Interesting facts about Phosphorus in steel

PMelting point 44 °CPhosphorus

Is usually regarded as a steel parasite, as P produces pronounced primary segregation on solidification of the melt and the possibility of secondary segregation in solid state due to the pronounced restriction of the γ-phase. As a result of the relatively low rate of diffusion, both in the α- and in the γ- crystal, segregation which has occurred can only be corrected with difficulty. As it is hardly possible to achieve homogeneous distribution of the P, an attempt is made to keep the P content very low and accordingly, with high grade steels, to strive for an upper limit of 0.03-0.05%. The extent of segregation cannot be determined with certainty. Even in the smallest quantities, P increases proneness to temper embrittlement. P embrittlement increases with the rise in C content, with rising hardening temperature, with grain size and with decrease of the ratio of reduction by forging. Embrittlement occurs as cold shortness and sensitivity to impact stress (tendency to brittle fracture). In low alloy structural steels with C contents of about 0.1%, P increases strength and corrosion resistance to atmospheric effects. Cu assists the improvement in corrosion resistance (rust resistant steels). In austenitic Cr-Ni steels, additions of P can cause increases in yield point and achieve precipitation effects.