Carbon [C]
Carbon is a non-metallic but important alloying element in all ferrous metals. It is always present in stainless steel and high temperature resistant metallic alloys.
It has very strong austenitizing properties and increases strength. It is found in low proportions (0.005 – 0.03%) in austenitic, ferritic, ferritic – austenitic types. In the martensitic type, it is found at a rate of 0.15 – 1.2%.
The main effect of carbon on corrosion resistance can be defined by the way it is present in the structure. If it combines with chromium to form chromium carbide, corrosion resistance can be compromised by the loss of some chromium from the solid solution in the alloy. This adverse effect of carbon can be caused by cooling too slowly after hot forming or annealing.
The result is an undesirable precipitate of chromium carbide. This reduces the corrosion resistance of the carbide precipitate due to the decrease in chromium content, making it susceptible to localized or intergranular corrosion.
Chromium [Cr]
Chromium is added to steels to increase their oxidation resistance. This resistance increases as the chromium addition increases. Corrosion resistance is due to the chromium-oxide passive layer, which is a self-repairing and protective layer on the surface of stainless steel.
Nickel [Ni]
Nickel has no direct effect on the passive layer in stainless steels but has a beneficial effect, especially in sulfuric acid media.
Austenitic stainless steels (Fe – C – Cr – Ni (Mo), unlike other alloys, show a wide range of mechanical properties. For example, these alloys show excellent ductility and toughness at very low and very high temperatures.
Nickel-based alloys are more resistant to corrosion than iron-based alloys when the passive layer is absent or localized/completely destroyed. For example, pitting corrosion progresses more slowly in nickel-based alloys.
Molybdenum [Mo]
This element, which is added up to 8% in stainless steels, is usually found in the range of 2 – 4%. Even at very low levels, it resists pitting corrosion and crevice corrosion in chlorine-containing environments. Molybdenum reduces the intensity of the oxidizing effect to protect and destroy the passive layer. It significantly increases localized corrosion resistance. It somehow increases the mechanical properties and enhances the ferritic microstructure.
However, molybdenum increases the risk of secondary phase formation in ferritic, duplex and austenitic stainless steels. In martensitic steels, it increases hardness in the tempering process at high temperatures due to its effect on carbide precipitation.
Niobium [Nb]
As long as corrosion resistance in stainless steels is of concern, the addition of Nb reduces the risk of intergranular corrosion in the heat affected zone. To reduce this risk, an appropriate amount of niobium is added depending on the level of carbon and nitrogen (ferritic types). With a fully balanced calculation, the theoretical formula for niobium is defined by %Nb ≥ 0.2 + 5 (%C + %N).
In ferritic stainless steels, the addition of this element is one of the most effective methods of improving thermal fatigue resistance.
Titanium [Ti]
Titanium is a strong ferrite and carbide former. This is effectively achieved by lowering the carbon content and promoting a ferritic structure. It is added to austenitic stainless steels with increased carbon content to improve intergranular corrosion resistance and mechanical properties at elevated temperatures.
In ferritic stainless steels, the addition of titanium improves toughness, formability and corrosion resistance. In martensitic stainless steels, the combination of titanium and carbon reduces hardness and increases tempering resistance. Titanium is the most stabilizing element used in stainless steel.
Manganese [Mn]
Manganese is usually added to stainless steels to improve hot ductility. It is an austenite stabilizer at low temperatures and a ferrite stabilizer at high temperatures. It increases nitrogen solubility and provides high nitrogen content in duplex and austenitic stainless steels.
As an austenite former, manganese can be replaced by some nickel in stainless steel.
