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The performance difference of flanges made of different materials in high temperature environment is mainly reflected in strength maintaining ability, oxidation resistance, creep resistance, thermal stability and medium compatibility, etc. The following analysis is carried out from the typical material categories:
I. Carbon steel flanges (e.g., Q235, 20# steel): the basic choice of low and medium temperature scenarios.
Strength at high temperatures is obviously attenuated.
The use temperature of carbon steel is usually not more than 425℃, and its yield strength will decrease by more than 30% when the temperature exceeds 350℃ (e.g., the yield strength of 20# steel drops from 245MPa to 180MPa at 400℃). When the temperature exceeds 350℃, its yield strength will decrease by more than 30% (e.g. 20# steel yield strength decreases from 245MPa to 180MPa at 400℃). If it continues to operate above 450℃, the grain size may be enlarged due to “pearlite spheroidization”, which will eventually lead to creep fracture.
Limited resistance to oxidation
carbon steel at 300 ℃ above the beginning of rapid oxidation, the surface of the formation of loose Fe ₃ O ₄ oxide layer, the higher the temperature the faster the oxidation rate (eg, 500 ℃ when the oxidation rate of 300 ℃ is 5 times). If the medium contains sulfur or water vapor, oxidation corrosion will be further aggravated.
Second, austenitic stainless steel flanges (304/316, etc.): the main high temperature corrosion scene
high temperature strength and oxidation resistance is better than carbon steel
304 stainless steel use temperature upper limit of about 870 ° C, 316L because of molybdenum elements, in the 650 ° C below can still maintain good strength (yield strength ≥ 120MPa), and oxidation resistance is significantly improved (800 ° C oxidation rate is lower than that of carbon steel 90%). 90% lower oxidation rate than carbon steel at 800℃).
Principle: The stability of the austenitic organization makes it less prone to pearlite spheroidization at high temperatures, and the chromium element (18%~20%) forms a dense Cr₂O₃ oxide film, preventing the diffusion of oxygen atoms.
Potential risks at high temperatures
Sensitized corrosion: If used for a long period of time in the 450~850°C range, 304 stainless steel may lead to intergranular corrosion due to carbide precipitation, which needs to be improved by “stabilization treatments” (e.g., titanium added to 321 stainless steel).
Creep limitation: above 650℃, the creep deformation rate of austenitic stainless steel accelerated, the design needs to reduce the permissible stress (such as 316L in 700℃ when the permissible stress is only 15% of room temperature).
Third, duplex steel flange (2205, 2507, etc.): high temperature and strong corrosive environment of the cost-effective choice
high-temperature mechanical properties between carbon steel and austenitic stainless steel
2205 duplex steel is usually ≤ 300 ° C, super duplex steel 2507 can be 350 ° C, and its yield strength at 300 ° C is still maintained at more than 400MPa (304 stainless steel 2 times), but higher than 350 ° C, the ferritic phase of the ferrite phase is accelerated, the design needs to reduce the allowable stress (such as 316L at 700 ° C the allowable stress is only 15% at room temperature). ℃, the ferrite phase creep resistance decreases, the strength of faster than the austenitic stainless steel.
Fourth, chromium and molybdenum steel flange (15CrMo, P91, etc.): high temperature and high pressure working conditions of the core materials
high temperature strength and creep resistance is significantly improved
15CrMo steel (chromium 1% ~ 1.5%, molybdenum 0.5%) can be used at a temperature of up to 550 ° C, 500 ° C, when the yield strength is still maintained at more than 200MPa; higher grade of P91 steel (9% chromium + 1% molybdenum) can be in the long term 650 ° C the following operation, creep fracture strength is more than the austenitic stainless steel. Operation, creep fracture strength is 15CrMo 2 times (such as 600 ℃ P91 100,000 hours of creep strength of 100MPa, 15CrMo only 40MPa).
V. Nickel-based alloy flanges (Inconel 625, Hastelloy C-276, etc.): the ultimate solution for extreme high-temperature corrosive environments
Ultra-high-temperature performance crushing other materials
Inconel 625 at 1093 ℃ can still maintain a tensile strength of more than 100MPa, Hastelloy C-276 can be oxidation-resistant at 1200 ℃ and below, and at high temperatures of the creep fracture life of up to 10 million. Creep fracture life of up to 100,000 hours or more (such as 800 ℃, C-276 creep strength is 5 times that of 316L).
Anti-temperature complex corrosion “all-rounder”
In high-temperature strong acid (such as 150 ℃ of concentrated sulfuric acid), fluorine / chlorine-containing media or high-temperature and high-pressure sulfur-containing oil and gas, nickel-based alloys due to the alloying system of high nickel (≥50%), high chromium (20% to 30%), high molybdenum (10% to 16%), can simultaneously resist oxidative corrosion, stress corrosion and intergranular corrosion. Intergranular corrosion. For example: coal chemical industry in the high temperature coal gasifier (temperature 650 ℃, containing H ₂ S / CO ₂), only nickel-based alloy flanges can meet the requirements of more than 20 years of service.