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Important Facts on
Stainless Steel
Nickel (Ni) Melting point 1453° C
With structural steels produces significant increase in notch toughness,
even in the low temperature range, and is therefore alloyed for increasing
toughness in case-hardening, heat-treatable and subzero toughness
steels. All transformation points (A1 – A2) are lowered by Ni; it is not a
carbide former. As result of pronounced extension of the Gamma phase,
Ni in contents of >7% imparts Austenitic structure to chemically resistant
steels down to well below room temperature. Ni on its own only makes the
steel rust resistant, even in high percentages, but in austenitic Cr-Ni
steels results in resistance to the effect of reducing chemicals. Resistance
of these steels in oxidizing substances is achieved by means of Cr. At temperatures
above 600° C, austenitic steels have greater high temperature
strength, as their recrystalization temperature is high. They are practically
unmagnetizable. Thermal and electrical conductivity are significantly reduced.
High Ni contents in precisely defined alloying ranges lead to physical
steels with certain physical properties, low thermal expansion (Invar
types).
Phosphorus (P) Melting point 44° C
Is usually regarded as a steel parasite, as P producers pronounced primary
segretation on solidification of the melt and the possibility of secondary
segretation in solid state due to the pronounced restriction of the gamma
phase. As a result of the relatively low rate of diffusion, both in the alphaand
in the Gamma crystal, segratation which has occured 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 limitof 0.03-
0.05%. The extent of segretation 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 in yield point and achieve precipitation effects.
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