Overview
Carbon steel is the most widely used engineering material — an iron-carbon alloy with less than 2% carbon and small amounts of manganese, silicon, and trace elements. It is inexpensive, readily available, easily fabricated, and weldable. Carbon steels are the default choice for structural steel, piping, pressure vessels, and general industrial equipment where corrosion is not a primary concern.
Common Grades
| Grade | Specification | Primary Use |
|---|---|---|
| A36 | ASTM A36 | Structural shapes (I-beams, angles, plate) |
| A53 Grade B | ASTM A53 | Welded/seamless pipe, plumbing, general |
| A106 Grade B | ASTM A106 | Seamless pipe for high-temperature service |
| A516-70 | ASTM A516 | Pressure vessel plate |
| A572-50 | ASTM A572 | HSLA structural (higher strength) |
| 1018 / 1020 | AISI/SAE | Cold-finished bar, machined parts |
| 1045 | AISI/SAE | Medium carbon, shafts, axles |
| 4140 | AISI/SAE | Cr-Mo alloy, high-strength shafts, bolts |
Mechanical Properties
| Property | Value |
|---|---|
| Density | 7,850 kg/m³ (0.284 lb/in³) |
| Young's Modulus | 200 GPa (29 × 10⁶ psi) |
| Yield Strength (A36) | 250 MPa (36,000 psi) |
| Ultimate Tensile (A36) | 400-550 MPa (58,000-80,000 psi) |
| Shear Modulus | 77 GPa |
| Poisson's Ratio | 0.29 |
| Hardness (A36) | 120-150 BHN |
| Elongation at break | 20-25% |
Thermal Properties
| Property | Value |
|---|---|
| Melting Point | 1425-1540°C |
| Thermal Conductivity (20°C) | 50 W/m·K |
| Specific Heat | 470 J/kg·K |
| Coefficient of Thermal Expansion | 11.7 × 10⁻⁶ /°C |
Applications
- Structural steel: Buildings, bridges, platforms, pipe supports (A36, A572)
- Process piping: A106-B for refineries, power plants; A53-B for general plumbing
- Pressure vessels: A516-70 plate for boilers, tanks, separators
- Sheet & plate: 0.5-100mm sheet for tanks, ductwork, equipment housings
- Fasteners: Grade 2, 5, 8 bolts (low, medium, high carbon)
- Shafting & machined parts: 1018, 1045, 4140 bars
Welding and Fabrication
Carbon steels are readily weldable with common processes (SMAW, GMAW, FCAW, GTAW). Key considerations:
- Low carbon (<0.25% C): Weldable without preheat for most thicknesses
- Medium carbon (0.25-0.55%): May require preheat (100-200°C) to prevent cracking
- High carbon (>0.55%): Difficult to weld; requires preheat and PWHT
Corrosion Protection
Carbon steel rusts rapidly in moist air, water, and outdoor environments. Always specify protection:
- Paint/epoxy coating for atmospheric service
- Galvanizing for outdoor/immersion
- Wrapping/coating for buried piping (FBE, coal tar)
- Cathodic protection for buried pipelines and tank bottoms
- Allow 1-3mm corrosion allowance in design thickness
Summary
Carbon steel (density 7850 kg/m³) is the default industrial material for structures, piping, and vessels when corrosion is manageable. A36 for structural use, A106-B for high-temperature piping, A516-70 for pressure vessels. Always specify corrosion protection and include corrosion allowance. See also [[steel-material-properties]] for detailed grade properties and [[pipe-material-selection]] for piping material comparison.