Stainless Steel Classifications
Comprehensive reference guide for stainless steel classifications, including martensitic, ferritic, austenitic, duplex, and precipitation-hardening stainless steels with their types and UNS equivalents.
Overview
Stainless steels are iron-based alloys containing between 10.5% to 30% Cr and achieve their stainless characteristic through the formation of an invisible and adherent chromium-rich oxide surface film.
Classification Categories
- Martensitic stainless steels: High strength, plain chromium class
- Ferritic stainless steels: Plain chromium category, good corrosion resistance
- Austenitic stainless steels: Chromium-nickel class, most commonly specified
- Duplex (ferritic-austenitic) stainless steels: Combined properties
- Precipitation-hardening stainless steels: Enhanced hardness through heat treatment
Common Alloying Elements
The metals used most commonly as alloying elements include chromium, nickel, and molybdenum. Other elements include copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, and selenium.
Selection Criteria
Selection is based on:
- Corrosion resistance
- Fabrication characteristics
- Availability
- Mechanical properties for specific temperature ranges
- Product cost
Austenitic Stainless Steels
The most commonly specified austenitic (chromium-nickel stainless class) stainless steels. Type 304 accounts for more than half of the stainless steel produced worldwide.
Ferritic Stainless Steels
Plain chromium stainless category offering general-purpose corrosion resistance. Type 409 is suitable for high temperatures with the lowest chromium content and thus least expensive. Type 430 is the most widely used ferritic stainless steel.
Martensitic Stainless Steels
Plain chromium stainless class with exceptional strength. Type 410 is the most widely used martensitic stainless steel, a low-cost, heat-treatable grade suitable for non-severe corrosion applications.