Chromium is the most important alloying element in austenitic stainless steel.
What is the role of Chromium in stainless steel? The corrosion resistance of austenitic stainless steel is mainly due to the fact that chromium in stainless steel promotes the passivation of steel and maintains the steel in a stable and passive state under the action of meeting material.
The effect of chromium in stainless steel structure
In austenitic stainless steel, chromium is an element that strongly forms and stabilizes the ferrite, narrowing the austenite zone, as the content of the steel increases, ferrite (δ) can appear in the austenitic stainless steel Organization, research shows 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, the minimum nickel content is required, about 8%. In this regard, the commonly used 18Cr-8Ni chromium-nickel austenitic stainless steel is the most suitable one for chromium-nickel content.
In austenitic stainless steels, with the increase of chromium content, the formation tendency of some intermetallic phases (such as δ phase) increases.
When the steel contains molybdenum, the chromium content will increase and x will form equal, as before as mentioned, the precipitation of σ, x phase not only significantly reduces the plasticity and toughness of the steel, but also reduces the corrosion resistance of the steel under certain conditions.
The increase of the chromium content in the austenitic stainless steel can make the martensite to hydrocarbon temperature (Ms ) Decreases, thereby improving the stability of the austenite matrix. Therefore, high-chromium (for example, more than 20%) austenitic stainless steel is difficult to obtain a martensite structure even after cold working and low temperature treatment.
Chromium is a strong carbide forming element, and it is no exception in austenitic stainless steel.
The effect of chromium on the performance
Generally comes, as long as the austenitic stainless steel pipe maintains a complete austenite structure without the formation of delta ferrite, etc., only improves content of chromium will not have a significant effect on the mechanical properties, and chromium will affect austenite.
The most important effect of stainless steel performance is corrosion resistance, mainly as follows:
chromium improves the performance of steel's oxidation resistance medium and acid chloride medium; under the combined action of nickel, molybdenum and copper, chromium improves the resistance of steel to some reducing media, organic acid, urea and alkaline media; chromium also improves the resistance of steel to localized corrosion, such as intergranular corrosion, pitting corrosion, crevice corrosion, and stress performance under certain conditions.
It has the greatest impact on the sensitivity of austenitic stainless steel intergranular, the factor is the carbon content in the steel, and other elements.
The role of chromium in stainless steel corrosion resistance with the experimental medium conditions and the actual use environment.
In the MgCl2 boiling solution, the role of chromium is generally harmful, but in aqueous media containing Cl- and oxygen, Under high temperature and high pressure water and stress corrosion conditions with pitting corrosion as the origin, increasing the chromium content in the steel is beneficial to stress corrosion resistance. Chromium can also prevent austenite stainless steel and alloys that are prone to appear between the grains due to the increased nickel content. The tendency of type stress corrosion, the effect of chromium on stress corrosion of open causticity (Nq0H) is also beneficial.
In addition to having an important influence on the corrosion resistance of negative-number austenitic stainless steel, chromium can also significantly improve the oxidation resistance, sulfidation resistance and resistance to melting salt corrosion of such steels.