care; 0.50-0.60 % demands preheat of 100-200 °C; above 0.60 % requires strong preheat, low-hydrogen consumables and often PWHT.
CE_IIW is intended for plain-carbon and low-alloy structural steels with carbon above about 0.18 %, where it correlates well with HAZ hardenability and preheat requirements. The Ito-Bessyo Pcm formula was developed for modern low-carbon (≤ 0.16 % C) high-strength steels such as HT780; it includes silicon and is more sensitive to cold-cracking tendency in this regime. Pcm > 0.30 % is the accepted threshold for high cold-cracking risk and is widely used in shipbuilding and pipeline welding procedure qualification.
Cold cracking (hydrogen-induced cracking) needs three simultaneous conditions: a hard martensitic HAZ, diffusible hydrogen and tensile residual stress. Preheating the parent metal to 100-200 °C slows the post-weld cooling rate of the heat-affected zone, suppressing martensite formation and lowering peak hardness. The longer time above 100 °C also lets diffusible hydrogen escape into the atmosphere before it can pool at hard microstructural sites. Easing two of the three conditions simultaneously cuts the cracking probability dramatically, which is why preheat is the cornerstone of welding procedures for hardenable steels.
A mill test certificate (MTC) for steel made to JIS, EN or ASTM normally includes CE / CEV / CET / Pcm columns alongside the chemical composition table. Welding structural grades such as SM490A, S355J2+N or A572-50 have a CE ceiling specified in the standard, and customers commonly add a tighter CE limit to the purchase specification. When you write a welding procedure (WPS), always read the as-rolled CE from the certificate, not just a textbook value, and use it to set preheat temperature, consumable class and heat input.
Real-world applications
Bridges and steel-frame buildings: Structural grades such as SM400/SM490/SM570 (JIS G 3106), S355J2+N (EN 10025) and A572-50 (ASTM) have an upper bound on CE_IIW written into the standard. For a bridge main girder or a tower main column, the site welding procedure picks heat input and preheat directly from the mill-certificate CE. When CE exceeds about 0.44 %, site practice (e.g. JASS 6 in Japan, AWS D1.5 for bridges) calls for preheat of 50-100 °C, and winter site welding in cold climates tightens that further.
Pressure vessels and process piping: Vessels in thermal and nuclear power plants (SA516, SA537) and high-temperature piping in refineries (Cr-Mo steels SA335 P22, P91) are qualified using both CE_IIW and Pcm. Heavily alloyed grades such as 2.25Cr-1Mo (P22) or 9Cr-1Mo-V (P91) can reach CE of 0.7-0.9 and require strong preheat (150-250 °C), strict interpass control and long PWHT cycles (700-760 °C for several hours).
Shipbuilding and offshore structures: Classification societies (NK, LR, ABS) cap Pcm for TMCP high-strength grades such as YP47-class plate. Cold cracks that escape NDE at the yard can later open as through-thickness fractures at sea, so shipyards manage Pcm around 0.20-0.24 % and police heat input and cooling carefully on thick-plate joints.
High-strength bolts and construction-machinery booms: Welding SCM440 or S45C parts often gives CE_IIW of 0.6-0.8, and without preheat the HAZ will almost certainly crack. Construction-machinery and axle manufacturers tie the CE value of every incoming plate to their preheating oven temperature, holding time and post-weld blanket cooling — the whole production line is structured around CE.
Common misconceptions and pitfalls
The most common mistake is "my CE is under the limit, so I don't need preheat". CE_IIW only captures one of the three legs of cold cracking — HAZ hardenability. If the electrodes were not properly dried, the plate is so thick that the natural cooling rate is faster than expected, or the joint is highly restrained so residual stresses are high, a steel with low CE can still crack. In practice you should look at CE together with plate thickness, joint geometry, hydrogen level and restraint, and use complementary tools like Yurioka's t8/5 cooling-time analysis, the CET index or the Pw cold-cracking parameter.
Next, "every carbon-equivalent formula gives the same answer". Several formulas are in active use — IIW CE, AWS CE, Yurioka's CEN, the German DIN CET, and Pcm — and they were calibrated for different grades and carbon ranges. A low-carbon high-strength steel can pass CE_IIW comfortably while failing Pcm, or vice versa. Always check which formula is mandated by the applicable code (AWS D1.1, EN 1011-2, JIS, ASME, class rules), and use that one. This simulator reports both IIW and Pcm side by side as a screening tool, not as a substitute for the contractually required formula.
Finally, "chemistry alone is enough". The real cold-cracking susceptibility also depends on micro-segregation, inclusions, plate thickness (mass effect) and any prior heat treatment. Two plates with the same nominal CE can have very different HAZ hardness if one is as-rolled and the other is normalised. For critical structures, use the CE-based preheat as a starting point and confirm it with an actual welding procedure qualification (PQR), HAZ hardness measurements (Vickers HV350 is a common upper limit) and, where it matters, a y-groove cold-cracking test (JIS Z 3158, ISO 17642-2). The calculation is the entry point, not the finish line.
How to Use
Enter carbon content (%) in the carbonPctNum field; typical structural steels range 0.15–0.45%
Input manganese (%), chromium+molybdenum+vanadium sum (%), and nickel+copper sum (%) using the corresponding numeric or range sliders
The simulator calculates IIW carbon equivalent (CE_IIW) using the formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, then derives Pcm cracking parameter and recommends preheat temperature based on plate thickness and cooling rate assumptions
Worked Example
High-strength structural steel plate (ASTM A514): C=0.28%, Mn=1.10%, Cr+Mo+V=0.80%, Ni+Cu=0.05%, Si=0.30%. CE_IIW = 0.28 + 1.10/6 + 0.80/5 + 0.05/15 = 0.28 + 0.183 + 0.16 + 0.003 = 0.626. Pcm ≈ 0.42. Carbon rating: 44.7% of CE from carbon content. Weldability: Moderate—requires 120–150°C preheat. Cold-cracking risk: Medium if cooling rate exceeds 10°C/s in 800–500°C range.
Practical Notes
CE_IIW >0.45 triggers preheat requirements; >0.55 demands controlled HEAT (hydrogen, energy input, ambient, thickness) and PWHT (post-weld heat treatment)
Pcm incorporates hardenability factors (Ni, Cu, Mo) for better prediction of martensitic hardness than CE alone—critical for thick sections above 25 mm
Manganese dominates CE in mild steels; reducing it from 1.5% to 1.0% cuts CE by ~0.083 and improves weldability without sacrificing strength
Verify input units match your material certificate (wt% vs. ppm); IIW formula assumes weight percent only