Which Steels Are Easier to Weld? Carbon Equivalent (CEV)

The weldability of steels is one of the most critical parameters in metallurgy that determines the structural performance of a material during and after manufacturing. It is directly dependent on the amount of carbon and alloying elements contained in the steel and how the material responds to welding chemistry. During welding, the area known as the “Heat Affected Zone” (HAZ) is exposed to high temperatures, melts, and then rapidly cools. When carbon content is high, this region transforms into a very hard, brittle, and crack-prone microstructure called martensite under mechanical stress. To evaluate the combined effect of alloying elements such as manganese, chromium, and molybdenum on this hardening tendency, the Carbon Equivalent (CEV) concept is used. The lower the calculated CEV value, the more resistant the microstructure is to thermal shocks during welding, the easier the material is to weld, and the lower the risk of delayed cold cracking caused by hydrogen and residual stresses. As a result, low-carbon steels with a CEV below 0.35% can be welded safely without preheating, while higher CEV values increase crack sensitivity and make preheating or post-weld stress relief heat treatment necessary.

Why Is Carbon Content So Important in Steel?

When steel is heated and cooled during welding, rapid changes occur in its crystal structure. Carbon is the primary element that directly influences both the speed and depth of these transformations. In high-carbon steels, the heat affected zone (HAZ) can form a very hard and brittle martensitic phase, which may eventually lead to cracking and separation of the welded joint over time.

Low-carbon steels (C ≤ 0.25%) have a much wider processing window. Preheating is usually not required or is very limited. Post-weld heat treatment (PWHT) is also rarely needed.

What Happens as Carbon Content Increases?

• 0.00% – 0.25% C (Low Carbon Steel): Ideal range for welding. Structural steels such as S235, S275, S355 fall into this category and can be welded without special precautions.
• 0.25% – 0.45% C (Medium Carbon Steel): Preheating is recommended. Steels such as C45 and Ck45 are included in this group. Crack risk increases significantly if not properly controlled.
• 0.45% – 0.80% C (High Carbon Steel): Welding becomes difficult. Preheating and PWHT are generally required.
• Above 0.80% C: Welding is extremely difficult and not recommended for most industrial applications.

What Is the CEV Formula and How Is It Calculated?

Carbon Equivalent (CEV) is a standard formula used to express the weldability of steel as a single numerical value.

Each element in this formula affects the hardening tendency of the weld zone at different levels. Alloying elements such as manganese (Mn), chromium (Cr), and molybdenum (Mo) are less influential than carbon individually, but their combined effect is significant.

CEV-Based Weldability Classification

How to Prevent Welding Cracks?

Weld cracks are generally classified into two main categories: hot cracks and cold cracks.

Cold Cracks (Hydrogen-Induced Cracks)

Cold cracks can appear hours or even days after welding.

• Susceptible microstructure (martensite)
• Dissolved hydrogen
• Tensile stresses

Applicable Preventive Measures

• Using low-hydrogen electrodes
• Drying electrodes properly
• Cleaning surfaces
• Applying preheating
• Ensuring slow cooling

Hot Cracks (Solidification Cracks)

Hot cracks occur during the solidification of the weld pool.

• Using clean steels with low sulfur (S) and phosphorus (P)
• Using high-manganese filler metals
• Avoiding narrow weld beads
• Optimizing welding speed

When Is Preheating Required for Steel Grades?

Preheating reduces the cooling rate by bringing the weld area to a specific temperature.

• S420 – S690: 100–200°C preheating recommended.
• Hardened steels: 50–150°C preheating required.
• C40, C45, C60: 150–250°C preheating recommended.
• Tool steels: 200–400°C preheating required.

When Is Post Weld Heat Treatment (PWHT) Required?

PWHT is applied to reduce residual stresses and lower hardness in the weld zone.

• Pressure vessels
• Thick welded sections
• High-hardness steels
• Components under dynamic loads
• Critical safety parts

Typical PWHT Parameters

• 580–650°C temperature
• 1 hour per 25 mm thickness
• Slow and controlled cooling

Comparison of Popular Steel Grades in Terms of Weldability

Frequently Asked Questions