How to choose stainless steel
How to choose stainless steel

Stainless Steel is short for stainless acid-resistant steel, air, steam, water and other weak corrosion medium or stainless steel; And the chemical corrosion resistant medium (acid, alkali, salt and other chemical etching) corrosion of steel called acid resistant steel.

Stainless steel refers to steel resistant to weak corrosive media such as air, steam, water, and chemically corrosive media such as acid, alkali, and salt. It is also called stainless acid-resistant steel. In practical applications, the steel resistant to weak corrosion medium is often called stainless steel, and the steel resistant to chemical medium corrosion is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Usually divided into metallographic organization:

Generally, ordinary stainless steel is divided into three categories according to the metallographic structure: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three types of basic metallographic structures, duplex steels, precipitation hardening stainless steels, and high-alloy steels with iron content less than 50% are derived for specific needs and purposes.

1. Austenitic stainless steel.

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

2. Ferritic stainless steel.

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

3. Martensitic stainless steel.

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

4. Austenitic-ferritic (duplex) stainless steel.

The matrix has both austenite and ferrite two-phase structure, and the content of less phase matrix is generally greater than 15%. It is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has high strength, intergranular corrosion resistance, chloride stress corrosion resistance and pitting corrosion resistance are significantly improved.

5. Precipitation hardening stainless steel.

The matrix is austenite or martensite, and can be hardened by precipitation hardening stainless steel. The American Iron and Steel Institute is marked with 600 series numbers, such as 630, which is 17-4PH.

Generally speaking, except for alloys, the corrosion resistance of austenitic stainless steel is relatively excellent. In a less corrosive environment, ferritic stainless steel can be used. In a mildly 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 the corrosion of stainless steel:

1. The content of alloying elements.

Generally speaking, steel with a chromium content of 10.5% 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 should be 8-10%, and the content of chromium should reach 18-20%. Such stainless steel will not rust under normal circumstances.

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

Large stainless steel factories with good smelting technology, advanced equipment and advanced technology can guarantee the control of alloy elements, the removal of impurities, and the control of billet cooling temperature. Therefore, the product quality is stable and reliable, with good internal quality and is not easy to rust. On the contrary, some small steel mills have backward equipment and backward technology. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.

3. External environment, dry and well-ventilated environment is not easy to rust.

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

How to deal with rust spots on stainless steel?

1. Chemical method

Use pickling cream 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 pollutants and acid residues, it is very important to rinse properly with clean water. After all treatment, re-polish with polishing equipment and seal with polishing wax. For those with slight rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean rag.

2. Mechanical method

Sand blasting, shot blasting with glass or ceramic particles, obliteration, brushing and polishing. It is possible to mechanically wipe away contamination from previously removed material, polishing material or obliterating material. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should ideally be properly cleaned under dry conditions. 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-polish with polishing equipment after mechanical cleaning and seal with polishing wax.

Stainless steel grades and properties commonly used in instruments

1. 304 stainless steel. It is one of the most widely used austenitic stainless steels. It is suitable for the manufacture of deep-drawn parts and acid pipelines, containers, structural parts, and various instrument bodies. It can also be used to manufacture non-magnetic, low-temperature equipment and part.

2. 304L stainless steel. In order to solve the serious intergranular corrosion tendency of 304 stainless steel under some conditions due to the precipitation of Cr23C6, the ultra-low carbon austenitic stainless steel is developed, and its resistance to intergranular corrosion in the sensitized state 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 equipment and parts that cannot be solid solution treated after welding. It can be used to manufacture various instrument bodies, etc.

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 better than that of 304 stainless steel.

4. 316 stainless steel. Adding molybdenum on the basis of 10Cr18Ni12 steel makes the steel have good resistance to reducing medium and pitting corrosion resistance. In seawater and various other media, the corrosion resistance is better than 304 stainless steel, and it is mainly used for pitting corrosion resistant materials.

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

6. 316H stainless steel. The internal branch of 316 stainless steel has a carbon mass fraction of 0.04%-0.10%, and its high temperature performance is better than that of 316 stainless steel.

7. 317 stainless steel. Pitting corrosion resistance and creep resistance are better than 316L stainless steel, used in the manufacture of petrochemical and organic acid corrosion resistant equipment.

8. 321 stainless steel. Titanium-stabilized austenitic stainless steel, adding titanium to improve intergranular corrosion resistance, and has good high-temperature mechanical properties, can be replaced by ultra-low carbon austenitic stainless steel. Except for special occasions such as high temperature or hydrogen corrosion resistance, it is not recommended for general use.

9. 347 stainless steel. Niobium-stabilized austenitic stainless steel, adding niobium to improve intergranular corrosion resistance, corrosion resistance in acid, alkali, salt and other corrosive media is the same as 321 stainless steel, good welding performance, can be used as corrosion-resistant materials and 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 Company of Finland. Its nickel mass fraction is 24%-26%, carbon mass fraction is less than 0.02%, and has excellent corrosion resistance. , has good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, phosphoric acid, and has good crevice corrosion resistance and stress corrosion resistance. It is suitable for sulfuric acid of various concentrations below 70°C, and has good corrosion resistance in acetic acid of any concentration and temperature under normal pressure and the mixed acid of formic acid and acetic acid. The original standard ASMESB-625 classified it as a nickel-based alloy, and the new standard classified it as a stainless steel. China only has a similar grade of 015Cr19Ni26Mo5Cu2 steel, and a few European instrument manufacturers use 904L stainless steel as the key material. For example, the measuring tube of the mass flow meter of E+H is made of 904L stainless steel, and the case of Rolex watches is also made of 904L stainless steel.

11. 440C stainless steel. Martensitic stainless steel has the highest hardness among hardenable stainless steel and stainless steel, with a hardness of HRC57. It is mainly used to make nozzles, bearings, valve cores, valve seats, sleeves, valve stems, etc.

12. 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 that of 304 stainless steel and 430 stainless steel. It is used to manufacture offshore platforms, turbine blades, valve cores, valve seats, sleeves, and valve stems. wait.

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