Spawalność stali S355 oraz S235 i S275

Weldability of S355, S275, and S235 Steel – Engineering Guidelines for WOST Profiles

High-strength structural steel requires a carefully planned joining process. Above all, the weldability of S355 steel as well as S235JR and S275JR grades depends on the carbon equivalent value (CEV). Therefore, for hot-rolled profiles with a wall thickness up to 20 mm, these grades are welded without preheating. Furthermore, proper selection of the arc heat input protects the heat-affected zone (HAZ) from the formation of brittle martensitic structures.

1. Weldability of S355 Steel vs. Carbon Equivalent (CEV)

Evaluating the weldability of steel profiles is based on analyzing their chemical composition. Therefore, the key parameter is the carbon equivalent CEV (Carbon Equivalent Value), calculated according to the International Institute of Welding (IIW) formula:

CEV = C + (Mn / 6) + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

For this reason, the EN 10025-2 standard, available through the Polish Committee for Standardization (PKN), precisely defines maximum CEV limit values. For hot-rolled sections from the WOST S.A. product range, such as equal angles or UPN channels, the allowable limits are:

S235JR: CEV ≤ 0.35% (for wall thickness t ≤ 20 mm)
S275JR: CEV ≤ 0.40% (for wall thickness t ≤ 20 mm)
S355J2: CEV ≤ 0.45% (for wall thickness t ≤ 20 mm)

As a result, maintaining CEV below 0.45% prevents the formation of hard martensite. Furthermore, using clean and dry filler metals effectively protects the weld from hydrogen diffusion and cold cracking.

2. Heat Input Control and the Weldability of S355 Steel

During arc welding of structural sections, the input heat causes microstructural changes in the steel. For this reason, the Heat-Affected Zone (HAZ) is divided into three main regions:

Coarse-grained zone: Heats up above 1100°C, causing austenite grain growth. Consequently, rapid cooling in this region promotes hardening.
Fine-grained zone: Heats up within the range of 900–1000°C. As a result, it exhibits very high impact toughness.
Partially transformed (recrystallized) zone.

Therefore, to maintain high mechanical properties of the joint, the weldability of S355 steel requires continuous control of the cooling time t_8/5. This value is controlled via the arc heat input E:

E = (k × U × I) / (v × 1000) [kJ/mm]

In the formula above, U represents arc voltage in volts [V], I is welding current in amperes [A], v is welding speed in [mm/s], and k is thermal efficiency factor (for MAG 135, k = 0.8). As a result, the recommended heat input for S355J2 profiles ranges from 1.0 to 2.5 kJ/mm.

3. Selection of Filler Metals and Shielding Gases: Weldability of S355 Steel in MAG and MMA Processes

An engineer preparing a Welding Procedure Specification (WPS) must properly select consumables. First and foremost, the filler metal must match the properties of the base material. This applies to products such as hot-rolled T-sections and special profiles.

Filler metals and shielding gases:

For S235 and S275 steels: Solid wire G3Si1 is used (acc. to EN ISO 14341-A).
For S355J2 steel: Solid wire G4Si1 or G3Si1 with higher metallurgical purity is recommended.
Additionally, for MAG welding, M21 class gas mixtures (Ar + 15–25% CO₂) perform best. As a result, the arc remains stable with minimal spatter.

4. Edge Preparation and the Weldability of S355 Profiles

The beveling method for joint edges depends mainly on the profile wall thickness t:

For t ≤ 4 mm: Welds are prepared square-edge (without beveling), with a root gap of 1.0–1.5 mm.
In turn, for t > 4 mm: Edges are V-beveled at an angle of α = 60°, with a root face b = 1–2 mm.

Above all, before assembly, the exact weight of components can be calculated using the WOST steel weight calculator. At the same time, dimensions and tolerances are available in the official product catalog.

Preheating:

Furthermore, profiles up to 20 mm thick do not require preheating provided the ambient temperature is above +5°C. However, at lower temperatures, the joint area must be preheated to 100–150°C.

5. Summary: Weldability of S355 Steel in WOST Practice

In conclusion, the weldability of S355 steel as well as S235JR and S275JR grades in WOST S.A. products is at an exceptionally high level. As a result, following these engineering guidelines guarantees welds that meet the requirements for EXC1–EXC3 execution classes under standards EN 10025-2, EN 1090-2, and EN ISO 15614-1.