Effects of alloying elements in Stainless Steels

What are all the effects of alloying elements in stainless steels?

The stainless steel properties vary according to the alloying elements and their ratios. The exact properties of the stainless steel grade depend on the mutual effect of alloying elements, heat treatments and to some extent, the impurities too. The following parts enumerate the alloying elements and the reasons they are present.

Carbon (C)

Carbon and iron are alloyed together to form steel. This process boosts the strength and hardness of iron. Heat treatment is not adequate to strengthen and harden pure iron, but when carbon is added, a wide range of strength and hardness is realized.

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. Moreover, High carbon content is not preferred in Ferritic and Austenitic stainless steels, specifically for welding purposes, due to the risk of carbide precipitation.

Manganese (Mn)

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.

The addition of manganese to steel improves hot working properties and boosts toughness, strength, and hardenability. Just like nickel, manganese is an Austenite forming element and has been traditionally used as a replacement for nickel in the AISI200 range of Austenitic stainless steels, for example, AISI 202 as a replacement for AISI 304.

Chromium (Cr)

This is the most important alloying element, and it gives stainless steels their primary corrosion resistance. Chromium is combined with steel to improve its resistance to oxidation.

Stainless steels have at least 10.5% chromium (usually 11 or 12%), which imparts a considerable level of corrosion resistance, compared to steels with a relatively lower percentage of chromium.

The resistance to corrosion is attributed to the formation of a passive, self-repairing layer of chromium oxide on the stainless steel surface. Chromium also increases the resistance to oxidation at high temperatures and promotes a ferritic microstructure.

Nickel (Ni)

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.

Large amounts of nickel - more than 8% - is added to high chromium stainless steels to produce the most crucial group of steels that are resistant to both heat and corrosion.

These include the Austenitic stainless steels that are characterized by 18-8 (304/1.4301), where nickel’s tendency to form austenite contributes to high strength and excellent toughness or impact strength, at both low and high temperatures. Nickel also significantly improves resistance to corrosion and oxidation.

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 (Mo)

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. When molybdenum mixed with chromium-nickel austenitic steels, it enhances resistance to crevice and pitting corrosion, particularly in sulphur and chlorides-containing environments.

Nitrogen (N)

Similar to nickel, nitrogen is an Austenite forming element and increases the Austenite stability of stainless steels. When nitrogen is mixed with stainless steels, yield strength is considerably enhanced along with increased resistance to pitting corrosion.

Nitrogen 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.

Copper (Cu)

In stainless steel, copper is often present as a residual element. This element is added to several alloys to create precipitation hardening characteristics or to improve corrosion resistance, predominantly in sulphuric acid and seawater conditions.

Adding of copper 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.

Titanium (Ti)

Titanium is often added to stabilize carbide, particularly when the material has to be welded. Titanium is a strong ferrite and carbide former, lowering the effective carbon content and promoting a ferritic structure in two ways.

In austenitic steels, titanium merges with carbon to form titanium carbides that are relatively stable and cannot be easily dissolved in steel, with increased carbon content it is added to increase the resistance to intergranular corrosion (stabilized grades). Still, 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.

When around 0.25 / 0.60% titanium is added, it causes the carbon to merge with titanium as opposed to chromium, avoiding a tie-up of corrosion-resistant chromium as intergranular carbides and the associated loss of corrosion resistance at the grain boundaries.

Phosphorus (P)

In order to improve machinability, phosphorus is often added with sulphur. While the presence of phosphorus in Austenitic stainless steels boosts strength, it has a detrimental effect on corrosion resistance and increases the material’s tendency to break during welding.

Sulphur (S)

Sulphur improves machinability when it is added in small quantities, but just like phosphorous, it has a negative effect on corrosion resistance and the subsequent weldability. Sulphur is added to certain stainless steels to increase their machinability.

Lower levels of sulphur can be added to decrease work hardening for improved formability. Slightly increased sulphur content also improves the weldability of steel.

Niobium (Nb)

Niobium is a strong ferrite and carbide former. Like titanium, it promotes a ferritic structure. Carbon stabilization is achieved by adding niobium to steel and performs in the same manner as titanium.

Besides, niobium strengthens alloys and steels for increased temperature service. 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.

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.

Also, silicon is typically employed as a deoxidizing (killing) agent in the steel melting process, and a small amount of silicon is used in most steels.

Cobalt (Co)

Cobalt is used in martensitic steels, where it increases hardness and tempering resistance, especially at higher temperatures. Some amount of the remaining cobalt will be present in the nickel used to make Austenitic stainless steels.

Calcium (Ca)

Calcium is added in small amounts to enhance machinability, without having any detrimental effect on other properties induced by selenium, phosphorus and sulphur.

Vanadium (V)

Vanadium forms carbides and nitrides at lower temperatures promote ferrite in the microstructure and increase 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 (W)

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

Zirconium (Zr)

Zirconium increases strength and limits grains sizes. Strength can be notably increased at very low temperatures (below freezing). Steel’s that include zirconium up to about 0.1% content will have smaller grains sizes and resist fracture.

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.

Lead (L)

Lead is almost insoluble in liquid or solid steel. Lead is sometimes added to carbon steels via mechanical dispersion during pouring to improve machinability.