Stainless Steel: The Definitive Guide to Grades, Properties, and Uses

Stainless Steel is used across every industry and in all walks of life, from domestic kitchens to high-performance corrosion-resistant structures. Its key property, corrosion resistance, is revered in manufacturing and design.

But not all stainless is the same, like any commercially available metal, there are multitudes of different grades, types and shapes available to both the engineer and hobbyist alike.

A Simple Guide to the Different Types of Stainless Steel

There are 4 different types of stainless steel: Austenitic, Ferritic, Martensitic and Duplex. Differences are more than just the chemical composition, but the molecular structure, meaning their properties can vary greatly.

Austenitic

These are the most commonly used types of stainless steel. They have very good corrosion resistance. Commonly used grades include 304 and 316.

Ferritic

Ferritic stainless steel is notable for its lack of Nickel, and as the name suggests, these grades are magnetic. 430 grade is probably the most widely used Ferritic stainless steel.

Martensitic

These types of stainless steel are the only ones that are heat-treatable. Martensitic grades are often considered high-performance grades, often used for surgical components and tools.

Duplex

Containing both ferrite and austenite, Duplex stainless steels can have both higher strengths than either of the two constituent groups, and just as good corrosion resistance. However, they are typically more expensive.

Key Properties of Stainless Steel

Stainless steel is not the cheapest metal to use, nor is it the easiest to work with, so why is it still so common across almost every industry? These are the key properties of stainless steel that make it so useful.

Corrosion Resistance

The clue is in the name; stainless steels generally are very good at resisting corrosion. Be that is from atmospheric conditions such as rainwater, acidic environments, or marine applications, there is usually a grade of stainless steel that will suit the task.

In general, stainless steels don’t rust because of their high chromium content, typically over 10.5%. The chromium reacts with oxygen at the surface of the metal and forms a protective layer of chromium oxide, preventing further corrosion.

High Strength

Whilst it is not as strong, pound-for-pound, as materials such as Titanium, stainless steel can be as strong as the highest grades of structural carbon steel. This means it can be used for structures such as bridges, platforms, and pressure equipment, all with corrosion resistance built in.

Workability

Just like regular carbon steel, stainless steel is very easy to manipulate, machine and weld, with the right equipment. Processes such as TIG welding and laser cutting apply very well to stainless steel, enabling cost effective manufacture of complex parts.

Stainless Steel Applications

One of the best qualities of stainless steel is how widely it can be used, from pharmaceutical to automotive. Here are some standout applications:

Food & Beverage Production

Wherever food is manufactured or processed, the surfaces, machines and tools that come into contact with it need to be sterile and clean. Because stainless steel needs no coatings, no paint, plating or powder coating, it significantly lowers the risk of contamination in the product.

Stainless steel can also be polished to produce a ‘hygienic’ finish that allows for thorough washing down of processing equipment such as hoppers, conveyors, etc.

Medical Instruments

For the same reasons listed above, cleanliness, stainless steel is widely used in the medical and pharmaceutical industries, too.

Not only can it be used to process products intended for human consumption, but it can also be used for tools and complex medical devices, thanks to its great manufacturability and mechanical properties.

Construction and Architecture

Whilst stainless steel is more expensive than mild steel, it is still often used on large scale construction projects, either for architectural or maintenance purposes.

One of many examples of this is the Chrysler Building in New York. It is clad in stainless steel panels to give an impressive Art Deco appearance, but also because it requires minimal maintenance.

Choosing the Right Stainless Steel: Factors to Consider

Material selection is important for any part, especially so if corrosive or harsh environments are involved. Incorrect selection could mean a part lasts only one-tenth of its design life and causes significant cost and time delays when it needs to be replaced.

Strength

Material strength is one of the most important factors; if a part is made too weak, it will fail prematurely, if it is too strong, there’s a good chance it will be needlessly expensive to manufacture.

The common grades of stainless steel are all around the same ballpark for tensile strength, but there are some standout examples, such as 440C, a martensitic grade, that, when heat-treated, can be up to 3x stronger than 316.

Corrosion/Environmental Resistance

It’s important to understand that not all corrosion acts in the same way, and not all stainless steels are best suited to certain types.

For example, 316 grade stainless steel is perfect for marine applications, being very resistant to saltwater corrosion, but 304 is far better at resisting nitric acid, especially at high temperatures.

Cost

In many industries, this is the biggest factor in material selection. Not only should you consider the material itself – duplex grades are more expensive than austenitic, for example – you have to consider the cost of manufacturing and sourcing the material.

Commonly used grades such as 316 and 304 are available in every shape, size and thickness you could require, but getting the same in a martensitic grade will be difficult, and the lesser used sections will be even more expensive. Furthermore, with martensitic grades

Stainless Steel Care: Extending Its Life

Stainless steel is pretty durable, but like any product, it can be improved and prolonged by care and maintenance.

Many companies make coatings specifically for stainless steel to enhance their chemical resistance. The most commonly used of these tends to be an epoxy or polyurethane paint finish. These coatings can increase resistance to sulphur dioxide, acid and alkali environments.

In a domestic setting, stainless steel utensils, cookware and surfaces are pretty resilient to most cleaners, but it’s good to avoid highly abrasive tools, for the risk of damaging the surface finish – but check the manufacturer’s recommendations before commencing.