Stainless steels are steels that contain a minimum of 10% chromium and are more resistant to corrosion than normal steels.  Stainless steels can vary in composition from a simple alloy of iron and chromium to complex alloys containing chromium, nickel, and various other elements in small quantities.

There are three main classifications of stainless steels distinguished based on their composition and internal structure. These types are austenitic, ferritic, and martensitic.

Austenitic steels are alloys containing 16-26% Chromium and 6-22% Nickel. They are non-magnetic and have excellent corrosion resistance. They are not hardenable by heat treatment. However, they can develop high strength even from light cold working. They have excellent weldability and formability, notable cryogenic properties, and a good hygiene factor. They are identified in the AISI 300 series.

Ferritic steels are alloys that contain 12-30% chromium without nickel. They are ferro-magnetic in nature and possess good resistance to corrosion and fair weldability. They are identified in the AISI 400 series. They are less ductile than austenitic steels and are not hardenable by heat treatment.

Martensitic steels are alloys that contain 11-14% Chromium without nickel but with a slightly higher carbon content compared to the austenitic and ferritic stainless steels. They are ferro-magnetic in nature and hardenable by heat treatment. They have moderate corrosion resistance, poor weldability, and are identified in the AISI 400 series.

Grades

Steels can be separated based on specific grades or types. Some of the most common are type 304, type 316, type 410, and type 430.

Type 304 is the most commonly produced stainless steel, accounting for more than half of all stainless steel production. It is an austenitic grade that withstands ordinary corrosion in architecture, is durable in typical food processing environments, and resists most chemicals.

Type 316 is austenitic steel containing molybdenum, giving it greater resistance to various types of deterioration and corrosion.

Type 410 is the most widely used martensitic stainless steel. It is high strength, low-cost, and heat-treatable and is suited for non-severe corrosion applications.

Type 430 is the most widely used ferritic stainless steel, offering standard corrosion resistance. It is often used in decorative applications.

Specifications

Selecting metal alloys requires an analysis of the desired dimensions and specifications. Dimensions to consider include:

• Outer diameter (OD)
• Inner diameter (ID)
• Overall length
• Overall thickness

Other specifications of importance (based on application) include product shape, tensile strength, yield strength, melting point, conductivity, corrosion resistance, ductility, and malleability. These properties differ based on the forming method and alloy composition.

Applications

Stainless steel’s corrosion resistance makes it ideal for a variety of applications. Some of these include cookware, cutlery, hardware, surgical instruments, appliances, industrial equipment, and structural alloys for the automotive and aerospace industries.

Image credit:

High Performance Alloys, Inc.