Short answer: Carbon equivalent (CE) converts a steel’s alloy content into a single number that predicts weldability — below about 0.35 the steel welds without preheat, above roughly 0.45 it needs preheat and low-hydrogen consumables, and above 0.55 it risks hydrogen cracking without preheat plus post-weld heat treatment.
At a Glance
What Carbon Equivalent Means
Carbon is not the only element that hardens the heat-affected zone — manganese, chromium, molybdenum, nickel, copper and vanadium all contribute. Carbon equivalent rolls them into one figure that behaves like an effective carbon content, so a fabricator can judge cracking risk from the mill certificate alone (see Mill Test Certificate Guide). A high CE means the zone beside the weld transforms to hard, brittle martensite as it cools, and that structure plus dissolved hydrogen is what cracks.
The most used formula is the IIW expression: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Thin-section and low-carbon steel is often assessed with Ito-Bessyo Pcm instead, which weights carbon more heavily and suits modern Hsla Steel Guide grades.
Reading the Number
Below 0.35 the steel welds freely — typical of Low Carbon Vs High Carbon Steel mild grades, Q235 Vs A36 Steel and most En 10025 Structural Steel S235/S275. Between 0.35 and 0.45 preheat becomes prudent as thickness rises, which covers much of S355 (see S355 Vs S275 Steel) and Astm A36 Vs A572 Steel A572. Between 0.45 and 0.55 — the world of 4140 Vs 4340 Steel et Boron Steel Guide — preheat of 100–200 °C plus low-hydrogen electrodes is mandatory. Above 0.55, treat the joint as crack-prone: preheat, control interpass temperature, and stress relieve afterwards per Stress Relieving Vs Annealing.
Thickness and restraint matter as much as chemistry, because both slow cooling and raise residual stress. Hydrogen is the third leg of the triangle — see Metal Hydrogen Embrittlement Guide for why damp electrodes and oily plate cause delayed cracking days after welding.
Practical Welding Control
Work the problem in order: get the real chemistry from the certificate, compute CE, then set preheat from CE and thickness. Bake or use vacuum-packed low-hydrogen consumables per Welding Consumables Guide, clean the joint of scale and grease, and keep interpass temperature inside the procedure window. Practice for the common families is in How To Weld Carbon Steel et How To Weld Stainless Steel; joining unlike metals is a different problem entirely, covered in Dissimilar Metal Welding Guide.
Verify with Metal Ndt Guide — magnetic-particle or ultrasonic inspection after the weld has cooled at least 48 hours, since hydrogen cracking is delayed. Common flaw signatures are catalogued in Weld Defects Guide.
Get a Quote
Tell HXSCO your requirements and we will respond with a competitive quote and lead time.
Questions fréquemment posées
Carbon equivalent (CE) is a single number that converts a steel’s carbon, manganese, chromium, molybdenum, vanadium, nickel and copper content into an effective carbon value used to predict weldability and hydrogen cracking risk.
The common IIW formula is CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Thin low-carbon steels are often assessed with the Ito-Bessyo Pcm formula instead.
Above roughly 0.45 CE preheat of 100-200 °C plus low-hydrogen consumables is needed; above 0.55 add interpass control and post-weld heat treatment. Below 0.35 usually needs no preheat.
Not directly. CE predicts weldability and hardenability, not strength. Strength is reported as yield and tensile values on the mill certificate.
Yes. HXSCO supplies structural and alloy steel with EN 10204 mill certificates listing full chemistry so you can compute CE before welding.
Source Weldable Steel from HXSCO
HXSCO supplies structural and alloy steel with full certified chemistry so you can verify carbon equivalent before you weld. Send your grade, thickness and quantity for a quote.