Categories and Types of Stainless Steels: Compositions and Properties

04-Aug-2020

The article is about an alloy, a material without which perhaps it won’t be possible to pass a single day in our life. Yes, we are talking about stainless steel and as the title suggests the key points that will be described in short here (this is the 1^st Part of this article):

• What is stainless steel (alloy steel);
• Metallurgical classification of stainless steels into various types and categories;
• Stainless steel composition
• Important properties of the various categories of stainless steel, and their specific uses  

Types and Categories of Stainless steel

Stainless Steel is a highly durable alloy of iron containing about 10 – 30 per cent chromium, 0.3 – 1.0 percent carbon and some other metals as minor constituents, possessing excellent oxidation, corrosion and fatigue resistance properties. Stainless steel has also an aesthetic appeal, excellent lusture, low wear, high strength and durability.

Also Read: Difference between Stainless Steel, Carbon Steel, and Alloy Steel

CLASSIFICATIONS – What are the different grades of stainless steel

Stainless Steels are usually grouped into 5 metallurgical categories (grades):

1. Austenitic Stainless Steels
2. Ferritic Stainless Steels
3. Martensitic Stainless Steels
4. Duplex Stainless Steels
5. Precipitation hardening Stainless Steels

Austenitic Stainless Steels (Compositions, Properties)

These steels have an austenitic crystal structure formed through the use of austenitizing elements like nickel, manganese and nitrogen. These steels have austenitic structure from room temperature to temperature below melting range and hence, can not be hardened by heat treatment.

Austenitic Stainless Steels possess the highest corrosion resistance of all Stainless Steels and also have the greatest strength and resistance at high temperatures. These steels are capable of retaining ductility at temperatures as low as zero degree.

Compositions of Austenitic Stainless Steels:

• Chromium: 16 – 26%
• Nickel: 4 – 22%
• Carbon: 0.03 – 0.25%
• Manganese: 2 – 10%
• Silicon: 1 – 2%

The purpose of adding nickel in austenitic grades is to increase density, co-efficient of thermal expansion and also corrosion resistance. Austenitic Stainless Steels have superior impact strength and toughness as compared to those of Ferritic Stainless Steels. The addition of manganese (Manganese Nickel Austenitic Stainless Steels) helps in achieving even higher strength than only nickel bearing grades over a wide range of temperatures. The yield strength also increases by nearly 40 percent while offering greater resistance to stress corrosion cracking carbide precipitation and pitting.     

Ferritic Stainless Steels (Compositions, Properties)

Ferritic Stainless Steels are basically chromium based alloys and are generally ferritic at all temperatures although some of the grades exhibit austenitic structure at high temperatures and can transform to Martensite.

Compositions of Ferritic Stainless Steels:

• Chromium: 11 – 26%
• Carbon: 0.08 – 20%
• Manganese: 1 – 1.5%
• Silicon: 1 – 2%

In annealed condition, these steels show fully ferritic structure at room temperature. Ferritic Stainless Steels are not strengthened by heat treatment. In annealed condition, Ferritic Stainless Steels develop maximum softness, ductility and corrosion resistance.

Ferritic Stainless Steels are ferro-magnetic, ductile and malleable. But at elevated temperatures mechanical properties of Ferritic Stainless Steels are relatively inferior to the Austenitic Stainless Steels. Ferritic stainless steels are more corrosion resistance than Martensitic Stainless Steels. 

Continued to 2nd Part: Grades and Series of Stainless Steels - Compositions and Uses