Carbon steel is steel that has properties made up mostly of the element carbon, and which relies upon carbon content for its structure. The most perfect carbon structure in the world is a diamond, which is 100% carbon. Carbon is present in all steel and is the principal hardening element, determining the level of hardness or strength attainable by quenching. It raises tensile strength, hardness, resistance to wear and abrasion as the carbon content of steel is increased. It lowers ductility, toughness and machinability.
Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn.
Plain carbon steels - very low content of alloying elements and small amounts of Mn. Most abundant grade of steel is low carbon steel – greatest quantity produced; least expensive. Not responsive to heat treatment; cold working needed to improve the strength. Good Weldability and machinability.
For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.
Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition.
Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.
High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%.
High Carbon content provides high hardness and strength. Hardest and least ductile. Used in hardened and tempered condition Strong carbide formers like Cr, V, W are added as alloying elements to from carbides of these metals. Used as tool and die steels owing to the high hardness and wear resistance property
Ultrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce micro structures that consist of ultra fine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.
Carbon steel Properties
High carbon steel will be any type of steel that contains over 0.8% carbon but less than 2.11% carbon in its composition. The average level of carbon found in this metal usually falls right around the 1.5% mark. High carbon steel has a reputation for being especially hard, but the extra carbon also makes it more brittle than other types of steel. This type of steel is the most likely to fracture when misused.
Cold Drawn carbon steel is typically numbered with the prefix “10” in the AISI numbering system,
followed by two numbers that represent the nominal percentage of carbon in the product (up to 100%). For example, C1018 has 0.18% carbon, while C1045 has 0.45%.
Generally carbon adds hardness to the material which improves wearability. For carbon contents above 0.30%, the product may be direct hardened. Carbon steel beneath this level typically require carburizing when heat treated in which carbon molecules are introduced so that a hardened “skin” is able to be developed on the surface, or “case”. This is where the concept of case hardening is found.
Carbon is maximized at less than 1.00% of steel because for levels above this percentage material can become brittle. Generally, the higher the carbon content, the more difficult carbon steel is to machine.
Effects of Alloying Elements
Manganese – strength and hardness; decreases ductility and weldability; effects hardenability of steel.
Phosphorus – increases strength and hardness and decreases ductility and notch impact toughness of steel.
Sulfur - decreases ductility and notch impact toughness weldability decreases. Found in the form of sulfide inclusions.
Silicon – one of the principal deoxidizers used in steel making. In low-carbon steels, silicon is generally detrimental to surface quality.
Copper – detrimental to hot-working steels; beneficial to corrosion resistance (Cu>0.20%)
Nickel - ferrite strengthener; increases the hardenability and impact strength of steels.
Molybdenum - increases the hardenability; enhances the creep resistance of low-alloy steels
Carbon Steel Uses
High carbon steel remains popular for a wide variety of uses. This type of steel is preferred in the manufacturing of many tools such as drill bits, knives, masonry nails, saws, metal cutting tools, and woodcutting tools. These materials may be used for stampings, forgings, seamless tubes, and boiler plate. Typical uses are in automobile body panels, tin plate, and wire products.
The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.