How to Check the Quality of Stainless Steel

How to tell the quality of stainless steel can be an uphill task without the right information. It is even cumbersome when you are importing stainless steel sheets, plates, rods, tubes, or pipes in bulk for your fabrication needs.

The best part – this article explores various you can use to determine stainless steel quality.

Table of Contents

Method 1: Stainless Steel Visual Inspection

Stainless Steel Sheet Color

Although this process is not accurate, it can help you identify stainless steel.

In most cases, stainless steel is inherently silver or silver-grey. However, this may vary depending on the stainless steel surface finishing.

Usually, depending on the stainless steel surface finish, the color may vary slightly. Again, it makes it difficult to conclusively determine the stainless steel quality.

Key Alloying Elements Determining Color Variation	Possible Color Effects
Chrome-nickel stainless steel after pickling	Silver white
Chrome stainless steel after pickling	Grayish stainless steel
Chrome-nitrogen-manganese stainless steel after pickling	Silver white thou the color is lighter

As you can see, there is a slight variation in the stainless steel color.

Furthermore, if you are importing stainless steel material from a reputable manufacturer, they will have labels. These labels indicate the stainless steel grade.

Again, through these labels, you can determine the best quality of stainless steel for your application.

In short – visually inspecting stainless sleet will give you a “rough” idea of whether the metal is good quality or not. It is not accurate.

Using Magnet to Check Stainless Steel Quality

Whether the magnetism of stainless steel determines the quality or not is also an interesting factor to evaluate. Remember, stainless steel composition has both magnetic and non-magnetic elements.

Besides, stainless steel exists as:

Even as you try to distinguish stainless steel qualities of grades using magnets, the process has some drawbacks. Therefore, making accurate deductions on the qualities of stainless steel is virtually impossible.

Still, here are critical factors to note when using magnetism to determine the quality of stainless steel:

Ferritic stainless steels are magnetic
Martensitic stainless steels are magnetic
Duplex stainless steels are magnetic
Austenitic stainless steels are generally non-magnetic

However, there is a limit when dealing with austenitic stainless steel. The non-magnetic property only holds when they are in a softened state.

With this knowledge, you can separate softened austenitic SS from other stainless steel grades. But remember, you will not know the chemical composition of each austenitic stainless steel.

Additionally, cold-working austenitic SS will make them partially magnetic. Of course, partial ferromagnetism will be unevenly distributed. A similar phenomenon is common through special thermal treatment.

Verdict: With magnetism, you can only confirm is stainless steel is austenitic or not.

Testing Stainless Steel Quality using Mechanical Methods

Stainless Steel Mechanical Properties

There are certain mechanical techniques you can use to examine the qualities of stainless steel. Theoretically, each metal has specific mechanical properties such as tensile strength, hardness, elongation, or modulus of elasticity.

So at this point, you will conduct some tests. Through these tests, you determine whether the stainless steel quality meets the recommended mechanical properties.

Let’s look at a few tests:

Hardness – austenitic stainless steel has>165 Brinell while martensitic SS has> 280 Brinell, and Ferritic varies from 180 to 250 Brinell. Remember, these values are applicable after heating stainless steel grade to 1800^oF followed by water quenching.
Heating martensitic stainless less makes the grades more scratch resistant. However, this test does not confirm whether it is a low or high-carbon steel.
Tensile test – it is a destructive you will use to determine the mechanical properties of stainless steel

Stainless Steel Dimensional Inspection

Pipe Dimensions

Dimension is an integral quality aspect when analyzing stainless steel. For example, for sheet metal sheets, you can check the thickness, length, and width.

The best quality of stainless steel should have uniform dimensions with acceptable tolerances. You can coordinate measuring machines (CMM) or other tools like vernier calipers.

Stainless Steel Gauge

Check Qualities Of Stainless Steel through Chemical Reaction

Before you start any chemical reaction, you should ensure the surface is free from:

Coating
Scales
Grease

You can perform several tests, whose outcome can detect stainless steel. However, they may not conclusively give specific quality.

Chemical Reaction Testing Stainless Steel Quality	Observation on Determining Quality of Stainless Steel and Grade (Where Possible)
Reaction with water after prolonged exposure	No visible reaction for most stainless steel For salty water, stainless steel 316 still maintains corrosion resistance properties, unlike other grades such as 304. 304 SS is not seawater resistant.
Rubbing with copper sulfate	No color change for stainless steel
Effects of sulphuric acid	302 SS and 304 SS – a strong attack that is evident through the dark surface and green crystals 316 SS – a slow attack with a possible light brown surface
Hydrochloric acid	302 SS – rapid reaction 304 – fast attack 316 SS experiences a very slow attack 321 – fast attack 410 – rapid reaction 416 and 430 – reacts with spoiled egg odor 430 FSe – reacts with a garlic odor
Molybdenum testing	It can distinguish stainless steel with high and low molybdenum
Manganese testing	You can use it to distinguish the 200 series SS for specialized applications
Sulphur	Perfect choice to identify free machining stainless steels such as the 303 and 430F

Testing Alloying Elements in Stainless Steel

The best quality of stainless steel should have standardized chemical composition. Unfortunately, analyzing the chemical composition of various elements is not an easy task.

The best part – it gives accurate information about the stainless steel qualities of grades. In today’s industrial setups, Spectro metal analyzers play integral roles when it comes to this analysis.

Through Spectro metal analyzers, you can classify various stainless steel grades. Among the machines you can use to analyze stainless steel grades are: