Elements used in Stainless Steel
Steel is an alloy of iron and carbon, generally with a number of other elements
What is an alloy?
An Alloy is a mix of two or more element at least one of which is a metal. The atoms of each element link to each forming a crystalline structure which become tightly “interwoven” with crystals or atoms of the other element in the alloy.
In a compound, the individual atoms of each element are chemically bonded to atoms of another element by sharing or exchanging one or more electrons.
The individual atoms in an alloy retain their chemical properties while in a compound they lose their individual chemical properties and the compound acquires a new set.
The advantage of combining elements in an alloy is that you can change the “mechanical” properties of the metal. These include such things as its resistance to corrosion, strength, response to heat (or cold), hardness or ductility.
All steel contains carbon
People often use the words iron and steel as though they mean the same thing. They are not!
Steel is an alloy of iron and carbon. It is actually quite difficult to produce pure iron since carbon in the form of coal or coke is used in the production process and gets incorporated into the iron as it is extracted from the ore.
The challenge is to produce steel with the right quantity of carbon to produce the properties we desire.
In most stainless steels the carbon content is less than 0.10%, but the precise control of the amount has a dramatic effect on the properties. Increasing the amount of carbon will make the steel harder and improve its strength at high temperature. Decreasing the amount improves its ductility and makes it easier to form. While the greater strength at high temperatures may be desirable, the extra carbon makes the steel more prone to “carbide precipitation.” which can result in stress corrosion in the presence of certain corrosive chemicals – particularly salt solutions.
The problem is that achieving a desired property may have a detrimental effect on other desired properties. Fortunately, the adverse effects can often be negated by the addition of other elements.
Manganese is essential in the production of steel; it acts as a mild deoxidising agent to remove both oxygen and sulphur from the molten ore.
However, the manganese remaining after the production of the steel confers some useful properties and, for specific applications, essential properties. While sulphur is generally regarded as an impurity, in combination with manganese in the right proportions it improves machinability by forming globules of manganese sulphide.
Manganese assists the distribution of carbon through the steel. In addition, by combining with sulphur in the steel it inhibits the formation of Iron Sulphide during welding which tends to make the weld brittle.
Chromium is the key component in stainless steel. To be described as stainless the steel must contain at least 10.5% chromium.
Chromium is highly reactive with oxygen and in contact with the air almost immediately forms a microscopically thin but hard layer of chromium oxide. It is so thin that it is effectively transparent but prevents the penetration of more oxygen thereby preventing the oxidation of the underlying iron.
This layer is impenetrable by many other ordinarily corrosive chemicals.
The chromium oxide layer can be polished to produce attractive surface finishes and improve the resistance to corrosion by making it more difficult for potentially corrosive materials to adhere to the surface.
While the surface it hard it is still possible to scratch it. Fortunately, in the presence of oxygen, the surface will “heal” itself almost immediately. However, if there is no oxygen available, it cannot repair itself. Consequently, if it is to be used in an environment where there is a danger of scratching, additional elements are required to ensure suitable corrosion resistance.
As well as affecting the surface of the steel chromium also increases yield strength and hardenability of steel.
Silicon is used in the production of steel to help to remove bubbles of oxygen from the molten steel. Silicon improves the strength of steel and increases its resistance to oxidation at high temperatures.
It may be added to steel containing molybdenum to improve corrosion resistance to sulphuric acid.
Also it can help to prevent stress corrosion cracking at high temperatures.
Nickel is a another important component in stainless steels.
It helps to increase ductility and toughness, even at high strength levels and enables these properties to be retained even at cryogenic temperatures. It increases the resistance to corrosion of the particularly where the passive layer of chromium oxide may be damaged.
This is particularly beneficial in conditions where the steel may be subject to abrasion but has no exposure to oxygen.
One of the problems of nickel is that it suffers from major price volatility which impacts on the price of the resulting steel.
Molybdenum increases hardenability and strength at elevated temperatures but its major benefit is the increase in corrosion resistance it provides. It forms carbides very easily and as a consequence tends to reduce the formation of chromium carbide. This spares the chromium in conditions where carbide formation is likely and preserves the passive chromium oxide layer. It also forms a smaller grain size than chromium carbide. This increases the resistance of the steel to stress corrosion cracking.
An additional benefit is to increase creep strength. When a metal is subject to a continuing stress below yield strength it will gradually take up the shape of the bend being induced and will not spring back – this is known as creep. Creep can eventually lead to burst rupture.
Titanium performs a variety of functions. It helps to keep “grain” size small thereby reducing the danger of intergranular corrosion and improving machining qualities. It transforms inclusions to globular form (from elongated) which improves strength, at higher temperatures and improves machining. It also increases toughness and ductility.
It is particularly useful in steels that are to be used in the chromium carbide temperature range between 425oC and 850oC since it readily forms carbides itself and as a consequence leaves the chromium available for the formation and repair of the passive layer.
In addition, it resists the formation of austenite in steel with a high chromium content and helps to make martensitic steel less hard or hardenable.