Introduction
Material selection is one of the most consequential decisions in mechanical design. The wrong material choice affects strength, machinability, corrosion resistance, and cost. This article covers the materials most commonly used in mechanical engineering and the key properties to consider when selecting them.
Structural Steel
Mild Steel (e.g., ISO S235 / JIS SS400)
The most widely used structural material. Weldable, machinable, low cost. Yield strength approximately 235 MPa. Used for frames, brackets, plates, and general structural components where high strength is not required. Rusts without surface treatment.
Medium Carbon Steel (e.g., ISO C45 / JIS S45C)
Higher strength and hardness than mild steel. Can be quenched and tempered for further strength increase. Yield strength 370–490 MPa (normalized). Used for shafts, gears, and components requiring better mechanical properties. Less weldable than mild steel.
Stainless Steel
Austenitic (Type 304 / ISO X5CrNi18-10)
Excellent corrosion resistance, good formability and weldability. The standard choice for food processing, chemical handling, and environments with moisture. Non-magnetic. Relatively expensive. Yield strength approximately 210 MPa.
Austenitic (Type 316 / ISO X5CrNiMo17-12-2)
Higher corrosion resistance than 304 due to molybdenum addition. Use in marine, saltwater, or highly corrosive chemical environments. More expensive than 304.
Aluminum Alloys
| Grade (ISO / equiv.) | Properties | Typical Use |
|---|---|---|
| A6061 (ISO AlMg1SiCu) | Good strength, machinability, weldable | Structural parts, brackets, housings |
| A6063 (ISO AlMg0.7Si) | Excellent extrudability, good anodizing | Extrusions, frames, architectural parts |
| A7075 (ISO AlZn5.5MgCu) | Very high strength, not weldable | Aerospace, high-load structural |
Aluminum is approximately 1/3 the density of steel. When weight is critical, aluminum allows significant weight reduction. It does not rust, but requires anodizing for wear resistance.
Cast Iron
Gray cast iron has excellent vibration damping and compressive strength. Used for machine tool beds, engine blocks, and housings. Brittle in tension — not suitable for impact or tensile-dominant applications. Cannot be welded easily.
Engineering Plastics
| Material | Key Property | Use Case |
|---|---|---|
| MC Nylon (PA6) | Good wear resistance, self-lubricating | Gears, slides, wear pads |
| PTFE | Very low friction, chemical resistance | Seals, bushings, slide plates |
| POM (Delrin) | High stiffness, dimensional stability | Precision parts, cams, guides |
Material Selection Checklist
- What loads does the part carry? (tensile, compressive, shear, fatigue)
- What is the operating environment? (temperature, humidity, chemicals)
- What manufacturing processes will be used? (machining, welding, forming)
- What is the cost target?
- What surface treatment or coating will be applied?
FAQ
Q. What is the most common steel used in mechanical design?
A. S235 (mild steel) for structural and general fabricated parts; C45 for shafts and higher-strength components; SUS304/316 stainless for corrosion-critical applications.
Q. When should I use aluminum instead of steel?
A. When weight reduction is important and the loads can be carried by aluminum’s lower strength. Also when corrosion resistance without coating is needed, or when the part will be anodized for wear or appearance.
Q. How do I find the mechanical properties of a specific material?
A. Check the material manufacturer’s data sheet, ISO/ASTM/JIS material standards, or a mechanical engineering handbook. For common materials, engineering reference databases (MatWeb, etc.) provide searchable property tables.
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