Stainless steel is an iron-based alloy known for its excellent resistance to rust, corrosion, and staining. It is widely used in household items, medical instruments, construction, automobiles, food processing, and aerospace industries.
Stainless steel is defined as a steel alloy containing a minimum of 10.5% chromium. The chromium forms a thin, invisible protective layer called chromium oxide on the surface, which prevents rusting and corrosion.
Unlike ordinary steel, this protective layer is self-healing. If the surface is scratched, exposure to oxygen reforms the protective layer automatically.
| Element | Typical Percentage | Purpose |
|---|---|---|
| Iron (Fe) | Balance | Base metal |
| Chromium (Cr) | 10.5–30% | Corrosion resistance |
| Nickel (Ni) | 0–35% | Toughness and ductility |
| Carbon (C) | 0.03–1.2% | Strength and hardness |
| Molybdenum (Mo) | 0–7% | Chloride resistance |
Ordinary steel rusts because iron reacts with oxygen and moisture to form flaky iron oxide. In stainless steel, chromium reacts with oxygen to form a stable chromium oxide layer.
This layer:
Austenitic stainless steel is the most widely used type, accounting for nearly 70% of all stainless steel production.
Applications: Kitchenware, food processing equipment, medical instruments, chemical tanks
Ferritic stainless steels contain chromium but little or no nickel.
Applications: Automotive exhaust systems, washing machines, decorative panels
Martensitic stainless steels are known for their hardness and strength.
Applications: Knives, surgical instruments, shafts, valves
Duplex stainless steel combines both austenitic and ferritic structures.
Applications: Offshore oil and gas, desalination plants, heat exchangers
These steels achieve extremely high strength through heat treatment.
Applications: Aerospace components, turbines, high-performance mechanical parts
| Grade | Type | Key Feature | Typical Use |
|---|---|---|---|
| 304 | Austenitic | General-purpose | Kitchenware |
| 316 | Austenitic | Marine and medical grade | Implants, coastal use |
| 430 | Ferritic | Low cost | Appliances |
| 410 | Martensitic | Hard and strong | Cutlery |
Stainless steel magnetism depends on its crystal structure:
Note: The magnet test is not a definitive method for identifying stainless steel grade.
Food-grade stainless steels such as 304 and 316 are:
Because of these properties, stainless steel is widely used in hospitals, kitchens, and pharmaceutical industries.
“Stainless steel is strength that resists time, corrosion, and wear.”
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