Характеристики нержавеющей стали и ее классификация

There are numerous types of stainless steel, which can be classified into several categories based on their microstructure at room temperature:

: Examples include 304, 321, 316, 310, etc. The 200 series represents a low-nickel, cost-effective stainless steel, also belonging to the austenitic category.

: Examples include 430, 420, 410, etc. Austenitic stainless steel is non-magnetic or weakly magnetic, while martensitic or ferritic stainless steel is magnetic.

For hardware products, factors such as rust resistance, hardness, and machinability must be considered. The rust resistance, heat resistance, and toughness of 201, 202, 301, 304, and 316 stainless steels improve sequentially. The densities corresponding to 202, 304, and 316 are 7.74, 7.93, and 7.98, respectively. 316 stainless steel plate, designated under American standards, is a type of stainless, heat-resistant, and corrosion-resistant steel, belonging to the austenitic stainless steel family. It is superior to 304 stainless steel and corresponds to 0Cr17Ni12Mo2 under Chinese national standards (GB).

Commonly used stainless steel plates come in two main types: 201 and 304, which differ in composition. Among them, 304 stainless steel plate is the most widely used as a stainless and heat-resistant steel, being an austenitic stainless steel. The 400 series represents martensitic stainless steel. 304 stainless steel is non-magnetic, while the 400 series is magnetic. However, some 304 stainless steel may be attracted by a magnet due to slight magnetism (austenitic stainless steel may exhibit slight magnetism under cold working or improper smelting conditions). Compared to the 400 series, 304 stainless steel has a higher nickel content, around 8-10%.

The 400 series of stainless steel has an internal microstructure of tempered martensite, exhibiting magnetic properties and can be attracted by a magnet. Austenitic stainless steel is non-magnetic, with 304# stainless steel being a typical representative. However, non-magnetic austenitic stainless steel cannot achieve high strength through conventional heat treatment and is not used for manufacturing knife blades, only for knife handles and stands. Therefore, the presence or absence of magnetism cannot be simply used to determine whether a material is stainless steel.

The 400 series of stainless steel has a lower chromium content than the 300 series but does not suffer from carbon deposition issues like the 300 series. It can be heat-treated and is suitable for working environments with temperatures up to 1200°F (approximately 393°C), increasing its hardness. The ferritic and martensitic stainless steels in the 400 series, such as 410, generally have better rust resistance and corrosion resistance than 304 in certain special applications where high hardness is required but excellent rust resistance and corrosion resistance are not necessary. In such cases, 410 may be superior to 304. The 400 series of stainless steel can be hardened through heat treatment. Currently, the 400 series is generally used for lower-end tableware products, while 304 is used for higher-end products.

When brown rust spots (or patches) appear on the surface of stainless steel pipes, people are often surprised and assume that "stainless steel does not rust, and if it does, it is not stainless steel, possibly indicating a problem with the steel quality." This is a one-sided and erroneous view stemming from a lack of understanding of stainless steel. Stainless steel can rust under certain conditions.

Stainless steel possesses the ability to resist atmospheric oxidation—i.e., stainlessness—as well as corrosion in media containing acids, alkalis, and salts—i.e., corrosion resistance. However, its corrosion resistance varies depending on its chemical composition, processing state, usage conditions, and the type of environmental medium. For example, 304 stainless steel pipes have excellent rust resistance in dry and clean atmospheres but will rust quickly when moved to coastal areas with high salt content in the sea fog. In contrast, 316 stainless steel pipes perform well in such environments. Therefore, not all stainless steels are corrosion-resistant and rust-free in any environment.

Stainless steel obtains its rust resistance through the formation of a thin, stable, and dense chromium-rich oxide film (protective film) on its surface, which prevents the continued penetration of oxygen atoms and further oxidation. Once this film is continuously damaged for some reason, oxygen atoms from the air or liquid will continuously penetrate, or iron atoms in the metal will continuously separate out, forming loose iron oxide and causing continuous rusting on the metal surface. This surface film can be damaged in various ways, with the following being common in daily life: