Stainless steel refers to air, steam, water and other weak corrosive media and acids, alkalis, salts and other chemical corrosive media corrosion of steel, also known as stainless acid-resistant steel. In practice, often weak corrosive media resistant steel called stainless steel, and chemical media resistant steel called acid-resistant steel. Because of the differences in the chemical composition of the two, the former is not necessarily resistant to chemical media corrosion, while the latter are generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
Usually, according to the metallurgical organisation, the common stainless steel is divided into three categories: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel. On the basis of the basic metallurgical organisation of these three categories, duplex steels, precipitation-hardening stainless steels and high-alloyed steels containing less than 50% iron have been derived for specific needs and purposes.
Divided by metallurgical organisation:
01
Austenitic stainless steel.
Stainless steels in which the matrix is dominated by a face-centred cubic crystal structure of austenitic organisation (CY phase), which is non-magnetic, and which are strengthened (and may result in a certain degree of magnetism) mainly by cold working. The American Iron and Steel Institute labels them with numbers from the 200 and 300 series, such as 304.
02
Ferritic Stainless Steel.
A stainless steel whose matrix is dominated by a body-centred cubic crystal structure of ferrite organisation ((a-phase)), which is magnetic, and which generally cannot be hardened by heat treatment, but which can be slightly strengthened by cold working. American Iron and Steel Institute to 430 and 446 for the label.
03
Martensitic stainless steel.
The matrix is martensitic organisation (body-centred cubic or cubic), magnetic, through heat treatment can adjust its mechanical properties of stainless steel. The American Iron and Steel Institute labels them with the numbers 410, 420 and 440. Martensite has an austenitic organisation at elevated temperatures, which can be transformed into martensite (i.e. hardened) when cooled to room temperature at an appropriate rate.
04
Austenitic-ferritic (duplex) type stainless steel.
The matrix has both austenitic and ferrite two-phase organisation, in which the content of the lesser phase matrix is generally greater than 15%, magnetic, can be strengthened by cold working of the stainless steel, 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel high strength, resistance to intergranular corrosion and chloride stress corrosion and pitting corrosion are significantly improved.
05
Precipitation hardening stainless steel.
Stainless steel whose matrix is austenitic or martensitic in organisation and which can be hardened by precipitation hardening treatment. The American Iron and Steel Institute labels it with a 600 series number, such as 630, which is 17-4PH.
In general, in addition to alloys, the corrosion resistance of austenitic stainless steel is superior, in less corrosive environments, you can use ferritic stainless steel, in mildly corrosive environments, if the material is required to have high strength or high hardness, you can use martensitic stainless steel and precipitation hardening stainless steel.
What kind of stainless steel is not easy to rust?
There are three main factors that affect the rusting of stainless steel:
1, the content of alloying elements.
Generally speaking, the chromium content of 10.5% steel is not easy to rust. The higher the content of chromium and nickel corrosion resistance is better, such as 304 material nickel to be in the content of 8-10%, chromium content of 18-20%, such stainless steel in general is not rust.
2, the smelting process of the manufacturer will also affect the corrosion resistance of stainless steel.
Smelting technology is good, advanced equipment, advanced technology, large stainless steel plant both in the control of alloying elements, the removal of impurities, billet cooling temperature control can be guaranteed, so the product quality is stable and reliable, good intrinsic quality, not easy to rust. On the contrary, some small steel plant equipment backward, backward technology, smelting process, impurities can not be removed, the production of products will inevitably rust.
3, the external environment, the climate is dry and well ventilated environment is not easy to rust.
The air humidity, continuous rainy weather, or air containing acidity and alkalinity of the environment is easy to rust. 304 stainless steel, if the surrounding environment is too poor is also rust.
Stainless steel rust spots how to deal with?
1,Chemical method
With pickling paste or spray to assist its rusted parts to re-passivate the formation of chromium oxide film to make it regain the ability to resist corrosion, after pickling, in order to remove all the pollutants and acid residues, it is very important to carry out a proper rinse with water. After everything is processed and re-polished with polishing equipment, it can be closed with polishing wax. For local slight rust spots can also be used 1:1 petrol, oil mixture with a clean rag to wipe off the rust spots can be.
2,Mechanical methods
Sandblasting cleaning, cleaning with glass or ceramic particle blasting, obliteration, brushing and polishing. Mechanical methods have the potential to wipe away contamination caused by previously removed materials, polishing materials or obliterated materials. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should preferably be formally cleaned under dry conditions. The use of mechanical methods only cleans its surface and does not change the corrosion resistance of the material itself. Therefore, it is recommended to re-polish the surface with polishing equipment and close it with polishing wax after mechanical cleaning.
Commonly used stainless steel grades and properties
1, 304 stainless steel. It is one of the austenitic stainless steels with large application and widest use, suitable for manufacturing deep-drawn moulding parts and acid pipelines, containers, structural parts, various types of instrument bodies, etc. It can also manufacture non-magnetic, low-temperature equipment and parts.
2, 304L stainless steel. In order to solve the Cr23C6 precipitation caused by 304 stainless steel in some conditions there is a serious tendency to intergranular corrosion and the development of ultra-low carbon austenitic stainless steel, its sensitised state of intergranular corrosion resistance is significantly better than 304 stainless steel. In addition to slightly lower strength, other properties with 321 stainless steel, mainly used for corrosion-resistant equipment and components that can not be welded solution treatment, can be used to manufacture various types of instrumentation body.
3, 304H stainless steel. 304 stainless steel internal branch, carbon mass fraction in 0.04%-0.10%, high temperature performance is better than 304 stainless steel.
4, 316 stainless steel. In 10Cr18Ni12 steel based on the addition of molybdenum, so that the steel has good resistance to reducing media and pitting corrosion resistance. In seawater and various other media, corrosion resistance is better than 304 stainless steel, mainly used for pitting corrosion resistant materials.
5, 316L stainless steel. Ultra-low carbon steel, with good resistance to sensitised intergranular corrosion performance, suitable for the manufacture of thick cross-section size of welded parts and equipment, such as corrosion-resistant materials in petrochemical equipment.
6, 316H stainless steel. 316 stainless steel internal branch, carbon mass fraction of 0.04%-0.10%, high temperature performance is better than 316 stainless steel.
7, 317 stainless steel. Pitting resistance and creep resistance is better than 316L stainless steel, used in the manufacture of petrochemical and organic acid corrosion resistant equipment.
8, 321 stainless steel. Titanium stabilised 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. In addition to high temperature or hydrogen corrosion resistance and other special occasions, the general situation is not recommended.
9, 347 stainless steel. Niobium stabilized austenitic stainless steel, niobium added to improve intergranular corrosion resistance, corrosion resistance in acid, alkali, salt and other corrosive media with the 321 stainless steel, welding performance is good, can be used as corrosion-resistant materials and can be used as a heat-resistant steel, mainly used in thermal power, petrochemical fields, such as the production of containers, pipelines, heat exchangers, shafts, industrial furnaces, furnace tubes and furnace thermometers and so on.
10, 904L stainless steel. Super fully austenitic stainless steel, is Finland Otokumpu (OUTOKUMPU) company invented a super austenitic stainless steel, its nickel mass fraction of 24% to 26%, carbon mass fraction of less than 0.02%, excellent corrosion resistance, non-oxidising acids such as sulphuric, acetic, formic and phosphoric acid has a very good corrosion resistance, and at the same time has a good resistance to crevice corrosion and resistance to stress corrosion properties. It is suitable for various concentrations of sulphuric acid below 70℃, and has good corrosion resistance to acetic acid and mixed acid of formic acid and acetic acid of any concentration and any temperature under normal pressure. The original standard ASMESB-625 attributes it to nickel-based alloys, and the new standard attributes it to stainless steel. China has only a similar grade 015Cr19Ni26Mo5Cu2 steel, a small number of European instrumentation manufacturers of key materials used in 904L stainless steel, such as E + H mass flowmeter measuring tube is the use of 904L stainless steel, Rolex watch cases are also used in 904L stainless steel.
11, 440C stainless steel. Martensitic stainless steel, hardenable stainless steel, stainless steel in the highest hardness, hardness HRC5
