How to Make Sure Your Stainless Steel is the Correct Grade

March 18, 2021
In this post, you'll read:There is an incredible assortment of grades of stainless steel. If you are not sure which grade best fits your needs, check out this guide.

Table of Contents

Differences in Alloy Composition

There is an incredible assortment of grades of stainless steel. If you are not sure which grade best fits your needs, check out this guide. 304 is the most commonly used form of stainless steel, especially in the “304 18-8” type. This is a shorthand name that denotes the 18 percent chromium and 8 percent nickel in the alloy’s composition. 

316 grade contains much more chromium than 304, and it is a better choice for projects that must withstand more corrosion. 316 stainless steel is often the only responsible choice in environments with higher moisture and saline in the air, like coastal regions. The higher price of 316 means that you may want to verify that you are receiving the right alloy. 

201 and 202 stainless steel are cheaper alternatives that do not contain enough chromium for stainless steel’s usual corrosion resistance. Whether you think a supplier has sent you a 200 family steel on accident or to cut their own costs, it can set your mind at ease to test your metal. 

This is an image of stainless steel chemical compoistion form

Verifying your stainless steel’s composition can be an expensive undertaking, but it does not need to be. There are many ways to make sure your stainless steel is 201/202, 304, or 316 grade.  

Mill Test Certificate

A lot of the stress of testing your stainless steel can be avoided if you deal directly with a factory. Then, either the metal is manufactured on-site or sourced directly from a mill. Mills are required to issue a certificate for each batch of metal that shows the elemental composition of the alloy. You can confirm that an alloy falls inside the grade parameters you need by reviewing the mill test certificate. 

Any company working with stainless steel should have these on hand and be willing to provide clients with a copy when one is requested. Do not be afraid to ask for this extra verification; it shows you understand what you are purchasing and are metal savvy. High-quality mills and stainless steel factories where people take pride in their work will be happy to give you whatever documentation you would like. 

Mill test certificates are typically produced using large-scale spectrometers that are able to reveal all inclusions in an alloy with startling accuracy. Spectrometers are expensive and highly technical; even NASA uses spectrometers to measure the chemical components of celestial bodies. These machines work by emitting light at something and then collecting the light waves it produces. 

This is an image of mill test certificate.

By analyzing the frequencies of light waves that have come into contact with something, a spectrometer reading can provide in-depth and accurate information about the object. It can even be used to measure the molecular structure of atmospheric air. Each substance and element gives off its own unique light wave frequency. 

The Importance of Testing

Finding a reliable supplier in a saturated marketplace can be a daunting task. That’s why we posted a guide to finding a stainless steel supplier in China. Many suppliers are able to send samples in advance of a whole order. Testing this sample is a great way to ensure that the supplier is trustworthy and provides accurate alloys. 

The larger your order or more functional the steel needs to be, the more important testing becomes. These tests are also a smart choice if you have a facility that needs some metal replaced. Understanding what type of stainless steel is in place now can help you match existing structures or analyze why the old alloy failed. 

Check the Color

One general test for stainless steel is to compare its color with another sample. Visual testing is one of the most basic nondestructive tests that is used for determining metal specifications. 

Stainless steel of the 304 grade is in the 300 family of stainless steel grades. The 300 family is full of austenitic alloys, which is an alloy group based on the shape of the molecules. Austenitic stainless steel has a yellow tinge to the natural color. This contrasts with the ferritic alloys of the 400-grade family, which have a blue metallic tinge. 

This is an image of stainless steel.

The downside of a color check test is that 304 and 316 look fairly similar, and you may not be able to reliably determine subtle differences between the two. 

Handheld Analyzer

While this may not be the best option for every client, getting a handheld alloy analyzer can be a huge help for checking a variety of metals. Handheld analyzers can be expensive, but they provide flexible, repeatable testing for many alloys. 

Another benefit of an electronic metal testing device is that they are nondestructive. Some methods of testing metal will alter or destroy part of the surface, as you will be testing the metal’s reaction with different other compounds. Handheld metal analyzers are made to test for the presence of heavy metals in an assortment of locations. These devices can also be used during production to test metals or to check the level of metals in soil. Testing stainless steel sheets is a very simple and easy use of these handy devices. 

This is an image of handheld metal analyzer.

Larger and more intensive electronic testing devices exist but might be overkill as a metal consumer. High-tech testing devices include radiographic machines and ultrasonic tests. These both usually require large, expensive technology that also makes it more difficult to test metals that have already been put into a larger piece like architecture. If you have concerns about metal sheets that you have already put into place, a light, handheld testing device will make the process much easier. The only upside to these alternative electronic options is the vast amount of detail and certainty they provide. 

Corrosion Resistance Tests

A common and simple test for determining if stainless steel is truly 304 or 316 is a corrosion resistance test. Corrosion resistance tests can provide a lot of information about an alloy, especially if you are comparing two samples that may not match. 

Unfortunately, corrosion resistance tests are destructive, and this type of testing should only be done on a sample of metal that can be discarded after. The two easiest corrosion resistance tests for stainless steel involve sulphuric acid and hydrochloric acid. 

If you are comparing 304 and 316-grade stainless steel, checking the corrosion is a good choice if you have some metal to spare. Be careful if you use these dangerous compounds to test your metal; the reaction may even create hazardous fumes. 

Sulphuric Acid Test

Sulphuric acid is one of the options for testing the corrosion resistance of stainless steel. 316 grade will resist sulphuric acid a lot better than 304 grade. Sulphuric acid will quickly eat away at 304-grade stainless steel in a chemical reaction that leaves behind a dark surface. 

304 grade and sulphuric acid will also react and create green crystals. If you have 316, it should only slowly react to sulphuric acid. The surface of 316-grade stainless steel will turn a brown color after this test. 

This is an image of acid solution.

Hydrochloric Acid Test

Another option for a chemical corrosion resistance test is hydrochloric acid. Hydrochloric acid will rapidly deteriorate 304 grade and cause gas to erupt from the reaction. This acid will only wreck 316-grade steel very slowly, thanks to the higher chromium content. 

Avoid These Problems

The best way to avoid the need to test new stainless steel orders is to work with a dependable supplier. FTSSS has been a reliable stainless steel supplier for eight years, and our quality assurance team works diligently to ensure every order is correct. To hear more about our process or get a custom quote for your project, contact our team at sales@ftsss.com or give us a call at +86-137-986-86781. 

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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