Family
Carbon steel
Carbon, manganese and silicon, and only residual amounts of anything else: strength and hardenability climb with carbon, ductility and weldability fall.
What defines the carbon steel family?
Carbon steel is steel that contains up to about one percent carbon, with manganese and silicon for deoxidation and strength. Other elements occur only in residual amounts. The absence of deliberate alloying additions makes carbon steel the cheapest and most widely produced class of steel. It also allows much of its behaviour to be predicted from one value on the certificate.
Carbon content defines the main property ranges within this family. Strength, hardness and attainable hardness after quenching increase with carbon content. Ductility, formability and weldability decrease. No other constituent in plain carbon steel changes its properties to the same extent. The family is therefore divided at two carbon levels rather than by alloy chemistry. Its certificate composition provides a direct indication of bar behaviour.
The Laxcon Steels grade reference lists 93 carbon steel grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.
Carbon content and properties
Steel containing less than roughly 0.25 percent carbon is primarily a forming and welding material. It does not harden usefully by quenching. It bends, draws and welds without a procedure. Structural sections, plate, sheet and general fabrications commonly use this material. The trade term mild steel describes this low-carbon part of the same family.
Steel containing roughly 0.25 to 0.6 percent carbon is used for shafting and machined components. Light sections respond to quenching and tempering. Flame or induction treatment can harden selected areas. The material also machines predictably. EN 8 contains 0.36 to 0.44 percent carbon and is a reference grade in this range. It combines useful strength with weldability when preheat is applied. Typical applications include shafts, spindles, keys, studs and general turned parts.
Steel containing more than roughly 0.6 percent carbon is used for wear parts and springs. Attainable hardness increases, while ductility and toughness decrease. Welding becomes impractical, and the risk of quench cracking increases. EN 9 contains 0.5 to 0.6 percent carbon and lies at this boundary. Its applications favour hardness and wear resistance rather than shock resistance.
Weldability follows the same progression. Application of the International Institute of Welding carbon equivalent formula to the midpoints of published composition limits gives values of about 0.53 for EN 8 and about 0.66 for EN 9. The approximate threshold is 0.45. Above this value, preheat, low-hydrogen consumables and interpass temperature control become mandatory rather than advisable. Low-carbon structural grades remain below this threshold and are therefore used for welded structures.
Hardenability and section depth
Carbon steel is limited by hardening depth rather than strength. Carbon controls the attainable hardness of martensite but has little effect on the depth at which martensite forms. Without alloying elements to delay competing transformations, the centre of a thick bar cools too slowly during quenching. It transforms into ferrite and pearlite and remains soft even when the surface meets the specified hardness. A surface hardness test can therefore pass while the core remains capable of yielding.
Three responses address this limitation. The section can remain light enough for the quench to act through its full depth, as in a small shaft made from a medium-carbon grade. Flame or induction hardening can place martensite along a track or journal while leaving the body unchanged. This is a principal use of EN 8 and EN 9. Alternatively, an alloy grade such as EN 19 or EN 24 can provide greater hardenability. Longer soaking or a more severe quench cannot overcome the cooling-rate limit imposed by the section.
Grade designation systems
Two designation systems cover most carbon steel bar. In the BS 970 system, a code such as 080M40 begins with the manganese content multiplied by one hundred. A following letter identifies supply to a chemical analysis, a hardenability requirement or a mechanical property specification. The final digits state the mean carbon content multiplied by one hundred. Older EN numbers from the 1955 edition, including EN 8 and EN 9, remain in workshop use and correspond to these codes on a one-for-one basis.
In the SAE and AISI system, a four-digit designation begins with 10 for plain carbon steel, 11 for resulphurised free-cutting grades or 12 for resulphurised and rephosphorised grades. The final two digits state the mean carbon content multiplied by one hundred. The designation 1045 therefore identifies a plain carbon steel containing 0.45 percent carbon. Its expected behaviour follows from its position within the carbon range.
Questions about carbon steel grades
What is the difference between mild steel and carbon steel?
Mild steel is the low-carbon end of the same family, below roughly 0.25 percent carbon, where the steel is formable and weldable and will not harden usefully in a quench.
Can carbon steel be hardened?
Medium and high carbon grades can, in light sections. The limit is hardenability rather than hardness: a thick bar will harden at the surface and not at the centre, which is what an alloy grade such as EN 19 or EN 24 is bought to fix.
What do the EN numbers mean on carbon steel bar?
They are the old British Standard designations, EN 8 and EN 9 among them, still in daily commercial use in India for engineering bar even though the standard that defined them has been withdrawn. The equivalents reference reads them across to their SAE and DIN counterparts.
Why does weldability fall as carbon rises?
Because the heat-affected zone quenches itself against the cold parent metal. More carbon means a harder, more crack-prone martensite forms there, which is why higher-carbon grades need preheat and a post-weld temper.
Carbon steel equivalents across standards
Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.
Where the carbon steel family is not the right answer
Carbon steel is unsuitable when a heavy section requires uniform strength. It lacks the necessary hardenability, and processing cannot restore it. Alloy steel is used for this requirement.
Carbon steel is also unsuitable where corrosion forms part of the service conditions. It has no chromium in solution to produce a passive film. In wet or humid conditions, protection depends entirely on coating, plating or oil. A hardened wear surface corrodes as readily as a soft surface.
High-carbon steel is unsuitable for welded or shock-loaded parts. It is also not a substitute for a dedicated spring steel. Springs require fatigue resistance and a high elastic limit, which are properties of silicon-bearing spring grades rather than plain carbon steel. Low-carbon steel is unsuitable where hardness or wear resistance is required because its carbon content is insufficient for useful quench hardening.
Carbon steel grades in detail
Reviewed 2026-08-18. The full composition table for all 93 carbon steel grades is on the grades reference, which covers 500+ grades in 11 families.