What Are the Types of Stainless Steel?

Stainless Steel is a family of corrosion-resistant iron-based alloys containing chromium as the principal alloying element responsible for the formation of a protective passive surface layer. Depending on chemical composition, microstructure and heat treatment, stainless steels are classified into different metallurgical families with distinct mechanical, corrosion-resistant, magnetic and fabrication characteristics.

The main types of stainless steel are austenitic, ferritic, martensitic and duplex stainless steel. Precipitation-hardening stainless steels are also commonly treated as a separate stainless steel family because they develop high strength through controlled heat treatment.

Within these stainless steel types are many individual grades, such as 304, 316, 321, 410, 430, 2205 and 2507. The appropriate grade depends on factors such as corrosion resistance, temperature, strength, weldability, magnetic behavior and the intended application.

There is no single number that applies to every classification system. When stainless steel is grouped according to its primary metallurgical structure, the commonly recognized families are austenitic, ferritic, martensitic and duplex stainless steels. Precipitation-hardening stainless steel is also widely identified as a separate family.

This is why searches for “what are the 4 types of stainless steel?” and “what are the 5 types of stainless steel?” can produce different answers. A four-family classification generally refers to the principal structural groups, while a five-family classification adds precipitation-hardening stainless steel as a distinct category.

There are also specialized classifications and subgroups within these families. Therefore, the number of stainless steel types and grades should not be confused with the number of individual commercial grades.

Stainless steel classification is primarily based on metallurgical structure, which is influenced by alloy composition and processing. The major stainless steel types can be summarized as follows:

• Austenitic stainless steel: Primarily austenitic structure with high chromium and nickel or other austenite-stabilizing elements.
• Ferritic stainless steel: Chromium-containing stainless steels with a ferritic structure and generally magnetic behavior.
• Martensitic stainless steel: Stainless steels that can be hardened through heat treatment to achieve higher strength and hardness.
• Duplex stainless steel: Stainless steels containing a combination of austenitic and ferritic phases.
• Precipitation-hardening stainless steel: Stainless steels designed to achieve high strength through controlled precipitation-hardening treatments.

Within each category, individual grades are identified by designation systems such as the AISI/SAE numbering system, UNS designations and applicable ASTM or other material specifications.

Austenitic stainless steel is the largest and most widely used stainless steel family. These alloys typically contain substantial chromium together with nickel or other elements that stabilize the austenitic structure.

Austenitic stainless steels are known for their corrosion resistance, formability, weldability and toughness. Most common grades are generally non-magnetic in the fully annealed condition, although cold working can introduce some magnetic response in certain grades.

Common Austenitic Stainless Steel Grades

Ferritic stainless steel has a ferritic crystal structure and is generally magnetic. These stainless steels are primarily chromium-based and normally contain lower nickel levels than conventional austenitic grades.

Ferritic grades offer useful corrosion resistance, good resistance to oxidation and, depending on grade and environment, good resistance to stress corrosion cracking. Their properties vary significantly between individual grades.

Common Ferritic Stainless Steel Grades

Martensitic stainless steel is characterized by a martensitic structure and is generally capable of being hardened through heat treatment. These grades are selected when higher strength, hardness and wear resistance are important.

Martensitic stainless steels are generally magnetic. Their corrosion resistance is typically lower than that of highly alloyed austenitic grades, although corrosion performance varies according to the specific grade and service environment.

Common Martensitic Stainless Steel Grades

Duplex stainless steel contains both austenitic and ferritic phases. This mixed microstructure gives duplex grades a combination of properties associated with both stainless steel families.

Duplex stainless steels provide higher strength than many conventional austenitic grades and strong resistance to several forms of corrosion, including chloride-related stress corrosion cracking in suitable environments.

Common Duplex Stainless Steel Grades

Precipitation-hardening stainless steel, also called PH stainless steel, is designed to achieve high strength through a controlled heat-treatment process that forms strengthening precipitates within the alloy.

These grades combine useful corrosion resistance with high mechanical strength and are used where a combination of strength, dimensional stability and corrosion performance is required.

Examples of Precipitation-Hardening Stainless Steel

• 17-4 PH: Common precipitation-hardening stainless steel used for high-strength components.
• 15-5 PH: PH stainless steel used in applications requiring high strength and useful corrosion resistance.

The terms stainless steel type and stainless steel grade describe different levels of classification. A type identifies the broader metallurgical family, while a grade identifies a specific alloy composition and associated requirements.

Ferritic and martensitic stainless steels are generally magnetic because of their crystal structures. Duplex stainless steels are also magnetic because they contain a significant ferritic phase.

Most fully annealed common austenitic stainless steels, such as 304 and 316, are generally considered non-magnetic. However, cold working can increase magnetic response in some austenitic stainless steels by producing deformation-induced martensite.

Selecting the correct type of stainless steel requires more than choosing a grade based only on its chromium content. The material should be evaluated according to the actual service conditions and applicable engineering requirements.

• Corrosion environment: Identify exposure to chlorides, acids, alkalis, moisture and other process chemicals.
• Temperature: Consider both operating and design temperatures.
• Strength: Determine the required tensile strength, yield strength, hardness and mechanical performance.
• Weldability: Consider welding requirements and post-weld service conditions.
• Magnetic requirements: Confirm whether magnetic or generally non-magnetic behavior is required.
• Fabrication: Consider forming, machining, welding and heat-treatment requirements.
• Applicable specification: Select a grade that complies with the required ASTM, ASME, EN, UNS or other applicable material specification.
• Application: Match the material to the actual process, equipment or component requirements.

For stainless steel pipes and tubes, material selection should also consider the required pipe type, nominal size, outside diameter, wall thickness, schedule, length, manufacturing standard, testing and documentation.

Different stainless steel families are used for pipe and tube products depending on the application and applicable specification. Austenitic stainless steel pipes, particularly grades such as 304 and 316, are widely used for general corrosion-resistant piping and process applications.

Duplex and Super Duplex stainless steel pipes are selected for applications requiring higher strength and enhanced resistance to demanding corrosive environments. Other stainless steel families are used for specific temperature, mechanical or application requirements.

Kamal Steels supplies stainless steel pipe and tube products in various grades, dimensions and specifications according to project and customer requirements.

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