CARBON and STEELS
Post 365 ⇒ by Gautam Shah →
Iron as a metal is very ancient material. It was difficult to process (smelt), unlike materials with lower melting temperatures, such as copper and its alloys. Iron is rarely obtainable in pure form. The impurities in iron derive from the ore, and carbon through the smelting process. Carbon is one of the most important of impurities, varying between 0.002% and 2.1%. Presence of Carbon makes the Iron up to 1000 times a harder material. Technically more than 90 per cent of all steels are carbon steels. Presence of small amounts of carbon changes the quality of steel. It affects strength, hardness, mechanical properties (machining, forming, etc.). With very high percentage of carbon workability and impact strength are reduced, whereas with lower carbon content hardness and tensile strength are higher.
Crude iron or Pig iron metal is produced in a furnace, by mixing ore with coke. The high carbon content of crude iron can be further reduced by refining it with air or oxygen, to turn it into steel. A carbon content metal is commonly called Cast Iron. The carbon content of cast iron is 2.1 percent or more. Gray cast iron is relatively soft. It can be easily machined and welded. It is used for engine cylinder blocks, pipe, and machine tool structures. White cast iron is hard, brittle, but not weldable. When annealed, it becomes malleable cast iron. Malleable cast iron can be welded and machined. It is ductile material. Ductile cast iron is sometimes called nodular or graphite cast iron. It is ductile malleable and weldable.
Besides carbon other elements present are, manganese, silicon, copper, nickel, chromium, molybdenum, vanadium, tungsten, tin, niobium, zirconium, and non metals like sulphur, phosphorus. These materials mostly find their way through the scrape that is partly used for steel making or through an intentional quality markup. The additions of these materials take steel to the category of Alloy steel. Such alloying elements are added to gain properties like better strength, hardness, durability, or corrosion resistance. These are often called specialty steels.
Adjusting the carbon content is most common tool to control quality of steel. Other quality determinant is the rate at which the steel is cooled. Steel properties are also modified by heat treatments, mechanical working it at hot or cold temperatures and by adding other alloying elements besides carbon.
Steel with high carbon content is hard and strong, but not ductile enough for common uses. In carbon steels, the higher carbon content lowers the melting point and reduces weldability.
Low carbon steel has approximately 0.05% to 0.25% carbon content with other materials like manganese. Mild steel, is also known as plain-carbon steel or low-carbon steel. Its very common form of steel, and its material properties are adequate for many applications. It is ductile and malleable. It has a relatively low tensile strength, but is cheap and amenable to cold forming processes. Its surface hardness can be increased with carburizing. It is used for structural steel.
Medium carbon steel has approximately 0.29% to 0.54% carbon content (with 0.60 to 1.65% manganese content). It shows good wear resistance and used for large parts, forging and car parts.
High carbon steel has approximately 0.55% to 0.95% carbon content (with 0.30 to 0.90% manganese content). It is very strong material and used for springs and high-strength wires.
Ultra high carbon steel has approximately 2.5–3.0% carbon content. These steels that can be tempered to great hardness and used cutting tools, knives, axles or punches. Steel with a carbon content above 2.14% is considered cast iron.
Hardened steel usually refers quenched or quenched and tempered steel. Silver steel or high-carbon bright steel, gets its name from its appearance, due to the high carbon content. Silver steel is used for cutting edges and axle components.