Experience Sharing：Stainless Steel Material Identification

Stainless steel is a common high-grade alloy steel with the characteristics of stainless, acid resistance, high temperature resistance, wear resistance and austenite generally non-magnetic. Most of the stainless steels we see in daily life are 302, 304 and other similar grade nickel-chromium stainless steels or chromium stainless steels with 1-4Cr13 (i.e. "stainless iron"). In China, there are more than 100 varieties (steel grades) of stainless steel, and the elements and contents in the steel grades are different, and their performance and value are also very different.

Some customers will have questions: If I am an amateur, how can I know that the material of the product (equipment) given to me by the manufacturer meets my requirements (steel grade)? Juye metal tells you here that the identification of stainless steel must be carefully studied, the steel grade imprint must be clarified, and comprehensive tests and other methods can be used to correctly identify it.

1. Check the quality certificate and verify the mark on the steel or packaging

The quality certificate is the supplier's confirmation and guarantee of the inspection results of the batch of products. It not only states the name, specification, number of delivered pieces, weight and delivery status of the material, but also must state all the inspection results of the specified guarantee items.

2. Physical identification method

For products without quality certificates and where the marks on the steel or packaging are no longer obvious or have fallen off, it is necessary to master the physical identification method, that is, when the steel type (grade) is unknown, according to the inherent physical and chemical properties of the commodity (including the whole material, residual material, waste material and other physical objects), with the help of simple appliances, use the senses to determine whether it is stainless steel and which type of stainless steel. 

(1) Color identification

The surface color of pickled stainless steel is silvery white and smooth: chromium-nickel stainless steel is silvery white with a jade color; chromium stainless steel is white with a slightly grayish luster; chromium-manganese-nitrogen stainless steel is similar to chromium-nickel stainless steel but slightly lighter. The surface color of stainless steel that has not been pickled: chromium-nickel steel is brownish white, chromium steel is brownish black, and chromium-manganese-nitrogen is black (these three colors refer to the colors of heavier oxidation). The surface of cold-rolled chromium-nickel stainless steel that has not been annealed is silvery white with a reflective surface.

(2) Magnet Identification

Magnets can basically distinguish between two types of stainless steel, because chromium stainless steel can be attracted by magnets in any state; nickel-chromium stainless steel is generally non-magnetic in the annealed state, and some will be magnetic after cold processing. However, high manganese steel with a higher manganese content is non-magnetic; the magnetic situation of chromium-nickel-nitrogen stainless steel is more complicated: some are magnetic, some are non-magnetic, and some are non-magnetic on the longitudinal surface but magnetic on the transverse surface. Therefore, although magnets can basically distinguish between chromium stainless steel and chromium-nickel stainless steel, they cannot correctly distinguish some special types of steel, let alone specific steel grades.

(3) Copper sulfate identification

Remove the oxide layer on the steel, put a drop of water on it, and rub it with copper sulfate. If it does not change color after rubbing, it is generally stainless steel; if it turns purple-red, it is non-magnetic for high manganese steel, and magnetic for ordinary steel or low-alloy steel.

(4) Grinding identification

When grinding stainless steel with a grinder, observe the sparks. If the sparks are streamlined and have more and denser knots, it is high manganese steel or manganese-nitrogen steel with a high manganese content; if there is no knot, it is chromium steel or chromium-nickel stainless steel.

(5) Annealing method identification

If the cold-processed chromium-nickel stainless steel is magnetic, a small piece can be put in the fire and let it cool naturally or placed in water (annealing). Generally speaking, the magnetism will be significantly weakened or completely disappear after annealing. However, some chromium-nickel stainless steels containing more ferrite elements are magnetic even in the hot-processed state.

(6) Stainless steel identification reagent

Stainless steel identification reagents are used to judge and distinguish stainless steel by observing the color change characteristics of stainless steel. "Color change" is often related to the presence of specific elements such as nickel (Ni), molybdenum (Mo), and manganese (Mn) in the tested steel. At present, there are many types of stainless steel identification reagents on the market, but they are actually similar. They can be basically divided into two types, one does not require batteries, and the other requires batteries.

Test method (battery-free reagent): Drop the stainless steel identification reagent on the surface of stainless steel, and observe the color change of the reagent after two minutes to distinguish the stainless steel grade. Stainless steel identification reagents that do not require batteries are often not subdivided. The use of this type of small steel identification reagent is mainly to measure the color change after the droplet is dropped on the stainless steel surface, and then distinguish it through the chromatogram. For example, there is a stainless steel identification reagent called "304" on the market, which is marked with four standard colors, corresponding to four stainless steel grades - 201, 202, 301, and 304, of which 201 corresponds to dark red, 202 corresponds to red, 301 corresponds to light red, and 304 corresponds to colorless or light yellow. These colors refer to the colors presented by the stainless steel identification reagent when used for testing.

There are many types of identification reagents on the market that require the use of batteries, such as "Mo2, low Ni, Ni2, Ni4, Ni6, Ni8, Ni14, Ni20, Ni40, Ni60", etc. Some products use "N" instead of "Ni" on the label. These products can be used alone or in combination to determine the approximate content (percentage) of the corresponding elements of nickel (Ni), molybdenum (Mo), and manganese (Mn) in steel, and then distinguish steel according to relevant standards.

Of course, if you want a more accurate test result, you still need professional spectral analysis.

The above is what Juye Metal shared with you today about several ways to identify good and poor stainless steel materials. Of course, the best way to correctly distinguish stainless steel grades is to conduct in-depth investigations and research and ask relevant units to conduct tests and judgments. If you have better detection and identification methods, welcome comment and add.