Isothermal transformation temperature and time of Austempered Ductile Iron

As a key factor affecting the microstructure and properties of austempered ductile iron, the range of isothermal quenching temperature (TA) is 260 ~ 400 ℃. When TA > 320 ℃, the matrix structure of austempered ductile iron can be coarse acicular ferrite (no carbide) + more high carbon austenite; when TA < 320 ℃, the matrix structure of austempered ductile iron can be fine acicular ferrite (containing carbide) + a small amount of high carbon austenite.

Isothermal transformation holding time (TA) is usually used to ensure the nucleation and growth of ferrite and the diffusion and migration of carbon atoms. If t a is too short, the isothermal transformation reaction of γ – α B is not complete, only a small amount of ferrite is formed, and the carbon content of the untransformed austenite is low, so martensitic transformation reaction occurs easily in the process of air cooling, at the same time, it is easy to work hardening under working conditions, resulting in the performance degradation of austempered ductile iron If it is too long, the high carbon austenite will further decompose into lath ferrite and cementite containing carbide, which will reduce the strength and toughness of austempered ductile iron.