People usually think that the valve of stainless steel and will not rust. If it does, it may be a problem with the steel. This is a one-sided misconception about the lack of understanding of stainless steel, which can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation—that is, rust resistance, and also has the ability to corrode in media containing acids, alkalis, and salts—that is, corrosion resistance. However, the size of its anti-corrosion ability is changed with the chemical composition of its steel itself, the state of protection, the conditions of use and the type of environmental media.
Stainless steel is usually divided into:
Usually, according to the metallographic structure, ordinary stainless steel is divided into three categories: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three basic metallographic structures, for specific needs and purposes, dual-phase steels, precipitation-hardening stainless steels and high-alloy steels with an iron content of less than 50% are derived.
1. Austenitic stainless steel.
The matrix is dominated by austenite structure (CY phase) of face-centered cubic crystal structure, non-magnetic, and is mainly strengthened by cold working (and may lead to certain magnetic properties) stainless steel. The American Iron and Steel Institute is designated by numbers in the 200 and 300 series, such as 304.
2. Ferritic stainless steel.
The matrix is dominated by the ferrite structure ((a phase) of the body-centered cubic crystal structure, which is magnetic and generally cannot 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 martensitic structure (body-centered cubic or cubic), magnetic, and its mechanical properties can be adjusted by heat treatment. The American Iron and Steel Institute is designated by the numbers 410, 420 and 440. Martensite has an austenite structure at 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 (duplex) stainless steel.
The matrix has both austenite and ferrite two-phase structure, and the content of the 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, dual-phase steel has high strength, and the resistance to intergranular corrosion and chloride stress corrosion and pitting corrosion are significantly improved.
5. Precipitation hardening stainless steel.
The matrix is austenite or martensitic structure and can be hardened by precipitation hardening. The American Iron and Steel Institute is marked with a 600 series number, such as 630, which is 17-4PH.
Generally speaking, in addition to 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 For strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used.
Common stainless steel grades and properties
01 304 Stainless Steel
It is one of the most widely used and widely used austenitic stainless steels. It is suitable for the manufacture of deep-drawn parts and acid pipelines, containers, structural parts, various instrument bodies, etc. It can also be used to manufacture non-magnetic, low-temperature equipment and part.
02 304L Stainless Steel
In order to solve the problem of ultra-low carbon austenitic stainless steel developed due to the precipitation of Cr23C6 causing serious intergranular corrosion tendency of 304 stainless steel under some conditions, its sensitized state intergranular corrosion resistance is significantly better than that of 304 stainless steel. Except for the slightly lower strength, other properties are the same as 321 stainless steel. It is mainly used for corrosion-resistant equipment and components that cannot be subjected to solution treatment after welding, and can be used to manufacture various instrument bodies.
03 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.
04 316 Stainless Steel
Adding molybdenum on the basis of 10Cr18Ni12 steel makes the steel have good resistance to reducing medium and pitting corrosion. In seawater and various other media, the corrosion resistance is better than 304 stainless steel, mainly used for pitting-resistant materials.
05 316L Stainless Steel
Ultra-low carbon steel has good resistance to sensitized intergranular corrosion and is suitable for the manufacture of welded parts and equipment with thick section dimensions, such as corrosion-resistant materials in petrochemical equipment.
06 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.
07 317 Stainless Steel
The pitting corrosion resistance and creep resistance are better than 316L stainless steel, which is used in the manufacture of petrochemical and organic acid corrosion resistant equipment.
08 321 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 generally not recommended for use.
09 347 Stainless Steel
Niobium-stabilized austenitic stainless steel, adding niobium to improve intergranular corrosion resistance, the 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 material and anti-corrosion 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 kind of super austenitic stainless steel invented by OUTOKUMPU in Finland. , It has good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid and phosphoric acid, and also has good resistance to crevice corrosion and stress corrosion resistance. It is suitable for various concentrations of sulfuric acid below 70°C, and has good corrosion resistance in acetic acid and mixed acid of formic acid and acetic acid at any concentration and temperature under normal pressure.
11 440C stainless steel
Martensitic stainless steel has the highest hardness among hardenable stainless steels and stainless steels, with a hardness of HRC57. Mainly used to make nozzles, bearings, butterfly valve cores, butterfly 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 above 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, butterfly valve (valve cores, valve seats, sleeves, valve stems) wait.
In valve design and selection, various systems, series, and grades of stainless steel are often encountered. When selecting, the problem should be considered from multiple perspectives such as specific process medium, temperature, pressure, stressed parts, corrosion, and cost.
Post time: Jul-20-2022
