What Is Mild Steel?
Quick Answer: Mild steel (also called low-carbon steel) is carbon steel with a carbon content between 0.05% and 0.25% by weight. It is the most widely used steel type in piping systems, structural applications, and general fabrication due to its excellent weldability, ductility, and low cost.
Carbon Steel Classification by Carbon Content
| Category | Carbon Range | Hardness (HB) | Weldability | Typical Use |
|---|---|---|---|---|
| Mild / Low-carbon | 0.05 - 0.25% | 120-170 | Excellent; no preheat required below 25 mm | Piping, structural, pressure vessels |
| Medium-carbon | 0.25 - 0.50% | 170-250 | Good; preheat required above 12 mm | Shafts, gears, forgings |
| High-carbon | 0.50 - 1.00% | 250-400+ | Poor; preheat + PWHT always required | Springs, cutting tools, wire |
Mild steel accounts for approximately 85% of all steel production worldwide. In the oil and gas industry, it is the default material for non-corrosive, non-high-temperature piping systems.
Common Mild Steel Grades in Piping
| ASTM Standard | Grade | Carbon (max) | Yield Strength (min) | Application |
|---|---|---|---|---|
| ASTM A106 | Gr. B | 0.30% | 240 MPa (35 ksi) | Seamless pipe for high-temperature service |
| ASTM A105 | - | 0.35% | 250 MPa (36 ksi) | Forged flanges, fittings, valve components |
| ASTM A36 | - | 0.26% | 250 MPa (36 ksi) | Structural steel (beams, plates, angles) |
| ASTM A53 | Gr. B | 0.30% | 240 MPa (35 ksi) | Welded and seamless pipe for general service |
| ASTM A216 | WCB | 0.30% | 250 MPa (36 ksi) | Cast steel valves and fittings |
Properties of Mild Steel
| Property | Typical Value |
|---|---|
| Density | 7,850 kg/m3 (0.284 lb/in3) |
| Tensile strength | 400-550 MPa (58-80 ksi) |
| Yield strength | 240-280 MPa (35-40 ksi) |
| Elongation | 20-30% (in 50 mm gauge length) |
| Modulus of elasticity | 200 GPa (29,000 ksi) |
| Melting point | ~1,500°C (2,732°F) |
| Thermal conductivity | 50 W/m-K |
| Max service temperature | 427°C (800°F) per ASME B31.3 for A106 Gr. B |
Weldability
The primary advantage of mild steel in piping fabrication is weldability. Low carbon content means:
- Low carbon equivalent (CE), typically 0.35-0.42; well below the 0.45 threshold where hydrogen cracking becomes a concern
- No preheat required for wall thicknesses up to 25 mm (1”) per ASME B31.3
- Post-weld heat treatment (PWHT) required only when wall thickness exceeds 19 mm (3/4”) per ASME B31.3, or when specified by project requirements
- Compatible with all common welding processes: SMAW (E7018), GTAW (ER70S-2), GMAW, FCAW, and SAW
Limitations
Mild steel has no meaningful corrosion resistance. Without protective coatings, painting, or cathodic protection, it corrodes rapidly in atmospheric exposure (0.1-0.5 mm/year depending on environment), seawater (~0.3 mm/year), and any process fluid containing CO2, H2S, or acids. Temperature limits are also a constraint: above 427°C, carbon steel loses strength and is subject to graphitization; above 260°C in hydrogen service, it is subject to high-temperature hydrogen attack (HTHA) per API RP 941.
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