How to calculate the carbon equivalent of C235–C390 steel and evaluate welding conditions
The carbon equivalent of construction steel is calculated from the actual chemical composition of the melt in order to pre-evaluate its weldability and determine whether additional technological work is required for welded joints. The Eq value itself does not replace the welding technology: the result must be considered together with the thickness of the metal, the stiffness of the assembly, the content of diffusive hydrogen, the temperature of the metal and the heat input.
For construction rolled products of strength classes C235–C390, the initial data is taken from the quality document for a specific smelting. Using an average grade composition or a reference value reduces the meaning of the calculation: two batches of the same strength class may have different actual chemical composition and, consequently, different carbon equivalents.
What does the carbon equivalent show?
Carbon increases the strength and hardenability of steel, but at the same time increases the likelihood of the formation of solid structures in the zone of thermal influence. Manganese, chromium, molybdenum, vanadium and a number of other alloying elements also affect hardenability. The carbon equivalent formula reduces their effect to a conditional carbon content and gives a single calculated indicator.
The higher the EQ, the more carefully it is necessary to assess the risk of cold cracks. This risk is formed by the simultaneous action of three groups of factors:
- sensitive structure — high cooling rate promotes the formation of solid hardening components;
- Hydrogen comes from wet electrodes, dirt, rust, condensate, and protective materials.;
- tensile stresses occur due to the shrinkage of the seam, the high rigidity of the assembly, the welding sequence and the external fastening of the parts.
The carbon equivalent characterizes mainly the metallurgical component of the risk. It does not show the actual hydrogen content, the degree of bonding, or the actual thermal cycle.
Formulas with eq according to GOST 27772-2021
GOST 27772-2021 establishes a formula for calculating the carbon equivalent of rolled products for building steel structures:
With eq = C + Mn/6 + Si/24 + Cr/5 + Ni/40 + Cu/13 + V/14 + P/2,
Where C, Mn, Si, Cr, Ni, Cu, V and P are the actual mass fractions of the elements indicated in the quality document, %.
By agreement of the manufacturer of rolled products with the customer, the standard allows for an extended version that takes into account niobium and molybdenum.:
With eq = C + Mn/6 + Si/24 + Cr/5 + Ni/40 + Cu/13 + (V + Nb)/14 + Mo/4 + P/2.
You should not use the extended formula arbitrarily. In the order and the quality document, it should be clear that
