What is secondary austenitization in heat treatment？

The two-phase structure in duplex stainless steel changes with the increase of heating temperature. When the temperature exceeds 1300 ℃, some duplex stainless steels may have a ferrite structure. When the stability of the ferrite is poor, some ferrite at the boundary of the ferrite crystal will be converted into austenite during future cooling processes, called secondary austenite. The formation mechanism and form of secondary austenite vary depending on different cooling rates. The secondary austenite formed at high temperatures is achieved through nucleation and growth, which is a diffusion transformation. Research has found that secondary austenite formed at high temperatures mostly nucleates at the dislocation of ferrite and grows along the subcrystalline boundaries of ferrite, thus possessing the structural characteristics of Weinsteinite in terms of microstructure. Compared with the surrounding ferrite, the secondary austenite formed at high temperature has a higher nickel content and lower chromium content, and the composition formed in the matrix is not uniform.
The secondary austenite formed in the lower temperature range, such as 300-650 ℃, has the characteristic of non diffusion transformation and belongs to the shear transformation of martensite. This type of secondary austenite is usually not obtainable when cooled by high temperature water. Another scenario is the temperature range of 600-8000 ℃ and tissue analysis σ When phases or carbides occur, the nickel rich and chromium poor areas formed around them will also transform into secondary austenite. Therefore, some people classify the formation of secondary austenite as a ferrite analysis reaction, which is the product of the analysis reaction α One → g+Y2.
Regardless of the method used to form secondary austenite, it will cause non-uniformity in the composition of the new alloy and have a negative impact on corrosion resistance.