Sulfur is kept low (≤0.045%) in H-beams to preserve weldability. High sulfur forms iron sulfides, which melt at welding temperatures, causing cracks in welds (hot cracking). This weakens joints, risking structural failure. Low sulfur levels prevent this, ensuring sound welds. For applications requiring extensive welding (e.g., bridge girders), ultra-low sulfur (≤0.030%) grades are used. Sulfur also reduces ductility, making welded areas brittle. Manufacturers control sulfur via desulfurization processes during steelmaking, using fluxes to remove sulfur from molten metal. Weldability tests (e.g., cruciform tests) verify that beams can be welded without defects, ensuring joint strength matches the beam's.