Effect of austempering process on tensile properties of Austempered Ductile Iron

Several samples with diameter of 7 mm are taken from the Φ 50 mm ductile iron test bar for isothermal quenching treatment under different process parameters. T γ is 920 ℃, t γ is 1 h and 2 h, Ta is 280 ℃, 330 ℃ and 380, Ta is 1.5 h, and finally air cooled to room temperature. The tensile properties of austempered ductile iron obtained by different austempering processes.

After treatment at the same austenitizing process parameters (920 ℃ / 1H or 920 ℃ / 2H), with the increase of TA, RM, Rp0.2, yield ratio and e of Austempered Ductile Iron gradually decrease, while a and Z gradually increase. At the same time, when t γ is 920 ℃, the tensile properties of ADI obtained by holding for 1 h is better than that by holding for 2 h. The specific performance is that RM, Rp0.2, yield ratio and E are higher, a and Z are larger. This is mainly because, on the one hand, the higher the TA, the smaller the temperature gradient of high temperature austenite from quenching to isothermal transformation temperature, and the lower the rate of transformation reaction of γ C → α B + γ HC, resulting in the decrease of nucleation rate of α B, and the coarsening of α B structure in the matrix of austempered ductile iron On the other hand, it can be seen that the carbon content in high temperature austenite after austenitizing for 2 h is slightly higher than that of holding for 1 h, but the difference is not significant With the increase of t γ, the grain size of high temperature austenite increases, and the acicular or plate-like ferrite obtained after isothermal transformation of austenite increases, and the microstructure of austenite becomes coarser. The growth and coarsening of austenite structure is the main reason for the poor tensile properties of austempered ductile iron.