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Introduction to High Performance Ferritic Stainless Steel

Attention:     Issuing time:2020-03-23 10:15
Introduction to High Performance Ferritic Stainless Steel
 
In addition to a small amount of stabilized carbides and nitrides, high-performance ferritic stainless steels have a complete ferrite microstructure.
 
The unique characteristic of this ferrite structure is its good resistance to chloride stress corrosion fracture, but its toughness is limited. Large cross-sections or grain effects and the precipitation of brittle secondary phases can further reduce its toughness. Due to the limited toughness, these steel grades are usually not produced into medium and heavy plates. These steel grades were developed to obtain stress corrosion cracking and pitting resistance superior to 18-8 stainless steel at a lower price than high nickel austenitic alloys. They are generally only used for heat exchanger tube materials or sheets. Heat treatment does 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 order of increasing chloride pitting resistance.
 

 

Steel UNS Grade Category Chemical composition 
C N Cr Ni Mo Cu other Corrosion resistance equivalent
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-type ferritic stainless steels, such as E-BRITE 26-1, have similar local corrosion resistance to 316 stainless steel, but have much better resistance to stress corrosion cracking than the latter. This good stress corrosion cracking performance makes it useful for hot concentrated alkali solutions and chloride-containing solutions.
 
F2 ferritic stainless steels, including stainless steels, including SEA-CURE, are designed for localized seawater corrosion at room temperature and are widely used in seawater cooling condensers for power plants. Due to 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, nickel in these steels reduces the resistance to chloride stress corrosion cracking and increases sensitivity to the formation of harmful secondary phases. All of these ferritic stainless steels are resistant to stress corrosion cracking in the sodium chloride test solution, but because of their nickel content of 0.5% to 4.2%, they may not be able to resist the stress corrosion cracking of the magnesium chloride test solution.
 
F3 ferritic stainless steel is similar to A6 high-performance austenitic stainless steel. AL29-4-2 in this category is designed to achieve the highest level of comprehensive performance of ferritic stainless steel. It also has good local resistance. Corrosion and acid resistance.
 
Ferritic stainless steel (400 series) contains 15% to 30% chromium and has a body-centered cubic crystal structure. This type of steel generally does not contain nickel, and sometimes contains a small amount of elements such as Mo, Ti, Nb, etc. This type of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent resistance to stress corrosion. , Water vapor, water and oxidizing acid parts. Ferritic stainless steel is not only relatively low and stable, but also has many unique features and advantages. It has been proven that in many applications where austenitic stainless steel (300 series) was originally considered to be used, ferritic stainless steel is an extremely Excellent alternative material. Ferritic stainless steel does not contain nickel. The main elements are chromium (> 10%) and iron. Chromium is a particularly corrosion-resistant element of stainless steel and its price is relatively stable.
 
Stainless steel with a chromium content of 12% to 30% and a body-centered cubic lattice of ferrite as the matrix structure at high and normal temperatures. This type of steel generally does not contain nickel, and some contain a small amount of elements such as molybdenum, titanium or niobium, and has good oxidation resistance, corrosion resistance and resistance to chloride corrosion cracking. Ferritic stainless steel can be divided into three types of low chromium, medium chromium and high chromium according to chromium content. According to the purity of steel, especially the content of carbon and nitrogen impurities can be divided into ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. . Ordinary ferritic stainless steels have the disadvantages of low temperature and room temperature brittleness, notch sensitivity, high intergranular corrosion tendency, and poor weldability. Although such steels developed earlier, they have been greatly restricted in industrial applications. These deficiencies of ordinary ferritic stainless steel are related to the purity of the steel, especially the higher content of interstitial elements such as carbon and nitrogen in the steel. As long as the carbon and nitrogen in the steel are sufficiently low, for example, not more than 150 × 10 ~ 250 × 10, the above disadvantages can be basically overcome. Since the 1970s, due to the development of smelting technology, especially vacuum metallurgy and secondary refining processes, high-purity ferritic stainless steel with carbon + nitrogen ≤150 ~ 250 × 10 has been produced, making this type of steel widely used in industry. .
 
 
 
Classification

Generally can be divided into two categories of ordinary ferritic stainless steel and high-purity ferritic stainless steel.
 
Ordinary ferrite
These steels include low, medium and high chromium content. Low chromium ferritic stainless steel, containing about 11% to 14% chromium, such as 00Cr12, 0Cr13Al in China. American AISI400, 405, 406MF-2 (see table). This steel has good toughness, plasticity, cold deformability and weldability. Because the steel contains a certain amount of chromium and aluminum, it has good oxidation resistance and rust resistance. 405 can be used as petroleum refining towers, tank linings, steam turbine blades, high temperature sulfur corrosion resistant devices, etc. 400 is used for home and office appliances. 409 is used in automobile exhaust muffler systems and cold and warm water pipes. Medium chromium ferritic stainless steel, chromium content is 14% ~ 19%, such as China's 1Cr17, 1Cr17Mo. American AISI429, 430, 433, 434, 435, 436, 439. This type of steel has good rust and corrosion resistance. Its work hardening coefficient is small (n≈2), it has good deep drawing performance, but the ductility is poor. 430 is used as parts for building decoration, car decoration, kitchen equipment, gas burner and nitric acid industrial equipment. 434 is used for exterior decoration of automobiles and buildings. 439 is used as hoses for gas water heaters, coal and gas pipelines. High chromium ferritic stainless steel contains 19% ~ 30% chromium, such as Cr18Si2, Cr25 in China, and AISI442, 443, 446 in the United States. This type of steel has good oxidation resistance. 442 is discontinued in the atmosphere, the upper limit temperature is 1035 ° C, and the maximum continuous use temperature is 980 ° C. 446 has better oxidation resistance.
 
High purity ferrite
This type of steel contains very low carbon and nitrogen; high chromium, molybdenum, titanium, niobium and other elements. Such as China's 00Cr17Mo, 00Cr18Mo2, 00Cr26Mol, 00Cr30Mo2 foreign 18-2, Cr26Mol, 25Cr-5Ni-4Mo-Nb, MoNiT, Al29-4, Al29-4-2. This type of steel has good mechanical properties (especially toughness), welding properties, resistance to intergranular corrosion, pitting corrosion, crevice corrosion, and excellent stress corrosion cracking resistance. For example, 18-2 has good corrosion resistance in nitric acid, acetic acid, and NaOH, and the pitting corrosion resistance in 3% NaCl and FeCl3 is equivalent to or exceeds 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 acids, oxidizing acids, and strong bases. Good pitting resistance in strong chloride media. No stress corrosion cracking in chloride, hydrogen sulfide, excessive sulfuric acid and strong alkali. 30Cr-2Mo has higher resistance to pitting and crevice corrosion while maintaining resistance to stress corrosion. Steel with a small amount of nickel improves the performance in reducing acids.
 
 
 
Corrosion resistance
(1) Uniform corrosion. Chromium is the most easily passivated element. In the atmospheric environment, iron-chromium alloys with chromium content above 12% can be self-passivated. In the oxidizing medium, the chromium content can be passivated above 17%. In some aggressive media, high chromium and adding molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
 
(2) Intergranular corrosion. Ferritic stainless steels have the same intergranular corrosion as austenitic stainless steels, but the sensitization treatment and the heat treatment to avoid this corrosion are just the opposite. Ferritic stainless steel is susceptible to intergranular corrosion from rapid cooling above 925 ° C, and the state susceptible to intergranular corrosion (sensitized state) can be eliminated by short-term tempering at 650 ~ 815 ° C. The intergranular corrosion of ferritic steel is also the result of chromium deficiency due to carbide precipitation. Therefore, reducing the carbon and nitrogen content in steel and adding elements such as titanium and niobium can reduce the sensitivity of intergranular corrosion.
 
(3) Pitting and crevice corrosion. Chromium and molybdenum are the most effective elements to improve the pitting and crevice corrosion resistance of stainless steel. As the chromium content increases, the chromium content in the oxide film also increases, and the chemical stability of the film increases. Molybdenum is adsorbed on the active metal surface in the form of MoO4, which suppresses the dissolution of the metal, promotes repassivation, and prevents the damage of the film. Therefore, high chromium and molybdenum ferrite stainless steels have excellent pitting and crevice corrosion resistance.
 
(4) Resistance to stress corrosion cracking. Due to the characteristics of the microstructure, the ferritic stainless steel is corrosion-resistant in the medium where the austenitic stainless steel generates stress corrosion cracking.
 
 
 
Mechanical properties
Ferritic stainless steel cannot be strengthened by heat treatment because it has no phase change. Generally used after 700 ~ 800 ℃ annealing. Due to the almost solid solution strengthening effect of iron-chromium atomic phase, the yield strength and tensile strength of ferritic stainless steel are slightly higher than those of low carbon steel, and the ductility is lower than that of low carbon steel.
 
Ordinary ferritic stainless steel is prone to brittleness: (1) Brittleness at room temperature. Ordinary ferritic stainless steel is sensitive to notches. The brittle transition temperature is above room temperature except for low chromium (such as 405). The higher the amount of chromium, the greater the cold brittleness. This kind of cold brittleness is related to interstitial elements such as carbon and nitrogen in steel, while ultrapure ferritic steels have very low carbon due to interstitial elements such as carbon and nitrogen, which can obtain good toughness, and the brittle transition temperature can be reduced below room temperature.
 
(2) High temperature embrittlement. Ordinary ferritic stainless steel is heated to above 927 ℃ and then rapidly cooled to room temperature, the plasticity and toughness are significantly reduced. This high temperature embrittlement is related to the rapid precipitation of carbon (nitrogen) compounds on the grain boundaries or dislocations at a temperature of 427 ~ 927 ° C. Reducing the carbon and nitrogen content in the steel (using ultrapure technology) can greatly improve this brittleness. In addition, the ferrite steel coarsens its grain capacity when heated to above 927 ° C. The coarse grains will make the plasticity and toughness of the steel deteriorate.
 
(3) Formation of σ-phase. According to the iron-chromium phase diagram (see Figure 1), at a temperature of 500 to 800 ° C, a 40% to 50% chromium-containing alloy will form a single phase σ, and an alloy containing less than 20% or greater than 70% chromium will form α + σ Biphasic organization. σ-phase formation significantly reduces the ductility and toughness of steel. So this kind of steel should not be used for a long time at 500 ~ 800 ℃.
 
(4) Brittleness at 475 ° C. High chromium (> 15%) ferritic steel will be strongly brittle when kept at 400 ~ 500 ℃. The time required for this embrittlement is shorter than the precipitation of the σ phase. For example, 0.080C-0.4Si-16.9Cr steel is kept at 450 ° C for 4 hours, and the impact toughness at room temperature is almost reduced to zero. The degree of embrittlement increases with the increase of chromium content, but the toughness can be restored by treatment above 600 ° C. The 475 ° C embrittlement is due to the precipitation of the chromium-rich α ′ phase. Such steels should avoid heating near 475 ° C.
 
 

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