Stainless steel is an abbreviation for stainless acid resistant steel. It is resistant to air, steam, water and other weakly corrosive media or stainless steel. It is called stainless steel; it will be chemically resistant to chemicals (acid, alkali, salt, etc. Corrosion of the steel is called acid-resistant steel.



Stainless steel refers to steels resistant to weak corrosive media such as air, steam, and water, and chemically etched media such as acids, alkalis, and salts, and also called stainless acid-resistant steels. In practical applications, steels that are resistant to weak corrosive media are often referred to as stainless steels, and steels that are resistant to chemical corrosion are known as acid-resistant steels. Because of the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally non-rust. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Common categories:

Usually divided into metallographic organizations:

Generally, according to the metallurgical structure, ordinary stainless steel is classified into three types: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Based on these three types of basic metallurgical structures, dual-phase steels, precipitation-hardening stainless steels, and high-alloy steels containing less than 50% iron were derived for specific needs and purposes.



1. Austenitic stainless steel.

The matrix is mainly composed of an austenite (CY phase) with a face-centered cubic crystal structure, is non-magnetic, and is mainly strengthened by cold working (and may cause some magnetic properties) in stainless steel. The American Iron and Steel Institute is marked with 200 and 300 series numbers, such as 304.



2, ferritic stainless steel.

The matrix is mainly composed of a ferrite structure with body-centered cubic crystal structure (a phase), which is magnetic and can not generally be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute is marked with 430 and 446.



3, martensitic stainless steel.

The matrix is a martensite (body-centered cubic or cubic), magnetic, stainless steel that can be adjusted by heat treatment to adjust its mechanical properties. The American Iron and Steel Institute is marked with numbers 410, 420, and 440. Martensite has an austenite structure at a high temperature, and when cooled to room temperature at an appropriate rate, the austenite structure can be transformed into martensite (ie, hardened).



4. Austenitic-ferritic (dual phase) stainless steel.

The matrix has both austenite and ferrite two-phase structure, in which the content of less phase matrix is generally greater than 15%, there is magnetic, stainless steel that can be strengthened by cold working, and 329 is a typical duplex stainless steel. Compared with austenitic stainless steels, dual-phase steels have high strength, resistance to intergranular corrosion, chloride stress corrosion resistance, and pitting corrosion.



5, precipitation hardening stainless steel.

The substrate is austenitic or martensitic and can be hardened by precipitation hardening stainless steel. The American Iron and Steel Institute is marked with 600 series numbers, such as 630, that is, 17-4PH.



In general, the corrosion resistance of austenitic stainless steels is excellent except for alloys. Ferritic stainless steels can be used in environments with low corrosiveness. In mild corrosive environments, materials are required to be high. For strength or high hardness, martensitic stainless steel and precipitation hardened stainless steel can be used.



Thickness difference

because the steel mill machinery in the rolling process, the roller heat occurs a slight deformation, resulting in deviation of the thickness of the rolled out of the board, the general thickness of the middle thin. When measuring the thickness of the board, the national regulations should measure the middle part of the board.


What kind of stainless steel is not easy to rust?

There are three main factors that affect stainless steel corrosion:

1, the content of alloying elements.

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



2. The smelting process of the production company will also affect the corrosion resistance of stainless steel.

The large stainless steel plant with good smelting technology, advanced equipment and advanced technology can ensure the control of the alloy elements, the removal of impurities, and the control of the cooling temperature of the billet. Therefore, the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. Conversely, some small steel plants are behind and the process is behind. During the smelting process, impurities cannot be removed and the products will inevitably rust.



3, the external environment, climate, dry and ventilated environment is not easy to rust.

Humidity in the air, continuous rainy weather, or air in areas with high pH, it is easy to rust. 304 stainless steel, if the surrounding environment is too bad it will rust.



How to deal with rust in stainless steel?

1, chemical methods

The use of a pickling paste or spray to assist the re-passivation of the corroded part to form a chromium oxide film restores its resistance to corrosion. After pickling, it is important to flush properly with water in order to remove all contaminants and acid residues. After everything is processed, it is polished again with a polishing device and closed with a polishing wax. For those with slight rust on the spot, a 1:1 mixture of petrol and oil can be used to wipe off rust with a clean rag.



2, mechanical method

Blast cleaning, shot blasting with glass or ceramic particles, annihilation, brushing and polishing. Using mechanical methods it is possible to wipe off the contamination caused by previously removed materials, polishing materials or annihilation materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned in a dry condition. The mechanical method can only clean the surface and cannot change the corrosion resistance of the material itself. It is therefore recommended to re-polish with a polishing machine after mechanical cleaning and close it with a polishing wax.



Instrument commonly used stainless steel grades and performance

1,304 stainless steel. It is one of austenitic stainless steels with a large amount of application and the widest range of applications. It is suitable for the manufacture of deep-drawn parts and acid-conveying pipes, containers, structural parts, and various types of instrument bodies. It can also produce non-magnetic and cryogenic equipment. part.



2, 304L stainless steel. In order to solve the ultra-low carbon austenitic stainless steel developed due to the presence of Cr23C6 precipitated 304 stainless steel under severe conditions, the sensitization resistance to intergranular corrosion is significantly better than that of 304 stainless steel. In addition to the slightly lower strength, the other properties are the same as 321 stainless steel. It is mainly used for corrosion resistant equipment and components that cannot be solution treated after welding, and can be used to manufacture various types of instrument bodies.



3, 304H stainless steel. The internal branch of 304 stainless steel has a carbon mass fraction of 0.04%-0.10%, and its high temperature performance is superior to 304 stainless steel.



4,316 stainless steel. Add molybdenum on the basis of 10Cr18Ni12 steel, so that the steel has a good resistance to reducing media and resistance to pitting corrosion. Corrosion resistance is superior to 304 stainless steel in seawater and various other media and is mainly used for pitting resistant materials.



5, 316L stainless steel. Ultra-low carbon steel, with good resistance to intergranular corrosion resistance, is suitable for the manufacture of welded parts and equipment with thick section dimensions, such as corrosion resistant materials in petrochemical equipment.



6, 316H stainless steel. Internal branch of 316 stainless steel, carbon content of 0.04% -0.10%, high temperature performance is better than 316 stainless steel.



7,317 stainless steel. Resistance to pitting and creep resistance is superior to 316L stainless steel for the manufacture of petrochemical and organic acid corrosion resistant equipment.



8,321 stainless steel. Titanium-stabilized austenitic stainless steels with added titanium to improve the resistance to intergranular corrosion and have good high temperature mechanical properties can be replaced with ultra-low carbon austenitic stainless steels. In addition to high-temperature or anti-hydrogen corrosion and other special occasions, the general situation is not recommended.



9, 347 stainless steel. 鈮 Stabilized austenitic stainless steel, antimony stainless steel added to improve resistance to intergranular corrosion, corrosion resistance in corrosion media such as acids, alkalis, and salts with 321 stainless steel, good welding performance, can be made of corrosion resistant materials and can be made resistant The use of hot steel is mainly used in thermal power and petrochemical fields, such as making containers, pipes, heat exchangers, shafts, furnace tubes in industrial furnaces, and furnace tube thermometers.



10, 904L stainless steel. Super complete austenitic stainless steel is a super austenitic stainless steel invented by OUTOKUMPU of Finland. Its nickel content is 24% to 26%, carbon content is less than 0.02%, and its corrosion resistance is excellent. It has good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid and phosphoric acid, and has good resistance to crevice corrosion and stress corrosion resistance. Suitable for various concentrations of sulfuric acid below 70 °C, at any pressure at any concentration, any temperature of acetic acid and acid mixed with acetic acid in the corrosion resistance is also very good. The original standard ASMESB-625 was attributed to nickel-base alloys, and the new standard classifies them as stainless steel. China only has an approximate grade of 015Cr19Ni26Mo5Cu2 steel. The key material of a few European instrument manufacturers uses 904L stainless steel. For example, the measuring tube of the E+H mass flowmeter uses 904L stainless steel, and the case of the Rolex watch also uses 904L stainless steel.



11, 440C stainless steel. Martensitic stainless steel, the highest hardness in hardenable stainless steel, stainless steel, hardness HRC57. Mainly used for making nozzles, bearings, valve spools, valve seats, sleeves, stems, etc.



12, 17-4PH stainless steel. Martensitic precipitation hardened stainless steel, hardness HRC44, with high strength, hardness and corrosion resistance, can not be used for temperatures higher than 300 °C. It has good corrosion resistance to atmosphere and dilute acid or salt. Its corrosion resistance is the same as that of 304 stainless steel and 430 stainless steel. It is used to manufacture offshore platforms, turbine blades, valve plugs, valve seats, sleeves, valve stems. Wait.



In the pump valve industry, combined with the commonality and cost issues, the conventional austenitic stainless steel selection order is 304-304L-316-316L-317-321-347-904L stainless steel, of which 317 is less, 321 is not recommended, 347 for high temperature corrosion, 904L individual manufacturers default material just some of the elements, the design is generally not the main movable selection 904L.



In the design and selection of pump valves, there are usually occasions in which the material and the material of the pipeline are different. Especially in the high-temperature conditions, special attention must be paid to whether the choice of material satisfies the design temperature and design pressure of the process equipment or pipeline. For example, the pipeline is High-temperature chrome-molybdenum steel, and the instrument selection of stainless steel, then it is very likely to be a problem, must go to the temperature and pressure table of the relevant material.



In the design selection, often encountered a variety of different systems, series, grades of stainless steel, the selection process should be based on the specific process media, temperature, pressure, force components, corrosion, cost and other considerations.