The Alloying Elements of Stainless Steel

Chromium, Cr

The main alloying element in all types of stainless steel, and normally present in between 10 and 25 % (by weight). The invisible passive fi lm of the steel mainly consists of chromium oxides, and in general, the corrosion resistance in most types of media (in particular towards pitting and crevice corrosion) increases with increasing Cr content.

 

The formation of the passive layer is at its best during aerobic (oxygenated) conditions. Ferrite stabilizing element, and mechanically, the yield strength increases with increasing Cr, and so does the heat resistance and the resistance towards high-temperature scaling.

 

 

Molybdenum, Mo

Added to the steel from 0.8 to 6.2 %. Even better than Cr in order to form a protective passive layer and even small amounts of Mo improve the corrosion resistance signifi cantly, in particular in acid, anaerobic environments.

 

Works positively against all types of corrosion. Like Cr, Mo is a ferrite stabilizing element which increases the mechanical strength of the steel.

Carbon, C

Harmful element which, apart from the martensites, should be kept as low as possible. Normal limits are < 0.08 % or 0.07 for most types; < 0.03 % for low carbon types. For martensites, C is usually kept in the range of 0.12-1.2 %; the higher C, the harder the steel after hardening. During heat treatment at temperatures in between 500 and 850 ºC (i.e. welding), C binds Cr (= sensitization) which may lead to intergranular corrosion.

 

This is the main reason why most steel types are made as low-carbon (such as EN 1.4306, 4307, 4404 and 4435). C is a very strong austenite stabilizer, and low content of carbon must be compensated by an increase in Ni in order to keep the austenite structure. This is particularly evident in the case of 4435.

 

Nitrogen, N

Present up to 0.5 %. Useful element which enhances the passivity reaction, even in very small amounts. In practice, however, it is very hard to add N to the molten metal. Frequently used in high-end austenites and duplex steel types. N is the only austenite stabilizer which increases the Pitting Resistance Equivalent (PREN) of the steel (factor 16).

 

 

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 sulfi des (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, fi ne machining steel qualities may contain 0.15-0.35 % (i.e. 4305 = AISI 303). S forms manganese sulfi des which tend to make the steel short-chipped and thus reduce tool wear during milling. Consequently, fi ne 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 streng