Phosphorus segregation coefficient
The phosphorus segregation coefficient is the ratio of phosphorus concentration in the solid state to that in the liquid state during solidification. Since it is well below one, phosphorus accumulates heavily in the last solidifying residual melts.
Physical-Chemical Mechanism of Action
The phosphorus segregation coefficient describes how strongly phosphorus concentrates during solidification from the liquid melt into the solid metal crystals. Since its value is clearly below one (normally between 0.1 and 0.3), phosphorus highly enriches in the last solidifying residual melts.
Metallurgical Relevance & Risks in Welding
A high phosphorus segregation coefficient can lead to increased phosphorus concentration at the grain boundaries of the weld. This can impair the toughness and resistance of the metal, as phosphorus has an embrittling property. In addition, it can cause porosity and increased susceptibility to hot cracking.
Workshop Practice & Action Guidelines
To minimize the negative effects of the phosphorus segregation coefficient, low phosphorus content should be preferred when selecting welding consumables. The melting time and temperature should be set so that cooling is uniform and no rapid solidification occurs. For steels that are difficult to weld, the use of fillers with a lower phosphorus segregation coefficient can be considered.