Development of Nanobainitic Microstructures in Carbo-Austempered Cast Steels: Heat Treatment, Microstructure and Properties

Abstract

Carburizing implies the existence of a carbon gradient from the surface to the core of the steel, which in turn will affect both the critical temperature for austenite formation and the kinetics of the bainitic transformation during the austempering treatment. Therefore, for future development of carbo-austempered steels with nanobainitic microstructures in the case, it is key to understand the effect of such carbon gradient has on the final microstructure and the mechanical properties reached by the heat treatments used. This work was divided into two parts, firstly two alloys with similar carbon content to those at the surface and center of the carburized steel were used to establish the optimal heat treatment parameters and to study bainite transformation kinetics by high resolution dilatometry. In a second step, a carburized alloy is produced and subjected to the designed heat treatments, in order to evaluate the microstructure and mechanical properties developed. Results thus obtained are compared with those obtained in the same carburized alloy after following the most common quench and temper treatment.

Effect of isothermal temperature and load on wear of nanobainitic cast steel

“…The details of the process to select the heat treatment conditions can be found in Ref. 12,13 Samples for microstructural examination were prepared using standard metallographic techniques with a final diamond 1 µm polish and etched with 2% Nital. The microstructure was examined using a JEOL-JSM 6490LV field emission scanning electron microscope (SEM) operating at 20 kV.…”

“…[1][2][3][4][5][6][7][8] Most of the studies on nanobainitic steels have been performed in rolled/ forged materials, but recently some interest in replicating the nanobainitic microstructure in cast steels has emerged since it will allow to fabricate parts with complex shapes that sometimes cannot be produced by plastic deformation. [9][10][11][12][13] The information available on wear behaviour of cast nanobainitic steels is limited, yet some insight can be taken from the reported literature on wear behaviour of rolled/forged nanobainitic steels.…”

Electron beam hardening of nanobainitic steel

“…Their tensile strength reaches 2.5 GPa with hardness in the range of 600-700 HV and elongation in the range of 5%-30%. [4][5][6][7][8] Nanobainite is also characterised by better wear resistance to abrasive wear compared to tempered martensite and bottom bainite with comparable or higher hardness. Due to the possibility of avoiding the separation of carbides, which cause embrittlement of the steel, and the very favourable combination of mechanical and functional properties, nanobainitic steels with Si are considered a very promising material.…”

The Impact of Retained Austenite on the Mechanical Properties of Bainitic and Dual Phase Steels

“…There are numerous chemical compositions and processing methods that can be used, but choosing the proper amount and type of elements for the material is extremely challenging, especially for bainitic rails, that will be subjected to natural cooling following rolling. The same is true for thermal processing, i.e., it is possible to modify the treatment time, the austenitization and austempering temperatures, and the cooling time, among other parameters (Ref [13][14][15][16][17]. Therefore, it is clear that many factors affect the selection of a material for the rails that is both suitable and enhanced relative to previous materials.…”