Segregation band
Banded concentration of alloying elements in the microstructure caused by forming cast segregations. It causes directional variation in the component's mechanical properties.
Segregation streaks form during steel solidification when accompanying elements such as sulfur, phosphorus, or manganese accumulate between the crystals. Rolling stretches these accumulations into thin bands that behave like internal notches. During welding, the streaks are dangerous because tensile stresses perpendicular to the plate surface can tear them open – this is called lamellar tearing. Particularly thick plates and steels with higher sulfur content are susceptible. Hydrogen also accumulates at the streaks and aggravates the problem. In the workshop, if segregation is suspected, check the mill certificate or perform an ultrasonic test. Weld with low-hydrogen filler materials, keep the heat input moderate, and allow the component to cool slowly. If it gets critical, deposit a soft buttering layer before welding the load-bearing joint.