BS S99D Steel (1.6745)
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About BS S99D:
BS S99D is a British aerospace-grade low-alloy chromium-molybdenum steel. It is similar to AISI 4130 but produced to stricter British standards for high-strength structural applications. Supplied in the normalized and tempered condition, BS S99D offers high strength, good impact toughness, and excellent weldability, making it ideal for aerospace and defense components.
Chemical Composition:
| Element | Content (%) |
|---|---|
| Carbon (C) | 0.28 – 0.33 |
| Manganese (Mn) | 0.40 – 0.60 |
| Silicon (Si) | 0.10 – 0.35 |
| Chromium (Cr) | 0.80 – 1.10 |
| Molybdenum (Mo) | 0.15 – 0.25 |
| Phosphorus (P) | ≤ 0.035 |
| Sulfur (S) | ≤ 0.035 |
| Iron (Fe) | Balance |
Mechanical Properties:
| Property | Value |
|---|---|
| Tensile Strength | 750 – 850 MPa |
| Yield Strength (0.2%) | ≥ 620 MPa |
| Elongation (50 mm) | ≥ 16% |
| Charpy Impact Strength | ≥ 35 J |
| Hardness (Brinell) | 220 – 260 HB |
Physical Properties:
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Modulus of Elasticity | ~205 GPa |
| Thermal Conductivity | ~42.7 W/m·K (at 100°C) |
| Specific Heat Capacity | ~460 J/kg·K |
| Melting Point | ~1430 – 1460 °C |
| Thermal Expansion | ~12.2 µm/m·°C (20–100°C) |
| Electrical Resistivity | ~210 nΩ·m |
Heat Treatment:
- Normalizing: 870 – 900°C (air cool)
- Tempering: 550 – 650°C (adjust for required properties)
Applications:
- Aircraft structural components
- Landing gear parts
- Brackets, struts, and clevises
- Welded assemblies for defense and aerospace
- High-integrity mechanical parts requiring good weldability and toughness
Corrosion Resistance
- Moderate under dry, indoor conditions
- Poor in humid, marine, or chemically aggressive environments