Phosphorus Segregation Limit
Critical concentration threshold of phosphorus at phase boundaries above which intercrystalline fractures occur at low temperatures. Compliance with this limit is mandatory for highly stressed steels.
Phosphorus is undesirable in steel because it tends to segregate, i.e., it enriches at the grain boundaries. If you exceed the limit at which phosphorus can no longer be bound in the structure, the material becomes brittle and intergranular cracks can occur, especially in cold conditions or under high stress. For critical components, you have to watch the phosphorus content. When welding, you normally don't introduce extra phosphorus, but the base material must be clean. Check the mill certificates and stick to the specified analysis, then it'll be fine. If in doubt, have the material tested before processing.