The effects of alloying elements

Different alloying elements have particular effects on the properties of stainless steel. It is the combined effect of all the alloying elements, heat treatment, and, to some extent, impurities that determine the property profile of a specific steel grade. It should be noted that the effect of the alloying elements differs to some extent between the different types of stainless steel.  


Aluminum (Al)

If added in substantial amounts aluminum improves oxidation resistance and is used in certain heat-resistant grades for this purpose. In precipitation hardening steels, aluminum is used to form the intermetallic compounds that increase the strength in the aged condition.


Carbon (C)

Carbon is a powerful austenite former that also significantly increases mechanical strength. In ferritic grades, carbon greatly reduces both toughness and corrosion resistance. In martensitic grades, carbon increases hardness and strength but decreases toughness.


Cerium (Ce)

Cerium is one of the rare earth metals (REM) and is added in small amounts to certain heat-resistant grades to increase resistance to oxidation at high temperatures.


Chromium (Cr)

Chromium is the most important alloying element since it gives stainless steel its general corrosion resistance. All stainless steels have a Cr content of at least 10.5%. Additionally, the corrosion resistance increases the higher chromium content. Chromium also increases the resistance to oxidation at high temperatures and promotes a ferritic microstructure.

 

Cobalt (Co)

Cobalt is used in martensitic steels, where it increases hardness and tempering resistance, especially at higher temperatures.


Copper (Cu)

Copper improves corrosion resistance to certain acids and supports an austenitic microstructure. It can also be added to decrease work hardening in grades designed for improved machinability. Additionally, it can also be added to improve formability.


Molybdenum (Mo)

olybdenum significantly increases the resistance to both uniform and localized corrosion. It slightly increases mechanical strength and highly promotes a ferritic microstructure. However, molybdenum also enhances the risk for the formation of secondary phases in ferritic, duplex, and austenitic steels. In martensitic steels, molybdenum increases the hardness at higher tempering temperatures due to its effect on carbide precipitation.


Manganese (Mn)

Manganese is generally used to improve hot ductility. Its effect on the ferrite/austenite balance changes with temperature: at low-temperature, manganese is an austenite stabilizer, but at high temperatures, it will stabilize ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in duplex and austenitic stainless steels. Manganese, as an austenite former, can also replace some of the nickel in stainless steel.


Nickel (Ni)

Nickel generally increases ductility and toughness. The main reason for adding nickel is to promote an austenitic microstructure. It also reduces the corrosion rate in the active state and is therefore advantageous in acidic environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase strength. Adding nickel in martensitic grades, combined with reducing carbon content, improves weldability.


Niobium (Nb)

Niobium is a strong ferrite and carbide former and promotes a ferritic structure, just like titanium. In austenitic steels, niobium is added to improve the resistance to intergranular corrosion (stabilized grades). Additionally, it enhances mechanical properties at high temperatures. In ferritic grades, niobium and/or titanium is/are sometimes added to improve toughness and to minimize the risk for intergranular corrosion. In martensitic steels, niobium lowers hardness and increases tempering resistance. In the US, niobium identifies as Columbium (Cb).


Nitrogen (N)

Nitrogen is a very powerful austenite former that also significantly improves mechanical strength. It also enhances resistance to localized corrosion, especially in combination with molybdenum. In ferritic stainless steel, nitrogen heavily reduces toughness and corrosion resistance. In martensitic grades, nitrogen increases both hardness and strength but reduces toughness.


Silicon (Si)

Silicon increases resistance to oxidation, both at high temperatures and in strongly oxidizing solutions at lower temperatures. It promotes a ferritic microstructure and increases strength.

 

Sulfur (S)

Sulfur is added to certain stainless steels to increase their machinability. At the levels present in these grades, sulfur slightly reduces corrosion resistance, ductility, weldability, and formability. At Outokumpu the trademark PRODEC (PRODuction EConomy) is used for some grades with balanced sulfur levels for improved machinability. Lower levels of sulfur can be added to decrease work hardening for improved formability. Slightly increased sulfur content also improves the weldability of steel.


Titanium (Ti)

Titanium is a strong ferrite and carbide former that lowers the effective carbon content and promotes a ferritic structure in two ways. By adding Titanium, the resistance to intergranular corrosion (stabilized grades) increases in austenitic steels, as well as carbon content and mechanical properties at high temperatures are improved. In ferritic grades, titanium is added to improve toughness, formability, and corrosion resistance. In martensitic steels, titanium lowers the martensite hardness in combination with carbon and increases temper resistance. In precipitation hardening steels, titanium is used to form the intermetallic compounds that are used to increase strength.


Tungsten (W)

Tungsten is present as an impurity in most stainless steels, although it is added to some special grades, for example the superduplex grade 4501, to improve pitting corrosion resistance.

 

Vanadium (V)

Vanadium forms carbides and nitrides at lower temperatures, promotes ferrite in the microstructure, and increases toughness. It increases the hardness of martensitic steels due to its effect on the type of carbide present. It also increases tempering resistance. It is only used in stainless steels that can be hardened.

 

Glossary

Toughness
The ability to absorb energy in the plastic range.

Active
(1) A state of a metal that is corroding without significant influence of reaction product.
(2) A lower or more negative electrode potential.