Sulfide shape control
Sulfide shape control transforms elongated manganese sulfides in steel into globular inclusions by adding calcium or cerium to the melt. This makes the steel tougher in the transverse direction and reduces cracking in the weld zone.
Physical-Chemical Mechanism of Action
In regular steel, sulfur combines with manganese to form manganese sulfide (MnS). During hot rolling, these soft sulfides are stretched into flat platelets or stringers. These inclusions act like internal notches and weaken the steel perpendicular to the rolling direction. Calcium and cerium bond sulfur more strongly than manganese. Adding them to the melt produces hard calcium or cerium sulfides. These sulfides don't deform during rolling and stay as small spheres. The steel becomes more uniform in all directions.
Metallurgical Relevance & Welding Risks
Stringer-type sulfides mainly cause lamellar tearing in the heat-affected zone. These cracks occur when shrinkage stresses pull perpendicular to the plate surface and tear open the flat inclusions. T-joints, fillet welds, and thick plates with high sulfur content are especially at risk. Spherical sulfides stop this crack propagation because they don't create planar weak points. At the same time, calcium and cerium tie up free sulfur. This reduces the risk of hot cracking in the weld. Without sulfide shape control, the steel remains anisotropic and crack-prone.
Shop Practice & Action Guidelines
For heavily loaded welded structures, always use steels with controlled sulfide shape. Ask the supplier for a Z-quality certificate per EN 10164 (for example Z25 or Z35). Check the heat analysis: sulfur below 0.005 %, calcium around 20–40 ppm, Ca/S ratio greater than 1.5. Weld these steels with normal heat input, but avoid extreme overheating of the plate surface. For unknown old steel with stringer sulfides, no preheat reliably prevents lamellar tearing – here, only additional plate thickness testing or replacement with modern fine-grain steel helps.