Five types of ductile cast irons (DCIs) were fabricated by crystallizers with different inner diameters, as well as five different austempered ductile cast irons (ADIs) after the same isothermal quenching process. The effects of amount, diameter, and morphology of graphite on the mechanical properties of DCI and ADI and the effect of the original as-cast microstructure on the microstructure after austempering were studied. The microstructures were characterized by optical microscopy, scanning electron microscopy, and X-ray diffraction. Their mechanical properties were examined by tensile, U-shaped impact, and hardness tests. As the diameter of the crystallizer increases from 60 mm to 150 mm, the diameter of the nodular graphite increases from ~10 to ~50 μm, and the nodularity rate decreases from 100 to 70%. The average ultimate tensile strength increases from ~500 MPa in the as-cast state to 1100 MPa in the austempered state and the hardness increases from ~180 HB to 400 HB. The elongation in cast state decreases from 11 to 4.6% and the elongation in ADI state decreases from 7 to 4.5%. Through the research in this paper, it can be seen that ADIs with different matrix microstructures can be obtained from different original as-cast microstructures through the same isothermal quenching process, and different casting crystallizers can be selected according to different performance requirements, which can reduce the nitrite pollution and reduce cost.

In this study, ultra-high-carbon steels with 1.4% carbon content alloyed with three different aluminum contents, 2.0%, 4.0% and 6.0%, were studied on their tempering stability and temperature resistance. The results showed that the addition of Al significantly enhanced the tempering stability and temperature resistance of ultra-high-carbon steel. The addition of Al inhibited the transformation of ε-carbide to cementite, suppressed the transition of martensite to ferrite and thus, endowed ultra-high carbon steels to maintain very high hardness during tempering within a wide range of temperature up to 500 °C. The present work provides a useful basis on which to develop bearing steel materials with low density and high hardness.