How to choose a stainless steel grade?

December 28, 2020
In this post, you'll read:It might surprise you to learn that there are over 150 different grades of stainless steel. Each of these stainless steel grades has varying properties. So how are you going to choose the best stainless steel grade?

Table of Contents

It might surprise you to learn that there are over 150 different grades of stainless steel. Each of these stainless steel grades has varying properties. That is because stainless steel is an alloy. If a different proportion of raw materials are combined, the stainless steel produced will have other properties. So how are you going to choose the best stainless steel grade? To help you make the best choice, we have first listed the different types of stainless steel.

What are the different types of stainless steel? 

1. Austenitic 

You can create austenitic stainless steel by adding austenitic chemicals. Examples of austenitic chemicals are nickel, manganese, and nitrogen. Austenitic stainless steel has a face-centered cubic crystalline structure. You can get this structure by adding chemicals to its composition. These chemicals allow austenitic stainless steel to maintain its face-centered cubic crystalline structure. Otherwise, it will revert to its ferrite form. Without these chemical additions, you will have ferritic stainless steel. 

Austenitic stainless steel grades are further broken down into two subgroups, the 200 series and the 300 series. The 300 series uses nickel to achieve their austenitic state. On the other hand, the 200 series substitutes nickel with manganese and nitrogen. The more common series is the 300 series. In it, type 304 is the most widely used stainless steel. You can easily find it in your kitchen as utensils and cooking equipment. The next most common type is type 316, which has added acid resistance due to molybdenum. Overall, austenitic stainless steel is a popular choice due to its corrosion resistance. One typical example is stainless steel screws, which are an essential component of many machines and structures.  

This is an image of stainless steel utensils.

2. Ferritic

Ferritic stainless steel is free from austenitic chemicals. This makes them much cheaper than their 300 series counterparts. Since Nickel prices tend to fluctuate, the demand for ferritic stainless steel is becoming more commonplace. Ferritic stainless steel has a body-centered cubic crystalline structure. This type of stainless steel contains low amounts of carbon, making them magnetic and not hardenable. However, ferritic stainless steel is less corrosion resistant and is not as strong or stiff as austenitic stainless steel. If you use them for hardware or interior construction, then ferritic stainless steel will help you save on costs.

3. Duplex

You can guess what this type of stainless steel is by its name. Duplex stainless steel is named after its two-phased metallurgical structure. They are both austenite and ferrite. This type of stainless steel is engineered to be used in more demanding environments. It is more corrosion resistant and more robust than the commonplace type 304 or 316. Strength wise, it is about twice as strong as your regular austenitic or ferritic stainless steel. It is also cheaper than pure austenitic stainless steel that contains nickel. Many large-scale projects in the petrochemical industry utilize Duplex grade stainless steel. This is due to its cost optimization. If you need higher corrosion resistant grade steel, you can duplex stainless steel containing a higher chromium level. Chromium levels affect stainless steels’ resistance to corrosion. Duplex grade stainless steel also has ferritic properties, which means that it has stress corrosion cracking resistance. Stress corrosion cracking resistance protects stainless steel from acidic environments and varying temperatures. This is a common problem that austenitic stainless steel faces. 

4. Martensitic

Martensitic stainless steels contain chromium, carbon, and other elements such as nickel and sulfur. They are incorporated into martensitic stainless steel to produce different properties. You can opt for different grades of martensitic stainless steel, depending on your needs. Generally, they are strong and durable during hardened conditions. In other words, they can be hot worked. You can add sulfur into its composition to also make them more machinable. They are also magnetic during both annealed and hardened conditions. These types of stainless steel are less chemically resistant than austenitic stainless steel. However, their hardness makes them the right choice for products like knives, where sharpness is essential. You can also find them in other cutlery, medical equipment, and stationery.

This is an image of stainless steel knife.

Factors to consider when choosing the stainless steel grade

1. The operating environment for the stainless steel product

The reason that many people choose stainless steel products is because of their corrosion resistance. But, different grades of stainless steel have different levels of corrosion resistance. This means that if the environment is more likely to cause your stainless steel product to rust, you will need to think of the type of stainless steel to resist such corrosion. Stainless steel with more chromium is more corrosion resistant. This means that austenitic and duplex stainless steel is a good option as they contain nickel. All you have to do is opt for the grade that includes the right amount of chromium.

This is an image of stainless steel handrail.

2. Consider your budget

If you are buying cutlery or kitchen equipment, the price differences might not affect you. But, money becomes a concern on large scale projects. Both ferritic and martensitic stainless steel are the more affordable options due to their low nickel content.

This is an image of calculating money.

3. Do you need stainless steel to be machined?

You can machine most grades of stainless steel. Stainless steel grades that contain sulfur are more machinable than their counterparts. Some examples of stainless steel grades that compose of sulfur are austenitic grade 304 and ferritic grade 416. 

4. How much weight does the stainless steel need to hold

The tensile strength of the stainless steel will affect how much weight it can hold. This is important where safety is a concern. Stainless steel that has been hardened and stress relieved will have a higher tensile strength. An example is a martensitic grade 420 stainless steel. The tensile strength of grade 420 stainless steel is originally not as high. After undergoing the processes of hardening and stress relieving, its tensile strength almost triples. Stainless steel grades that have higher tensile strength are optimal choices for heavy loads.

5. How much heat do you need your stainless steel to withstand

You can quickly test the temperature required to choose the right stainless steel grade. You can measure the highest and lowest temperature of the working environment you want to use stainless steel. Subsequently, look at the stainless steel’s melting point to see if it is suitable for the environment. If the melting point is lower than the working environment’s highest temperature, you should not use it.

This is an image of stainless stel architecture.

6. Do you need your stainless steel to be welded or formed?

Hot creaking and stress corrosion cracking are the most common problems you face when you weld stainless steel. For example, you should not use most martensitic stainless grades as they contain high carbon levels. Austenitic stainless steel is the right choice for welding and forming as they have lower carbon content. However, austenitic stainless steel is prone to stress cracking. To prevent stress cracking of austenitic stainless steel, you can incorporate minute amounts of ferrite. In other words, duplex stainless steel is a good fit if you need both weldability and formability. 

Different stainless steel grades and their typical applications 

1. Austenitic 300 series 

Typical grades are 304 and 316. You would generally use them for medical equipment, food containers, and architectural purposes. Its high corrosion resistance level also makes it suitable for marine equipment such as seawater heat exchangers.

2. Ferritic 400 Series

The ferritic 400 series of stainless steel are ideal for high-temperature environments such as the petrochemical industry and the exhaust system for automobiles. 

3. Martensitic 400 Series

The martensitic 400 series of stainless steel are often used for utensils, cooking applications, and multi-purpose tools. This is because they need sharpness, which comes from hardenability. 

4. Duplex series

These contain both the austenitic and ferritic properties of stainless steel and are commonly used for demanding environments. For example, chloride and acid intensive environments such as reactors and pipework are usually made from duplex grade stainless steel. You can also find them in refineries and chemical tanks.

This is an image of stairs.

Conclusion 

As you can see, there are many different grades of stainless steel. It would help if you only chose the stainless steel grade after analyzing its applications. If you are still unsure which grade of stainless steel you should go for, you can always contact our experts at FTSSS. Our experienced team will guide you to make the best choice possible.

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.

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