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What is high performance ferritic stainless steel?

What is high performance ferritic stainless steel?

High performance ferritic stainless steel has complete ferrite microstructure besides a small amount of stabilized carbide and nitride. The unique characteristic of this ferrite structure lies in its good resistance to chloride stress corrosion cracking, but its toughness is limited. Large section or grain effect and the precipitation of brittle secondary phase will further reduce its toughness. Due to the limited toughness, these grades of steel are usually not produced into medium and thick plates. The purpose of developing these steel grades is to obtain stress corrosion cracking resistance and pitting resistance superior to 18-8 stainless steel at a price lower than that of high nickel austenitic alloy. They are generally used only for tubular materials or sheets of heat exchangers, and heat treatment will not harden them, but in the annealed state, they show higher strength than many austenitic stainless steels. The following table lists the main forged and rolled ferritic steels in the order of increasing chloride pitting resistance.

[lwptoc]

Grade
UNS
Type
C
N
Cr
Ni
Mo
Cu
Other

Corrosion

resistance

equivalent

Type 444
S44400
F1
0.025
0.035
17.5~19.5
1
1.75~2.50
Ti,Nb
23
26-1S
S44626
0.06
0.04
25.0~27.0
0.5
0.75~1.50
Ti
27
E-BRITE 26-1
S44627
0.01
0.015
25.0~27.0
0.5
0.75~1.50
Nb
27
MONIT
S44635
F2
0.025
0.035
24.5~26.0
3.5~4.5
3.5~4.5
Ti,Nb
36
SEA-CURE
S44660
0.03
0.04
25.0~28.0
1.0~3.5
3.0~4.0
Ti,Nb
35
AL29-4C
S44735
F3
0.03
0.045
28.0~30.0
1
3.6~4.2
Ti,Nb
40
AL29-4-2
S44800
0.01
0.02
28.0~30.0
2.0~2.5
3.5~4.2
40
  • F1 ferritic stainless steel, such as e-brite26-1, has similar resistance to local corrosion as 316 stainless steel, but its resistance to stress corrosion cracking is much better than the latter. This kind of good stress corrosion fracture property makes it suitable for hot concentrated alkali solution and chloride solution.
  • F2 ferritic stainless steel, including sea-cure, is designed to resist local corrosion of seawater at normal temperature and is widely used in seawater cooling condensers of power plants. Because of their high chromium content, moderate molybdenum and nickel content, they also have good corrosion resistance to strong organic acids and oxidizing or medium reducing inorganic acids. However, the nickel in these steel grades reduces the resistance to chloride stress corrosion cracking and increases the sensitivity to the formation of harmful secondary phases. All of these ferritic stainless steels can resist stress corrosion cracking in NaCl test solution, but due to the nickel content of 0.5% – 4.2%, they may not be able to resist stress corrosion cracking in NaCl test solution.
  • F3 type ferritic stainless steel is like A6 type high performance austenitic stainless steel. Al29-4-2 in this type is designed to achieve the highest comprehensive performance of ferritic stainless steel. It has good local corrosion resistance and acid corrosion resistance.

The chromium content of ferritic stainless steel (400 Series) is 15% ~ 30%, which has a BCC crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to manufacture parts resistant to atmospheric, water vapor, water and oxidative acid corrosion. The price of ferritic stainless steel is not only relatively low and stable, but also has many unique characteristics and advantages. It has been proved that ferritic stainless steel is a very excellent alternative material in many application fields that originally thought that only austenitic stainless steel (300 Series) can be used. Ferritic stainless steel does not contain nickel, and its main elements are chromium (> 10%) and iron. Chromium is a special corrosion-resistant element of stainless steel , its price is relatively stable.
Stainless steel with chromium content of 12% ~ 30% and ferrite with BCC lattice as matrix structure at high temperature and normal temperature. This kind of steel generally does not contain nickel, some contain a small amount of molybdenum, titanium or niobium and other elements, which has good oxidation resistance, corrosion resistance and chloride corrosion cracking resistance. Ferritic stainless steel can be divided into low chromium, medium chromium and high chromium according to the chromium content. According to the purity of steel, especially the content of carbon and nitrogen impurities, it can also be divided into ordinary ferritic stainless steel and ultra pure ferritic stainless steel. Ordinary ferritic stainless steel has many disadvantages, such as low temperature and room temperature brittleness, notch sensitivity, high tendency of intergranular corrosion, poor weldability and so on. These shortcomings of ordinary ferritic stainless steel are related to the purity of steel, especially the high content of interstitial elements such as carbon and nitrogen in steel. As long as the carbon and nitrogen in the steel are low enough, for example, not more than 150 × 10 ~ 250 × 10, the above disadvantages can be basically overcome. After 1970s, due to the development of smelting technology, especially vacuum metallurgy and secondary refining technology, high-purity ferritic stainless steel with carbon + nitrogen ≤ 150 ~ 250 × 10 has been produced, which has been widely used in industry.

202003201584688629211098 - What is high performance ferritic stainless steel?

Classification

Generally, it can be divided into ordinary ferritic stainless steel and high-purity ferritic stainless steel.

Ordinary ferrite

This kind of steel includes low, medium and high chromium content. Low chromium ferritic stainless steel, containing about 11% ~ 14% chromium, such as 00Cr12, 0Cr13Al in China. Aisi400, 405, 406mf-2 in the United States (see table). This kind of steel has good toughness, plasticity, cold deformation and weldability. The steel contains a certain amount of chromium and aluminum, so it has better oxidation resistance and rust resistance. 405 can be used as oil refining tower, tank lining, steam turbine blade, high temperature sulfur corrosion resistance device, etc. 400 for home and office appliances, etc. 409 is used for automobile exhaust silencing system, cold and warm water pipes, etc. The chromium content of medium chromium ferritic stainless steel is 14% ~ 19%, such as 1Cr17 and 1cr17mo in China. AISI429, 430, 433, 434, 435, 436, 439 in the United States. This kind of steel has good rust resistance and corrosion resistance. Its work hardening coefficient is small (n ≈ 2), it has good deep drawing performance, but its ductility is poor. 430 is used for building decoration, automobile decoration, kitchen equipment, gas burner and parts of nitric acid industrial device, etc. 434 for exterior decoration of automobiles and buildings. 439 used as the hose of gas water heater, coal, gas pipeline, etc. High chromium ferritic stainless steel contains 19% ~ 30% chromium, such as Cr18si2 and Cr25 in China and AISI442, 443 and 446 in the United States. This kind of steel has good oxidation resistance. 442 is used continuously in the atmosphere, the upper limit temperature is 1035 ℃, and the maximum continuous use temperature is 980 ℃. 446 has better antioxidant activity.

High purity ferrite

This kind of steel contains very low carbon, nitrogen, high chromium, molybdenum, titanium, niobium and other elements. For example, 00cr17mo, 00Cr18Mo2, 00Cr26Mol, 00Cr30mo2 of China, 18-2, Cr26mol, 25cr-5ni-4mo-nb, monit, Al29-4 and Al29-4-2 of foreign countries. This kind of steel has good mechanical properties (especially toughness), weldability, intergranular corrosion resistance, pitting corrosion resistance, crevice corrosion resistance and excellent stress corrosion cracking resistance. For example, 18-2 has good corrosion resistance in nitric acid, acetic acid and NaOH. In 3% NaCl and FeCl3, the pitting resistance is equal to or more than 18-8 austenitic steel, and the SCC resistance is far more than 18-8 steel. 26crmo steel is resistant to corrosion in many media, especially in organic acid, oxidizing acid and strong alkali. It has good pitting resistance in strong chloride medium. There is no stress corrosion cracking in chloride, hydrogen sulfide, excessive sulfuric acid and strong base. 30cr-2mo has higher resistance to pitting corrosion and crevice corrosion while maintaining stress corrosion resistance. Steel with a small amount of nickel can improve its properties in reducing acid.

Corrosion resistance

  1. Uniform corrosion. Chromium is the most easily passivated element. In the atmospheric environment, the Fe Cr alloy with more than 12% chromium content can be passivated by itself. In the oxidizing medium, when the chromium content is more than 17%, it can be passivated. In some aggressive medium, high chromium and molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
  2. Intergranular corrosion. The intergranular corrosion of ferritic stainless steel is the same as austenitic stainless steel, but the sensitization treatment and the heat treatment to avoid the corrosion are just the opposite. Ferritic stainless steel is easy to suffer intergranular corrosion when quenched from 925 ℃ or above, but the state of easy to suffer intergranular corrosion (sensitized state) can be eliminated after tempering at 650 ~ 815 ℃ for a short time. Intergranular corrosion of ferritic steel is also the result of chromium deficiency caused by carbide precipitation. Therefore, the sensitivity of intergranular corrosion can be reduced by reducing the content of carbon and nitrogen and adding elements such as titanium and niobium.
  3. Pitting and crevice corrosion. Chromium and molybdenum are the most effective elements to improve the pitting and crevice corrosion resistance of stainless steel. With the increase of chromium content, the content of chromium in the oxide film increased, and the chemical stability of the film increased. Molybdenum is adsorbed on the active metal surface in the form of MoO4, which can inhibit the dissolution of the metal, promote the re passivation and prevent the damage of the film. Therefore, high chromium and molybdenum ferritic stainless steel has excellent pitting and crevice corrosion resistance.
  4. Stress corrosion cracking resistance. Due to the characteristics of microstructure, ferritic stainless steel is corrosion resistant in the medium of stress corrosion cracking of austenitic stainless steel.

Mechanical property

Ferritic stainless steel can not be strengthened by heat treatment because of no phase transformation. It is generally used after annealing at 700-800 ℃. The yield strength and tensile strength of ferritic stainless steel are slightly higher than those of low carbon steel, while the ductility of ferritic stainless steel is lower than that of low carbon steel.
Ordinary ferritic stainless steel is prone to brittleness:

  1. Room temperature brittleness. Ordinary ferritic stainless steel is sensitive to notch. The brittle transition temperature is above room temperature except for low chromium (such as 405). The higher the chromium content, the greater the cold brittleness. The cold brittleness is related to the interstitial elements such as carbon and nitrogen in the steel, while the ultra pure ferritic steel can obtain good toughness because of the very low carbon content of interstitial elements such as carbon and nitrogen, and the brittleness transition temperature can be reduced to below room temperature.
  2. High temperature embrittlement. The plasticity and toughness of ordinary ferritic stainless steel decreased significantly when it was quenched to room temperature after being heated above 927 ℃. This high temperature embrittlement is related to the rapid precipitation of carbon (nitrogen) compounds on the grain boundaries or dislocations at 427-927 ℃. The brittleness can be greatly improved by reducing the carbon and nitrogen content in the steel. In addition, when the ferrite steel is heated to 927 ℃ or above, the coarse grain will make the plasticity and toughness of the steel worse.
  3. σ – phase formation. When the temperature is 500 ~ 800 ℃, a single phase σ will be formed in the alloy containing 40% ~ 50% chromium, and a dual phase structure of α + σ will be formed in the alloy containing less than 20% or more than 70% chromium. σ – phase formation can significantly reduce the plasticity and toughness of steel. So this kind of steel should not be used at 500 ~ 800 ℃ for a long time.
  4. 475 ℃ brittleness. High chromium (> 15%) ferritic steel will be embrittled at 400 ~ 500 ℃. The time required for embrittlement is shorter than that for σ phase precipitation. For example, when 0.080c-0.4si-16.9cr steel is kept at 450 ℃ for 4h, the impact toughness at room temperature almost drops to zero. The degree of embrittlement increases with the increase of chromium content, but the toughness can be restored after treatment above 600 ℃. The embrittlement at 475 ℃ is due to the precipitation of chromium rich α′. This type of steel should avoid heating near 475 ℃.

Source: China Stainless Steel Pipe FittingsManufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Summary
what is high performance ferritic stainless steel - What is high performance ferritic stainless steel?
Article Name
What is high performance ferritic stainless steel?
Description
High performance ferritic stainless steel has complete ferrite microstructure besides a small amount of stabilized carbide and nitride.
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www.epowermetals.com
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