Nickel Alloy Valve Stem Inconel 625 Reactor Core Components Nuclear Industry

Nickle Alloy Valve Stem is not only a moving part and a load-bearing component during the valve opening and closing process, but also a sealing element. It is subjected to the impact and corrosion of the medium, as well as friction with the packing. Therefore, when selecting the valve stem material, it is necessary to ensure that it has sufficient strength, good impact toughness, anti-scoring properties, and corrosion resistance at the specified temperature. The valve stem is a wearing part, and attention should also be paid to the material’s machinability and heat treatment properties when selecting it.

The Valve Stem Process:

• The valve stem undergoes tensile, compressive, and torsional forces during the opening and closing of the valve, and it directly contacts the medium while also experiencing relative friction with the packing. Therefore, when selecting valve stem material, it is necessary to ensure that it has sufficient strength, good impact toughness, wear resistance, and corrosion resistance at the specified temperature.
• The joint between the valve stem and the ball, as well as the contact point between the valve stem and the valve body, should have an antistatic mechanism to prevent the accumulation of static electricity on the ball. The safety design of the valve stem should prevent it from being “blown out” under working pressure. A ring-shaped ring is placed on the flange for preventing blowout on the valve stem to reduce the friction coefficient.

​Materials:

• Copper Alloys The commonly selected grades are QA19-2 and HPb59-1-1. They are suitable for low-pressure valves with nominal pressure not exceeding 1.6MPa and temperature not exceeding 200 degrees.
• Carbon Steel Generally, A5 and 35 steel are selected, which have undergone nitriding treatment. They are suitable for ammonia valves with nominal pressure not exceeding 2.5MPa, and low to medium pressure valves with water, steam, and other media. A5 steel is suitable for valves with a temperature not exceeding 300 degrees; 35 steel is suitable for valves with a temperature not exceeding 450 degrees. (Note: Practical experience has shown that carbon steel valves with nitriding treatment do not effectively solve the corrosion resistance issue and should be avoided.)
• Alloy Steel Commonly selected materials include 40Cr, 38CrMoA1A, and 20CrMo1V1A. After chromium plating, 40Cr is suitable for water, steam, petroleum, and other media with nominal pressure not exceeding 32MPa and temperature not exceeding 450 degrees. 38CrMoA1A, after nitriding treatment, can withstand 10MPa pressure at a working temperature of 540 degrees and is commonly used in power station valves. 20CrMo1V1A, after nitriding treatment, can withstand 14MPa pressure at a working temperature of 570 degrees and is also commonly used in power station valves.

• Generally selected materials include 2Cr13, 3Cr13, 1Cr17Ni2, and 1Cr18Ni12Mo2Ti. 2Cr13 and 3Cr13 stainless steels are suitable for water, steam, and weakly corrosive media with nominal pressure not exceeding 32MPa and temperature not exceeding 450 degrees. They can be strengthened through methods such as chromium plating and high-frequency quenching. 1Cr17Ni2 stainless steel valves can withstand corrosive media. 1Cr18Ni9Ti and 1Cr18Ni12Mo2Ti stainless acid-resistant steels are used in high-temperature valves with nominal pressure not exceeding 6.4MPa and temperature not exceeding 600 degrees, and they can also be used in stainless steel valves with temperatures not exceeding -100 degrees, especially in low-temperature valves. 1Cr18Ni9Ti can resist nitric acid and other corrosive media; 1Cr18Ni12Mo2Ti can resist acetic acid and other corrosive media. When used in high-temperature valves, 1Cr18Ni9Ti and 1Cr18Ni12Mo2Ti can be treated with nitriding to improve abrasion resistance.

• Bearing Chromium Steel GCr15 is selected and is suitable for ultra-high-pressure valves with nominal pressure not exceeding 300MPa and temperature not exceeding 300 degrees.There are many materials used for making valve stems, including 4Cr10Si2Mo martensitic heat-resistant steel and 4Cr14Ni14W2Mo austenitic heat-resistant steel.The valve stem nut directly bears the axial force of the valve stem and is in friction with the bracket and other valve parts. Therefore, in addition to having sufficient strength, the valve stem nut requires low friction coefficient, non-corrosion, and non-galling performance.

Nickel Alloy 625 (UNS NO6625) is a material withexcellent resistance to pitting, crevice, and corrosioncracking. This alloy is highly resistant in a wide range oforganic and mineral acids, and it exhibits good hightemperature strength. Excellent mechanical properties at both extremely lowand extremely high temperatures. Outstanding resistance to pitting, crevice corrosion, and intercrysta ine corrosion. Almost complete freedom from chloride induced stress corrosion cracking. High resistance to oxidation at elevated temperaturesup to 1050°C. Good resistance to acids, such as nitric, phosphoricsulfuric, and hydrochloric, as wel as to alkalis makespossible the construction of thin structura parts ofhigh heat transfer.

Applications

• Components where exposure to sea water and higrmechanical stresses are required.
• Oil and gas production where hydrogen sulfide anoelementary sulfur exist at temperature in excess of150°C.
• Components exposed to flue gas or in flue gasdesufurization pants.
• Flare stacks on offshore oil platforms.
• Hydrocarbon processing from tar-sand and oil-shale
• Recovery projects.

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Fabrication Data:

Alloy 625 can be easily welded and processed by standard shop fabrication practices, however because the high strength of the alloy, it resists deformation at hotworking temperatures.

• Hot Forming. The hot-working temperature range for Alloy 625 is 1650–2150°F (900–1177°C). Heavy working needs to occur as close to 2150°F (1177°C) as possible, while lighter working can take place down to 1700°F (927°C). Hot-working should occur in uniform reductions to prevent duplex grain structure
• Cold Forming. Alloy 625 can be cold-formed by the standard shop fabrication practices. The alloy should be in the annealed condition. Work hardening rates are higher than the austenitic stainless steels.
• Welding. Alloy 625 can be readily welded by most standard processes including GTAW (TIG), PLASMA, GMAW (MIG/MAG), SAW and SMAW (MMA). A post weld heat treatment is not necessary. Brushing with a stainless steel wire brush after welding will remove the heat tint and produce a surface area that does not require additional pickling.
• Machining. Alloy 625 should preferably be machined in the annealed condition. Since Alloy 625 is prone to workhardening, only low cutting speeds should be used and the cutting tool should be engaged at all times. Adequate cut depth is necessary to assure avoiding contact with the previously formed work-hardened zone.

Chemical Composition:

Physical Properties:

Thermal Properties:

Processing Flow Chart: