Steel is an alloy primarily composed of iron (Fe) and carbon (C). The carbon content typically ranges from 0.02% to 2.14% by weight. This small percentage of carbon greatly influences the hardness, strength, and ductility of the material.
1. Why Steel?
Steel is widely used because of its:
- High strength compared to its weight
- Versatility in manufacturing and shaping
- Recyclability (one of the most recycled materials in the world)
- Cost-effectiveness in production and use
2. Types of Steel (Based on Carbon Content)
Steel is categorized into the following types:
| Steel Type | Carbon Content | Characteristics | Examples |
| Low Carbon Steel (Mild Steel) | 0.02% – 0.30% | High ductility, easily weldable, used for general-purpose pipes | ASTM A36, API 5L Gr. B |
| Medium Carbon Steel | 0.30% – 0.60% | Higher strength, lower ductility, used for stronger pipes | API 5CT J55, K55 |
| High Carbon Steel | 0.60% – 1.00% | Very hard, difficult to weld, used in tools | Not commonly used for pipes |
| Ultra-High Carbon Steel | 1.00% – 2.14% | Brittle, very hard, used in cutting tools | Not used in pipes |
👉 Most steel pipes are made from low and medium-carbon steel due to their balance of strength and ductility.
3. Alloy Steel & Stainless Steel
- Alloy Steel: Contains additional elements (Mn, Cr, Mo, Ni, etc.) for improved properties.
- Example: API 5CT L80, P110 (used in oil & gas pipes)
- Stainless Steel: Contains at least 10.5% Chromium (Cr) for corrosion resistance.
- Example: ASTM A312 TP304, TP316 (used in chemical processing pipes)
4. Mechanical Properties of Steel
Steel pipes must meet mechanical property requirements based on their application.
| Property | Definition | Example in Pipes |
| Tensile Strength (MPa, psi) | Maximum stress before breaking | Determines pipe pressure rating |
| Yield Strength (MPa, psi) | Stress at which permanent deformation starts | Important for load-bearing capacity |
| Ductility (% Elongation) | Ability to stretch before breaking | High ductility prevents sudden failure |
| Hardness (Brinell, Rockwell, Vickers) | Resistance to indentation | Affects wear resistance |
| Toughness (Impact Test, Joules) | Ability to absorb energy before fracture | Crucial in low-temperature environments |
👉 Different steel grades have different mechanical properties, which determine their suitability for various pipe applications.
