Low-Alloyed Spring Steel: Nanostructure and Strength After Austempering

Carbide-free bainite microstructures were developed in 0.6 wt.%–2 wt.% Si spring steel via austempering at 250 °C. Heat treatment included austenization with subsequent isothermal holding at 250 °C, varying the holding duration to 1.0, 1.5, or 2.0 h with final cooling in water. X-ray diffraction, SEM investigation, tensile test, and hardness measurement were employed to study the microstructure and phase compositions of the samples. It was found that nanostructured bainite developed in the experimental steel. The distribution of distances between phase borders was determined via digital processing of SEM micrographs, and the mode distance was found to be 30 nm. The analytical estimation of possible strengthening showed that the yield strength of the nanobainite obtained should be in the gigapascal range, and the tensile testing results demonstrated that a 2 GPa yield strength was developed in the sample after isothermal treatment at 250 °C for 1 h. Investigations of the fracture surface and microstructure of the cross-section near the fracture zone confirmed the ductile mode of failure.