The effects of alloying elements on stainless steel
The different alloying elements have specific effects on the properties of a 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 certain steel grade. It should be noted that the effect of the alloying elements differs to some extent between the different types of stainless steel.
This is the most important alloying element and it gives stainless steels their basic corrosion resistance. All stainless steels have a Cr content of at least 10.5% and the corrosion resistance increases the higher chromium content. Chromium also increases the resistance to oxidation at high temperatures and promotes a ferritic microstructure.
The main reason for adding nickel is to promote an austenitic microstructure. Nickel generally increases ductility and toughness. 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. In martensitic grades adding nickel, combined with reducing carbon content, improves weldability.
Molybdenum significantly increases the resistance to both uniform and localized corrosion. It slightly increases mechanical strength and strongly 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 it increases the hardness at higher tempering temperatures due to its effect on carbide precipitation.
Copper enhances corrosion resistance to certain acids and promotes an austenitic microstructure. It can also be added to decrease work hardening in grades designed for improved machinability. It may also be added to improve formability.
Manganese is generally used to improve hot ductility. Its effect on the ferrite/austenite balance varies 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.
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.
Carbon is a strong austenite former that also significantly increases mechanical strength. In ferritic grades carbon strongly reduces both toughness and corrosion resistance. In martensitic grades carbon increases hardness and strength, but decrease toughness.
Nitrogen is a very strong austenite former that also significantly increases mechanical strength. It also increases resistance to localized corrosion, especially in combination with molybdenum. In ferritic stainless steels nitrogen strongly reduces toughness and corrosion resistance. In martensitic grades nitrogen increases both hardness and strength but reduces toughness.
Titanium is a strong ferrite and carbide former, lowering the effective carbon content and promoting a ferritic structure in two ways. In austenitic steels with increased carbon content it is added to increase the resistance to intergranular corrosion (stabilized grades), but it also increases mechanical properties at high temperatures. In ferritic grades titanium is added to improve toughness, formability, and corrosion resistance. In martensitic steels titanium lowers the martensite hardness by combining with carbon and increases tempering resistance. In precipitation hardening steels, titanium is used to form the intermetallic compounds that are used to increase strength.
Niobium is a strong ferrite and carbide former. Like titanium, it promotes a ferritic structure. In austenitic steels it is added to improve the resistance to intergranular corrosion (stabilized grades), but it also enhances mechanical properties at high temperatures. In ferritic grades niobium and/or titanium is 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 it is designated Columbium (Cb).
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.
Cobalt is used in martensitic steels, where it increases hardness and tempering resistance, especially at higher temperatures.
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.
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.
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.
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.