Several tensile test bars with the sizes of Φ 7mm × 90mm, Φ 12mm × 100mm and Φ 17mm × 110mm (made by wire cutting) were cut from the Φ 50mmtest bar for isothermal quenching at 920 ℃ / 2H + 280 ℃ / 1.5h. The tensile properties of Austempered Ductile Iron with different wall thickness were studied.
When the casting wall thickness δ is 7 mm, the yield strength Rp0.2 and yield ratio of ADI obtained by isothermal transformation are the highest; when δ is 12 mm, the RM, a, Z and e of austempered ductile iron are the largest. But generally speaking, the mechanical properties of ADI obtained by austempering for the castings with δ 7 mm and δ 12 mm are better than those with δ 17 mm. The main reason is that the hardenability of ductile iron used in this test can be 65 mm at t γ = 920 ℃ through formula calculation. It can be seen that when δ is 7mm, 12mm and 17mm, it can be hardened. Under the same austempering process parameters, when δ = 7mm, the time required for quenching is short, and the time for undercooled austenite to be in isothermal transformation process is long enough. Combined with the analysis results of fine structure of austempered ductile iron, it can be seen that δ = 7mm under this process parameter There is no carbide in the matrix structure of austempered ductile iron obtained by casting. Therefore, it can be determined that the isothermal transformation of undercooled austenite does not involve the second adverse reaction in the process of isothermal transformation. If the isothermal transformation time is long enough, the transformation process of undercooled austenite is between the first reaction and the second reaction, which is in the “process window” to make the transformation of austenite More responsive. At the same time, when δ = 12mm, the quenching time of casting is shorter, the time of undercooled austenite in isothermal transformation process is longer, and the transformation process of undercooled austenite is still in the “process window”. However, when δ = 17 mm, it takes longer time for the casting to harden, and the time for undercooled austenite to undergo isothermal transformation is shorter, and the transformation process of undercooled austenite is still in the first reaction stage.