Effect of as cast microstructure on hardness of Austempered Ductile Iron
The hardness test results of Austempered Ductile Iron with different as cast microstructure after treatment at 920 ℃ / 2H + 280 ℃ / 1.5hr shows that when the modulus Ms = 0.25mm, ADI has the largest hardness, which is 47.5hrc; when the modulus Ms = 1.25mm, the hardness of austemperedis the minimum, which is 45.8hrc. With the increase of MS, the hardness of austempered ductile iron decreases gradually, but the change range is not large. This is mainly related to the grain size of the matrix structure of austempered ductile iron. The finer the grain size is, the larger the number of grain boundaries per unit area is, the higher the hardness of the matrix is. On the one hand, with the increase of MS, the amount of pearlite in as cast microstructure tends to decrease, and the amount of high temperature austenite obtained after austenization decreases and the grain size increases, which leads to the decrease of the amount of austenite and the increase of grain size after isothermal transformation. On the other hand, although the size of austenite needle obtained by Ms = 0.25cm ductile iron is the shortest, that of MS = 0.50cm is the second, and that of ductile iron with MS = 0.75cm, Ms = 1.00cm and Ms = 1.25cm is longer, the grains of austenite obtained by ductile iron with different as cast microstructure are smaller at lower isothermal transformation temperature (280 ℃).
Effect of austempering process on hardness of Austempered Ductile Iron
The hardness test results of Austempered Ductile Iron with different austempering parameters were obtained. It can be seen from the table that the hardness of austempered ductile iron decreases gradually with the increase of isothermal transformation temperature Ta after treatment under the same austenitizing process parameters (920 ℃ / 1H or 920 ℃ / 2H). The main reason is that when TA is high, the volume fraction of high carbon austenite in the matrix of austempered ductile iron increases, while the volume fraction of acicular ferrite decreases. At the same time, when t γ is 920 ℃, the hardness of austempered ductile iron obtained by holding for 2 h is similar to that obtained by holding 1 h, indicating that the effect of austenitizing holding time on matrix hardness of austempered ductile iron is little.