Kinetics of formation of different structures in steel 25Kh2NMFA

Conclusions

• 1. The investigation of austenite decomposition in steel 25Kh2NMFA showed that the intermediate transformation is the basic transformation both under isothermal and continuous cooling conditions.

• 2. In the process of the intermediate transformation the austenite partially decomposes with formation of martensitic α phase (α_M), i.e., "bainite without carbides" is formed. The dispersed carbides in bainite observed by electron microscopic analysis are precipitated due to secondary processes associated with selftempering of α_M phase. The quantity of carbides increases substantially when the intermediate transformation temperature is lowered.

• 3. Upper bainite has a distinct granular structure. The mother phase of this bainite is α_M phase almost free of self-tempering carbides, with regions of A_re or M+A_re evenly distributed, which are visible in a light microscope at large magnifications.

  Under isothermal conditions upper bainite is formed at temperatures above 350°C, but in a broad range of cooling rates during continuous cooling (1000-1 deg/min).

• 4. Lower bainite is characterized by a small quantity (<10%) of evenly distributed regions of A_re or M+A_re and, as a consequence, a somewhat granular structure. In the mother α solid solution of lower bainite there are large numbers of dispersed, evenly distributed carbides from self-tempering.

Lower bainite is formed during isothermal holding below 350°C and during continuous cooling at rates >1000 deg/min.