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stainless steel

Stainless steel

Stainless steel is the abbreviation of stainless acid-resistant steel, which is resistant to air, steam, water, and other weak corrosive media or stainless steel; The steel that is resistant to chemical corrosion medium (acid, alkali, salt, and other chemical etching) is called acid-resistant steel.

Stainless steel refers to the weak corrosive medium such as air, steam, and water and the chemical corrosive medium such as acid, alkali, and salt, also known as stainless acid-resistant steel.

ss sheet

 In practical application, a steel resistant to weak corrosive medium is often called stainless steel, while steel resistant to a chemical medium is called acid-resistant steel. Because of the difference in chemical composition, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. 

According to the metallographic structure, it is generally divided into three parts, The corrosion resistance of SS depends on the alloying elements contained in the steel.

Generally, according to the metallographic structure, ordinary stainless steel is divided into three types: austenitic Stainless, ferritic stainless, and martensitic stainless.

 On the basis of these three kinds of basic metallographic structures, dual-phase steel, precipitation hardening stainless, and high alloy steel with less than 50% iron content are derived for specific needs and purposes.

stainless steel bar

Austenitic stainless steel.

SS whose matrix is mainly austenite with face-centered cubic crystal structure and non-magnetic is mainly strengthened (and may cause certain magnetism) by cold working. The American Iron and Steel Association is designated by 200 and 300 series numbers, such as 304.

Ferritic stainless steel.

SS whose matrix is mainly ferrite with body-centered cubic crystal structure, which is magnetic and can not be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Association is marked 430 and 446.

Martensitic stainless steel.

Material stainless whose matrix is martensite (BCC or BCC), magnetic, and whose mechanical properties can be adjusted by heat treatment. The American Iron and Steel Association is numbered 410, 420, and 440. 

Martensite has an austenite structure at high temperatures. When it is cooled to room temperature at an appropriate rate, the austenite structure can be transformed into martensite

Austenitic ferrite (duplex) stainless steel.

The matrix is composed of austenite and ferrite, in which the content of less phase matrix is generally more than 15%, which is magnetic and can be strengthened by cold working. 329 is typical duplex steel. 

Compared with austenitic stainless, dual-phase steel has higher strength, higher resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion.

Precipitation hardening stainless steel.

The stainless whose matrix is austenite or martensite and can be hardened by precipitation hardening. American Iron and Steel Association is indicated by 600 series numbers, such as 630, i.e. 17-4PH.

Generally speaking, in addition to alloy, austenitic stainless steel has excellent corrosion resistance. In a low corrosive environment, ferritic stainless can be used. In the mild corrosive environment, martensitic stainless and precipitation hardening stainless steel can be used if high strength or hardness is required.

Thickness differentiation

  • Because in the rolling process of steel plant machinery, the roller is slightly deformed when heated, which leads to the deviation of plate thickness, generally the middle thickness is thin on both sides. When measuring the thickness of the plate, the state stipulates that the middle part of the plate head should be measured.
  • According to the market and customer demand, tolerance is generally divided into large tolerance and small tolerance: e.g

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.

For, 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 is 8-10%, and the content of chromium is 18-20%. In general, such stainless steel will not rust.

2. The smelting production enterprises affect the corrosion resistance

Large stainless plants with good smelting technology, advanced equipment, and advanced technology can ensure the control of alloy elements, impurity removal, and billet cooling temperature, so the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. 

On the contrary, some small steel plants are backward in equipment and technology. In the smelting process, impurities cannot be removed, and the products will inevitably rust.

3. External environment, dry climate, good ventilation environment is not easy to rust.

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

How to deal with stainless steel rust?

1. Chemical methods

Using acid pickling paste or spray to assist the rust part to be passivated to form chromium oxide film to restore its corrosion resistance. After pickling, it is very important to rinse properly with clean water in order to remove all pollutants and acid residues. 

After all, treatment, use polishing equipment to polish again and seal with polishing wax. For those with slight local rust spots, a 1:1 gasoline and engine oil mixture can be used to wipe the rust spots with a clean rag.


2. Mechanical method

Blast cleaning shot peening with glass or ceramic particles, annihilation, brushing and polishing. It is possible to remove the contamination caused by previously removed materials, polishing materials, or annihilation materials by mechanical means. 

All kinds of pollution, especially foreign iron particles, can be the source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should be properly cleaned under dry conditions. 

The use of mechanical methods can only clean the surface, can not change the corrosion resistance of the material itself. Therefore, it is suggested that after mechanical cleaning, the polishing equipment should be used to re-polish and the polishing wax should be used to seal.

stainless steel piping

Common stainless steel grades and performance of instruments

304 stainless.

  • As one of the most widely used austenitic stainless steels, it is suitable for manufacturing deep drawing forming parts, acid conveying pipes, containers, structural parts, various instrument bodies, etc., as well as non-magnetic and low-temperature equipment and parts.

304L stainless.

  •  In order to solve the serious intergranular corrosion tendency of 304 stainless caused by Cr23C6 precipitation, the sensitized intergranular corrosion resistance of ultra-low carbon austenitic stainless is significantly better than that of 304 stainless. 
  • Except for slightly lower strength, other properties are the same as that of 321 stainless. It is mainly used for corrosion-resistant equipment and parts that need to be welded but can not be treated by a solid solution. It can be used for manufacturing all kinds of instrument bodies, etc.
ss flange

304H stainless.

  • The internal branch of 304 stainless steel, the carbon content in 0.04% – 0.10%, high-temperature performance is better than 304 stainless steel.

316 stainless.

  • 316 the addition of molybdenum to 10cr18ni12 steel makes the steel have good reducing medium resistance and pitting corrosion resistance. In seawater and other media, the corrosion resistance is better than 304 stainless steel, which is mainly used for pitting corrosion-resistant materials.

316L stainless.

  • 316L stainless. Ultra-low carbon steel, with good resistance to sensitized intergranular corrosion, is suitable for manufacturing thick section welding parts and equipment, such as corrosion-resistant materials in petrochemical equipment.

316H stainless.

  • 316H stainless. The high-temperature performance of 316 stainless is better than that of 316 stainless because of its internal branches and carbon content of 0.04% – 0.10%.

317 stainless.

  • 317 stainless. Pitting corrosion resistance and creep resistance are better than 316L stainless steel. It is used in petrochemical and organic acid corrosion-resistant equipment.

321 Stainless.

  • 321 stainless. Titanium stabilized austenitic stainless steel can be replaced by ultra-low carbon austenitic stainless steel because of its excellent intergranular corrosion resistance and high-temperature mechanical properties. It is not recommended to use except for special occasions such as high temperature or hydrogen corrosion resistance.

347 stainless.

  • 347 stainless. Niobium stabilized austenitic stainless steel can improve intergranular corrosion resistance by adding niobium. Its corrosion resistance in acid, alkali, salt, and other corrosive media is the same as that of 321 stainless.
  • It can be used as corrosion-resistant material and heat-resistant steel. It is mainly used in thermal power and petrochemical fields, such as making vessels, pipes, heat exchangers, shafts, furnace tubes in industrial furnaces, and furnace tube thermometers.
ss fittings

904L Stainless

  • 904L stainless. Super complete austenitic stainless steel is a kind of super austenitic stainless steel invented by Outokumpu company in Finland. Its nickel content is 24% – 26%, and its carbon content is less than 0.02%. 
  • It has excellent corrosion resistance. It has good corrosion resistance in nonoxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid, as well as good crevice corrosion resistance and stress corrosion resistance. It is suitable for various concentrations of sulfuric acid below 70 ℃. 
  • It is resistant to acetic acid of any concentration and temperature under normal pressure and the mixed acid of formic acid and acetic acid and has good corrosion resistance. The original standard ASME sb-625 classified it as nickel base alloy, and the new standard classified it as stainless steel. In China, there is only approximate brand 015cr19ni26mo5cu2 steel. 
  • A few European instrument manufacturers use 904L stainless steel as the key material. For example, the measuring tube of the E + H mass flowmeter is made of 904L stainless, and the case of the Rolex watch is also made of 904L stainless.

440C stainless

  • 440C stainless. The hardness of martensitic stainless, hardenable stainless, and stainless steel is the highest, and the hardness is hrc57. It is mainly used for making nozzle, bearing, valve core, valve seat, sleeve, valve stem, etc.

17-4 PH

  • 17-4 PH stainless. Martensitic precipitation hardening stainless, with the hardness of hrc44, has high strength, hardness, and corrosion resistance, and can not be used at temperatures higher than 300 ℃. It has good corrosion resistance to the atmosphere and diluted acid or salt. 
  • Its corrosion resistance is the same as 304 stainless and 430 stainless. It is used to manufacture offshore platforms, turbine blades, valve core, seat, sleeve, stem, etc.

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