Introduction

Stainless steel is a metal used extensively in industrial applications because it is strong, tough, and resistant to corrosion. This article will discuss the various types of stainless steel, their industrial applications, the advantages and disadvantages of stainless steel, and how to select the correct stainless steel for your needs.

What is Stainless Steel?

Stainless steel is a type of alloy steel that contains a minimum of 10.5% chromium. The chromium content in stainless steel provides resistance to corrosion, while the other alloying elements determine the grade of stainless steel. There are over 150 grades of stainless steel, each with unique physical and chemical properties. The most common grades of stainless steel are austenitic, ferritic, martensitic, and duplex.

Austenitic Stainless Steel

Austenitic stainless steels are the most widely used type of stainless steel. This class contains chromium and nickel as the primary alloying elements and are classified by their microstructure.

Austenitic stainless steels have austenite as their primary microstructure. This austenite is a face-centred cubic crystal structure formed by heat treatment at temperatures above 700°C.

The addition of nickel to chromium-rich alloys causes a suppression of the transformation of austenite to martensite. This results in stainless steel that is soft and ductile, with excellent corrosion resistance.

The most popular austenitic stainless steel grades are 304 and 316, which cover most commercial applications.

Ferritic Stainless Steel

Ferritic stainless steels are also chromium-containing alloys but have a different microstructure than austenitic stainless steels.

Ferritic stainless steels have ferrite as their primary microstructure. Ferrite is a body-centred cubic crystal structure formed by heat treatment at temperatures below 700°C.

The lower transformation temperature of ferritic alloys results in a less ductile and more brittle microstructure. However, this microstructure provides excellent resistance to corrosion and is used in many high-temperature applications.

The most common ferritic stainless steel grades are 430 and 434, which are often used in automotive applications for exhaust pipes and the building and construction industries.

Martensitic Stainless Steel

Martensitic stainless steels are similar to ferritic alloys but contain additional carbon. The addition of carbon allows martensitic stainless steels to be heat treated to high strengths.

As the name suggests, martensitic stainless steels have martensite as their primary microstructure. Martensite is a body-centred tetragonal crystal structure formed by heat treatment at temperatures below 700°C.

The martensitic microstructure is hard and brittle but can be tempered to improve ductility. Martensitic stainless steels are used in cutting tools and surgical instruments.

The most common grade of martensitic stainless steel is 410.

Duplex Stainless Steel

Duplex stainless steels are a class of alloys that contain a mixture of austenitic and ferritic stainless steel.

Duplex stainless steels have a microstructure which is a mixture of austenite and ferrite. This microstructure provides excellent resistance to corrosion and is used in many industrial applications.

The most common grades of duplex stainless steel are 2205 and 2507.

Industrial Applications of Stainless Steel

Stainless steel is used extensively in various industrial applications because of its strength, toughness, and resistance to corrosion. Some of the most common applications include:

  • Food Processing
  • Chemical Processing
  • Power Generation
  • Petroleum
  • Pharmaceuticals

Advantages of Stainless Steel

There are many advantages to using stainless steel, including:

Durability

Stainless steel is a durable material that can withstand even high temperatures. In addition, it has been proven to be strong and maintain its shape over time, making it perfect for equipment in industrial businesses.

Corrosion Resistance

The corrosion resistance of stainless steel is unmatched. This makes it perfect for environments where you need equipment that will be put through rigorous conditions on an everyday basis — like in food, medical, pharmaceutical, or chemical production.

Hygiene

Stainless steel is a popular material for hygienic applications because it’s easy to clean. Unlike other materials, such as wood or grooved plastic, which can trap dirt in their crevices, making it challenging to keep hygienically clean, stainless steel retains an even surface that makes hygienic maintenance easier.

Recyclability

Stainless steel can be recycled indefinitely, with a close to 100% recovery rate.

Disadvantages of Stainless Steel

There are also a few disadvantages to using stainless steel, including:

Cost

Stainless steel equipment may be more expensive than other materials, such as aluminium. However, the resale value of this material makes it an attractive choice for long-term investments that will pay off in years to come with lower maintenance costs and increased durability.

Fragile Surface Finish

While stainless steel has high mechanical strength, its surface finish can be challenging to keep clean and scratch-free. This may not be too big a problem in industrial applications, but when the equipment is highly visible to customers, extra cleaning is needed to maintain its surface finish.

How to finish stainless steel fabrications

Post-fabrication finishing is an essential step in ensuring the quality and longevity of stainless steel fabrications. There are a variety of post-fabrication finishing options available, including:

Mechanical Finishing (polishing)

Stainless steel fabrications are often finished by a manual or mechanical finishing process. Mechanical finishing is a cost-effective option that provides good corrosion protection, improves the appearance and increases the life of stainless steel fabrications.

Stainless steel can be polished to any level, from a dull, brushed finish right up to a mirror finish. But, of course, it all depends on the final application and desired result.

Electropolishing

Electropolishing is a more expensive option than mechanical finishing, but it provides the best corrosion protection and the most beautiful appearance. In electropolishing, the fabrication is placed in an acid bath, and a current is passed through the bath. The current removes metal from the surface of the fabrication, leaving a smooth, shiny surface.

Passivation

Passivation is a chemical process used to increase the corrosion resistance of stainless steel fabrications. Passivation is typically done after electropolishing and provides an extra layer of protection against corrosion. There are various passivation chemicals available, but all of them work by forming a thin layer of chromium oxide on the surface of the stainless steel. This layer protects the metal from corrosion.

For more information on surface finishing, please refer to this previous article.

Summary

Stainless steel is a popular material for a variety of industrial applications. It is strong, durable, and corrosion-resistant, making it a good choice for many applications. There are a few disadvantages to using stainless steel, but the advantages often outweigh the disadvantages. Mechanical finishing and electropolishing are two common post-fabrication finishing options that can be used to improve the appearance and corrosion resistance of stainless steel fabrications.

Pegasus Precision specialise in producing high-quality, precision, stainless steel sheet metal fabrications for the instrumentation industry