Austenitic, ferritic, martensitic stainless steel

• Austenitic stainless steels is the most common type of stainless steel. It is non-magnetic, has good corrosion resistance, and is easy to weld. Austenitic stainless steel is often used in foodservice applications, such as kitchen utensils and appliances. It is also used in building construction, marine applications, and chemical processing. Examples of austenitic stainless steels include AISI 304, AISI 316, and AISI 316L.
• Ferritic stainless steels is less common than austenitic stainless steel. It is magnetic, has good corrosion resistance, and is strong and tough. Ferritic stainless steel is often used in applications where strength and toughness are important, such as in the construction of bridges and other structures. It is also used in the manufacturing of gears, bearings, and other machine parts. Examples of ferritic stainless steels include 405, 430, 434.
• Martensitic stainless steels is the least common type of stainless steel. It is magnetic, has good corrosion resistance, and can be hardened by heat treatment. Martensitic stainless steel is often used in applications where high strength and hardness are required, such as in the manufacture of knives, tools, and other cutting instruments. Examples of martensitic stainless steels include AISI 410, 420, 431, 440..

Here is a brief overview of the three main types of stainless steel:

Here is a table that summarizes the key differences between the three types of stainless steel:

Property	Austenitic	Ferritic	Martensitic
Crystal structure	Face-centered cubic (FCC)	Body-centered cubic (BCC)	Body-centered tetragonal (BCT)
Magnetic properties	Non-magnetic	Magnetic	Magnetic
Corrosion resistance	Good	Good	Good
Strength	Moderate	Strong	Very strong
Toughness	Good	Good	Poor
Weldability	Easy	Easy	Difficult
Typical applications	Foodservice, building construction, marine applications, chemical processing	Construction, machine parts	Knives, tools, cutting instruments

stainless steel bolts and nuts – Photo by Pixabay on Pexels.com

Advantages and disadvantages

Austenitic stainless steel

• Advantages:
  • Good corrosion resistance
  • Easy to weld
  • Non-magnetic
  • Ductile
  • Tough
• Disadvantages:
  • Not as strong as ferritic or martensitic stainless steels
  • Can be more expensive than ferritic or martensitic stainless steels

Some applications of austenitic stainless steel include:

• Kitchen utensils and appliances
• Building construction
• Marine applications
• Chemical processing
• Medical devices
• Jewelry
• Coins

Ferritic stainless steel

• Advantages:
  • Strong
  • Tough
  • Resistant to pitting and crevice corrosion
  • Magnetic
  • Relatively inexpensive
• Disadvantages:
  • Not as corrosion resistant as austenitic stainless steel
  • Not as easy to weld as austenitic stainless steel

Some applications of ferritic stainless steel include:

• Bridges
• Other structures
• Gears
• Bearings
• Machine parts
• Surgical instruments
• Razor blades

medical instruments and surgeon performing a surgery – AISI 316, SUS316, SS316 austenitic stainless steel

Martensitic stainless steel

• Advantages:
  • Very strong
  • Hard
  • Resistant to wear and tear
  • Can be heat treated to achieve desired properties
  • Magnetic
• Disadvantages:
  • Not as corrosion resistant as austenitic or ferritic stainless steels
  • Difficult to weld
  • Brittle

Some applications of martensitic stainless steel include:

• Knives
• Tools
• Cutting instruments
• Springs
• Ball bearings
• Armor plate

The best type of stainless steel for a particular application will depend on the specific requirements of that application.

What is the difference between austenitic, ferritic and martensitic steels seen from the levels of the elements and what is the effect?

The main difference between austenitic, ferritic, and martensitic stainless steels is their crystal structure. Austenitic stainless steels have a face-centered cubic (FCC) structure, ferritic stainless steels have a body-centered cubic (BCC) structure, and martensitic stainless steels have a body-centered tetragonal (BCT) structure.

The crystal structure of a stainless steel determines its properties. Austenitic stainless steels are non-magnetic, have good corrosion resistance, and are easy to weld. Ferritic stainless steels are magnetic, have good corrosion resistance, and are strong and tough. Martensitic stainless steels are magnetic, have good corrosion resistance, and can be hardened by heat treatment.

The elements that are added to stainless steel to create these different crystal structures are:

• Austenitic stainless steels contain chromium and nickel. The chromium provides corrosion resistance, and the nickel stabilizes the FCC structure.
• Ferritic stainless steels contain chromium and molybdenum. The chromium provides corrosion resistance, and the molybdenum improves toughness.
• Martensitic stainless steels contain chromium, carbon, and other elements such as manganese, silicon, and vanadium. The chromium provides corrosion resistance, the carbon allows the steel to be hardened by heat treatment, and the other elements improve strength and toughness.

The effect of the different crystal structures on the properties of stainless steel is as follows:

• Austenitic stainless steels are non-magnetic, have good corrosion resistance, and are easy to weld. This makes them ideal for applications where corrosion resistance is important, such as foodservice, building construction, and marine applications.
• Ferritic stainless steels are magnetic, have good corrosion resistance, and are strong and tough. This makes them ideal for applications where strength and toughness are important, such as bridges, gears, and bearings.
• Martensitic stainless steels are magnetic, have good corrosion resistance, and can be hardened by heat treatment. This makes them ideal for applications where high strength and hardness are required, such as knives, tools, and cutting instruments.

What is the difference between austenitic, ferritic and martensitic steels in terms of their carbon content and what is the effect?

The carbon content of austenitic, ferritic, and martensitic stainless steels is different, and this has a significant impact on their properties.

• Austenitic stainless steel have a very low carbon content, typically less than 0.2%. This is because carbon atoms disrupt the face-centered cubic (FCC) crystal structure that is responsible for the corrosion resistance and other desirable properties of austenitic stainless steels.
• Ferritic stainless steels have a higher carbon content, typically between 0.08% and 0.20%. This carbon content is not high enough to disrupt the body-centered cubic (BCC) crystal structure that is responsible for the corrosion resistance and other desirable properties of ferritic stainless steels.
• Martensitic stainless steels have a high carbon content, typically between 0.10% and 1.2%. This carbon content is high enough to form martensite, a hard and brittle phase that is responsible for the high strength and hardness of martensitic stainless steels.

The following table summarizes the carbon content and properties of the three main types of stainless steel:

Type	Carbon Content (%)	Properties
Austenitic	< 0.2	Non-magnetic, good corrosion resistance, easy to weld
Ferritic	0.08-0.2	Magnetic, good corrosion resistance, strong and tough
Martensitic	0.1-1.2	Magnetic, good corrosion resistance, can be hardened by heat treatment

The best type of stainless steel for a particular application will depend on the specific requirements of that application.

Here are some examples of applications for each type of stainless steel:

• Austenitic stainless steel are commonly used in foodservice, building construction, and marine applications.
• Ferritic stainless steels are commonly used in bridges, gears, and bearings.
• Martensitic stainless steels are commonly used in knives, tools, and cutting instruments.

It is important to note that there are many grades of stainless steel within each category. The properties of a particular grade of stainless steel will depend on its exact chemical composition.