How to select the material of stem and stem nut?
What is a stem?
The valve stem is not only a moving part, a stressed part, but also a seal in the process of valve opening and closing. At the same time, the valve stem is impacted and corroded by the medium, and it also has friction with the packing. Therefore, when selecting the valve stem material, the valve rod must have enough strength, good impact toughness and corrosion resistance under the specified temperature. The valve rod is vulnerable, and the machining performance and heat treatment performance of the material should be paid attention to when selecting.
The commonly used materials of valve stem are as follows:
- ① Copper alloy: QA19-2 and HPb59-1-1 are generally selected. It is suitable for low pressure valves with nominal pressure less than or equal to 1.6Mpa and temperature less than or equal to 200 ℃.
- ② Carbon steel: generally choose Q275, 35# steel, after nitriding treatment, suitable for ammonia valve with nominal value less than or equal to 2.5MPa, low and medium pressure valve of water, steam and other media. Q275 steel is suitable for the valve whose temperature does not exceed 300 ℃; 35# steel is suitable for the valve whose temperature does not exceed 450 ℃. Note: it has been proved by practice that the valve stem made of carbon steel can not solve the problem of corrosion resistance and should be avoided.
- ③ Alloy steel: 40Cr, 38CrMoAlA, 20crmo1v1a and other materials are generally selected. After chromium plating, 40Cr is suitable for water, steam, oil and other media with nominal pressure less than or equal to 32Mpa and temperature less than or equal to 450 ℃. After nitriding, 38CrMoAlA can withstand 10MPa pressure at 540 ℃ and is commonly used in power station valves. After nitriding, 20crmo1v1a can withstand 14MPa pressure at 570 ℃ and is commonly used in power station valves.
- ④ Stainless steel: 2Cr13, 3Cr13, 1Cr17Ni2, 1cr18ni12mo2ti and other materials are generally selected. 2Cr13 and 3Cr13 stainless steels are suitable for water, steam and weakly corrosive medium with nominal pressure less than or equal to 32Mpa and temperature less than or equal to 450 ℃, and the surface can be strengthened by chromium plating and high frequency quenching. 1Cr17Ni2 Stainless acid resistant steel is used for stainless steel valve and low temperature valve with nominal pressure less than or equal to 6.4Mpa and temperature from – 100 ℃ to 200 ℃. 1Cr18Ni9Ti and 1cr18ni12mo2ti stainless and acid resistant steels are used in high temperature valves with nominal pressure less than or equal to 6.4Mpa and temperatures less than or equal to 600 ℃, as well as stainless steel valves and low-temperature valves with temperatures less than or equal to – 100 ℃. 1Cr18Ni9Ti can be resistant to corrosive media such as nitric acid; 1cr18ni12mo2ti can be resistant to corrosive media such as acetic acid; when 1Cr18Ni9Ti and 1cr18ni12mo2ti are used in high temperature valve, nitriding treatment can be used to improve the scratch resistance.
- ⑤ Shaft multiplication chromium steel: GCr15 is selected, which is suitable for ultra-high pressure valves with nominal pressure less than or equal to 300mpa and temperature less than or equal to 300 ℃.
There are many materials used to make valve stem, such as 4Cr10Si2Mo martensitic heat-resistant steel, 4Cr14Ni14W2Mo austenitic heat-resistant steel, etc.
What is a stem nut?
The valve stem nut and the valve stem are matched by threads, which directly bear the axial force of the valve stem, and are in the friction with the valve parts such as the bracket. Therefore, in addition to a certain strength, the stem nut also requires small friction coefficient, no rust, no seizure with the valve stem and other performance.
The stem nut is usually made of the following materials:
- ① Plastic: Nylon 66 and nylon 1010 are used to make valve stem nut, which has the advantages of corrosion resistance, small friction coefficient, good processability and low cost. But because of its lower strength than metal, it is only used in low pressure, small caliber valve.
- ② Copper alloy: copper alloy is not rusty, has a small friction coefficient, has a certain strength and toughness, is currently the valve stem nut commonly used material. Zcuzn38mn2pb2 cast brass is often used for low-pressure valves with nominal pressure less than or equal to 1.6Mpa, ZCuAl10Fe3 Wuxi bronze for valves with nominal pressure less than or equal to 6.4Mpa, and zcuzn25al6fe3mn3 cast brass is often used for high-pressure valves with nominal pressure greater than 6.4Mpa.
- ③ Steel: when the stem nut of electric valve needs high hardness and ammonia medium has corrosion to copper, steel is used instead of copper. Under the condition of not causing stem seizure, 35,40 high-quality carbon steel and 1Cr13, 1Cr18Ni9, Cr17Ni2 and other stainless steel are often used to make stem nut. The general rule that the hardness of the stem nut is lower than that of the stem should be observed in the selection, so as to avoid premature wear and seizure.
Source: Network Arrangement – China Valve Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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