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Effect of Chromium on Stainless Steel

2019/6/23 18:47:13 Viewers:
Chromium is the most important alloy element in austenitic stainless steel. The main reason for obtaining the stainless and corrosion resistance of austenitic stainless steel is that chromium promotes the passivation of steel and keeps the steel stable and passive under the action of mass transfer. Effect of Chromium on Microstructure: In austenitic stainless steel, chromium is an element that strongly forms and stabilizes ferrite, reducing austenitic zone. With the increase of steel content, ferrite (delta) structure can appear in austenitic stainless steel. The research shows that in chromium-nickel austenitic stainless steel, when carbon content is 0.1% and chromium content is 18%, the required nickel content for obtaining stable single austenitic structure. In this regard, the commonly used 18Cr-8Ni austenitic stainless steel with chromium content is the most suitable one. In austenitic stainless steel, with the increase of chromium content, the formation tendency of some intermetallic phases (e.g. Delta phase) increases. When molybdenum is contained in steel, the chromium content will increase and the formation of_equivalence will also occur. As mentioned earlier, the precipitation of_and_phase not only significantly reduces the plasticity and toughness of steel, but also reduces the corrosion resistance of steel under some conditions, and the extraction of chromium content in austenitic stainless steel. High martensitic hydrocarbon transfer temperature (Ms) decreases, which improves the stability of austenite matrix. Therefore, it is difficult to obtain martensite structure of austenitic stainless steel with high chromium (for example, more than 20%) even after cold working and low temperature treatment.

Chromium is a strong carbide forming element, and it is no exception in austenitic stainless steel. Cr23C6 is a common chromium carbide in austenitic stainless steel. When molybdenum or chromium is contained in steel, carbides such as Cr6C can also be seen. Their formation will have an important impact on the properties of steel under certain conditions. In general, as long as austenitic stainless steel retains complete austenitic structure without formation of delta ferrite, only increasing chromium content in steel will not have significant effect on mechanical properties. The greatest effect of chromium on the properties of austenitic stainless steel is corrosion resistance, which is mainly manifested as follows: chromium improves the properties of oxidation resistance medium and acid chloride medium of steel; Under the combined action of molybdenum and copper, chromium improves the resistance of steel to some reducing medium, organic acid, urea and alkali medium, and chromium also improves the resistance of steel to local corrosion, such as intergranular corrosion. Point corrosion, crevice corrosion and stress corrosion properties under certain conditions. The most important factor affecting the susceptibility to intercrystalline corrosion of austenitic stainless steel is the carbon content in the steel. The effect of other elements on Intercrystalline Corrosion depends mainly on the dissolution and precipitation behavior of carbides. In austenitic stainless steel, chromium can increase the solubility of carbon and reduce the dilution of chromium. Therefore, increasing the chromium content is beneficial to the intercrystalline corrosion resistance of austenitic stainless steel. Effectively improve the pit corrosion resistance and crevice corrosion resistance of austenitic stainless steel. When molybdenum or molybdenum and nitrogen exist in the steel at the same time, the effectiveness of chromium is greatly enhanced. Although the resistance of molybdenum to pit corrosion and crevice corrosion is about 3 times of chromium and 30 times of chromium according to the research, a lot of studies have shown that if there is no chromium or low chromium content in austenitic stainless steel, the resistance of molyb Point corrosion and crevice corrosion will be lost or not significant enough.

The effect of chromium on stress corrosion resistance of austenitic stainless steel varies with the experimental medium conditions and the actual use environment. In MgCl2 boiling solution, the effect of chromium is generally harmful. However, under the conditions of stress corrosion with Cl-and oxygen-containing water, high temperature and high pressure water and pitting corrosion as its origin, increasing the content of chromium in steel is beneficial to stress corrosion resistance, and at the same time, chromium can be prevented. The tendency of intergranular stress corrosion in austenitic stainless steels and alloys is prone to occur due to the increase of nickel content, which is also beneficial to the stress corrosion of cracking (NaOH) and the effect of chromium. In addition to the important influence of chromium on the corrosion resistance of austenitic stainless steels, it can also significantly improve the anti-oxidation, anti-sulfuration and anti-molten salt corrosion properties of such steels.