Regardless of the stainless Steel Plate or the heat-resistant steel plate, the austenitic steel plate has the best comprehensive performance, and has sufficient strength, excellent plasticity and low hardness, which is one of the reasons why they are widely used. Austenitic stainless steels are similar to most other metallic materials. Their tensile strength, yield strength and hardness increase with decreasing temperature; plasticity decreases with decreasing temperature. Its tensile strength is relatively uniform in the temperature range of 15~80 °C. More importantly: as the temperature decreases, the impact toughness decreases slowly and there is no brittle transition temperature. Therefore, stainless steel maintains sufficient plasticity and toughness at low temperatures.
The heat resistance of stainless Steel Pipe stainless steel is heat resistance, which means that it has the performance of oxidation resistance or gas medium corrosion resistance, that is, thermal stability, and at the same time, it has sufficient strength and heat strength at high temperature.
Carbon effect: Carbon is an element in austenitic stainless steel that strongly forms and stabilizes austenite and enlarges the austenite. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is a kind of interstitial element. Solid solution strengthening can significantly improve the strength of austenitic stainless steel. Carbon can also improve the corrosion corrosion resistance of austenitic stainless steel in high concentration chloride (such as 42% MgCl2 boiling solution), but in austenitic stainless steel Carbon is often considered a harmful element, mainly due to the formation of high chromium Cr23C6 carbon with chromium in steel under some conditions in stainless steel and corrosion resistant applications (such as welding or heating at 450~850 °C). The compound thus leads to the depletion of local chromium, which reduces the corrosion resistance of the steel, especially the resistance to intergranular corrosion. Therefore, the newly developed chromium-nickel austenitic stainless steel since the 1960s has a carbon content of less than 0.03% or 0.02%. For carbon type, it can be known that as the carbon content decreases, the intergranular corrosion sensitivity of steel decreases. When the carbon content is less than 0.02%, the most obvious effect is obtained. Some experimental pearls also indicate that carbon also increases chromium austenite. Stainless steel point corrosion tendency The harmful effects of carbon, not only in the austenitic stainless steel smelting and in the process should be controlled as low as possible carbon content, but also in the subsequent heat, cold processing and heat treatment process to prevent carbonation of stainless steel surface, and chromium-free carbonization Material precipitation.
Effect of chromium: Chromium is the most important alloying element in austenitic stainless steel. The corrosion and corrosion resistance of austenitic stainless steel is mainly due to the fact that chromium promotes the passivation of steel and the steel under the action of the mass. The result of maintaining a stable and passive state. ○1 The effect of chromium on the structure: In austenitic stainless steel, chromium is an element that strongly forms and stabilizes the iron body, shrinking the austenite region, and austenitic stainless steel as the content in the steel increases. Ferrite (δ) structure may occur, and studies have shown that in chromium-nickel austenitic stainless steel, when the carbon content is 0.1% and the chromium content is 18%, in order to obtain a stable single austenite structure, nickel is required. The lowest content is about 8%. In this regard, the commonly used 18Cr-8Ni type chrome-nickel austenitic stainless steel is the most suitable one with chromium content and nickel content ratio. With austenitic stainless steel, with chromium When the content is increased, the formation tendency of some intermetallic phases (such as δ phase) increases. When the steel contains molybdenum, the content of chromium increases and the formation of bismuth is equal. As mentioned above, the precipitation of σ and χ phase is not only Significantly reduce the ductility and toughness of steel, and also reduce the corrosion resistance of steel under some conditions Sex, the increase of chromium content in austenitic stainless steel can reduce the martensite hydrocarbon temperature (Ms), thereby improving the stability of the austenite matrix. Therefore, high chromium (such as more than 20%) austenitic stainless steel even after cold working It is also difficult to obtain martensite structure with low temperature treatment..
Chromium is a strong carbide forming element, and it is no exception in austenitic stainless steel. The common chromium carbide in austenitic stainless steel is Cr23C6; when the steel contains molybdenum or chromium, it can also be found in the expired Cr6C and other carbides. The formation of the steel will have an important effect on the properties of the steel under certain conditions. ○2 The effect of chromium on the performance: generally, as long as the austenitic stainless steel maintains a complete austenite structure without the formation of δ ferrite, etc. Increasing the chromium content in steel does not have a significant effect on the mechanical properties. The most important effect of chromium on the properties of austenitic stainless steel is corrosion resistance. The main performance is: chromium improves the performance of steel oxidation resistant medium and acid chloride medium; Nickel and the combination of molybdenum and copper, chromium improves the resistance of steel to some reducing media, organic acids, urea and alkali media; chromium also improves the local corrosion resistance of steel, such as intergranular corrosion, pitting corrosion, crevice corrosion and some of these conditions The performance of the lower stress gymnasium: The most influential factor on the sensitivity of the austenitic stainless steel intercrystalline gymnasium is the carbon content in the steel. The effect of other elements on the intergranular stadium is mainly due to its dissolution of carbides. In the austenitic stainless steel, chromium can increase the solubility of carbon and reduce the depletion of chromium. Therefore, increasing the chromium content is beneficial to the intergranular corrosion resistance of austenitic stainless steel. Chromium is very effective. Improve the pitting corrosion resistance and crevice corrosion performance of austenitic stainless steel. When molybdenum or molybdenum and nitrogen are present in the steel, the effectiveness of chromium is greatly enhanced, although according to the research on the resistance of molybdenum stadium and crevice corrosion Chromium is about twice as much as nitrogen is 30 times that of chromium, but a lot of research, if there is no chromium or chromium content in austenitic stainless steel, the pitting corrosion and crevice corrosion of molybdenum and nitrogen will be lost or not significant.
The effect of chromium on the stress corrosion resistance of austenitic stainless steel varies with the experimental medium conditions and the actual use environment. In the MgCl2 boiling solution, the effect of chromium is generally harmful, but in the aqueous medium containing Cl- and oxygen, Under the condition of stress corrosion of high temperature and high pressure water and pitting corrosion, increasing the chromium content in steel is beneficial to stress corrosion resistance. At the same time, chromium can prevent the intergranular type which is easy to appear due to the increase of nickel content in austenitic stainless steel and alloy. The tendency of stress corrosion, the corrosion of the caustic (NqOH) stress corrosion, the effect of chromium is also beneficial. In addition to the important influence on the corrosion resistance of negative-nano-steel stainless steel, chromium can significantly improve the oxidation resistance and sulfurization resistance of this steel. Anti-melting salt corrosion and other properties.
1 Nickel impact on tissue
Nickel is an element that is strongly one million and stabilizes austenite and enlarges the austenite phase. To obtain a single austenitic structure, the minimum nickel content required for steel containing 0.1% carbon and 18% chromium is approximately 8%, this is the basic score of the most famous 18-8 chromium-nickel austenitic stainless steel. In the austenitic stainless steel, with the increase of nickel content, the residual ferrite can be completely eliminated, and the formation of σ phase is significantly reduced. At the same time, the temperature of martensite trans-hydrocarbon is reduced, and even λ→M phase transition may not occur, but the increase of nickel content will reduce the solubility of carbon in austenitic stainless steel, so that the tendency of carbide precipitation is enhanced.
2 nickel effect on performance
The effect of nickel on the mechanical properties of austenitic stainless steels, especially chromium-nano-negative stainless steels, is mainly determined by the influence of nickel on the austenite stability. Within the range of nickel content in which martensite transformation may occur in steel, With the increase of nickel content, the strength of steel decreases, the sheet plasticity increases, and the toughness (including extremely low temperature toughness) of chromium-nickel austenitic stainless steel with stable austenitic structure is very good, so it can be used as low temperature steel, which is well known. The chromium-manganese austenitic stainless steel with stable austenitic structure, the addition of nickel can further improve its toughness. Nickel can also significantly reduce the cold work hardening tendency of austenitic stainless steel, mainly due to the increased austenite stability and reduction. It also eliminates the martensite transformation during cold working, and the cold work hardening effect of austenite itself is not obvious. The effect of cold working hardening tendency of stainless steel, nickel reduces the cold work hardening rate of austenitic stainless steel, and lowers the room temperature and low temperature of steel. The effect of strength and plasticity determines that the increase of nickel content is beneficial to the cold forming properties of austenitic stainless steel and the improvement of nickel. The amount can also reduce or even eliminate the δ ferrite in the austenitic stainless steel of 18-8 and 17-14-2 type chrome nickel 9 tongs, thereby improving its hot workability, however, the reduction of δ ferrite to these steels Unfavorable solderability will increase the tendency of soldering hot cracked wire. In addition, nickel can significantly improve the hot workability of chromium-manganese-nitrogen (chromium-manganese-nickel-nitrogen) austenitic stainless steel, thereby significantly increasing the yield of steel. In the stainless steel, the addition of nickel and the increase of the nickel content lead to an increase in the thermodynamic stability of the steel, so the austenitic stainless steel has better rust and oxidation resistance properties, and with the increase of nickel content The performance of the reductive medium is further improved. It is worth noting that nickel is the only important element to improve the austenitic stainless steel to resist the transgranular stress corrosion of many media. The corrosion resistance of nickel to austenitic stainless steel in various acid media The effect of the need to point out that under some conditions of high temperature and high pressure water, the increase of nickel content leads to an increase in the intergranular stress corrosion sensitivity of steel and alloy, but this adverse effect will be due to the increase of chromium content in steel and alloy. It is reduced or suppressed. With the increase of nickel content in the magnetic austenitic stainless steel, the critical carbon content of the intergranular corrosion is reduced, that is, the intergranular corrosion sensitivity of the steel is increased, and the pitting corrosion resistance of the austenitic stainless steel is The effect of crevice corrosion is not significant. In addition, nickel also improves the high temperature oxidation resistance of austenitic stainless steel, which mainly improves the composition, structure and performance of chromium oxide film with nickel, and the higher the nickel content. The more harmful, mainly due to the low melting point nickel sulfide at the grain boundary in steel. In general, simple chromium-nickel (and chromium-manganese-nitrogen) austenitic stainless steel is only used for rust and oxidation resistance. Under the conditions of use of medium (such as nitric acid, etc.), molybdenum is added to steel as an important alloying element in austenitic stainless steel to further expand its use range. The role of molybdenum is mainly to increase the steel in reducing medium (than H2SO4, H3PO4). , as well as some organic acid and urea environment) corrosion resistance, and improve the resistance of steel to pitting corrosion and crevice corrosion.
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