Based on the chemical compositions, Steel can be categorized into four basic groups:
- Carbon Steel
- Alloy Steel
- Stainless Steel
- Tool Steel
CARBON STEEL: Carbon steel is the most utilized steel in the industries and accounts for more than 90% of the total steel production. Based on the carbon content, Carbon steels are further classified into three groups.
- Low Carbon Steel/Mild Steel
- Medium Carbon Steel
- High Carbon steel
Carbon content is given in the table below:
|S.No.||Type of carbon steel||Percentage of Carbon|
|1||Low Carbon Steel/Mild steel||Upto 0.25%|
|2||Medium Carbon Steel||0.25% to 0.60%|
|3||High Carbon steel||0.60% to 1.5%|
ALLOY STEEL: In alloy steel, varying proportions of alloying elements are used, in order to achieve desired (improved) properties such as weldability, ductility, machinability, strength, hardenability and corrosion resistance etc. Some of the most used alloying elements and their effects are as follows;
- Manganese – Increases strength and hardness, decreases ductility and weldability
- Silicon – Used as deoxidizers used in steel making process
- Phosphorus – Increases strength and hardness and decreases ductility and notch impact toughness of steel.
- Sulfur –Decreases ductility, notch impact toughness and weldability. Found in the form of sulfide inclusions.
- Copper –improved corrosion resistance
- Nickel – Increases hardenability and Impact strength of steels.
- Molybdenum – Increases hardenability and enhances the creep resistance of low-alloy steels
STAINLESS STEEL: Stainless steels – Stainless steel is an alloy steel with 10.5% Chromium (Minimum). Stainless steel exhibits extraordinary corrosion resistance due to formation of a very thin layer of Cr2O3 on the surface. This layer is also known as passive layer. Increasing the amount of Chromium will further increase the corrosion resistance of the material. In addition to Chromium Nickel and Molybdenum are also added to impart desired (or improved) properties. Stainless steel also contains varying amounts of Carbon, Silicon and Manganese.
Stainless steels are further classified as
- Ferritic Stainless Steels
- Martensitic Stainless Steels
- Austenitic Stainless Steels
- Duplex Stainless Steels
- Precipitation-Hardening (PH) Stainless Steels
- Ferritic: Ferritic steels consists of Iron-Chromium alloys with body-centered cubic crystal structures (BCC). These are generally magnetic and cannot be hardened by heat treatment but can be strengthened by cold working.
- Austenitic: Austenitic steels are most corrosion resistant. It is non-magnetic and non heat-treatable. Generally austenitic steels are highly weldable.
- Martensitic: Martensitic stainless steels are extremely strong and tough but not as corrosion-resistant as the other two classes. These steels are highly machinable, magnetic and heat-treatable.
- Duplex Stainless Steels: Duplex stainless steel consists of a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel ( i.e Ferrite + Austenite). Duplex steels are about twice as strong as austenitic or ferritic stainless steels.
- Precipitation-Hardening (PH) Stainless Steels: Precipitation-Hardening (PH) Stainless Steels possess Ultra high strength due to precipitation hardening.
TOOL STEEL: Tool steels have high carbon content (0.5% to 1.5%). Higher carbon content provides higher hardness and strength. These steels are mostly used to make tools and dies. Tool steel contains various amounts of tungsten, cobalt, molybdenum, and vanadium to increase the heat and wear resistance and durability of the metal. This makes tool steels very ideal for using as cutting and drilling tools.