A Comprehensive Guide to Super Austenitic Stainless Steel: 254SMO (UNS S31254)
What is 254SMO (UNS S31254)?
254SMO (UNS S31254, F44, 1.4547) is a super austenitic stainless steel. Because of its high molybdenum content, it has high resistance to spot corrosion and crevice corrosion. This brand of stainless steel is developed for use in halogenated environments such as seawater. 254SMO also has good corrosion resistance. Especially in the acid containing halide, the steel is superior to ordinary stainless steel. Its C content is less than 0.03%, so it is called pure austenitic stainless steel (< 0.01% is also called super austenitic stainless steel).
Super stainless steel is a kind of special stainless steel. First of all, it is different from ordinary stainless steel in chemical composition. It refers to a kind of high alloy stainless steel with high nickel, high chromium and high molybdenum. Among them, 254SMO containing 6% Mo is more famous. This kind of steel has very good local corrosion resistance. It has good pitting resistance (PI ≥ 40) and better stress corrosion resistance under the conditions of sea water, inflation, gap and low-speed scouring. It is a substitute material for Ni based alloy and titanium alloy. Secondly, in terms of high temperature or corrosion resistance, it has better high temperature or corrosion resistance, which is irreplaceable for 304 stainless steel. In addition, the microstructure of special stainless steel is a stable austenite microstructure.
Because 254SMO (S31254, F44, 1.4547) is a kind of special stainless steel with high alloy, its manufacturing process is quite complex. Generally, people can only rely on traditional processes to manufacture this special stainless steel, such as pouring, forging, rolling, or seamless steel pipes can be made by center punching, cold rolling or cold drawing.
Chemical composition of 254SMO (s31254, f44, 1.4547)
| Element | Wrought | Castings |
|---|---|---|
| Carbon | ≤0.020 | ≤0.025 |
| Chromium | 19.5-20.5 | 19.5-20.5 |
| Nickel | 17.5-18.5 | 17.5-19.5 |
| Molybdenum | 6.0-6.5 | 6.0-7.0 |
| Nitrogen | 0.18-0.22 | 0.180-0.240 |
| Copper | 0.50-1.00 | 0.50-1.00 |
| Sulfur | ≤0.010 | ≤0.010 |
| Phosphorus | ≤0.030 | ≤0.045 |
| Silicon | ≤0.80 | ≤1.00 |
| Manganese | ≤1.00 | ≤1.20 |
| Iron | Balance | Balance |
Mechanical Properties of 254 SMO
254SMO WNRF flanges 300# 3″ SCH80, ASTM A182 F44(S31254)
| Tensile Strength, min. | Yield Strength, min. | Elongation, min. | Hardness, max. | |||
|---|---|---|---|---|---|---|
| ksi | Mpa | ksi | Mpa | % | Brinell HBW | Rockwell |
| 95 | 655 | 45 | 310 | 35 | 223 | 96 HRBW |
*The values are furnished on the basis of 254SMO plates.
Physical Properties of 254 SMO
| Modulus Elasticity | Thermal Conductivity | Specific Heat | Density | Melting Range | M.P |
|---|---|---|---|---|---|
| 106psi | Btu/hft°F | Btu/lb°F | g/cm3 | °C | – |
| 29 | 7.5 | 0.120 | 8.1 | 1320-1390 | 1.003 |
*All values are measured at room temperature.
Corrosion resistance of 254SMO (s31254, f44, 1.4547)
254SMO (s31254, f44, 1.4547) stainless steel is a kind of super austenitic stainless steel with high corrosion resistance. It is developed for the environment of halide and acid, and widely used in high concentration chloride ion medium, seawater and other harsh working conditions. 254SMO is much better than other stainless steel in various industrial occasions of acid medium, especially in the acid containing halide. In some cases, it can be compared with Hastelloy alloy and titanium. Low carbon content and high molybdenum content make it have better resistance to spot corrosion and crevice corrosion, excellent resistance to intergranular corrosion. It is a high cost-effective stainless steel, widely used in chemical industry, desulfurization and environmental protection at home and abroad.
- A large number of field experiments and extensive use experience show that even at a slightly higher temperature, 254SMO has a very high resistance to crevice corrosion in seawater, and only a few kinds of stainless steel have this performance.
- 254SMO’s corrosion resistance in acid solutions and oxidizing halide solutions, such as those required for bleaching production in the paper industry, is comparable to that of the most corrosion-resistant nickel base and titanium alloys.
- Because 254SMO has high nitrogen content, its mechanical strength is higher than other kinds of austenitic stainless steel. In addition, 254SMO has high ductility, impact strength and good weldability.
- The high molybdenum content of 254SMO can make it have a higher oxidation speed during annealing, so it has a rougher surface than the ordinary stainless steel after pickling. However, it has no adverse effect on the corrosion resistance of the steel.
Welding material 254SMO (s31254, f44, 1.4547)
Welding wire: ERNiCrMo-3, welding rod: enicrmo-3.
Applications of 254SMO (s31254, f44, 1.4547)
- Ocean: marine structure of sea environment, desalination, mariculture, heat exchange of sea water, etc.
- Environmental protection: flue gas desulfurization device of thermal power generation, waste water treatment, etc.
- Energy field: atomic power generation, comprehensive utilization of coal, tidal power generation, etc.
- Petrochemical Industry: oil refining, chemical equipment, etc.
- Food field: salt making, soy sauce brewing, etc.
- High concentration chloride environment: papermaking industry, various bleaching devices
Metallographic structure of 254SMO
254SMO is face centered cubic lattice structure. In order to obtain austenite structure, 254SMO is annealed at 1150-1200 ℃. In some cases, there may be traces of metal intermediate phases (χ and α phases) in the center of the material. But in general, they have no adverse effect on impact strength and corrosion resistance. When placed in the range of 600-1000 ℃, these phases may precipitate on the grain boundary.
Product Forms and Relative Standards
A floating ball valve with ASTM A182 F44 body, Maersk Oil, Kazakhstan
| Product Form | Standard |
|---|---|
| Plate, Sheet, Strip | ASTM A240,A480 |
| Bar, Billet | ASTM A276, A479 |
| Seamless and Welded Pipe | ASTM A312, A358, A409, A813, A814 |
| Seamless and Welded Tube | ASTM A249, A269, A270 |
| Fittings, Forgings | ASTM A182, A403, A473 |
| Bolting, Nuts | ASTM A193, A194 |
| Casting | ASTM A351, A743, A744 |
*254SMO is designated differently in different product forms: for wrought pipe fittings, it is ASTM A403 WPS31254; for forgings, it is ASTM A182 F44; for castings, it should be CK3MCuN or UNS J93254.
Forming process of UNS S31254 180 degree short radius elbow
The UNS S31254 180 degree short radius elbow used for high-temperature oil pyrolysis furnace is currently manufactured using a casting process. The elbow manufactured by this process has low tissue density, high temperature medium is prone to intergranular corrosion, poor wear resistance, and short service life. A company in the United States commissioned our company to develop and manufacture a 180 degree short radius elbow using UNS S31254 steel pipe forming to replace the cast-formed elbow. Our company used two molding processes, hot extrusion molding and cold extrusion molding, to trial produce R=1D 180 degree elbows. We studied 180 degree short radius elbows manufactured using different processes and ultimately used 180 degree short radius elbows formed by cold extrusion. The samples were tested by a subsidiary of Tianxiang Group (UK) commissioned by a certain American company to meet the standards and requirements of a certain American company. The 180 degree short radius elbow has high corrosion and high temperature resistance performance, which can meet the quality requirements of UNS S31254 180 degree short radius elbow for high-temperature oil pyrolysis furnaces and greatly improve its service life. The UNS S31254 180 degree short radius elbow made of steel pipes developed by our company was successfully used in a pyrolysis furnace of a certain project in the United States in July 2016. The company conducted thickness measurement and metallographic testing on this batch of pipe fittings every year, and the actual service life was greatly improved compared to the elbow manufactured by the casting process.
Our company’s commonly used specification for pyrolysis furnaces is UNS S31254 180 degree elbow R=1D Φ 60.3 x 5.54; elbows manufactured using both hot and cold forming methods were tested.
1. Material re-inspection
The design and manufacturing standard for 180 degree short radius elbows is ASME B16.9. We choose ASTM A312 UNS S31254 seamless steel pipe with the specification of Φ 60.3x 6.0 (cold formed) and Φ 48.3 x6.0 (hot formed) material. After the materials entered the factory, the chemical composition and mechanical properties of the two specifications of materials were retested. The chemical composition, mechanical properties, and retest results in the material certificate meet the requirements of relevant standards.
2. Forming research content and key technologies adopted
We used preforming and secondary forming methods to achieve the final forming of a 180 degree short radius elbow in the cold extrusion molding process. The analysis model and forming limit of short radius elbows were studied, and the effects of axial feed rate, main size parameters of elbows, material forming performance, lubrication conditions, etc. on process parameters and forming results (such as wall thickness, stress, strain, etc.) were studied. Propose a prediction method and process for defects such as large wall thickness reduction and intergranular cracks during the forming process. And research the cold extrusion secondary forming process. Reasonably establish the matching relationship between the amount of advance and the speed of advance. During the extrusion process, intermediate heat treatment is used to reduce the wall thickness reduction and occurrence of intergranular crack defects during the material forming process. After the secondary forming, solid solution heat treatment is used to refine the grains, eliminate stress, and obtain a uniform austenite structure.
The relationship between the determination of heating and insulation methods, appropriate expansion ratio, insulation temperature and insulation device, pushing speed and wall thickness reduction in the hot extrusion molding process was studied. After hot forming, the same solid solution heat treatment process is used as after cold forming.
We use secondary forming for cold forming, with 1.5D preforming for bending. After forming, we use solid solution heat treatment to restore material properties. The secondary forming uses a special mold with the same bending radius as the required short-radius elbow. After secondary forming, we use solid solution heat treatment. The pre-forming adopts a mold with a large bending radius. In contrast, the secondary forming adopts a special mold with the same bending radius as the required short-radius elbow, greatly reducing wall thickness reduction and improving ellipticity.
The use of intermediate heat treatment reduces the occurrence of intergranular crack defects after deformation, eliminates stress concentration, and ensures the quality of secondary molding.
Multiple sets of external shaping blocks are used to ensure uniform stress on the elbow and prevent damage during shaping.
3. Comparison of the size and performance of elbows completed using cold and hot forming processes
3.1 Shape diagram of 180 degree short radius elbow after forming (see Figure 1-4) and size situation (see Tables 1 and 2)
Figure.1 Cold-Formed Products
Figure.2 Thermoformed Products
Table.1 Measurement Results of Outer Diameter and Wall Thickness of Elbows Manufactured by Cold Forming Process
| Item | Outside diameter | Wall thickness | ||
| A-end | B-end | A-end | B-end | |
| 1 | 60.17 | 59.69 | 5.73 | 5.93 |
| 2 | 60.21 | 60.11 | 5.39 | 5.29 |
| 3 | 60.11 | 59.73 | 5.52 | 5.5 |
| 4 | 59.97 | 60.01 | 5.79 | 5.73 |
Table.2 Measurement results of outer diameter and wall thickness of elbows manufactured by hot forming process
| Item | Outside diameter | Wall thickness | ||
| A-end | B-end | A-end | B-end | |
| 1 | 60.68 | 60.42 | 5.85 | 6.01 |
| 2 | 60.87 | 60.49 | 5.5 | 5.62 |
| 3 | 60.61 | 60.8 | 5.13 | 5.29 |
| 4 | 60.82 | 60.92 | 5.88 | 5.93 |
Figure.3 Cold formed short radius elbow profile
Figure.4 Hot formed short radius elbow profile
The outer diameter and wall thickness of 180 degree short radius elbows manufactured by hot and cold forming processes meet the requirements of ASME B16.9 standards.
3.2 Microstructure of 180 degree short radius elbows after cold and hot forming
Figure.5 Metallography of hot-formed short-radius elbows
The metallography of the elbow manufactured by the hot forming process is: austenite is roughly equiaxed, and there is a continuous carbide network at the grain boundaries, as shown in Figure 5. The metallographic structure of the elbow manufactured by the cold-forming process is shown in Figure 6, which includes austenite with a small amount of intergranular carbide precipitation.
Figure.6 Metallography of Cold-Formed Short-Radius Elbows
3.3 Microstructure of 180 degree short radius elbows after intergranular corrosion after cold and hot forming (ASTM A262 E method)
The microstructure of the elbow manufactured by the hot forming process shows a groove structure after intergranular corrosion, as shown in Figure 7.
The microstructure of the elbow manufactured by the cold forming process after intergranular corrosion shows a stepped structure, as shown in Figure 8.
Figure.7 Intergranular corrosion test of hot-formed short radius elbow
Cascade structure is defined as having only steps between particles, while trench structure is defined as one or more particles surrounded by the trench.
Figure.8 Intergranular corrosion test of cold-formed short radius elbow
4. Conclusion
Compared with 180 degree short radius elbows manufactured through cold forming and hot forming processes, hot forming can cause sensitization of UNS S31254 material within the sensitization range. The microstructure exhibits obvious carbide precipitation (sensitization) at grain boundaries. These carbides are chromium-rich (Cr23C6), and their formation leads to chromium depletion near grain boundaries, which locally reduces corrosion resistance and makes them more susceptible to intergranular corrosion. It was reflected in ASTM A262 E testing that the microstructure has a groove structure, meaning that the region around the grain boundaries has been effectively chemically attacked.
The microstructure of the 180 degree short radius elbow formed by cold forming is an austenite equiaxed grain structure without obvious grain boundary precipitation, and the microstructure exhibits a hierarchical structure. Adopting appropriate cold forming and solid solution heat treatment processes can better ensure the quality of similar products.
Author: Lu Hengping, Ask Lin Xian
