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Chapter 2: The Alloying Elements of Stainless Steel
Silicon, Si
Normally added as an unwanted pollution from the ceramic melting pots at the steel works. Ferrite
stabilizer and normally present in a concentration less than 1.0 %. In the normal range, Si has no
big effect on the corrosion resistance of the steel, but is useful in high-temperature austenites, such
as 4828 and 4841.
Manganese, Mn
Like Si, Mn is normally present as a pollution element in the steel. Concentration level normally < 2
%. In the “AISI 200” series (i.e. 4372), however, Mn is used as a cheap nickel substitute Mn and may
reach 7.5 %. Improves the hot rolling properties of the steel and increases the mechanical strength
slightly. Mn is an austenite stabilizer with no big effect on the corrosion resistance of the steel apart
from binding sulphur as highly harmful manganese sulfides (MnS).
Sulphur, S
Unwanted pollution and extremely harmful to the corrosion resistance. Normally, S is kept lower
than 0.015 % (0.030 for rods). However, fine machining steel qualities may contain 0.15-0.35 % (i.e.
4305 = AISI 303). S forms manganese sulfides which tend to make the steel short-chipped and thus
reduce tool wear during milling. Consequently, fine machining types are much better than normal
austenitic steel types as regards most types of machining. The downside is that i.e. 4305 is significantly
less corrosion resistant than the normal 4301, and sulphur alloyed steel types are unsuitable
for welding as well as pickling.
Phosphorous, P
Like S, an unwanted pollution. However, slightly less damaging to the corrosion resistance. The
limit of most standards is < 0.045 %. In most cases, however, the content is much lower.
Copper, Cu
0-2 %. Increases the corrosion resistance in acid, anaerobic environments (i.e. sulfuric acid) by
increasing the effect of the cathodic hydrogen evolution, thereby making the acid more oxidizing (=
anodic protection). I.e. 904L (4539) contains 1.2-2.0 % Cu and is particularly adapt for handling sulfuric
acid. Cu increases the mechanical strength moderately.
Titanium / Niobium, Ti / Nb
Important elements and usually present up to 0.80 %. Both Ti and Nb bind the potentially harmful C
and thereby prevent sensitization and intergranular corrosion. The effect of adding Ti and Nb corresponds
to the use of low-carbon steel, and 4541 and 4571 can usually be substituted by 4306/07 and
4404 – and vice versa. It is usually a question of tradition with the Germans preferring the Ti stabilized
steel types while most others prefer low-carbon. Mechanically, Ti and Nb alloyed steel types
are marginally stronger than the corresponding low-carbon steel while the Ti steels are harder to
polish and may give yellowish welds in combination with nitrogen containing gasses. In ferritic steel
types (i.e. 4509, 4521, 4526), Ti and/or Nb stabilize the steel and prevent grain growth, thus making
the steel weldable without damaging the ability to be polished / electro polished.
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