Glossary

Carbon steel is an iron–carbon alloy in which carbon is the primary hardening element, typically ranging from about 0.05% to 1.0% carbon and up to roughly 2.1% by definition, with only small amounts of other elements present—about ≤1.65% manganese, ≤0.60% silicon, and ≤0.60% copper. When alloying elements exceed those limits the steel is classified as an alloy steel rather than plain carbon steel. Increasing carbon content raises strength and hardness but reduces ductility, toughness, and weldability. As carbon rises, the microstructure shifts from mostly ferrite/pearlite at low carbon to finer pearlite and, when heat treated, martensite at higher carbon levels.

Within this family, low-carbon (mild) steels at about 0.05–0.30% carbon are easily formed and welded but cannot be through-hardened by quenching; they are often strengthened by cold work or given a hard surface by case hardening. Common examples include AISI 1008, 1018, and 1020, used for sheet, plate, brackets, and low-strength fasteners such as A307/Grade 2-type bolts. Medium-carbon steels at roughly 0.30–0.60% carbon respond well to quench-and-temper heat treatment for higher strength while retaining moderate weldability; AISI 1045 is typical, used for shafts, gears, and many higher-strength fasteners (for example, many Grade 5-type bolts are medium-carbon and quenched and tempered). High-carbon steels around 0.60–1.0% carbon (up to about 1.2% in some specifications) can be made very hard and wear-resistant after heat treatment but have lower toughness and weldability; examples include 1060, 1080, and 1095, used for springs, cutting edges, and wear parts.

Heat treatment is central to tuning properties. Annealing or normalizing is used to soften material and refine grain. Quenching and tempering of medium- and high-carbon grades produce a martensitic structure that is then tempered to balance strength and toughness. Low-carbon grades generally are not through-hardened; when a hard surface is needed they are case-hardened (e.g., carburized or nitrided) so the core remains tough while the surface gains wear resistance.

Because plain carbon steels rust readily, they are commonly protected with finishes such as paint, oil, phosphate, electro-zinc, zinc-flake, and hot-dip galvanizing. Care is required when plating high-strength fasteners (approximately ≥1000 MPa tensile strength) to avoid hydrogen embrittlement.

In fastener terms, A307/SAE Grade 2 hardware is typically low-carbon steel and low strength; SAE Grade 5 fasteners are typically medium-carbon steel that has been quenched and tempered for higher strength; and SAE Grade 8 fasteners are usually alloy steel rather than plain carbon steel, even though they are also quenched and tempered. In short, carbon steel is the workhorse of steels—cost-effective, versatile, and highly tunable via carbon content and heat treatment—chosen when strength and formability at low cost are priorities, with surface protection applied wherever corrosion resistance is required.