Carbon steel

Family overview

Carbon steel is iron and carbon with small amounts of manganese and silicon. It differs from alloy steel in that alloying elements such as chromium, molybdenum and nickel are not added on purpose. Its properties depend heavily on carbon content and heat treatment condition, so the same grade can differ widely in strength and toughness depending on whether it is used as rolled or quenched and tempered.

This family has two groups. Carbon steel for machine structural use (S10C to S55C, JIS G 4051) is mostly machined or forged from bar into machine parts such as shafts, gears and bolts. Carbon steel for cold heading (SWCH series) is a wire grade matched to cold heading, in which wire is pressed in a die at room temperature to form heads, and it is used for fasteners such as bolts and nuts.

In the grade names, S stands for steel and C for carbon, and the number in the middle (the 45 in S45C) is taken from a representative carbon content. A larger number means more carbon. SWCH denotes wire for cold heading and uses the same numbering; the final A indicates aluminum-killed steel and K indicates killed steel.

The more carbon the steel contains, the higher the hardness and strength obtained by quenching, and the lower the ductility, toughness, weldability and cold workability. For this reason, lower-carbon S10C and S20C are often used for parts that are carburized to harden only the surface, medium-carbon S35C and S45C for shafts and gears that are quenched and tempered to combine strength and toughness, and higher-carbon S48C, S50C and S55C for parts that need surface hardness and wear resistance. Carbon steel has low hardenability, however (the depth to which quenching hardens the steel), so a large section does not harden to the core; for large sections or parts that need core strength, alloy steels are compared as well.

Role of the main elements

• Carbon (C): the key element that sets the properties of carbon steel. The more there is, the higher the hardness and strength after quenching, but the lower the ductility, toughness, weldability and cold workability, and the greater the risk of quench cracking.
• Manganese (Mn): aids deoxidation and combines with sulfur to form manganese sulfide, which prevents cracking during hot working (hot shortness). It also raises strength and, to a degree, hardenability.
• Silicon (Si): used mainly for deoxidation; dissolved in the steel, it raises strength slightly. Among cold heading steels, aluminum-killed (A) grades tend to keep the silicon upper limit low to reduce deformation resistance.
• Phosphorus (P) and sulfur (S): treated as impurities, with upper limits set by the standard. Phosphorus gathers at grain boundaries and makes the steel brittle at room temperature and below; sulfur remains as sulfide inclusions that lower toughness and cold workability. Grades to which sulfur or lead is added on purpose to improve machinability are covered under free-cutting steel.
• Aluminum (Al): a deoxidizing element that removes oxygen from the steel. The aluminum nitride it forms with nitrogen restrains grain growth and keeps the grains fine. Fully deoxidized steel is called killed steel, and steel deoxidized with aluminum is called aluminum-killed steel.
• Chromium (Cr), nickel (Ni) and copper (Cu): in carbon steel these are residual elements that enter with the raw material rather than deliberate additions, and the standard sets upper limits. Steel in which such elements are added on purpose to raise hardenability and strength is alloy steel.

Heat treatment and delivery condition

• Delivery condition: bars of carbon steel for machine structural use are used in the hot-rolled (black-surface), normalized, annealed or quenched-and-tempered condition. They are also used as peeled bar with the surface machined off, as cold-drawn bar and as ground bar. Hardness and machinability change with the condition, so it is specified at the time of ordering and confirmed on the mill certificate.
• Normalizing and annealing: normalizing heats the steel and cools it in air, which evens out the structure coarsened by rolling and refines the grain. Annealing cools the steel slowly, which softens it and relieves internal stress. A low-carbon steel that is too soft forms long chips, so the normalized condition is sometimes better, while for a higher-carbon steel the annealed or spheroidize-annealed condition (a softening treatment that turns carbides into spheres) is sometimes better.
• Quenching and tempering: grades of medium carbon content or above (S35C and up) are quenched and then tempered to balance strength and toughness. Because carbon steel has low hardenability, water quenching, with its fast cooling, is often used, and the more carbon the steel contains, the greater the risk of quench cracking and distortion. The larger the section, the less the core hardens, so the result depends on part size (mass effect).
• Induction hardening: only the surface is heated quickly and then cooled, so the surface hardens while the interior keeps its toughness. Some carbon is needed to reach surface hardness, so it is often applied to higher-carbon grades such as S45C, S50C and S55C; the depth of the hardened layer depends on the heating conditions and the grade.
• Carburizing and carbonitriding: low-carbon grades such as S10C and S20C do not harden much by quenching alone, so carbon (carbon and nitrogen in carbonitriding) is diffused into the surface before quenching. The surface becomes hard and the interior keeps its toughness, but carbon steel has lower hardenability than alloy steel, so the core strength is on the low side.
• Decarburization and residual stress: a decarburized layer (a surface layer that has lost carbon) can form on hot-rolled bar, so for parts where surface hardening or fatigue strength matters, an allowable depth is specified or the layer is removed as machining allowance. Cold-drawn bar is stronger than rolled bar through work hardening and tends to have better size, straightness and surface condition, but residual stress remains, so the bar can bend when only one side is cut, as in keyway machining.
• SWCH wire condition: cold heading wire is pickled and drawn, annealed when needed, and often given a phosphate coating and lubricant treatment that reduce friction and galling against the die. Higher-carbon grades are more likely to receive spheroidizing annealing, which reduces cracking during heading, and even within one grade the strength varies widely with the drawing reduction and the annealing condition.
• Heat treatment after heading: the lower-carbon A grades (SWCH6A to SWCH22A) are used without heat treatment or with the surface hardened by carburizing or carbonitriding, and the higher-carbon K grades (SWCH25K and SWCH45K) are often quenched and tempered to raise strength for high-strength fasteners.

Standards cross-reference

Composition range

Confirm the composition range against the applicable standard.

Typical applications

• Low-carbon steel for machine structural use (S10C, S20C): pins, bushings, cams and small gears that are carburized or carbonitrided, parts that require welding, and general machine parts that do not need high strength.
• Medium-carbon steel for quenching and tempering (S35C, S45C): general machine parts such as shafts, gears, levers, couplings and bolts that are quenched and tempered, and jigs, fixtures and mold bases.
• Higher-carbon steel (S48C, S50C, S55C): shafts, gears, splines, rollers and cams whose surfaces are induction hardened, and keys and pins that need wear resistance. These grades are less suited to welding and cold forming.
• Forging material (S20C, S35C, S45C, S50C and others): small and medium parts such as flanges, couplings and levers that are shaped by forging and then heat treated and machined.
• Low-carbon cold heading steel (SWCH6A, SWCH10A, SWCH18A, SWCH22A): bolts, nuts, rivets, pins and small screws made by cold heading, including screws used without heat treatment and tapping screws carburized to harden the surface.
• Higher-carbon cold heading steel (SWCH25K, SWCH45K): high-strength fasteners such as bolts and studs that are quenched and tempered after heading.

Products covering these grades

• Cold & hot forging materialsS20C · S35C · S45C · S50C
• CHQ wire · cold heading quality wireSWCH series
• Cold-drawn bar · bar-to-barS20C · S35C · S45C · S50C
• Peeled barS35C · S45C · S50C · S55C
• Q&T barS45C · S50C
• Engineering steel · hot-rolled barS10C · S20C · S35C · S45C · S50C · S55C

Grades not listed on a product page (S48C): send us the standard, size and quantity, and we will confirm availability. Quote & technical inquiry