Chemical simbol : P
Is usually regarded as a steel parasite as
P produces pronounced primary segregation an solidification of the melt and the
possibility of secondary segregation in solid state due to the pronounced
restriction of the gamma phase.
AS the result of the relatively low rate
of diffusion, both in the alpha- and in the gamma crystal segregation which has
occurred can only be corrected with difficulty. As it is hardly possible to
achieve homogeneous 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
determind with certainly.
Even in the smallest quantities, P
increase proneness to temper 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
structure steel with C contents of about 0.1%, P increases strength and
corrosion resistance to atmospheric effect. Cu assists the improvement in
corrosion resistance (rust resistance steel).
In austenitic Cr-Ni steels, additions of P
can cause increases in yield strength and achieve precipitation
effects.
Sulphur
Produces the most pronounced segregation
of all steel accompanying elements.
Iron sulphide, leads to re shortness or
hot shortness, as the low melting point sulphide eutectics surround the grains
in reticular fashion, so that only slight cohesion of the latter occurs and
during hot forming the grain boundary tend to break down. This is further
increased by the action of oxygen.
As sulphur possesses a considerable
affinity for manganese, it is combined in the form of Mn sulphide, as this is
the least dangerous of all existing inclusions, being present distributed in the
point form in the steel. Toughness in transverse direction is reduced
significantly by S.
Sulphur is added intentionally to steels for automatic
machining up to 0.4% as the friction on tool cutting edge reduced by the tool.
In addition, short chips occur when free-cutting steels are
machined.
Sulphur increase susceptibility to welding
crack.