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

Oscar Ríos-Diez,Jose A Jimenez,Claudia Serna-Giraldo,Ricardo Aristizábal-Sierra,Carlos Garcia-Mateo

doi:10.3390/met10050635

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.

Highlights

In recent decades, nanobainitic steels have been the subject of countless research papers and reviews, especially on topics related to the mechanism of transformation, mechanical performance, optimization of the processing routes and tempering resistance of the microstructure
From the relative change in length (RCL) vs. temperature curves, Figure 1, on heating and cooling segments, Ac1, Ac3 temperatures were calculated following the procedures described by Garcia-De
The results showed that, regardless of the austenitization T, in terms of strength (YS and UTS), ductility (TE) and impact energy, the microstructures obtained by isothermal heat treatment performed much better than those obtained by the more traditional quench and temper (Q&T) treatment

Summary

Introduction

Nanobainitic steels have been the subject of countless research papers and reviews, especially on topics related to the mechanism of transformation, mechanical performance, optimization of the processing routes and tempering resistance of the microstructure. According to Bhadeshia et al [1,2], bainite transformation is best described as a diffusionless and displacive reaction in which nucleation happens by a paraequilibrium mechanism, where only C diffuses, and growth occurs with no change in the chemical composition of the parent and product phases. The typical chemical composition range for nanobainitic steel is (0.6–1.0)C-(1.5–2.0)Si-(0.7–2.0)Mn-(0.4–1.7)Cr wt.%, which ensures both the necessary hardenability to avoid any undesirable transformation to ferrite or pearlite, and the occurrence of the bainitic reaction at low transformation temperatures

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Methods

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Conclusion