Effect of prior martensite on bainite transformation and microstructure of high-carbon nano-bainitic steel

Bainite transformation kinetics,microstructure,and mechanical properties were comparably investigated by X-ray diffraction,scanning electron microscopy,electron backscatter diffraction,transmission electron microscopy observation,and tensile and impact tests for two treatments:with and without quenching before isothermal bainite transformation at 300°C.Both the incubation time and growth stage of bainite transformation are accelerated by the introduction of prior martensite,especially the former.In addition,the bainite microstructure around the prior martensite is refined and shows a similar orientation with the martensite due to the strain field caused by the existence of prior martensite.The finer bainite microstructure and prior martensite improve the strength and hardness of the high-carbon nano-bainitic steel.Although the presence of prior martensite and low misorientation relationship between the martensite and the adjacent bainite microstructure are prejudicial to impact toughness,the nanoscale bainite microstructure can inhibit the crack propagation and then improve the impact toughness of the high-carbon nano-bainitic steel.

Notably Accelerated Nano-Bainite Transformation via Increasing Undissolved Carbides Content on GCr15Si1Mo Bearing Steel

In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 ℃, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.

Effect of Composition and Processing Parameters on the Formation of Nano-Bainite in Advanced High Strength Steels

The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechanical processing.The microstructure for both steels consisted of bainite.The microstructural characteristics of bainite,such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers,as a function of steel composition and processing was studied using transmission electron microscopy (TEM).It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing.Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution.The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.