Carbon Steel

What is Carbon Steel

Carbon steel is an alloy of iron and carbon in which carbon and manganese are the principal strengthening elements, and in which no minimum content is specified for chromium, cobalt, nickel, molybdenum, niobium, titanium, tungsten, vanadium, zirconium or other elements added to obtain a defined alloying effect. In practice the carbon range that matters for engineering runs from about 0.05 percent up to roughly 1.0 percent. Below that range the metal behaves like soft iron; above about 2.1 percent it becomes cast iron, which is no longer forgeable. Carbon is the single most powerful and economical strengthener: each tenth of a percent raises hardness and strength while lowering ductility, toughness and weldability.

The defining feature of carbon steel, compared with stainless or alloy steel, is the absence of significant chromium. Without chromium it has no passive oxide film, so it rusts in the presence of moisture and oxygen and must be protected by coating or environmental control in wet service. The trade-off is cost and availability: plain carbon steel is the cheapest engineering metal per unit of strength and is stocked in every product form, from bar, steel plate, sheet and coil to structural sections, wire rod and steel pipe, by mills and service centers worldwide.

Carbon steel is the dominant share of global steel output. According to the World Steel Association, world crude steel production reached about 1.885 billion tonnes in 2024. The vast majority of that tonnage is carbon and low-alloy steel rather than stainless or specialty alloy. Two production routes split the output: the integrated blast furnace plus basic oxygen furnace (BF-BOF) route accounted for roughly 70 percent in 2024, while the scrap-melting electric arc furnace (EAF) route accounted for about 29 percent, a share that has been rising year on year as the industry decarbonizes.

Metallurgically, room-temperature carbon steel is a mixture of two phases: soft, ductile ferrite (nearly pure iron) and hard, layered pearlite (alternating ferrite and iron carbide, Fe3C, also called cementite). The proportion of pearlite rises with carbon content, which is why higher-carbon grades are stronger and harder but less ductile. Heat treatment in a heat treatment furnace exploits this chemistry: heating above the critical temperature forms austenite, and the cooling rate then decides whether the steel ends up soft (slow-cooled pearlite), tough (normalized fine pearlite), or hard and brittle (fast-quenched martensite). The amount of carbon present sets the ceiling on how hard the steel can be made, which is the basis of the classification in the next chapter.

Four engineering attributes drive carbon steel selection: strength (yield and tensile), ductility and toughness, hardenability, and weldability. These move in opposition as carbon rises. A procurement engineer rarely buys the strongest steel available; instead the task is to choose the lowest-carbon grade that meets the load case, because lower carbon means easier welding, better toughness and lower fabrication risk.

Classification by Carbon Content

The most useful first cut divides plain carbon steel into three classes by carbon percentage. The boundaries are conventional rather than absolute, and different references shift them by a few hundredths, but the engineering behavior in each band is consistent: weldability and ductility fall, while strength, hardness and wear resistance rise, as carbon increases. The table below summarizes the three classes and their characteristic uses.

Low carbon steel, commonly called mild steel, contains roughly 0.05 to 0.25 percent carbon. It is the most-used class by tonnage because it combines ductility, formability and excellent weldability at the lowest cost. It cannot be appreciably hardened by quenching (too little carbon to form martensite) but it can be surface-hardened by carburizing, in which carbon is diffused into the skin and then quenched to give a hard case over a tough core. Typical grades include AISI 1008, 1010, 1018 and 1020, and structural plate grade ASTM A36. Applications span automotive body panels, general fabrication, structural framing, wire, nails and low-stress machined parts.

Medium carbon steel at roughly 0.30 to 0.60 percent carbon is the strength-versus-toughness compromise class. It responds well to quench-and-temper heat treatment, reaching tensile strengths in the 600 to 900 MPa range while retaining useful ductility, and to selective flame or induction surface hardening. AISI 1045 is the archetype, widely used for shafts, gears, axles, crankshafts, bolts and forged components. Welding requires more care than mild steel: preheat is usually needed to avoid a brittle heat-affected zone, as discussed under carbon equivalent in Chapter 5.

High carbon steel above about 0.60 percent carbon is specified where hardness and wear resistance dominate. Quenching produces very high hardness, but the steel is brittle and difficult to weld, so it is usually used in the hardened-and-tempered or as-rolled condition rather than welded. AISI 1095 (about 0.90 to 1.0 percent carbon) is the classic spring and blade steel, also used for cutting tools, knife blades, music wire and dies. Below high carbon proper, the 0.55 to 0.75 percent band is often singled out as a spring-steel range. Above roughly 1.0 percent, steels overlap with the tool and die steel families that add alloying elements for hot hardness and dimensional stability.

Beyond carbon, the AISI/SAE 1xxx family is subdivided by sulfur and manganese for machinability and processing. The 10xx series is plain carbon with up to 1.00 percent manganese; the 11xx series adds sulfur (resulfurized) to break up chips for free machining, for example 1117 and 1144; the 12xx series adds both sulfur and phosphorus (resulfurized and rephosphorized) for the best machinability, for example 12L14 with lead; and the 15xx series carries higher manganese up to 1.65 percent for greater hardenability at a given carbon level. Choosing 11xx or 12xx over 10xx is a fabrication decision driven by machining volume, not a strength decision.

Grades and Mechanical Properties

The AISI/SAE four-digit code (standardized in SAE J403) packs the essentials into four numerals: the first digit gives the alloy class (1 for carbon steel), the second a subclass, and the last two the nominal carbon content in hundredths of a percent. So 1018 is plain carbon steel with about 0.18 percent carbon, 1045 with about 0.45 percent, and 1095 with about 0.95 percent. The matching UNS designation prefixes G and adds a trailing zero, so 1045 is UNS G10450. The table below lists chemistry for the three benchmark grades that bracket the low, medium and high carbon classes.

Mechanical properties depend as much on the delivered condition (hot rolled, cold drawn, normalized, annealed, or quenched and tempered) as on chemistry, so a grade is only fully specified together with its temper. Cold drawing raises yield and tensile strength and surface finish at the cost of ductility; normalizing refines grain for consistent toughness; annealing softens for machining; quench-and-temper develops the highest strength in medium and high carbon grades. The table below gives representative tensile, yield and hardness values for the three benchmark grades in common delivered conditions. Treat them as typical reference figures, not as guaranteed minimums; the governing material standard and mill certificate are authoritative for any purchase.

Reading the trend across the table makes the carbon effect concrete. As carbon climbs from 0.18 percent (1018) to 0.95 percent (1095), tensile strength roughly doubles and Brinell hardness more than doubles, while elongation collapses from the 15 to 25 percent range typical of cold-drawn 1018 down to single digits in hot-rolled 1095. This is why a knife maker reaches for 1095 and a structural fabricator reaches for low carbon: each is buying the property that carbon content delivers and accepting the loss of the property it sacrifices.

Two practical notes follow. First, hardenability (how deep a section will harden on quenching) is limited in plain carbon grades because they lack the alloying elements that slow the transformation to soft phases. A water-quenched 1045 bar hardens at the surface but stays soft at the core beyond a modest diameter, which is exactly why thick, fatigue-critical parts move to alloy steels such as 4140. Second, residual elements matter: copper and tin picked up from scrap in EAF steel can degrade hot ductility and surface quality, so deep-draw and fatigue-critical grades specify limits on these tramp elements in addition to the headline carbon and manganese.

Standards and Product Forms

Carbon steel is bought against a material standard that fixes chemistry, mechanical property minimums, tolerances and test requirements for a specific product form. Two families dominate: the North American ASTM (and the parallel ASME SA-series for pressure equipment) system, and the European EN system maintained by CEN. The AISI/SAE grade codes from Chapter 3 describe chemistry for bar and forging stock, while ASTM and EN specifications govern plate, sheet, coil, structural shapes and pressure vessel material. The table below maps the most-specified carbon steel standards to their product form and headline property.

ASTM A36 is the default North American structural carbon steel, named for its minimum yield of 36 ksi (about 250 MPa) with tensile strength of 400 to 550 MPa. It is supplied as plate, bar and rolled shapes for building frames, bridges and general fabrication, and is prized for low cost and excellent weldability, with a carbon content low enough (around 0.25 to 0.29 percent depending on thickness) that no preheat is normally required for thin sections.

ASTM A1011 covers hot-rolled carbon, structural and high-strength low-alloy sheet and strip in coil, in thicknesses up to about 6 mm (0.230 inch). The structural (SS) grades are designated by minimum yield, so A1011 SS Grade 36 mirrors A36 yield at 36 ksi but in light-gauge formable sheet for automotive parts, brackets and drums. ASTM A516, and its ASME twin SA-516, is the most widely specified plate for welded pressure vessels and boilers at moderate and lower temperatures; Grade 70 is the common choice, with notch toughness controlled by normalizing and impact testing for cold service.

EN 10025-2 is the European standard for hot-rolled structural steel. The grade code reads directly: the S prefix means structural, the number is the minimum yield strength in MPa for thicknesses up to 16 mm (235, 275, 355, 420, 460), and the suffix encodes Charpy impact toughness, where JR is 27 J at +20 degrees Celsius, J0 at 0 degrees, and J2 at -20 degrees. S235JR is the general-purpose grade closest to ASTM A36, with 360 to 510 MPa tensile, while S355 (470 to 630 MPa tensile) is the higher-strength workhorse that corresponds to ASTM A572 Grade 50. Cross-standard substitution should always be confirmed against chemistry and impact requirements, not assumed from a yield number alone.

Carbon steel reaches the buyer in a defined product form, and the form constrains both the achievable property and the applicable tolerance standard. Hot-rolled bar and plate carry a mill scale and looser dimensional tolerance; cold-drawn bar offers tighter size, better finish and higher yield strength from work hardening; sheet and coil are sold by gauge and surface class for forming; structural sections (I-beam, channel, angle, hollow section) come to dimensional standards such as the EN 10025 / EN 10210 families. Specifying the standard, grade, form, dimension and surface condition together is what turns a generic material into a procurable line item.

Key Specification Parameters

Reading a carbon steel spec sheet or mill test certificate is a core procurement skill. A certificate lists chemistry, mechanical results and processing condition, but only a handful of parameters truly drive whether a grade fits the application: carbon content, yield and tensile strength, elongation and toughness, hardness, carbon equivalent, and surface or coating condition. Each is decoded below.

Carbon content is the master variable. It sets the strength ceiling, the hardness achievable by quenching, and, in the opposite direction, ductility and weldability. Always read it together with manganese, which acts as a secondary strengthener and deoxidizer and shifts the effective hardenability at a given carbon level. Yield strength is the stress at which permanent deformation begins and is the number most structural and pressure designs are sized against; tensile (ultimate) strength is the maximum stress before fracture. Both are quoted in MPa (or ksi); 1 ksi is about 6.895 MPa.

Elongation (percent strain at break over a gauge length) is the headline ductility figure: low carbon grades reach 15 to 25 percent or more, while hardened high carbon can fall to single digits. For impact and cold service, Charpy V-notch toughness (joules at a stated temperature) is specified separately, because a steel can be strong yet brittle. The EN JR/J0/J2 suffixes and ASTM supplementary impact tests exist precisely to certify toughness at temperature. Hardness, reported in Brinell (HB), Rockwell (HRB/HRC) or Vickers (HV), is a fast proxy for strength and wear resistance and is the practical acceptance test for heat-treated parts; the rough rule that tensile strength in MPa is about 3.45 times Brinell hardness holds for many carbon steels.

Carbon equivalent (CE or CEV) rolls carbon and alloying content into one weldability index. The IIW formula for carbon and carbon-manganese steel is:

• CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
• CE below 0.40%: readily weldable, no preheat needed for plate under 20 mm.
• CE 0.40 to 0.60%: preheat usually required, especially above 0.50 percent.
• CE above 0.60%: preheat mandatory, with hydrogen control to prevent cold cracking.

Low carbon grades like 1018 and A36 sit at or below 0.40 and weld freely; medium carbon 1045 and high carbon 1095 push well above the preheat threshold, which is why fabricators favor low carbon for welded assemblies. Surface and coating condition closes the list because bare carbon steel rusts: certificates and orders should state mill finish, pickled-and-oiled, shot-blasted, galvanized, or painted, since this determines both corrosion life and how the part can be further fabricated or coated.

One spec-sheet trap deserves emphasis: a grade name without a condition is incomplete. The same 1045 can be supplied annealed at about 180 HB for machining or quench-and-tempered above 50 HRC for wear, a more than threefold spread in hardness. Always pair grade with delivered condition, and for critical parts require the actual mill test certificate rather than a generic datasheet, since the certificate reports the heat-specific chemistry and tested mechanicals that the warranty rests on.

Selection Decision Factors

To convert the preceding chapters into a specific purchase, follow the decision sequence below. The recurring selection error is over-specifying carbon or strength, which buys fabrication difficulty (preheat, cracking risk, poor machinability) that the application never needed. The default discipline is to choose the lowest-carbon grade that meets the load case.

1. Load case and strength: Establish the required yield and tensile strength from the structural or mechanical design, then map to the lowest class that clears it: low carbon / A36 / S235 for general structure, medium carbon 1045 for stressed shafts and gears, high carbon 1095 for springs and edges. Size against yield strength, not tensile.
2. Ductility and toughness: Confirm the elongation and, for cold or impact service, the Charpy toughness needed. If the part sees low temperature or shock, specify an impact-tested grade (EN J2, or ASTM with supplementary impact) rather than trusting strength alone.
3. Weldability and carbon equivalent: If the part is welded, compute or request the carbon equivalent. Below 0.40 percent weld freely; between 0.40 and 0.60 plan for preheat and procedure qualification; above 0.60 reconsider the grade or the joint. This step often vetoes a higher-carbon choice for fabricated assemblies.
4. Heat treatment and hardenability: Decide whether the part needs through-hardening, surface hardening (carburizing for low carbon, induction or flame for medium carbon) or no heat treatment. For thick sections needing deep, uniform hardness, recognize that plain carbon hardenability is limited and an alloy grade may be required instead.
5. Standard, grade and product form: Pin the governing specification (ASTM, ASME SA, EN), the grade, and the form (plate, bar, sheet, coil, section, wire, pipe) with its dimensional and surface class. A complete line item names all of these plus the delivered condition.
6. Corrosion protection: Because carbon steel rusts, specify the protection system up front: an industrial coating such as paint, hot-dip galvanizing, zinc-rich primer, oiling, or, for outdoor unpainted service, switch the material to weathering or stainless steel. The coating choice affects cost, weldability and downstream finishing.
7. Production route and cleanliness: For deep-draw, fatigue-critical or surface-critical parts, address steel cleanliness and tramp-element limits (copper, tin) and, where relevant, the BF-BOF versus EAF route and ladle refining, since these affect inclusion content and formability beyond the headline grade.
8. Total cost of ownership: Weigh material price against fabrication cost (machinability, preheat, weld rejection), coating life, and the cost of failure. The cheapest bar is not the cheapest part if it cracks on welding or rusts out in service before the design life.

One last dimension that buyers underweight is supply continuity and certification: a credible mill test certificate traceable to the heat, EN 10204 inspection documentation (2.1 / 2.2 / 3.1 / 3.2 as the application demands), consistent dimensional tolerances batch to batch, and a service center able to hold stock and cut to size. For structural and pressure work, third-party certified material (EN 10204 3.1 or higher) is not optional. Large global mills and regional service centers stocking ASTM, ASME SA and EN grades to certificate are the reliable sources for projects where a material non-conformance can stop a production line or fail an inspection.