How to Identify Stainless Steel Materials 304 and 316

1. Color identification

After pickling, the surface color of stainless steel is white and bright. The color of chromium-nickel stainless steel is white and like a jade; The color of chrome stainless steel is white and a bit gray, it also have a weak luster; The color of chromium-manganese-nitrogen stainless steel is similar to that of chromium-nickel stainless steel, but it is lighter.
The surface color of unpickled stainless steel: Chromium-nickel steel is brown-white, chromium steel is brown-black, and chromium-chromium-manganese is black (these three colors refer to the heavier oxidized color). The surface color of cold-rolled unannealed chromium-nickel stainless steel is silver-white and reflective.

2. Identification with magnets

Magnets can be leveraged to distinguish between two types of stainless steel. Because the chromium stainless steel can be attracted by the magnet in any state. The chromium nickel stainless steel is generally non-magnetic in the annealed state; after the cold working, some of them will be magnetic. However, manganese-rich steels with higher manganese content are non-magnetic. Chromium-nickel-nitrogen stainless steels have more complex magnetic properties: some are non-magnetic, some are magnetic, and some have a non-magnetic vertical surface and a magnetic transversal surface. Therefore, although magnets can basically distinguish between chromium stainless steel and chromium-nickel stainless steel, they cannot correctly distinguish some special-property steel grades, let alone distinguish specific grades or types of stainless steel.

3. Identification with copper sulfate

Remove the oxide layer on the steel, put a drop of water and rub with copper sulfate. If it is not discolored after rubbing, it is generally stainless steel; If it turns purple, it can be one of the two results: 1) if it is non-magnetic, it is high manganese steel; 2) if it is magnetic, it is generally ordinary steel or low alloy steel.

For steel grades of special property, we can adopt the following three methods for identification.

4. Grinding spark identification

Grinding spark identification is to grind the stainless steel on the grinder and observe the spark. For example, if the spark is streamlined and has more dense knots, it is high manganese or manganese nitrogen steel with higher manganese content; if there is no knot, it is chrome steel or chrome nickel stainless steel.

5. Annealing identification

If the cold-worked chromium-nickel stainless steel is magnetic, two methods can be used to verify it: take a small piece and burn it to red, then 1) let it cool naturally; 2) put it into water. In general, the magnetism will be significantly reduced or completely disappear after annealing. However, some chromium-nickel stainless steels, such as Cr18Ni11Si4AlTi steel and Cr21Ni5Ti steel are magnetic even in the hot working state, because they contain more ferrite elements and a considerable part in their internal structure are ferrite.

6. Chemical qualitative identification

Chemical qualitative identification is a method of identifying whether magnetic stainless steels contains nickel. The method is to dissolve small pieces of stainless steel in aqua regia, dilute the acid solution with clean water, add ammonia water, and then gently inject the nickel reagent. If a red fluffy substance floats on the liquid surface, it means that the stainless steel contains nickel. If there is no red fluffy material, it proves lack of nickel in the stainless steel. Since the nickel content in stainless steel is relatively low with only a few percent, it may not be easy to perceive or determine the content. Generally, many times of standard sample experiments need to be done to determine the content.