If you asked a metallurgy professor the definition of steel, he might define it as “An alloy of carbon and Iron with the carbon content not less than 2% theoretically; exceeding which would be called Iron.”
And If I asked myself the definition, I’d surely just say the first few words(the only part I’d remember). However, just by understanding the basic definition of steel, we could tell a lot about the type of steels. Since the professor defined steel with its carbon content not exceeding 2%, the below figure illustrates it more clearly.
The above figure shows the percentage of carbon in Iron due to which the type of steel changes. The above classification is done based on practical values. Theoretically, the percentage of carbon extends to 2%(hence the void between High carbon steel and Cast Iron). Also, the reason for the carbon percentage only till 6.67% is because, if the carbon content exceeds it, the material we obtain will not be any use to us.
Basically, Steel is composed of the following elements:-
->Iron(Fe)- Majority of steel is composed of Iron.
->Carbon(C)- Ranges from 0.008% to 2%. Steel properties are because of the function alloy(Fe,C).
->Manganese(Mn)- Almost 1%. It acts as a deoxidiser in steel.
->Silicon(Si)- Almost 0.3%. Its presence increases strength of the material.
->Sulphur(S)- Less than 0.05%.
->Phosphorous(P)- Also less than 0.05%.
Apart from this, other elements like Chromium and Tungsten are added to improve the properties of steel making it an Alloy steel.
Note:- Steel is present in many phases depending on the temp and percentage of carbon by mass. The below picture shows us the different phases of steel. The properties of steel majorly depend on these phases..
( image taken from wikipedia )
It okay even if you don’t understand the above picture as the basic knowledge needed for steel is discussed further.
How is Steel produced?
The flow chart below shows us the different processes the raw materials are subjected to for making steel.
As seen above, Pig Iron is the basic raw material for making Iron or Steel (Iron is the major component of steel remember?). The steel produced from either of these furnaces i.e. Bessemer or Open Hearth (depending on our production cost) is then sent for alloying to improve the steel properties. The types of steels are produced according to the properties required for its use.
The different types of alloying elements and their effects are shown below( based on this, the steels are further classified under alloy steels depending on the percentage of alloying element ).
The above elements are added in specific percentages depending on the type of alloying element we need.
The alloy steels are Cr steel, Mn steel, Mo steel, Ni-Cr steel, Ni-Cr-Va steel etc. And application steels are structural steel, spring steel, stainless steel, tool steel, ball bearing steel etc. The use of the application steels is self explanatory from their name. Different types of steel are invented every year with the help of modern technology however the basic elements like shown above, will always be the main constituents of these steels.
Terms in Heat Treatment of Steel
Heat treatment is done on steel to improve the properties. Just like we add elements to form alloy steels, the same in heat treatment, the steel is subjected to different processes to change its phase( steel is composed of many phases; hence its widely used for many applications. Refer to Iron-Iron Carbide diagram for its phases.)
Annealing– Heating the material to above its critical temperature for a specific period and cooling it. It is done to soften the steel and improve its machinability.
Hardening- A mechanism in which the dislocation slips are prevented. This is done for tool steel to prevent fracture.
Tempering- Performed after hardening. It basically is used to remove the excess hardness.
Surface hardening- As the name implies, it hardens the surface. It involves infusion of additional carbon into the surface (steel is composed of C and Fe. So increasing content of C on surface makes its surface harder).
Carburizing- Making metal harder by increasing the carbon content. Surface hardening is a part of this.
Nitriding- Introduces nitrogen into the surface of steel. In this nitrogen is added in ferrite condition whereas in carburising, carbon is added in the austenite condition of steel.
To conclude, we can say that steel is one of the most widely used materials in the world and research is still being done to expand its applications range.
If you still want to read more on steels (or to understand production of steel completely) , I’d suggest you to check out the following links.