Carbon Equivalent (CEq)
Compute a steel's carbon equivalent by the (simplified) IIW formula, CEq = C + Mn/6 + Cr/5 + Ni/15, weighting the alloying elements' effect relative to carbon on the hardening and cracking tendency. It is the key weldability index: a CEq below 0.40 indicates easily weldable steel; above 0.45–0.50 it requires preheating and care to avoid cold cracking. Enter the carbon, manganese, chromium and nickel contents (%).
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Carbon equivalent (CEq)
How do you predict whether a steel will crack when it is welded? Through the carbon equivalent. The idea is elegant: every alloying element adds to the hardening (and to the cracking risk) in a way equivalent to a certain amount of carbon. The IIW formula weights those contributions: CEq = C + Mn/6 + Cr/5 + Ni/15 (simplified here; the full expression also brings in Mo, V and Cu). The result is a single index of weldability: CEq < 0.40 → readily weldable steel, with no special precautions; 0.40-0.45 → caution; > 0.45-0.50 → calls for preheating, hydrogen control and sometimes post-weld heat treatment to keep away the dreaded cold cracking (hydrogen induced) in the heat affected zone. It is the first number any welding engineer looks up before qualifying a procedure. Enter the C, Mn, Cr and Ni contents.
Related Tools
Welding Preheat Temperature
Estimate the preheat temperature for welding, Tp = 350·√(CE − 0.25), as a function of the steel's carbon equivalent (CE). Preheating reduces the cooling rate, giving hydrogen time to escape and preventing the formation of brittle martensite and cold cracks in the heat-affected zone. Steels with a high CE require more preheating. Enter the steel's carbon equivalent.
Tensile Strength from Brinell Hardness
Estimate a carbon steel's tensile strength (Rm) from the Brinell hardness, Rm ≈ 3.45·HB, in MPa. There is a remarkably robust empirical correlation between hardness and strength in steels, which lets you estimate strength from a hardness test — fast, cheap and almost non-destructive — instead of a tensile test. Useful in inspection and quality control. Enter the Brinell hardness (HB).
Martensite Fraction (Koistinen-Marburger)
Computes the fraction of austenite already transformed into martensite when quenching stops at a given temperature, using the Koistinen-Marburger equation, f = 1 − e^(−0.011·(Ms − Tq)), where Ms is the martensite start temperature and Tq the temperature at which the part stopped cooling. The result is the percentage of martensite formed — whatever is missing from 100 % stays as retained austenite, which is soft, dimensionally unstable and able to transform later in service, distorting the part. Because the exponent is linear in the temperature difference, 63 °C below Ms already converts half the austenite, but 209 °C are needed to reach 90 % and the end of the transformation is asymptotic, never exact — which is precisely why precision parts get a cryogenic treatment after quenching. Enter the steel Ms temperature and the quench stop temperature.
Weld Cooling Time t8/5
Computes how long the heat affected zone takes to cool from 800 °C to 500 °C, the t8/5 parameter of EN 1011-2, from the heat input, the preheat temperature and the joint shape factor. The formula multiplies the term (6700 − 5 × preheat temperature) by the heat input, by the difference between the reciprocals of (500 − T₀) and (800 − T₀), and by the shape factor tabulated in the standard, which is 1.0 for a bead deposited on a plate and drops to about 0.9 for a butt weld and 0.67 for a fillet weld on a T-joint. Austenite decomposes in that range, so t8/5 decides the microstructure of the joint: cooling too fast forms martensite and opens the door to cold cracking, cooling too slowly coarsens the grain and destroys impact toughness, and most structural steels call for something between 5 and 25 seconds. The three-dimensional heat flow equation was adopted, valid when the plate is thick relative to the weld bead; in thin plate the flow is two-dimensional and t8/5 grows with the square of the heat input rather than in proportion to it. Enter the heat input, the preheat temperature and the joint shape factor.
Equivalent Stiffness (Springs in Parallel)
Calculate the equivalent stiffness of two springs in parallel, k_eq = k₁ + k₂, by adding the individual stiffnesses. The result, in the same unit (N/m), is always larger than the largest stiffness — springs in parallel are stiffer, since they share the load under the same displacement and the forces add. This is the case of mounts, isolators and supports placed side by side carrying the same component. Reducing spring assemblies to an equivalent stiffness is the first step to compute a vibrating system's natural frequency. Enter the two stiffnesses.
Mold Fill Time
Calculate the fill time of a casting mold, t = V ÷ Q, dividing the cavity volume V by the metal flow rate Q of the gating system. The result, in seconds, is the time to completely fill the mold with molten metal. It is a critical parameter: filling too slowly lets the metal cool and solidify before filling everything (cold shut, misrun defects), while too fast causes turbulence, gas entrapment, mold erosion and inclusions. The optimal time depends on the part's weight and thickness and the metal. Sizing the gating system for the right time is central to casting design. Enter the cavity volume and the flow rate.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.