Alloying elements in stainless steel
Alloying elements in stainless steel
When it comes to stainless steel, there are many grades to choose from. Depending on the alloying elements added, properties can vary considerably. It all comes down to choosing the most appropriate cost-effective option based on the requirements of the application.Let's see what alloying elements can be added and how they affect the final product.
Chromium
Chromium is the decisive alloying element in stainless steel. It gives steel its "stainless" properties. The passivation layer of chromium oxide together with the surface protection layer also prevents the diffusion of oxygen into the metal, thereby protecting the internal structure of the metal from corrosion. The chromium oxide ions are also similar in size to the steel molecules, resulting in a strong bond between the two. This allows the oxide ions to attach firmly to the surface under normal operating conditions. Steel needs to be at least 10.5% to be "stainless". However, to increase corrosion resistance, it is common to add more chromium. Chromium also acts as a ferrite stabilizer, leading to the formation of a ferrite microstructure in the alloy.
Nickel
Nickel is added to further improve corrosion resistance. It is also an austenite stabilizer, promoting the formation of austenite. An addition of 8-9% nickel results in a fully austenitic structure which provides excellent weldability. Further increasing the nickel percentage leads to better machinability and corrosion resistance.
Copper
Copper also acts as an austenite stabilizer, improving corrosion resistance and work hardening properties. It is added to produce stainless steel products suitable for the cold working conditions required for screws and nails.
siliconThe addition of silicon increases the resistance of stainless steel to high concentrations of nitric and sulfuric acids. It also promotes the formation of ferrite and makes the metal resistant to oxidation.
Nitrogen
Nitrogen is an austenite stabilizer which increases strength and resistance to localized corrosion. Local corrosion refers to pitting corrosion, crevice corrosion, intergranular corrosion and other phenomena.
Molybdenum
Molybdenum and tungsten improve general and local corrosion resistance. The former are ferrite stabilizers, so when used in austenitic alloys, must be balanced with austenite stabilizers to maintain the austenitic composition. Molybdenum also improves high temperature strength when added to martensitic stainless steels. The addition of tungsten and molybdenum also improves the above properties.
Manganese
Manganese increases the strength, toughness and hardenability of stainless steel. Adding manganese helps the metal perform better during thermal processing. Manganese also promotes the dissolution of nitrogen in stainless steel, so manganese can be added to replace nickel in stainless steel with nitrogen.
