254SMO is an
superaustenitic stainless steel designed for maximum resistance to
pitting and crevice corrosion. With high levels of chromium, molybdenum, and nitrogen, 254SMO is especially suited for high chloride environments such as brackish water
seawater pulp mill bleach plants and other high chloride process streams. 254SMO offers chloride resistance superior to that of Alloy
904L Alloy 20,
Alloy 825 and
Alloy G.
254SMO is compatible with the common austenitic stainless steels. It is often used
316L or
317L has failed by pitting, crevice attack, or
chloride stress corrosion cracking. In new construction, 254SMO has been found in many cases to be a technically adequate and much less costly substitute for
nickel based alloys and
titanium. 254SMO is substantially stronger than the common
austenitic grades, but is also characterized by high ductility and impact strength. 254SMO is readily fabricated and welded.
The high concentration of molybdenum,chromium and nitrogen made 254SMO has very good resistance to pitting and
crevice corrosion performance.
Copper improved
corrosion resistance in some of the acid . In addition, due to its high content of nickel,chromium and molybdenum, so that 254SMO have a good
stress corrosion cracking performance.
1.A lot of a wide range use of experience has shown that even in higher
temperature, 254SMO in the sea water is also highly resistant to
corrosion performance gap, only a few types of
stainless steel with this performance.
2.254SMO such as bleach paper required for the production of acidic solution and the solution halide oxidative corrosion resistance and
corrosion resistance can be compared to the most resilient in the base alloy of nickel and titanium alloys.
3.254SMO due to a high nitrogen content, so its
mechanical strength than other types of austenitic stainless steel is higher. In addition, 254SMO also highly scalable and impact strength and good weldability.
4.254SMO with high molybdenum content can make it higher rate of
oxidation in the
annealing , which after the acid cleaning with rough
surface than normal stainless steel is more common than the rough surface. However,has not adversely affected for the corrosion resistance of this steel .
Metallurgical structure
254SMO is face-centered cubic lattice structure. In order to obtain austenitic structure, 254SMO general annealing in the 1150-1200℃.In some cases, the material maybe with the traces of metal middle phase (χ phase and α-phase). However,their impact strength and
corrosion resistance are not adversely affected in normal circumstances. When placed in the range of 600-1000℃, they may phase in the grain boundary precipitation.
Corrosion resistance
254SMO with very low carbon content, which means that the danger by the heating caused of carbide precipitation is very small. Even in the 600-1000℃ after one hour sensitization still able to Strauss through the
intergranular corrosion test (Strauss Test
ASTMA262 order E).However, because of the high-alloy steel content. In the above-mentioned temperature range intermetallic phase with the possibility of metal in the grain boundary precipitation. These sediments do not make it occur
intergranular corrosion in the corrosive media applications, then,welding can be carried out without intergranular corrosion.But in the
heat of concentrated
nitric acid, these sediments may be caused intergranular corrosion in heat-affected zone. If ordinary stainless steel in the solution that containing chloride, bromide or iodide,it will show pitting, crevice corrosion or stress corrosion cracking form by localized corrosion. However, in some cases,the existence of halide will speed up the uniform corrosion.
Particularly in the non-oxidizing acid. In pure sulfuric acid, 254SMO with a much greater corrosion resistance than 316(common stainless steel),but with abated corrosion resistance compared to 904L (NO8904) stainless steel in high concentrations. In the sulfuric acid containing chloride ions , 254SMO with the largest corrosion resistance ability.The 316 can not be used for stainless steel in hydrochloric acid since it may occur localized corrosion and uniform corrosion, but 254SMO can be used in diluted hydrochloric acid in general temperatures,uder the border region do not need to worry about corrosion occurred. However, we must try to avoid the gap cracks. In the fluoride silicate (H2SiF4) and hydrofluoric acid (HF), the corrosion resistance of ordinary stainless steel is very limited, and 254SMO can be used in a very wide temperature and concentration.
254SMO Chemical composition:
| Alloy |
% |
Ni |
Cr |
Mo |
Cu |
N |
C |
Mn |
Si |
P |
S |
254SMO |
Min. |
17.5 |
19.5 |
6 |
0.5 |
0.18 |
- |
- |
- |
- |
- |
Max. |
18.5 |
20.5 |
6.5 |
1 |
0.22 |
0.02 |
1 |
0.8 |
0.03 |
0.01 |
254SMO Physical properties:
Density |
8.0 g/cm3 |
Melting point |
1320-1390 ℃ |
254SMO minimum mechanical properties in the room temperature:
Hot Working
Hot working should be carried out in the temperature range 1000-1200�C. Higher temperatures will reduce the workability. Fairly heavy scaling occurs at tempertaures exceeding 1150�C.
To ensure dissolution of possible precipitations of secondary phases from the hot forming, the subsequent heat treatment should take place at minimum 1150�C. The material should then be cooled as quickly as possible. A cooling rate that is too slow can result in reduced corrosion resistance.
Cold Working
254 SMO possesses very good cold formability. Bending, pressing and other forming operations which are used in fabrication can be carried out without difficulty. Practical experience obtained from the pressing of heat exchanger plates has been very favourable. The steel work hardens rapidly.
Welding
254 SMO possesses good weldability. When delivered, stainless steel sheets and plates have a homogeneous composition. Remelting of the parent metal, such as during welding without filler metal, may cause microscale variations in composition for elements such as chromium, nickel and particularly molybdenum. This phenomenon occurs in all highly alloyed stainless steels. These variations may reduce the pitting resistance of the weld. GTA and plasma arc welding without filler metal should therefore be avoided.
The material may not be subjected to abrasive contact with copper/brass items. Penetration of copper/brass into the grain boundaries can give rise to crack formation.
Avesta P12 or P16 welding consumables should be used for all welding methods.
Ignite the electrode in the joint since ignition burns beside the weld can give rise to corrosion attacks.
Heat treatment is normally not necessary after welding. However, welding without filler metal should be followed by solution annealing at 1150-1200�C and subsequent rapid cooling in order to achieve the best possible corrosion resistance.
To ensure optimum corrosion resistance the welds should be cleaned, preferably by pickling.
Applied field：
254SMO is a mult-purpose material can be used in many industrial applications:
1. Petroleum, petrochemical equipment, petro-chemical equipment, such as the bellows.
2. Pulp and paper bleaching equipment, such as pulp cooking, bleaching, washing filters used in the barrel and cylinder
pressure rollers, and so on.
3. Power plant flue gas desulphurization equipment, the use of the main parts: the absorption tower, flue and stopping plate, internal part,spray system.
4. At sea or
sea water processing system, such as power plants using
sea water to cool the thin-walled
Condenser, desalination of sea water processing equipment,can be applied even though the water may not flow in the device.
5. Desalination industries, such as
salt or desalination equipment.
6.
Heat exchanger, in particular in the working environment of chloride ion.