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Stainless steel knowledge you must know

Issuing time:2020-11-13 16:24Author:momo


Why is it called stainless steel?

Stainless steel is the abbreviation forstainless and acid-resistant steel. Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water, and chemically corrosive media such as acid, alkali, and salt. It is also called stainless acid-resistant steel. In practical applications, steel that is resistant tocorrosion by weak corrosive media is often called stainless steel, and steel that is resistant to chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is notnecessarily resistant to corrosion by chemical media, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Usually, according to the metal lographic structure, ordinary stainless steel is divided into three categories:austenitic stainless steel, ferritic stainless steel, and martensitic stainlesssteel. Based on these three basic metal lographic structures, for specific needs and purposes, dual-phase steels, precipitation hardening stainless steels and high-alloy steels with iron content less than 50% have been derived.

Generally speaking, in addition to alloys,the corrosion resistance of austenitic stainless steel is relatively excellent.In less corrosive environments, ferritic stainless steel can be used. In amildly corrosive environment, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used.

What kind of stainless steel is not easy to rust?

There are three main factors affecting stainless steel corrosion:

A.The content of alloying elements.

Generally speaking, when the content of chromium is 10.5%, the steel is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the content of nickel in 304 material is 8-10%, and the content of chromium is 18-20%. Such stainless steel will not rust under normal circumstances.

B.The smelting process of the manufacturer will also affectthe corrosion resistance of stainless steel.

A large stainless steel plant with good smelting technologies, advanced equipments and advanced technologies can guarantee the control of alloy ingelements, the removal of impurities, and the control of the cooling temperature of the billet, so the product quality is stable and reliable, the internal quality is good, and is not easy to rust. On the contrary, some small steel mills have backward equipments and technologies. During the smelting process,impurities cannot be removed, and the products produced will inevitably rust.

C.External environment. Stainless steel is not easy to rust in a dry and ventilated environment.

In areas with high air humidity, continuous rainy weather, or high PH inthe air, stainless steel is prone to rust. 304 stainless steel, if thesurrounding environment is too bad, it will rust.

How to deal with stainless steel rust spots?

A.chemical method.

Use pickling paste or spray to assist the re-passivation of the rusted parts to form a chromium oxide film to restore the corrosion resistance. After pickling, in order to remove all contaminants and acid residues, it is very important to rinse properly with clean water. After all treatments,re-polishing with polishing equipment and sealing with polishing wax. For those with slight rust spots, you can use 1:1 gasoline and engine oil mixture to wipe off the rust spots with a clean cloth.

B.Mechanical method.

Sandblasting, blasting with glass or ceramic particles, annihilation,brushing and polishing. It is possible to use mechanical methods to wipe off the pollution caused by the previously removed materials, polishing materials or oblique materials. All kinds of pollution, especially foreign ironparticles, can become a source of corrosion, especially in humid environments.Therefore, mechanically cleaning should be properly handled under dryconditions. The use of mechanical methods can only clean the surface, and cannot change the corrosion resistance of the material itself. Therefore, it is recommended to re-polishing with polishing equipment after mechanical cleaning and sealing with polishing wax.

Commonly used stainless steel grades and performance

A.304 stainless steel. It is one of the most widely used austenitic stainless steels. It is suitable for the manufacture of deep-drawing forming parts and acid pipes, containers, structural parts, various instrument bodies, etc.. It can also manufactured to non-magnetic, low-temperature equipments and parts.

B.304L stainless steel. In order to solve the problem of thesevere intergranular corrosion tendency of 304 stainless steel due to the precipitation of Cr23C6 under some conditions, the ultra-low carbon austenitic stainless steel developed, its sensitized state of intergranular corrosion resistance is significantly better than that of 304 stainless steel. Except for slightly lower strength, other properties are the same as 321 stainless steel. It is mainly used for corrosion-resistant equipments and parts that need to be welded but cannot be solid-solution treated, and can be used to manufacture various instrument bodies.

316 stainless steel. Molybdenum is added to the 10Cr18Ni12 steel to make the steel have good resistance to reducing media and pitting corrosion. In sea water and other various media, the corrosion resistance is better than 304 stainless steel,mainly used for pitting corrosion resistant materials.

316L stainless steel. Ultra-low carbon steel has good resistance to intergranular corrosion in the sensitized state. It is suitable for the manufacture of welding parts and equipments with thick section dimensions, such as corrosion-resistant materials in petrochemical equipments.

17-4PH stainless steel. Martensitic precipitation hardening stainless steel, with a hardness of HRC44, has high strength, hardness and corrosion resistance, and cannot be used at temperatures higher than 300°C. It has good corrosion resistance to the atmosphere and diluted acid or salt. Its corrosion resistance is the same as 304 stainless steel and 430 stainless steel. It is used to manufacture offshore platforms, turbine blades, valve cores, valve seats, sleeves, valve stems, etc.

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