Some basic knowledge about Stainless Steel

What is stainless steel？

Stainless steel is the abbreviation of rust resistant and acid resistant steel. It is called stainless steel with weak corrosion medium, such as air, steam and water, or stainless steel with rust, and the steel resistant to chemical corrosion medium (acid, alkali, salt and other chemical etching) is called acid resistant steel.

In practical applications, because of their differences in chemical composition, the former is not necessarily resistant to chemical corrosion, while the latter is generally non corrosive. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel.

Common classification:

Usually, according to metallographic structure, ordinary stainless steel can be divided into three types: austenitic stainless steel, ferritic stainless steel and martensitic stainless steel.

On the basis of these three basic metallographic structures, for specific needs and purposes, the double phase steel, precipitated hardened stainless steel and high alloy steel with less than 50% iron content have been derived.

1. Austenitic stainless steel.

A stainless steel based on the austenitic structure (CY phase) of a surface centered cubic crystal, which is not magnetic and is mainly reinforced by cold machining (and may lead to a certain magnetism). The American Steel Association marked 200 and 300 series numbers, such as 304.

2. Ferritic stainless steel.

The matrix is mainly composed of ferrite (a phase) in the body centered cubic crystal structure, which is magnetic, and can not be hardened by heat treatment, but cold processing can make the stainless steel slightly strengthened. The American Steel Association is marked by 430 and 446.

3. Martensitic stainless steel.

The matrix is martensite structure (body centered cubic or cubic), and has magnetic properties, which can adjust the mechanical properties of stainless steel by heat treatment. The American Steel Association marked 410, 420 and 440. Martensite has austenite structure at high temperature, and when austenite is cooled at room temperature, the austenite can be transformed into martensite.

4. AUSTENITIC FERRITIC (biphasic) stainless steel.

The matrix has both austenite and ferrite two phase structure, in which the content of the less phase matrix is generally more than 15%, it is magnetic, and the stainless steel which is strengthened by cold processing and 329 is the typical double phase stainless steel. Compared with austenitic stainless steel, dual phase steel has high strength, good resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion.

5. Precipitated hardened stainless steel.

The matrix is austenite or martensite and can be hardened by precipitation hardening. The American Steel Association has 600 series of digital markup, such as 630, 17-4PH.Generally speaking, the corrosion resistance of austenitic stainless steel is excellent except for alloy. Ferritic stainless steel can be used in low corrosive environment. In mild corrosive environment, martensitic stainless steel and precipitated hardened stainless steel can be used in the material with high strength or high hardness.

How to deal with rust stains on stainless steel?

1. Chemical methods

The rust area is repassivated by pickling paste or spray to form chromium oxide film to restore its corrosion resistance.After pickling, in order to remove all pollutants and acid residues, it is very important to rinse properly with clean water. After all the treatments are polished with polishing equipment and sealed with polished wax. For local slight rust, you can also use 1:1 gasoline and oil mixture to wipe rust spots with clean rag.

2. Mechanical method

The methods of cleaning rust spots are: sand blasting, cleaning with glass or ceramic particles, annihilation, scrubbing and polishing. By mechanical means, it is possible to wipe off the pollution caused by previously removed materials, polishing materials or annihilation materials.

All kinds of pollution, especially foreign iron particles, may be the source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned under dry conditions. The mechanical method can only clean up the surface, and can not change the corrosion resistance of the material itself.It is suggested that after polishing the machine, the polishing equipment should be re polished and polished.

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