When the steel type (No.) is unknown, we can determine whether it is stainless steel and what kind of stainless steel it belongs to with the help of simple instruments and senses according to the inherent physical and chemical properties of commodities (including whole materials, surplus materials, waste materials and other objects). It should be pointed out that the sensory identification can not distinguish the specific steel (type) number, but can basically distinguish three categories: chromium stainless steel, chromium nickel stainless steel (SUS304, 304) and chromium manganese nitrogen stainless steel (200 Series). The identification methods are as follows:
1. Color identification
Identify by the color change of stainless steel before and after pickling:
| Type | After Pickling | No Pickling (In case of serious oxidation) |
| Chromium nickel stainless steel | The color is silver white and jade | Brownish white |
| Chrome stainless steel | Slightly grayish white | Brownish black |
| Chromium manganese nitrogen stainless steel | Similar to chrome nickel stainless steel, slightly lighter | Black |
2. Identification with copper sulfate
The method is to remove the oxide layer on the steel, put a drop of water and wipe it with copper sulfate. If it does not change color after wiping, it is generally stainless steel; If it turns purple red, the non-magnetic is high manganese steel, and the magnetic is generally ordinary steel or low alloy steel.
3. Identification with magnet
Magnet can basically distinguish two types of stainless steel. Because chrome stainless steel can be attracted by magnet in any state; Chromium nickel stainless steel is generally non-magnetic in the annealed state, and some will be magnetic after cold processing. However, high manganese steel with high manganese content is non-magnetic; The magnetic condition of crni-n stainless steel is more complex: some are non-magnetic, some are magnetic, some are non-magnetic in the longitudinal plane and magnetic in the transverse plane.
Therefore, although the magnet can basically distinguish between chromium stainless steel and chromium nickel stainless steel, it can not correctly distinguish some steel types with special properties, let alone specific steel grades.
For steel grades with special properties, we also need to adopt the following three methods for identification.
1. Grind flower identification
Grinding identification is to grind stainless steel on a grinder and observe its spark. If the spark is linear and has more dense knots, it is high manganese steel or manganese nitrogen steel with high manganese content; If there is no knot, it is chromium steel or chromium nickel stainless steel.
2. Annealing identification
If the cold-worked chromium nickel stainless steel is magnetic, a small piece can be burned red in the fire and allowed to cool naturally or put into water (annealing). Generally speaking, the magnetism will be significantly weakened or completely disappeared after annealing. However, some chromium nickel stainless steels, such as cr18ni11si4alti steel and cr21ni5t, contain more ferrite elements, and a considerable part of their internal structure is ferrite. Therefore, it is magnetic even in the hot working state.
3. Chemical qualitative identification
Chemical qualitative method is an identification method to identify whether there is nickel in magnetic stainless steel. The method is: deeply dissolve a small piece of stainless steel in aqua regia, dilute the acid with purified water, neutralize it with ammonia, and then gently inject nickel reagent. If there is red velvet material floating on the liquid surface, it indicates that the stainless steel contains nickel; If there is no red velvet material, it proves that there is no nickel in stainless steel. (however, due to the low nickel content in stainless steel, it is generally only a few percent. The nickel content is not easy to be revealed or determined. Generally, it can only be mastered after multiple experiments with standard samples.