Stainless Steel Grades Explained

It’s no surprise that stainless steel is so often the material of choice in modern manufacturing. Its combination of the properties of steel – ductile, malleable, easy to weld, durable and strong for its weight – plus the added benefit of resistance to corrosion, make it unbeatable in many applications.

But stainless steel isn’t a single material. It’s a family of similar steels, all featuring added amounts of chromium when compared to standard steel. This is what makes it stainless – resistant to corrosion. But beyond that, there are many varieties and grades of stainless steel. Each has a different composition and different properties to suit it for specific uses.

To help designers, engineers, fabricators and purchasers ensure they get the right stainless steel for the job, there is a standard system of defining stainless steel grades. But what are stainless steel grades? How are they categorised? How do you compare stainless steel grades to decide which is right for your project?

Rapid Metals is one of the UK’s leading metal stockists, including the most commonly used grades of stainless steel. So let us demystify the intricacies of stainless steel grading in this blog.

What Are Stainless Steel Grades?

Stainless steel grades provide a standardised and informative way to describe the chemical makeup and the mechanical properties of stainless steel.

There are several different systems for grading and describing stainless steel in use around the world and across different industries, and there are also some systems that were used in the past but have been superseded.

Officially, the most commonly used grading standard is the Unified Numbering System (UNS), which is a six-digit system that has been adopted by the AISI (American Iron and Steel Institute), the Society of Automotive Engineers (SAE) and the ASTM (American Society for Testing and Materials).

Before that, each of these bodies had its own grading/specification system for stainless steel (and other key materials). The UNS incorporates parts of the older three-digit AISI system, developed in the 1930s, but with the extra pair of digits giving more information. It is still very common to refer to stainless steel by its AISI number outside the most technical and official documentation and situations.

For instance, the most commonly used grade of stainless steel worldwide was classified as ‘304’ under the AISI system. Under the UNS it’s S30400, while a low-carbon variant previously known as 304L is S30403. Because the UNS standard incorporates the older three-digit AISI grade, stainless steel is still widely described, specified, bought and sold under this grading, especially for the most common and versatile types.

How Many Different Grades of Stainless Steel Are There?

There are other grading and standard systems around the world, such as ISO (International Standards Organisation), EN (European Norm), BS (British Standard) and JIS (Japanese Industrial Standards). Every grade of steel sold will have an entry in all these standards, and any good supplier should be able to tell you which of any of these standards apply to its products.

But, around the world, you’re most likely to see stainless steel described by the three digits of the AISI system – ironically the one standard that isn’t officially maintained anymore!

Whichever grading system is used, there are over 150 defined types of stainless steel. However, only around 20 are in common use – the others are for specialised applications.

Under the AISI and UNS grades, stainless steels are grouped into five broad series or families, represented by the first digit of the grade number. All the metals in each series have the same microscopic crystalline structure and thus will have broadly similar mechanical properties. Within each series, the individual grades have varying compositions to give them different qualities to suit them for specific purposes.

Generally, the higher the grade within each family, the better the performance of the stainless steel in terms of corrosion resistance – but higher-grade metals will also be more expensive.

Stainless Steel Grading Explained

1. Austenitic Chromium-Nickel-Manganese Stainless Steel (200 Series)

These are lower-cost grades of stainless steel for use where high corrosion resistance isn’t required. These are ideal for indoor applications that won’t be regularly exposed to weather, moisture, extremes of temperature or corrosive environments. Typical uses include household appliances, furniture fittings and car interior parts.

2. Austenitic Chromium-Nickel Stainless Steel (300 Series)

This is the most widely used family of stainless steels. They usually have good corrosion resistance and are strong, non-magnetic, have good formability and are easy to weld.

Grade 303 stainless steel has sulphur in its composition, which makes it more readily machinable than many other grades of stainless steel. The downside is that the sulphur reduces their corrosion resistance – 303 stainless steel will still be far superior to mild steel, but not as durable or resistant as higher grades of stainless steel. It’s commonly used for making nuts and bolts, precision gears and other mechanical shafts.

Grade 304 stainless steel is the most commonly used stainless steel worldwide. It has reasonable machinability and, unlike 303 grade stainless steel, responds very well to welding. It has good corrosion resistance in all but the harshest usual environments. It’s highly versatile but is particularly used in kitchen equipment and fittings, food preparation machinery and medical environments.

Grade 316 stainless steel is alloyed with molybdenum, giving it improved corrosion resistance and durability even in marine and other corrosive environments. Its resistance to corrosion and pitting even when exposed to harsh chemicals also makes it ideal for applications in the paper, textile and photographic industries.

3. Ferritic & Martensitic Stainless Steel (400 Series)

Stainless steels in the 400 Series have more chromium but less nickel than the 200 and 300 Series grades. This makes them significantly cheaper to buy in bulk. They are magnetic, have good resistance to stress corrosion cracking and can be heat treated for higher strength. However, while they won’t fully corrode away like mild steel, 400 Series grades are prone to localised corrosion and surface pitting in damp or corrosive environments. Automotive exhaust systems, cutlery and culinary knives, surgical instruments and razor blades and industrial ball bearings are common uses of 400 Series stainless steel.

4. Martensitic Stainless Steel (500 and 600 Series)

These are specialist forms of chromium alloy, optimised for durability, strength and stability in high-temperature environments. The 500 Series gains these qualities through its chemical and structural composition, while the 600 Series stainless steel grades are also heat-treated and hardened. These are used in furnaces, turbines, automotive exhausts, aerospace, chemical processing and nuclear reactors.

Choosing the Right Stainless Steel Grade

The right grade depends on the intended environment – will the metal be exposure to moisture, salt or chemicals? Is the structural durability of the metal vital or is only the surface appearance important? Does the surface have to remain entirely free of corrosion?

What are the strength requirements? Are their hygiene considerations or regulations to consider? Can or should the metal be magnetic? And what is the budget or cost allowance?

For instance, where costs must be controlled but stainless steel will only be serving as a surface finish in an indoor environment, 430 stainless steel is ideal. But for a metal that can remain flawless and hygienic when regularly wetted and cleaned, then 304 is the obvious choice. Meanwhile, a section of railing for a yacht is best made out of 316 for its added corrosion resistance in salty environments.

Understanding Stainless Steel Grading

Rapid Metals stocks 303, 304 and 316 stainless steel – as you hopefully now appreciate, this is the right material for a huge variety of non-specialist situations. Our locations in the West Midlands can supply and deliver these stainless steels in many forms – angle, box section, flat, bar (square, round and hex), tube and sheet – cut to custom lengths and in quantities large or small.

We pride ourselves on being able to supply what our customers need, whatever the job. Don’t hesitate to contact us for more information about our stainless steel or our other products.