Many demanding applications require plants to operate at higher temperatures to increase efficiency and/or burn lower-quality fuels. Because of that, there is a demand for high-performance alloys that offer improved resistance to high-temperature corrosion.

The majority of high-temperature corrosion occurs when the temperature exceeds 932°F (500°C). Depending on the application and process conditions, the type of high-temperature phenomena encountered will be different. For example, the reaction of metals with hot gases containing H2S and SO2 may lead to sulfidation. Oxidation, on the other hand, derives from the reaction of metals with air forming an oxide layer or scale.

When the reaction of the metals is with hot gases containing carbon monoxide or hydrocarbons such as methane, ethane or propane, carbides will precipitate. The process is known as carburization. Lastly, when steels come into contact with gaseous mixtures containing nitrogen (N2), mixtures of N2 and hydrogen (H2) and cracked ammonia (NH3), hard and brittle nitrides will form. This is known as nitridation.


alloy chart

Fig. 1. Weight increase for Alleima 353 MA and other grades after 45 hours oxidation at different temperatures.


Alleima 353 MA™* (35Ni-25Cr-1.5Si-Ce-N) is an austenitic stainless steel grade with 25% chromium and 35% nickel with high addition of silicon. This grade is also alloyed with nitrogen and rare-earth metals (REM) such as cerium (Table 1).[1] These two elements are incorporated to improve the ability of the alloy to form a protective oxide film on the surface of the stainless steel. The addition of REM increases the diffusion of silicon to the surface of the base metal, creating a dual protective layer (chromium oxides, Cr2O3, and silicon oxides, SiO2).

table 1

Alloy 353 MA has been a popular upgrade from stainless steels, such as alloy 309 and alloy 310. Alloy 353 MA shows high creep strength with excellent oxidation, carburization and nitridation resistance. The alloy is rated for use in the temperature range of 1472-2102°F (800-1150°C). The combination of the chemical composition and good high-temperature mechanical properties contributes to longer tube life, reduces maintenance costs and allows for its utilization in a wider temperature range. In addition, this material shows an easy fabrication with good weldability.figure two

Fig. 2. Sigma-phase diagram for different stainless steel grades. *A modified Sanicro® 31HT is used at 550-700˚C to eliminate the risk of precipitation sigma phase. **253 MA, 353 MA are trademarks owned by Outokumpu.


Structural stability is another desirable property for high-temperature alloys, and alloy 353MA shows good structural stability at high temperatures (Fig. 2). Different alloys have different degrees of detrimental phase of precipitation depending on the time spent being exposed to the sensitive temperature range. The temperature at which they are more prone to form intermetallic phases is shown in Fig. 2. The gradient orange to dark blue color indicates the degree of precipitation, with dark blue being the highest degree of precipitation and orange being the lowest.

figure 3

Fig. 3. Creep rupture strength of some austenitic and ferritic stainless steels.


This figure shows that alloys 309 and 310 are more prone to sigma-phase precipitation at 800°C than Alleima 253 MA and, overall, the least prone to this kind of precipitation at this temperature is Alleima 353 MA. The data used to derive this figure demonstrated that a 1% sigma phase precipitates are expected for Alleima 353 MA after 7,000 hours of exposure to at 800°C, whereas precipitation is expected after 2,000 hours for Alleima 253 MA. The sigma-phase precipitation is expected to occur in less than 200 hours for alloy 310 and alloy 309. Alloy 4C54 (alloy 446 types 1) precipitates 1% sigma phase at for 650°C after less than 200 hours.

Figure 3 shows the creep rupture strength of some grades. Alleima 353 MA, Alleima 253 MA and Sanicro® 31HT show the best creep deformation performance at high temperature when compared to the 300-series austenitic stainless steels and the ferritic stainless steel, Alleima 4C54-446-1 and Al-leima 2C48-446-2.


Applications

Alleima 353 MA is a solution for diverse high-temperature applications such as recuperators, muffle tubes, boilers, ethylene furnaces, radiant cracking tubes, thermocouple protection and furnace applications including radiant protection tubes for electrically heated and gas-fired systems.[2]


Lime Lances

Lime lance tubes in vertical kilns typically suffer from cracking, reduction of wall thickness and scaling on the tube’s surface. These happen due to deposit of solids from the process and are typically affected by hot corrosion mechanisms highly dependent on the fuel type. Alleima 353 MA has excellent resistance for oxidation, carburization and nitriding.

Figure 4

Fig. 4. Carburization tests at 1050˚C in steam cracking tubes for ethylene production.


Finned Tubes for Ethylene Furnaces

The combined properties of high carburization and high oxidation resistance make Alleima 353 MA an excellent material for finned tubes in ethylene furnaces, contributing to longer production runs and reducing the need for decoking. Figure 4 shows the results from a carburization test performed at the independent National Physics Laboratory (NPL) in London. Alleima 353 MA has higher carburization resistance compared to the other common wrought grades tested. The Cr number below each illustration indicates the remaining free Cr in the metal matrix. Low numbers indicate a high degree of carburization. Alloy 353 MA also has very good creep resistance at high temperatures, which meets the de-sign requirements of steam cracking furnace tubes.[3]


Muffle Tubes

Muffles furnaces are most often used in wire drawing mills and Bundy tube production. However, they can also be found in razor-blade production and tube annealing. These muffle tubes are fed with different compositions of shielding gas (oxidizing, carburizing and nitriding conditions as well as in endo-thermic gas or environments containing hydrogen gas). The endogas will cause rapid carburization, which will reduce the mechanical strength of the tube. Alleima 353 MA is the most cost-effective material for this application. Here is a summary of type of environment and maximum temperature:

  • Hydrogen up to 677°F (1250°C)
  • Nitrogen up to 621°F (1150°C)
  • Cracked ammonia up to 593°F (1100°C)
  • Endogas up to 621°F (1150°C)
  • Air up to 677°F (1250°C)


Conclusion

Alleima 353 MA™ is an excellent candidate for most severe high-temperature environments in steel mills and other industries due to its high creep strength, very good resistance to isothermal and cyclic oxidation, good resistance to nitriding gases, and good structural stability at high temperatures. Its maximum operating temperature is around 2150°F (1175°C).


For more information: Luiza Esteves is currently working as a technical marketing engineer at Alleima with a focus on high-temperature applications and chemical process industries. She can be reached at luiza.esteves@alleima.com.

*Trademark information: 353 MA is a trademark owned by Outokumpu OY.

All images/graphics provided by Alleima.


References

  1. https://www.alleima.com/en/technical-center/material-datasheets/tube-and-pipe-seamless/alleima-353-ma/
  2. S-KA075-B-ENG, 01 2014 – Sandvik Materials Technology
  3. S-TU308-B-ENG.03.2018 – Sandvik Materials Technology