The effects of the alloying elements in Stainless Steels

The alloying elements each have a specific effect on the properties of the steel. It is the combined effect of all the alloying elements and, to some extent, the impurities that determine the property profile of a certain steel grade. In order to understand why different grades have different compositions a brief overview of the alloying elements and their effects on the structure and properties may be helpful. The effects of the alloying elements on some of the important materials properties will be discussed in more detail in the subsequent sections. It should also be noted that the effect of the alloying elements differs in some aspects between the hardenable and the non-hardenable stainless steels.

 

Chromium (Cr)

This is the most important alloying element in stainless steels. It is this element that gives the stainless steels their basic corrosion resistance. The corrosion resistance increases with increasing chromium content. It also increases the resistance to oxidation at high temperatures. Chromium promotes a ferritic structure.

 

Nickel (Ni)

The main reason for the nickel addition is to promote an austenitic structure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase the strength.

 

Molybdenum (Mo)

Molybdenum substantially increases the resistance to both general and localised corrosion. It increases the mechanical strength somewhat and strongly promotes a ferritic structure. Molybdenum also promotes the formation secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it will increase the hardness at higher tempering temperatures due to its effect on the carbide precipitation.

 

Copper (Cu)

Copper enhances the corrosion resistance in certain acids and promotes an austenitic structure. In precipitation hardening steels copper is used to form the intermetallic compounds that are used to increase the strength.

 

Manganese (Mn)

Manganese is generally used in stainless steels in order to improve hot ductility. Its effect on the ferrite/austenite balance varies with temperature: at low temperature manganese is a austenite stabiliser but at high temperatures it will stabilise ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in austenitic steels.

 

Silicon (Si)

Silicon increases the resistance to oxidation, both at high temperatures and in strongly oxidising solutions at lower temperatures. It promotes a ferritic structure.

 

Carbon (C)

Carbon is a strong austenite former and strongly promotes an austenitic structure. It also substantially increases the mechanical strength. Carbon reduces the resistance to intergranular corrosion. In ferritic stainless steels carbon will strongly reduce both toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels carbon increases hardness and strength. In the martensitic steels an increase in hardness and strength is generally accompanied by a decrease in toughness and in this way carbon reduces the toughness of these steels.

 

Nitrogen (N)

Nitrogen is a very strong austenite former and strongly promotes an austenitic structure. It also substantially increases the mechanical strength. Nitrogen increases the resistance to localised corrosion, especially in combination with molybdenum. In ferritic stainless steels nitrogen will strongly reduce toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels nitrogen increases both hardness and strength but reduces the toughness.

 

Titanium (Ti)

Titanium is a strong ferrite former and a strong carbide former, thus lowering the effective carbon content and promoting a ferritic structure in two ways. In austenitic steels it is added to increase the resistance to intergranular corrosion but it also increases the mechanical properties at high temperatures. In ferritic stainless steels titanium is added to improve toughness and corrosion resistance by lowering the amount of interstitials in solid solution. In martensitic steels titanium lowers the martensite hardness and increases the tempering resistance. In precipitation hardening steels titanium is used to form the intermetallic compounds that are used to increase the strength.

 

Niobium (Nb)

Niobium is both a strong ferrite and carbide former. As titanium it promotes a ferritic structure. In austenitic steels it is added to improve the resistance to intergranular corrosion but it also enhances mechanical properties at high temperatures. In martensitic steels niobium lowers the hardness and increases the tempering resistance. In U.S. it is also referred to as Columbium (Cb).

 

Aluminium (Al)

Aluminium improves oxidation resistance, if added in substantial amounts. It is used in certain heat resistant alloys for this purpose. In precipitation hardening steels aluminium is used to form the intermetallic compounds that increase the strength in the aged condition.

 

Cobalt (Co)

Cobalt only used as an alloying element in martensitic steels where it increases the hardness and tempering resistance, especially at higher temperatures.

 

Vanadium (V)

Vanadium increases the hardness of martensitic steels due to its effect on the type of carbide present. It also increases tempering resistance. Vanadium stabilises ferrite and will, at high contents, promote ferrite in the structure. It is only used in hardenable stainless steels.

 

Sulphur (S)

Sulphur is added to certain stainless steels, the free-machining grades, in order to increase the machinability. At the levels present in these grades sulphur will substantially reduce corrosion resistance, ductility and fabrication properties, such as weldability and formability.

 

Cerium (Ce)

Cerium is one of the rare earth metals (REM) and is added in small amounts to certain heat resistant temperature steels and alloys in order to increase the resistance to oxidation and high temperature corrosion.

 

The effect of the alloying elements on the structure of stainless steels is summarised in the Schaeffler-Delong diagram (Figure below). The diagram is based on the fact that the alloying elements can be divided into ferritestabilisers and austenite-stabilisers. This means that they favour the formation of either ferrite or austenite in the structure. If the austenite-stabilisers ability to promote the formation of austenite is related to that for nickel, and the ferrite-stabilisers likewise compared to chromium, it becomes possible to calculate the total ferrite and austenite stabilising effect of the alloying elements in the steel. This gives the so-called chromium and nickel equivalents in the Schaeffler-Delong diagram:

 

Chromium equivalent = %Cr + 1.5 x %Si + %Mo

Nickel equivalent = %Ni + 30 x (%C + %N) + 0.5 x (%Mn + %Cu + %Co)

 

In this way it is possible to take the combined effect of alloying elements into consideration. The Schaeffler- Delong diagram was originally developed for weld metal, i.e. it describes the structure after melting and rapid cooling but the diagram has been found to give a useful picture of the effect of the alloying elements also for wrought and heat treated material. However, in practice, wrought or heat treated steels with ferrite contents in the range 0-5% according to the diagram contain smaller amounts of ferrite than that predicted by the diagram. It should also be mentioned here that the Schaeffler-Delong diagram is not the only diagram for assessment of ferrite contents and structure of stainless steels. Several different diagrams have been published, all with slightly different equivalents, phase limits or general layout. The effect of some alloying elements has also been the subject of considerable discussion. For example, the austenite-stabilising effect of manganese has later been considered smaller than that predicted in the Schaeffler-Delong diagram. Its effect is also dependent on temperature.