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Stainless Steel: Definition, Properties, Classifications, Applications

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Stainless steel is a type of steel that contains at least 10.5% chromium, which forms a protective layer on the surface of the metal and prevents it from rusting.

Stainless steel is widely used in various industries and consumer goods because of its excellent properties, such as durability, resistance to high and low temperatures, ease of cleaning, environmental friendliness, and aesthetic appeal.

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What is Stainless Steel ?

Stainless steel is a type of steel that contains at least 10.5% chromium, which is the key element that gives it its corrosion resistance. Chromium reacts with oxygen in the air or water and forms a thin layer of chromium oxide on the surface of the metal. This layer, also known as the passive layer, acts as a barrier that prevents further oxidation and corrosion of the underlying metal. The passive layer can also self-heal if it is damaged by scratches or cuts, as long as there is enough chromium in the metal.

Differences between Stainless Steel and Other Types of Steel and Metals

  1. Stainless steel has higher rust and corrosion resistance. Carbon steel contains only iron and carbon and can easily rust when exposed to moisture and oxygen.
  2. Stainless steel has higher heat and cold resistance than most metals, which makes it suitable for applications requiring high or low temperatures
  3. Stainless steel has higher strength and hardness than most metals, which makes it durable and wear-resistant.
  4. Stainless steel has a higher aesthetic appeal than most metals because it can come in various finishes and colors, such as shiny, matte, brushed, or colored.

Composition of Stainless Steel

Stainless steel is primarily an alloy of iron, containing a minimum of 10.5% chromium by mass. Other elements such as nickel, manganese, silicon, carbon, and molybdenum are added to enhance specific properties. Chromium is crucial as it forms a passive oxide layer on the surface of stainless steel when exposed to oxygen, providing its resistance to corrosion.

Properties of Stainless Steel

-Excellent corrosion resistance

-Excellent mechanical properties

-High strength and durability

-Easy to clean and maintain

-Heat and fire resistance

-Magnetic and Non-Magnetic Properties

Types of Stainless Steel

Stainless steel is a type of steel that can have different microstructures, which are the arrangements of atoms and crystals within the metal.

The microstructure of stainless steel determines its mechanical properties, such as strength, hardness, ductility, toughness, etc., as well as its corrosion resistance, thermal resistance, magnetic properties, etc.

Based on the microstructure, stainless steel can be classified into four main categories: austenitic, ferritic, martensitic, and precipitation hardening.

1. Austenitic stainless steel

Austenitic stainless steels are the most common and widely used type of stainless steel, accounting for approximately 70% of global production. Containing high levels of chromium and nickel, it has high corrosion resistance, particularly to acidic and chloride environments, as well as high ductility, which means it is easy to form and weld.

Austenitic stainless steels have a face-centered cubic (FCC) crystal structure, resulting in high strength and toughness at both low and high temperatures. Austenitic stainless steel is non-magnetic in the annealed state but becomes slightly magnetic when cold worked.

Austenitic stainless steel is used in a variety of industrial and consumer products such as kitchenware, cutlery, cookware, appliances, sinks, faucets, utensils, medical equipment, surgical instruments, implants, and dental equipment.

According to the American Iron and Steel Institute (AISI) and the International Organization for Standardization (ISO), the grades and standards of austenitic stainless steel are 304, 316, 321, 347, 904L, and so on.

 

2. Martensitic stainless steel

Martensitic stainless steel is the second most common type of stainless steel, accounting for about 15% of global production. Martensitic stainless steels are the second most common type of stainless steel, accounting for about 15% of global production. Due to its tetragonal crystal structure, it has high strength and hardness, as well as high wear and tear resistance.

Martensitic stainless steels can be hardened by heat treatments such as quenching and tempering, thereby improving their mechanical properties. Martensitic stainless steels are magnetic and have lower corrosion resistance and ductility than austenitic stainless steels.

Martensitic stainless steels are used in a variety of industrial and consumer products such as turbines, cutting tools, multi-tools, etc.

The grades and standards of martensitic stainless steel are 410, 420, 440, etc. according to AISI and ISO standards.

 

3. Ferritic stainless steel

Ferritic stainless steel is the third most common type of stainless steel, accounting for about 10% of the global production. Ferritic stainless steel has a body-centered cubic (BCC) crystal structure, which gives it a high resistance to stress corrosion cracking, which is a type of failure that occurs when a metal is subjected to tensile stress and a corrosive environment.

Ferritic stainless steel also has a high resistance to oxidation and fire, as well as a low thermal expansion and contraction, which means it can maintain its dimensions and stability under temperature changes. Ferritic stainless steel is magnetic and has a lower ductility and weldability than austenitic stainless steel.

 

Ferritic stainless steel is used in various industries and consumer goods, such as automotive parts, petroleum and chemicals, heat exchangers, furnaces, etc.

Some examples of ferritic stainless steel grades and standards are 409, 430, 439, 441, 444, etc., according to the AISI and the ISO.

 

4. Duplex stainless steel

Duplex stainless steel is the fourth most common type of stainless steel, accounting for about 5% of the global production. Duplex stainless steel has a mixed crystal structure, which consists of equal amounts of austenite and ferrite. This gives it a combination of the properties of both austenitic and ferritic stainless steel, such as high strength, high corrosion resistance, high ductility, and high weldability.

Duplex stainless steel is also magnetic and has a low thermal expansion and contraction, which means it can maintain its dimensions and stability under temperature changes.

Duplex stainless steel is used in various industries and consumer goods, such as pipelines, pressure vessels, desalination plants, etc.

Some examples of duplex stainless steel grades and standards are 2205, 2507, 2304, etc., according to the AISI and the ISO.

 

5. Precipitation-hardening stainless steel

Precipitation-hardening stainless steel is the least common type of stainless steel, accounting for about 1% of global production. Precipitation-hardening stainless steel has a mixed crystal structure, which can be austenitic, ferritic, or martensitic, depending on the heat treatment and the chemical composition of the metal.

Precipitation-hardening stainless steel can be strengthened by a process called precipitation hardening, which involves heating and cooling the metal to form fine particles of a hard phase within the metal matrix. Precipitation-hardening stainless steel has high strength and hardness, as well as high corrosion resistance and ductility.

Precipitation-hardening stainless steel is used in various industries and consumer goods, such as aerospace, nuclear, energy, etc.

Some examples of precipitation-hardening stainless steel grades and standards are 17-4 PH, 17-7 PH, 15-5 PH, etc., according to the AISI and the ISO.

The applications of stainless steel

Some of the common applications of stainless steel are:

  •  Kitchenware: Stainless steel is widely used in kitchenware, such as cutlery, cookware, appliances, sinks, faucets, utensils, etc., because of its hygienic, durable, easy-to-clean, and aesthetic qualities. Stainless steel can also resist stains, odors, and flavors, which makes it ideal for food preparation and storage.

 

  •  Medical equipment: Stainless steel is also used in medical equipment, such as surgical instruments, implants, dental devices, etc., because of its biocompatible, sterile, and corrosion-resistant qualities. Stainless steel can also avoid infection and inflammation, which makes it suitable for human health and safety.

 

  •  Building and construction: Stainless steel is also used in building and construction, such as bridges, skyscrapers, monuments, railings, roofs, etc., because of its strong, durable, and weather-resistant qualities. Stainless steel can also withstand fire and earthquakes, which makes it reliable and safe for structural and architectural purposes.

 

  •  Transportation: Stainless steel is also used in transportation, such as cars, trucks, buses, trains, planes, ships, etc., because of its lightweight, fuel-efficient, and impact-resistant qualities. Stainless steel can also reduce noise and vibration, which makes it comfortable and smooth for travel and transport.

 

  •  Energy and environment: Stainless steel is also used in energy and environment, such as nuclear reactors, solar panels, wind turbines, pipelines, etc., because of its safe, reliable, and eco-friendly qualities. Stainless steel can also reduce carbon footprint and greenhouse gas emissions, which makes it sustainable and responsible for the environment and society.

Conclusion

In this article, we have explored the definition, composition, types, characteristics, and applications of stainless steel, and learned why it is such a versatile and high-performance material. 

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