Steel Man: March 2025

The precipitation behavior of carbide and nitride in duplex stainless steel has significant influence on its properties. There are mainly two types of carbides, M7C3 and M23C6, among which M7C3 precipitates along the phase boundary of ferrite and austenite in the temperature range of 950~1050℃, while M23C6 precipitates faster at lower than 950℃, especially at 800℃, M23C6 precipitates in 2507 steel less than 1 minute. The precipitation of M23C6 consumes ferrite and leads to the formation of secondary austenite structures. The nitrides are mainly Cr2N and CrN, among which Cr2N precipitates more in the ferritic phase, especially in the case of rapid cooling after high temperature solution, the amount of nitride precipitates increases. Although the nitride precipitates less in the welding heat affected zone, it has little effect on the toughness and corrosion resistance of the steel.

There are mainly two types of carbide precipitated phase: M7C3 and M23C6. In the duplex stainless steel with relatively high carbon content (≥0.03%), M7C3 carbides will be precipitated along the two-phase boundary of ferrite and austenite during thermal treatment in the temperature range of 950~1050℃. In the heat treatment at a temperature below 950℃, M23C6 carbide will be precipitated along the two-phase boundary, and its precipitation speed is very fast, such as 2507 steel at 800℃, its M23C6 carbide precipitation time is less than 1min, even if the rapid cooling method is difficult to avoid the generation of this carbide. It is first produced at the two-phase boundary of ferrite and austenite, and then precipitated at the grain boundary of ferrite and austenite, but it does not occur in ferrite and austenite. During the growth of M23C6 grains, the ferritic content of the neighboring region will be consumed, which leads to the transformation of the original ferritic region into secondary austenite structure, thus forming the aggregation region of M23C6 and γ phases.

There are two types of nitride precipitated phases, Cr2N and CrN, of which Cr2N is the most important precipitated form. Cr2N usually also contains Fe and Mo elements, but is structurally a type M2N nitride. 2507 type two-phase stainless steel after high temperature solid solution into water, Cr2N precipitation will occur at the grain boundary of the ferritic phase and inside the grain, mainly because the solubility of nitrogen in the ferritic phase is relatively low, and the N element is susaturated, so it will be easy to precipitate nitrogen in the case of rapid cooling, and the higher the temperature of solid solution, The amount of precipitation will also increase. It is found in the heat affected zone of 700-900 welded joints, and the precipitation of such nitrides is relatively rare, and the influence on the toughness and corrosion resistance of steel is not significant in general.