Nickel Alloy Stem 254 SMO 6MO Super Dupex F44 1.4547 UNS S31254
Nickel 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 forms of valve stem packing are as follows:
- Lip packing. Lip packing, due to its soft lips, will expand laterally and adhere to the retaining wall under the action of medium pressure. This expandable packing can be used in compression packing but cannot be made of relatively hard materials. The disadvantage of lip packing is that its sealing action is unidirectional. Most lip packings used for valve stems are made of pure polytetrafluoroethylene (PTFE) or filled PTFE. However, some are made of fiber-reinforced rubber or leather, mainly used in hydraulic applications. Most lip packings used for valve stems are made in a V-shape, which is convenient for installation and expansion.
- Gland packing. Gland packing is named for packing like O-rings. After installation, the side of this packing is compressed, and the material’s elastic deformation maintains its lateral preload force. When the medium enters the packing cavity from the bottom, the packing moves towards the gap between the valve stem and the support seat, thereby blocking the leakage path. When the pressure in the packing cavity drops again, the packing returns to its original shape.
- Thrust packing. Thrust packing consists of packing rings or gaskets placed between the valve bonnet and the valve stem shoulder, allowing the valve stem to move freely axially relative to the packing ring. The initial valve stem sealing can be provided by auxiliary shaft seals such as live-compaction packing or by a spring, which forces the valve stem shoulder to press against the thrust packing. Subsequent medium pressure can force the valve stem shoulder to contact the packing more closely.
- Sealing of diaphragm valve stems. The valve stem of a diaphragm valve is sealed by a flexible and pressure-bearing valve bonnet that connects the valve stem to the closing member. This seal can prevent any medium from leaking through the valve stem to the atmosphere as long as the diaphragm does not fail. The material of the diaphragm depends on the purpose of the valve and can be made of stainless steel, plastic, or rubber, among others.
UNS S31254 / 1.4547 / F44:
Material to UNS S31254 (and the other specifications listed below) is described as a 6% Mo super austenitic stainless steel. The steel combines moderate mechanical strength (typically over 300 MPa yield strength) and high ductility with excellent corrosion resistance in seawater and a variety of industrial environments. Typically the alloy has a PREn (Pitting Resistance Equivalent) of 42-44 which ensures that the resistance to pitting corrosion is high. In addition, the steel provides good resistance to crevice corrosion. Ambient and subzero temperature notch ductility is very good. These attributes mean that this high molybdenum stainless steel can be used successfully as an alternative to 300 series austenitic stainless steels (such as type 316) in applications where higher mechanical strength and/or enhanced resistance to pitting and crevice corrosion is required. This alloy possesses a lower yield strength than that of duplex stainless steel (and much lower than that of super duplex steel) and pitting resistance which is comparable to super duplex stainless steel (such as UNS S32760 / S32750).
Availability:
Bar, forgings, sheet, plate, pipe, tube, closed die forgings, flanges and welding consumables.
Related material specifications:
- UNS S31254 in various ASTM product form specifications
- EN 10088-3 1.4547 (Grade X1CrNiMoN20-18-7)
- NORSOK MDS R11 to R15, R17 & R18
- ASTM A182 F44
- NACE MR01-75 (latest revision) / ISO 15156
Machinability / Welding:
The machining and welding of this grade of super austenitic stainless steel presents no particular problems. Guidance notes are available upon request.
Chemical Composition:
| Element | C | Mn | Si | S | P | Cr | Ni | Mo | N | Cu |
| Minimum(%) | - | - | - | - | - | 19.5 | 17.5 | 6 | 0.18 | 0.5 |
| Max(%) | 0.02 | 1 | 0.7 | 0.01 | 0.03 | 20.5 | 18.5 | 7 | 0.25 | 1 |
Specifications:
| ASTM | S31254 / A182 / A276 / A479 / A240 / A312 / A312 |
| ASME | S31254 / SA182 / SA276 / SA479 / SA240 / SA312 / SA312 |
Fabrication And Heat Treatment:
| Machinability | 1. Stainless steel grade 254 SMO™ is quite tough to machine due to the extremely high work hardening rate and lack of sulfur content; 2. however using sharp tools, overpowered machine tools, positive feeds, good amount of lubrication, and slow speeds tend to provide good machining results.
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| Welding | 1. Welding of stainless steel grade 254 SMO™ requires filler material without which it results in poor strength properties. 2. Filler metals such as AWS A5.14 ERNiCrMo-3, and alloy 625 are recommended. 3. Electrodes used in the process, have to match with AWS A5.11 ENiCrMo-12.
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| Annealing | Annealing of this material should be performed at 1149-1204°C (2100-2200°F), which should be followed by a water quench. |
| Hot Working | 1. Forging, upsetting and other operations relating to this material can be performed at 982 – 1149°C (1800 – 2100°F). 2. It is recommended that temperatures do not exceed this range as it would result in scaling and reduction in the workability of the material. 3. To re-attain maximum corrosion resistant properties, it is advisable to perform post-process annealing.
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| Cold Working | 1. Cold working can be carried out using all the traditional methods; 2. however the process would be tough due to its high work hardening rate. 3. The result will provide the material with increased strength and toughness.
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| Hardening | 1. Stainless steel grade 254 SMO™ does not respond to heat treatment. 2. Hardening is possible only through cold reduction. |
Processing Flow Chart:
