ROLE OF ALLOYING ELEMENTS IN STAINLESS STEEL

CARBON

Carbon is always present in stainless steel. The amount of carbon is the key. In all categories except
martensitic, the level is kept quite low. In martensitic grade the level is deliberately increased to
obtain high strength and hardness. Heat treating by heating to a high temperature, quenching and
then tempering develops the martensitic phase.

Carbon can have an effect on the corrosion resistance. If the carbon is allowed to combine with
the chromium (to form chrome carbides), it may have a detrimental effect on the ability of the
“passive” layer to form. If, in localized areas, the chrome is reduced to below 10.5%, the layer will
not form.

CHROMIUM

Chromium is a highly reactive element and accounts for the “passive” nature of all stainless steels.
The resistance to the chemical effects of corrosion and the typical “rusting” (oxidation) that occurs
with unprotected carbon steel, is the direct result of the presence of chromium. Once the
composition contains at least 10.5% chromium, an adherent and insoluble surface film is
instantaneously formed that prevents the further diffusion of oxygen into the surface and prevents
the oxidation of the iron in the matrix. The higher the chromium level the greater the protection.

NICKEL

Nickel is the essential allying element in the 300 series stainless steel grades. The presence of
nickel results in the formation of an “austenitic” structure that gives these grades their strength,
ductility and toughness, even at cryogenic temperatures. It also makes the material non-magnetic.
While the role of nickel has no direct influence on the development of the “passive” surface layer,
it results in significant improvement in resistance to acid attack, particularly with sulfuric acid.

MOLYBDENUM

The addition of molybdenum to the Cr-Fe-Ni matrix adds resistance to localized pitting attack and
better resistance to crevice corrosion (particularly in Cr-Fe ferritic grades). It helps resist the
detrimental effects of chlorides (316 with 2% moly is preferred over 304 in coastal and de-icing
salt situations). The higher the molybdenum content (there are stainless steels at 6% moly), the
better the resistance to higher chloride levels.
MANGANESE

Generally manganese is added to stainless steels to assist in de-oxidation, during melting, and to
prevent the formation of iron sulfide inclusions which can cause hot cracking problems. It is also
a “austenite” stabilizer and when added in higher levels (from 4 to 15%) replaces some of the
nickel in the 200 series stainless steel grades.

SILICON & COPPER

Small amounts of silicon and copper are usually added to the austenitic stainless steels containing
molybdenum to improve corrosion resistance to sulfuric acid. Silicon also improves oxidation
resistance and is a “ferrite” stabilizer. In “austenitic stainless steels, high silicon contents improves
resistance to oxidation and also prevents carburizing at elevated temperatures (309 and 310 are
examples)

NITROGEN

In “austenitic” and “duplex” stainless steels, nitrogen increases the resistance to localized pitting
attack and inter-granular corrosion. Low carbon “austenitic” grades (designated with an “L” since
they contain less than 0.03% carbon), are suggested for welding operations, since the lower carbon
minimizes the risk of sensitization. The low carbon levels, however, tend to reduce the yield
strength. The addition of nitrogen helps to raise the yield strength levels back to the same level as
standard grades.

NIOBIUM

Niobium additions prevents inter-granular corrosion, particularly in the heat effected zone after
welding. Niobium helps prevent the formation of chrome carbides, that can rob the microstructure
of the required amount of chromium for passivation. In “ferritic” stainless steels the addition of
niobium is an effect way to improve thermal fatigue resistance.

TITANIUM

Titanium is the main element used to stabilize stainless steel before the use of AOD (Argon-
Oxygen Decarburization) vessels. When stainless steel is melted in air, it is difficult to reducing
the carbon levels. 302, the most common grade before AOD’s, was allowed to have a maximum
carbon level of 0.15%). At this high level, something was needed to stabilize the carbon and
titanium was the most common way. Titanium will react with the carbon to form titanium carbides
and prevent the formation of chrome carbides, that could affect the formation of the “passive”
layer. Today all stainless steel are finished in an AOD vessel and the carbons levels are generally
low due to the absence of oxygen. The most common grade today is 304 (with 0.08 max carbon,
although in reality the levels are lower).

SULFUR

Sulfur is generally kept to low levels as it can form sulfide inclusions. It is used to improve
machinability (where these inclusion act as “chip breakers). The addition of sulfur, however, does
reduce the resistance to pitting corrosion.

Source: The stainless Steel Information Center

RESISTANCE TO WEAR
CARBIDE FORMATION
HIGH TEMPERATURE
ALLOYING ELEMENT

MACHINABILITY

RESISTANCE TO
COOLING RATE
IMPACT VALUE

FORGEABILITY
ELONGATION
YIELD POINT

CORROSION
ELASTICITY
STRENGTH
HARDNESS

STABLITY

SCALING
Silicon ↑ ↑ ↑↑ ↓ ↓ ↑↑↑ ↑ ↓ ↓ ↓↓↓ ↓ ↓ ↓ –
Manganese in
perlit.steels ↑ ↑ ↑ • • ↑ • ↓ • ↓↓ ↑ ↓ • –
Manganese in
austenit.steels. ↓↓↓ ↑ ↓ ↑↑↑ – – – ↓↓ – – ↓↓↓ ↓↓↓ ↓↓ –
Chromium ↑↑ ↑↑ ↑↑ ↓ ↓ ↑ ↑ ↓↓↓ ↑↑ ↑ ↓ – ↓↓↓ ↑↑↑
Nickel in
perlit.steels ↑ ↑ ↑ • • – ↑ ↓↓ – ↓↓ ↓ ↓ ↓ –
Nickel in
austennit.steels ↓↓ ↑ ↓ ↑↑↑ ↑↑↑ – ↑↑↑ ↓↓ – – ↓↓↓ ↓↓↓ ↓↓ ↑↑
Aluminum – – – – ↓ – – – – – ↓↓ – ↓↓ –
Tungsten ↑ ↑ ↑ ↓ • – ↑↑↑ ↓↓ ↑↑ ↑↑↑ ↓↓ ↓↓ ↓↓ –
Vanadium ↑ ↑ ↑ • ↑ ↑ ↑↑ ↓↓ ↑↑↑↑ ↑↑ ↑ – ↓ ↑
Cobalt ↑ ↑ ↑ ↓ ↓ – ↑↑ ↑↑ – ↑↑↑ ↓ • ↓ –
Molybdenum ↑ ↑ ↑ ↓ ↑ – ↑↑ ↓↓ ↑↑↑ ↑↑ ↓ ↓ ↑↑ –
Copper ↑ ↑ ↑↑ • • – ↑ – – – ↓↓↓ • • ↑
Sulphur – – – ↓ ↓ – – – – – ↓↓↓ ↑↑↑ – ↓
Phosphorous ↑ ↑ ↑ ↓ ↓↓↓ – – – – – ↓ ↑↑ – –

↑=Increase ↓=Decrease • = constant - = not characteristic or unknown Several arrows =more intensive effect