Understanding the Different Grades of Stainless Steel

February 19, 2021
In this post, you'll read:Stainless steel is grouped into families and grades in order to make sense of each blend’s alloy composition and uses.

Table of Contents

To call a metal “stainless steel” is correct, but there is much more detail needed to decide if it is the best type of stainless steel for your project’s needs. Stainless steel is grouped into families and grades in order to make sense of each blend’s alloy composition and uses. 

At its core, “stainless” steels are alloys with an iron base, containing at least 11 percent chromium. Chromium gives stainless steel the heat resistance and rust resistance that are drawbacks of other iron-based alloys. With so many families and grades, it can be overwhelming to choose the best one. Work with your stainless steel supplier to ensure their engineers provide you with the optimal grade for your project and environment. 

The Alloy Composition of Stainless Steel

As with any type of steel, the largest inclusion in the alloy composition is, of course, iron. Iron is the building block for all steel alloys, and what separates their uses and values is what is added to the iron base. 

Chromium 

To qualify as stainless steel in most classification guides, an alloy must contain at least 11 percent chromium. Some metal grading bodies set the limit at other close amounts, such as 10.5 percent. Alloys with a chromium content of 11 percent or more have rust corrosion resistance and heat resistance. Chromium is unstable when exposed to oxygen, which causes stainless steel to form a “passive layer” on the surface that protects the metal below. Higher chromium percentages make stainless steel more lustrous and polishable, but chromium is also a brittle, hard metal. 

Nickel

A content of 8 percent or more of nickel in an alloy lends extra corrosion resistance to the alloy as a whole. Nickel is exceptionally workable, with both high malleability and ductility. This aids stainless steel to form more specific, thinner shapes. An alloy with high nickel content is stronger and conducts heat better. 

Molybdenum

Stainless steel grades that include molybdenum resist pitting corrosion. Some alloys will have a PREN given, which is the pitting resistance equivalent number. The higher an alloy’s PREN, the more it can resist. 316 grade stainless steel has a very high PREN number. 

Carbon

Carbon is present in some alloys of stainless steel and often at levels at or less than one percent. Carbon steel is a different iron-based alloy group than stainless steel, but the high strength, hardness, and resistance to wear that carbon lends to the metal means some overlap between the two types. Higher carbon content also makes the steel more difficult to weld, so some blends for welding projects have reduced carbon for this reason.  

Other Elements

Other elements take their place in different stainless steel alloys, often in trace amounts. They do not change the result as drastically as chromium, nickel, molybdenum, or carbon, but there are other niche reasons to use them. Other elements you may see in alloy ingredient lists include Sulfur, Silicon, Phosphorus, Nitrogen, Titanium, Niobium, Aluminium, Selenium, and Manganese.

Stainless Steel Classes

There are five classes of stainless steel based on their atomic crystalline structure. It is not as important as a consumer to understand these classes compared to the actual stainless steel grades. However, classes provide a good “broad strokes” view of what tasks are most suitable for particular steels. These classes are austenitic, ferritic, duplex, martensitic, and precipitation hardened. 

This class is the most widely used class of stainless steel. Its alloy composition works well for welding and has substantial corrosion resistance. Metalworkers cannot heat-harden austenitic steels, so it is not the best choice for tools, especially those that require an edge. 

Austenitic

Ferritic

Ferritic stainless steel is one of the most cost-effective stainless steel families. The lower price is the result of its lower nickel content. This makes ferritic steel less attractive, so it is mostly used in hidden applications like ducts or exhaust pipes. 

Duplex

A mixture of austenitic and ferritic phase metallurgic structures, duplex stainless steel, features the corrosion resistance of austenitic and the low nickel content of ferritic steel. The much higher chromium content, typically between 20 and 28 percent, offsets some of the cost benefits of including less nickel. However, the reduced nickel and improved strength offer cost savings. Duplex steel can hold its shape in thinner sheets than other classes. 

Martensitic and Precipitation Hardened 

These two classes are very similar and only differ on an atomic level. Both use higher carbon content in the alloy, so they are more suited for hardened steel brought to a fine point. They are used more in blades and cutting tools like scissors. 

Grading Systems

Stainless steel grades make it easier for manufacturers, producers, and consumers to communicate about the contents of the alloys they are working with or trading. There are many authoritative grading organizations, and your country may very well have its own organization providing its standards and definitions. Here are some of the preeminent groups grading steel and where they operate. 

GB

GB stands for the Guobiao Standard, which was established by the Standardization Administration of China. The GB standard helps businesses work with stainless steel suppliers in China and aids manufacturers by standardizing their products. 

SAE

SAE International is the Society of Automotive Engineers and was founded in 1905. They were one of the first large professional organizations to provide international industry standards. They established the earliest grades of stainless steel, many of which are still used today. 

ASTM

The American Society for Testing and Materials, or ASTM, is another international organization that promotes the creation and use of standards for a wide variety of materials. They currently have members from over 140 countries, and they operate offices around the world. 

UNS

UNS is the unified numbering system that is used for alloy designation in most of North America. Stainless steel alloys in the UNS all begin with the letter “S.” 

The Most Popular Grades of Stainless Steel 

Listing all the grades of stainless steel would be far more information than most people need, even over a lifetime of design and production choices. Instead, let’s look at a few of the most commonly used stainless steel grades and what their strengths and weaknesses are. The main two groups of grades are the 200 family and the 300 family. 

The 200 group of grades are less expensive due to their lower chromium and nickel content. While they will not hold up to heavy use or corrosive conditions, they function fine in decorative applications. The primary grades in the 200 group are the 201 and 202 grades. 

The 300 family of stainless steel grades are rated for food safety, while the 200 family is not. When planning for kitchen tools or surfaces, many countries’ regulations will limit the grade of stainless steel that can be used. To err on the side of caution, 304 should be the standard choice for any machinery or metal that could come into contact with food. The most common grades in the 300 group are 304 and 316. 

201 Stainless Steel 

201 grade is typically the least expensive option for stainless steel while still being considered “stainless.” It has a near bare minimum chromium content and reduced nickel. This makes it much cheaper than alternatives but also causes it to be susceptible to corrosion. 201, like 202, needs to be thicker to hold its form if actively used. For aesthetic purposes like stainless steel sheets for interior decorations, 201 should work just fine in many cases. 

202 Stainless Steel 

The 202 grade stainless steel has a higher chromium and nickel content than the 201 grade. This makes it more expensive than 201 but more resistant to corrosion. 202 is an excellent compromise between cost and quality, especially if 201 is not a good match. 

304 Stainless Steel

The most commonly used form of stainless steel is the 304 18-8. This is a shorthand name that denotes the 18 percent chromium and 8 percent nickel. This alloy composition is very cost-effective and provides excellent corrosion resistance from many common sources. 

One drawback of the 304 blends is the ability to be corroded by chloride and saline sources. This means it is not the best fit for applications near pools or outside uses near the coast. Seawater and chlorinated pool water will eventually cause pitting on the surface, which is small localized corrosion areas. Pitting can also be caused by salt usage in the winter to de-ice roads. 

316 Stainless Steel 

A slightly more expensive option for 300-series stainless steel is the 316 form. The 316 grade has 2 to 3 percent molybdenum, which gives it the corrosion-resistance that 304 lacks. The molybdenum makes 316 resistance to chlorides and industrial solvents. Pieces of 316 contain around 3 percent of molybdenum to provide it with resistance to pitting corrosion. 

Choosing a Grade of Stainless Steel 

For many projects, the budget will be one of the largest concerns. In that case, choosing a cheaper option like a grade from the 200 family can help you cut costs. If you are not sure a 200 grade will meet your needs or if you have a hard-to-define situation, reach out to the FTSSS team. The customer support team works closely with our engineers to ensure each customer receives the best grade of stainless steel for their specifications. 

Hi, I am Linda, FTSSS blog writer . I have been working on stainless steel  since I graduated from university. How time flies!  13 years have slipped away from me. ‘Stay hungry, Stay foolish.’ I am walking steadily on the road of seeking knowledge every day.

Ask For Free Samples

We will contact you within 1 working day, please pay attention to the email with the suffix “@ftsss.com”.

2023 Hottest Designs

More than 100+ popular patterns of stainless steel decorative sheet to help you win the market. 

You can get this catalog file for free as long as you leave your email address.