Aerospace engineering favors low-density alloys and composites for primary structure, yet low- and medium-carbon steel grades — SAE 1010, 1018, 1020, 1045 — remain standard for pins, shafts, fasteners, brackets, and gear housings on civil and military aircraft [S7]. For high-strength forged structural parts, low-alloy AISI 4340 is still the workhorse: 1500-1950 MPa (220-280 ksi) ultimate tensile strength after heat treatment, forged at 1065-1230 °C, with good gas/arc weldability and the same bake-out caution against hydrogen embrittlement that applies to any high-strength steel in propulsion service [S1].
Real carbon content is the gate, not the name. Carbon steel is an iron-carbon alloy with carbon as the principal strengthening element; grades run from low-carbon (≤0.25% C, formable) through medium (0.25-0.60% C, hardenable) to high-carbon (≥0.60% C, spring/wire). For aerospace, that maps onto a narrow band: low-carbon for formed brackets and ductile pins, medium-carbon for heat-treated wear parts, and the low-alloy 4340 family — a nickel-chromium-molybdenum steel, not a plain carbon steel — for the structural forgings that look like steel on a drawing but sit in their own spec envelope [S7][S1].
Specific grades and the operating envelope each one covers
SAE 1010 is the deep-draw, cold-formed grade: high formability, low strength, used for clips, washers, and thin-wall formed brackets where weight is irrelevant and a clean bend matters [S7]. SAE 1018 sits one step up in carbon and manganese, with the best machinability of the low-carbon set, so it shows up in pins, spacers, and precision-turned aerospace hardware [S7].
SAE 1020 balances formability against strength for brackets and low-load fittings, while SAE 1045 — roughly 0.43-0.50% C — is the medium-carbon workhorse for gears, axles, and hydraulic parts that need a through-hardened wear surface; it is routinely quenched and tempered to lift surface hardness into the 50-60 HRC range [S7]. Above 1045, you have left aerospace use cases for carbon steel and entered the low-alloy / tool-steel spec territory. The boundary is the carbon number, not the chemistry label.
Why carbon steel is FOR vs NOT FOR on an airframe
Carbon steel is FOR: non-primary secondary structure, internal brackets, hinges, pins, fasteners, machined bushings, gear and shaft components in landing-gear actuators and accessory gearboxes, hydraulic fittings, and any part where wear resistance, machinability, or local hardness matters more than a 30% weight saving [S4][S7]. Distribution stockists such as Avion Alloys and thyssenkrupp Aerospace cut and machine to AMS, MIL, and customer-proprietary specs in bar, billet, and plate form for exactly these roles [S4][S5].
Carbon steel is NOT FOR: primary load-bearing airframe structure, skins, spars, pressurized fuselage frames, wing boxes, engine disks, hot-section hardware, or cryogenic propellant tanks. Those slots are filled by aluminum 7075-T6 (Young's modulus 71.7 GPa, representative strength 503 MPa), titanium Ti-6Al-4V (113 GPa, 880 MPa), Inconel 718 (200 GPa, 1,035 MPa), and carbon fiber reinforced polymer laminates (modulus 70-230 GPa, strength 600-1,500 MPa) [S2].
4340 and the high-strength corner of the steel envelope
AISI 4340 sits in a different spec book from plain carbon steel. With ~0.40% C, 1.8% Ni, 0.8% Cr, and 0.25% Mo, it is a low-alloy steel that combines high hardenability with 1500-1950 MPa tensile strength after quenching and tempering — a range that no plain carbon steel can reach at the same section thickness [S1]. NASA Marshall's aerospace materials reference cites 4340 as a forging alloy for landing-gear and high-stress structural components, with forging windows of 1065-1230 °C and a mandatory hydrogen bake-out step because the same strength that makes 4340 useful makes it prone to hydrogen embrittlement [S1].
Stress-corrosion cracking is the second hard limit. 4340 at 1500-1950 MPa has "extremely poor resistance to SCC," which is why aerospace specs pair the alloy with surface treatments (shot peening, Cd or IVD coatings) and strict inspection intervals, and why the alloy is reserved for parts where the load path tolerates the inspection regime [S1]. In propulsion-side service, the same hydrogen-bake-out discipline applies to any high-strength ferrous part exposed to hydrogen-rich environments [S1]. Engineers who need comparable strength without the SCC penalty typically step to maraging steel (210 GPa modulus, 210 MPa representative strength) or to Inconel 718 (1,035 MPa at 200 GPa) [S2].
Comparison: plain carbon grades vs 4340 vs the non-ferrous alternatives
On a four-criterion decision table — strength-to-weight, max service temperature, corrosion/SCC resistance, machinability — plain carbon grades sit at the bottom of the strength band but at the top of the machinability column, which is why they still get bought. 4340 lifts strength to 1500-1950 MPa but inherits the SCC and hydrogen-embrittlement risk of any high-strength ferrous alloy, and its density penalty is identical to every other steel grade [S1][S2].
By contrast, 7075-T6 aluminum (503 MPa at 2.81 g/cm³) and Ti-6Al-4V (880 MPa at 4.43 g/cm³) give roughly the same specific strength as 4340 without the SCC risk, and CFRP pushes specific strength 3-5× higher than either — at the cost of impact tolerance, inspectability, and a different repair training pipeline [S2]. For a wear-and-fastener job, the choice is still a 10XX-series carbon steel; for a primary structural load path, it almost never is.
Specifications, standards, and where to source stock
Aerospace carbon and low-alloy steel is bought against a layered spec stack: the SAE/AISI grade designation defines the chemistry, an ASTM/ASME material specification (A213, A29, etc.) controls melting and form, an AMS or MIL spec adds the heat-treatment and inspection regime, and the OEM drawing adds the proprietary gates [S3][S6]. A buyer who only quotes "SAE 1045 bar" has not finished the spec; an aerospace drawing typically also calls out condition, hardness range, and traceability.
Stocking distributors carry that layered spec set. thyssenkrupp Aerospace and Avion Alloys both publish cut-to-size and mill-supported bar/billet inventory against national and OEM-proprietary aerospace specifications, with in-house machining to drawing [S4][S5]. For non-aerospace general fabrication, the same grade family shows up in copper-alloy selection guides where machinability and formability are the priorities — see this copper grade map for the parallel selection logic on a different alloy system.
Engineers specifying a wear part for a 2026 build should pin four data points on the drawing before release: the SAE/AISI grade (e.g. 1018, 1045, 4340), the spec line (AMS 6414 for 4340 bar is the common aerospace call-out), the required hardness or tensile range after heat treatment, and the hydrogen-bake-out / surface-protection step for any 4340 part. Those four are the difference between a part that flies for 30,000 hours and one that gets rejected at FAI.
Spec-level background on the components involved: carbon steel, and alloy steel.