Low Alloy Steel is a category of steel containing controlled additions of alloying elements—typically between 1% and 5% by weight—to enhance mechanical and physical properties beyond those of plain carbon steel. Common alloying elements include chromium, nickel, molybdenum, manganese, vanadium, silicon, and boron.
These additions improve strength, toughness, hardenability, fatigue resistance, wear resistance, and service life while maintaining reasonable weldability and cost efficiency. Low alloy steels are widely used in structural, mechanical, pressure-bearing, and energy-sector applications.
Metallurgical Classification
Structural Low-Alloy Steels
• Designed for load-bearing structures
• Improved strength-to-weight ratio
• Examples: HSLA steels
Heat-Treatable Low-Alloy Steels
• Used in quenched and tempered condition
• High strength and toughness
• Examples: AISI 4130, 4140, 4340
Pressure Vessel & Boiler Steels
• Designed for high-temperature and high-pressure service
• Enhanced creep resistance
• Examples: ASTM A387, ASTM A335
Typical Chemical Composition
| Element | Typical Range (%) | Function |
|---|---|---|
| Carbon (C) | 0.10 – 0.35 | Strength & hardness |
| Chromium (Cr) | 0.3 – 1.5 | Wear & corrosion resistance |
| Nickel (Ni) | 0.5 – 3.5 | Toughness & low-temp strength |
| Molybdenum (Mo) | 0.15 – 0.6 | Creep & high-temp strength |
| Manganese (Mn) | 0.5 – 1.6 | Hardenability |
| Vanadium (V) | 0.05 – 0.2 | Grain refinement |
| Silicon (Si) | 0.2 – 0.6 | Strength & oxidation resistance |
| Iron (Fe) | Balance | Base metal |
Key Mechanical Properties
| Property | Typical Range |
|---|---|
| Tensile Strength | 550 – 1,200 MPa |
| Yield Strength | 350 – 900 MPa |
| Hardness | 170 – 350 HB (up to 500+ HB after Q&T) |
| Elongation | 12 – 25% |
| Impact Toughness | Excellent |
| Fatigue Resistance | High |
| Density | ~7.85 g/cm³ |
| Modulus of Elasticity | ~200 GPa |
Heat Treatment & Processing
• Normalizing – Grain refinement and uniform properties
• Quenching & tempering – High strength with controlled toughness
• Annealing – Improved machinability and ductility
• Stress relieving – Reduces residual welding stresses
Weldability & Fabrication
• Generally weldable using standard processes
• Preheating and PWHT may be required for thick sections
• Good machinability in annealed or normalized condition
Common Grades
International: AISI 4130, 4140, 4340, 8620
ASTM: A387, A335, A182
EN: 25CrMo4, 42CrMo4
IS (India): EN8, EN19, EN24
Available Forms
✔ Plates & sheets
✔ Bars (round, square, flat)
✔ Structural sections
✔ Pipes & tubes (seamless / welded)
✔ Forged components (shafts, rings, discs)
✔ Billets, blooms & castings
Applications
Construction & Infrastructure: Bridges, flyovers, heavy structures
Machinery & Heavy Engineering: Gears, shafts, mining equipment, cranes
Automotive & Transport: Crankshafts, axles, suspension components
Oil & Gas / Energy: Pressure vessels, boilers, pipelines, drilling tools
Defence & Aerospace: High-strength structural and wear-resistant components
Why Choose Low Alloy Steel
Low alloy steel provides an optimal balance of strength, toughness, durability, and cost efficiency. Its adaptability through alloying and heat treatment makes it a preferred material for critical structural, mechanical, pressure, and energy-sector applications where safety and long service life are essential.