Unit 18.1: Stainless Steel
Engineering Materials → Engineering Materials → Ferrous Materials → Ferrous Materials → Special Steels | Author: admin | Mar 10, 2026
Introduction
Stainless steel is an alloy steel with high corrosion resistance, primarily due to chromium content (≥11%), which forms a passive oxide layer on the surface.
It is widely used in mechanical, structural, and chemical engineering applications where resistance to rust, oxidation, and chemical attack is essential.
Definition
Stainless Steel
Stainless steel is an alloy steel containing at least 11% chromium, often with nickel, molybdenum, or other elements, designed to provide high corrosion resistance and moderate to high strength.
Core Concept Explanation
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Chromium forms a thin, stable oxide layer on the steel surface, preventing corrosion.
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Additional alloying elements:
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Nickel: improves ductility, toughness, and corrosion resistance
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Molybdenum: enhances pitting and crevice corrosion resistance
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Stainless steel can be heat treated or cold worked depending on type.
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Used where corrosion resistance and strength are both required.
Types of Stainless Steel
| Type | Composition | Key Features | Applications |
|---|---|---|---|
| Austenitic | 16–26% Cr, 8–22% Ni | Non-magnetic, excellent corrosion resistance | Chemical vessels, kitchen utensils, pipelines |
| Ferritic | 10–18% Cr, <0.2% C | Magnetic, good corrosion resistance | Automotive parts, industrial equipment |
| Martensitic | 12–14% Cr, 0.1–1.2% C | Hard, can be heat treated | Cutting tools, knives, turbine blades |
| Duplex | 18–28% Cr, 4–8% Ni | High strength, excellent corrosion resistance | Pressure vessels, marine applications |
| Precipitation Hardening | 12–17% Cr, Ni + Cu/Al | Heat treatable, high strength | Aerospace components, shafts |
Properties / Characteristics
| Property | Description |
|---|---|
| Corrosion Resistance | Excellent (due to chromium oxide layer) |
| Strength | Moderate to high, depending on type |
| Hardness | High in martensitic and PH types |
| Ductility | Good in austenitic types |
| Toughness | Excellent in austenitic and duplex types |
| Heat Resistance | Good, varies with type |
| Magnetic Properties | Varies: austenitic non-magnetic, ferritic/martensitic magnetic |
Applications in Engineering
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Chemical and food processing equipment
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Kitchen utensils, cutlery
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Pressure vessels, pipelines, and tanks
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Automobile exhaust systems
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Aerospace and marine components
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Surgical instruments
Exam-Focused Points
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Chromium ≥11% is key for corrosion resistance
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Nickel improves ductility and toughness
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Types: Austenitic, Ferritic, Martensitic, Duplex, PH
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Applications: corrosion-resistant components, tools, and vessels
Common Exam Traps
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Confusing martensitic stainless steel with austenitic – martensitic can be hardened, austenitic cannot
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Forgetting chromium content requirement (≥11%)
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Assuming all stainless steels are non-magnetic – only austenitic type is non-magnetic
Example Competitive Exam Questions
Question: What is the minimum chromium content in stainless steel?
Answer: 11%
Question: Which stainless steel type is non-magnetic and highly ductile?
Answer: Austenitic stainless steel
Question: Give one application of martensitic stainless steel.
Answer: Cutting tools or knives
Question: Which element improves corrosion resistance in stainless steel besides chromium?
Answer: Nickel and molybdenum
Quick Revision Summary
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Chromium: ≥11% (key for corrosion resistance)
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Nickel: Improves ductility and toughness
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Types: Austenitic, Ferritic, Martensitic, Duplex, PH
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Properties: Corrosion-resistant, moderate to high strength, varying hardness
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Applications: Chemical vessels, tools, cutlery, pipelines, aerospace