The stem of Liaoyang valve is not only a moving part, a force receiving part, but also a sealing part during the opening and closing of the valve. At the same time, it is impacted and corroded by the medium, and it also generates friction with the filler. Therefore, when selecting the valve stem material, it must be ensured that it has sufficient strength, good impact toughness, scratch resistance, and corrosion resistance at the specified temperature. The valve stem is a vulnerable part, and attention should be paid to the machining performance and heat treatment performance of the material when selecting it.

 

Generally, A5 and 35 steels are selected, after nitriding treatment, they are suitable for ammonia valves with a nominal pressure less than or equal to 2.5MPa, and low and medium pressure valves for water, steam and other media. A5 steel is suitable for valves whose temperature does not exceed 300 degrees; 35 steel is suitable for valves whose temperature does not exceed 450 degrees. (Note: The actual route proves that the valve stem made of carbon steel nitriding can not solve the corrosion problem well and should be avoided.)

 

alloy steel:

 

Generally choose 40Cr, 38CrMoA1A, 20CrMo1V1A and other materials. After 40Cr is chrome-plated, it is suitable for water, steam, petroleum and other media with a nominal pressure of 32MPa or less and a temperature of 450 degrees or less. 38CrMoA1A has undergone nitriding treatment and can withstand a pressure of 10MPa at a working temperature of 540 degrees. It is often used in power station valves. After nitriding treatment, 20CrMo1V1A can withstand a pressure of 14MPa at a working temperature of 570 degrees, and is often used in power station valves.

 

Generally, 2Cr13, 3Cr13, 1Cr17Ni2, 1Cr18Ni12Mo2Ti and other materials are used. 2Cr13, 3Cr13 stainless steel is suitable for water, steam and weakly corrosive media with nominal pressure less than or equal to 32MPa and temperature less than or equal to 450 degrees. The surface can be strengthened by methods such as chrome plating and high frequency quenching. The 1Cr17Ni2 stainless steel valve and low temperature valve are resistant to corrosive media. 1Cr18Ni9Ti, 1Cr18Ni12Mo2Ti stainless, acid-resistant steel is used in high temperature valves with a nominal pressure of 6.4MPa or less and a temperature of 600 degrees or less, as well as stainless steel valves and low temperature valves with a temperature of -100 degrees or less. 1Cr18Ni9Ti can resist corrosive media such as nitric acid; 1Cr18Ni12Mo2Ti can resist corrosive media such as acetic acid. When 1Cr18Ni9Ti and 1Cr18Ni12Mo2Ti are used in high-temperature valves, nitriding treatment can be used to improve scratch resistance.

 

Stainless steel valve stems, especially those made of chromium 13 series steel, are often corroded on the surface in contact with the packing. This kind of corrosion often occurs in the storage stage before use, because the filler is saturated with water after the hydrostatic test. If it is put into use immediately after the hydraulic test, it will not corrode. Theoretically speaking, the corrosion of the stainless steel valve stem in the wet packing is due to the fact that the surface of the valve stem surrounded by the packing is in a deoxidized environment. This environment affects the activation and passivation properties of metals. Many small anodes are produced on the oxygen-deficient sensitive points on the surface of the stainless steel oxidation protection layer. Together with the large amount of residual passive metal that has an anodic effect, these anodes cause the metal to function as a galvanic cell. Graphite, which is usually used in the filler, acts as an anode material on the cathode field of the valve stem steel to enhance the current intensity of the galvanic cell, thereby greatly intensifying the corrosion of the original corrosion point.