The steel that does not rust in weak corrosive media such as air, water, and steam is stainless steel. Stainless steel is divided into five categories: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, austenitic-ferritic duplex stainless steel and precipitation hardening stainless steel. Among them, only austenitic stainless steel and a part of precipitation hardening stainless steel (austenitic precipitation hardening stainless steel) are non-magnetic and cannot be absorbed by magnets; other types of stainless steel are magnetic and can be absorbed by magnets.
Magnet identification method: Magnets can distinguish two types of stainless steel.
1. Chrome stainless steel can be attracted by magnets in any state;
2. Chromium-nickel stainless steel is generally non-magnetic in the annealed state, and some will be magnetic after cold working.
3. High-manganese steel with higher manganese content is non-magnetic; the magnetic situation of chromium-nickel-nitrogen stainless steel is more complicated: some are non-magnetic, some are magnetic, and some are non-magnetic in vertical and magnetic in transverse.
Therefore, although the magnet can basically distinguish between chromium stainless steel and chromium-nickel stainless steel, it cannot correctly distinguish some steel types with special properties, and cannot distinguish specific steel grades. It is very likely that the magnetic 400 series stainless steel products with lower price and better quality than 200 series are mistaken for fake and shoddy products and rejected, and the ones bought back may be non-magnetic and have poor corrosion resistance. Some 200 series stainless steel inferior products.
Copper sulfate identification method:
1. Use a non-polluting pickling passivation paste and a non-toxic cleaning solution with inorganic additives at room temperature for dipping, so as to achieve the purpose of whitening the natural color of stainless steel.
2. After processing, it basically looks like a dull color. Remove the oxide layer on the steel, put a drop of water, wipe it with copper sulfate, if it does not change color after wiping, it is generally stainless steel; if it turns purple: non-magnetic high manganese steel, magnetic is generally ordinary steel or low alloy steel.
Grinding identification:
Grinding identification is to grind stainless steel on a grinder and watch its sparks. If the spark is streamlined and has more dense knots, it is high manganese steel or manganese nitrogen steel with higher manganese content; if there is no knot, it is chrome steel or chrome-nickel stainless steel.
Annealing method identification:
Cold-worked chrome-nickel stainless steel, if there is magnetism, can take a small piece to burn red in the fire and let it cool naturally or put it in water (annealing). Generally speaking, the magnetism will be significantly weakened or disappeared after annealing. However, some chromium-nickel stainless steels, such as Cr18Ni11Si4AlTi steel and Cr21Ni5Ti steel, contain a lot of ferrite elements, and a considerable part of their internal structure is ferrite. Therefore, it is magnetic even in the hot-worked state.
Identification by chemical qualitative method:
The chemical qualitative method is an identification method to identify whether the magnetic stainless steel contains nickel. Inject nickel reagent. If there is a red velvet substance floating on the liquid surface, it means that the stainless steel contains nickel; if there is no red velvet substance, it means that there is no nickel in the stainless steel. (However, due to the low nickel content in stainless steel, which is generally only a few percent, the nickel content is not easy to reveal or determine how much, and it is generally necessary to use standard samples for many experiments to master it.)
The above identification methods illustrate that: to identify stainless steel with a sensor tube, not only a comprehensive test of several methods is required, but also the test results can only determine a certain type of stainless steel, but cannot determine which alloy elements and specific contents are contained in the steel. Therefore, the method of sensory identification is currently extremely imperfect, and some may be wrong, and there are many physical phenomena, which are only known but unknown, and need to be further explored.
