2 16 Corrosion Resistance Steels

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Slide 1: Introduction to Corrosion-Resistant Steels
Lecture Content:
Corrosion-resistant steels, often called stainless steels, are iron-based alloys containing at least 12% chromium. Chromium is the key element that provides corrosion resistance by forming a thin, protective oxide layer on the steel's surface. However, stainless steel does not resist all corrosive conditions equally. There are four main types: ferritic chromium steels, austenitic chromium-nickel steels, martensitic steels, and precipitation-hardenable stainless steels.
Example: Stainless steel is used in kitchen utensils and medical instruments due to its corrosion resistance.
Slide 2: Ferritic Chromium Steels
Lecture Content:
Ferritic chromium steels contain 12-27% chromium. Their corrosion resistance improves as the chromium content increases. Alloys with less than 12% chromium still show some corrosion resistance but can rust under certain conditions. The steel's quench hardenability (its ability to be hardened by cooling quickly) depends on both chromium and carbon content. High carbon levels improve hardenability up to 18% chromium, while lower carbon reduces this benefit around 13%.
Example: Ferritic steels are commonly used in automotive exhaust systems due to their moderate corrosion resistance and formability.
Slide 3: Austenitic Chromium-Nickel Steels
Lecture Content:
Austenitic stainless steels contain both chromium and nickel, maintaining an austenitic structure at room temperature. These steels cannot be strengthened by heat treatment but gain significant strength through cold working. They have excellent corrosion resistance, superior to ferritic steels, and are not magnetic unless cold worked. However, their high work hardenability makes them difficult to machine.
Example: Austenitic steels are used in chemical plants due to their high corrosion resistance in harsh environments.
Slide 4: Design Considerations for Corrosion-Resistant Steels
Lecture Content:
Designers must balance corrosion resistance with factors such as weldability, machinability, and cost. Higher chromium content improves corrosion resistance but makes the steel harder to weld and more expensive. Additionally, steels with very high chromium content become excessively hard, requiring careful selection for specific service conditions. To retain the austenitic structure in high-chromium steels, nickel is often added.
Example: In construction, low-chromium stainless steel is used for general-purpose applications, while high-chromium, high-nickel stainless steel is used for aggressive environments like marine applications.