Meaning
Accumulation of sulfur atoms at the grain boundaries of metals and alloys during solidification or subsequent thermal processing. This phenomenon of sulfur segregation occurs because sulfur has low solubility in the solid metal lattice, which drives it to migrate to high-energy boundary regions. The presence of sulfur at grain boundaries reduces the cohesive strength of the metal, making it susceptible to cracking.
This migration is particularly problematic in steel and nickel-based alloys used in high-temperature environments.
Intergranular Concentration
Thermodynamic driving forces cause sulfur to concentrate at internal interfaces. In sulfur segregation, the local concentration of sulfur at grain boundaries can be thousands of times higher than the bulk concentration. This localized concentration can occur even in alloys with extremely low nominal sulfur levels.
Mechanical Weakness
Embrittlement occurs when the metal is subjected to tensile stresses at elevated temperatures. The occurrence of sulfur segregation leads to hot shortness, a condition where the alloy loses its ductility at forging temperatures. This loss of ductility causes cracks to form along the grain boundaries during manufacturing or service.
Thermal Action
High temperatures accelerate the diffusion of sulfur toward the grain boundaries. Cooling rates from the annealing temperature also influence the final degree of sulfur segregation. Rapid cooling can suppress segregation by trapping sulfur atoms in the grain interiors, where they do less harm.
