Effect of second-step austempering temperature on mechanical damping of austempered ductile iron

Abstract

Austempered ductile iron(ADI) has good strength and toughness, which is widely used in aerospace, engineering machinery and automobile manufacturing. In the above application fields, the requirements of safety and comfort are gradually increasing, so ADI is required not only to have excellent mechanical properties, but also to have high damping properties. The performance of ADI is closely related to its heat treatment method. Therefore, it provides a theoretical basis for the application of ADI, and reveals its mechanism to some extent. The influence of two-step ADI’s mechanical properties and damping properties due to two-step austempering quenching temperature is the main content of this paper. The retained austenite with obvious coarsening of bundle ferrite and increased size is the experimental result obtained after the increase of austempering temperature in the second step. After that, when the austempering temperature in the second step increases, the retained austenite content will directly decrease. The tensile strength of ADI will increase first and then decrease with the increase of austempering temperature in the second step. And the value reaches the maximum of 1350MPa at 360°C. The yield strength of ADI, which reaches the maximum value (1252MPa) at 400 °C, will follow the increase of austempering temperature in the second step, and the elongation and impact energy of ADI will gradually decrease. The elongation and impact energy of ADI will gradually decrease with the increase of austempering temperature in the second step. The minimum values of 4.8% and 42J are obtained when the austempering temperature in the second step reaches 400 °C. The impact fracture morphology shows that the fracture mechanism will change from ductile fracture to brittle fracture with the increase of austempering temperature in the second step. The reason why the internal friction value (Q−1) of ADI with different austempering temperatures in the second step increases gradually is the increase of strain amplitude. There are obvious S-K peaks and Ge peaks, that is, the damping characteristics change with frequency.