Nanostructured Steels

Chapter 8 Nanostructured SteelsNanostructured metals with grain sizes smaller than 100 nm usually exhibit strengths which can be more than twice higher than their coarse-grained counterparts. The “smaller is stronger” effect is generally understood in terms of the Hall-Petch effect of grain size strengthening, or the capability of grain boundaries to obstruct the motion of dislocations as carriers of plastic deformation. Nanostructured steels take benefit of reductions in the grain size to show improved tensile strength, hardness and in-service properties. The steel nanostructures are usually multi-phase and hierarchical, maintaining or even improving the stress-ductility trade-off. The increasing demands for stronger, tougher, wear resistant and heat-tolerant materials have led to the development of new families of steels where the length scale that controls such properties is in the nanometer range. This chapter provides an overview of the current status of the most technologically relevant bulk nanostructured steels, describing the approaches to metallurgical design, processing routes, mechanical properties, in-use behavior and industrial applications.The authors acknowledge financial support from the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of a coordinate project (MAT2016-80875-C3-1-R) and the Research Fund for Coal and Steel of the European Commission under the contract SuperHigh (RFSR-CT-2014-00019)

A Constitutive Relationship between Fatigue Limit and Microstructure in Nanostructured Bainitic Steels

The recently developed nanobainitic steels show high strength as well as high ductility. Although this combination seems to be promising for fatigue design, fatigue properties of nanostructured bainitic steels are often surprisingly low. To improve the fatigue behavior, an understanding of the correlation between the nanobainitic microstructure and the fatigue limit is fundamental. Therefore, our hypothesis to predict the fatigue limit was that the main function of the microstructure is not necessarily totally avoiding the initiation of a fatigue crack, but the microstructure has to increase the ability to decelerate or to stop a growing fatigue crack. Thus, the key to understanding the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features that could act as barriers for growing fatigue cracks. To prove this hypothesis, we carried out fatigue tests, crack growth experiments, and correlated these results to the size of microstructural features gained from microstructural analysis by light optical microscope and EBSD-measurements. Finally, we were able to identify microstructural features that influence the fatigue crack growth and the fatigue limit of nanostructured bainitic steels

Ductility of Nanostructured Bainite

Nanostructured bainite is a novel ultra-high-strength steel-concept under intensive current research, in which the optimization of its mechanical properties can only come from a clear understanding of the parameters that control its ductility. This work reviews first the nature of this composite-like material as a product of heat treatment conditions. Subsequently, the premises of ductility behavior are presented, taking as a reference related microstructures: conventional bainitic steels, and TRIP-aided steels. The ductility of nanostructured bainite is then discussed in terms of work-hardening and fracture mechanisms, leading to an analysis of the three-fold correlation between ductility, mechanically-induced martensitic transformation, and mechanical partitioning between the phases. Results suggest that a highly stable/hard retained austenite, with mechanical properties close to the matrix of bainitic ferrite, is advantageous in order to enhance ductility

The Non-Steady State Growth of Pearlite outside the Hultgren Extrapolation

The goal of this paper is to analyse the effect of adding Al on the non-steady pearlite growth occurring in a Fe–C–Mn system. The results are discussed in terms of the partitioning of elements across the austenite/ferrite and austenite/cementite interfaces, and the modification of the pearlite driving force related to the change in carbon activity in austenite

High temperature performance of 316L steel reinforced by particle inoculation and processed by laser powder bed fusion

Grade 316 L is one of the most versatile austenitic stainless-steel products whose potential in laser powder bed fusion has been recently evaluated. A way of improving its properties is to add reinforcements, such as TiC nanoparticles, to promote dispersion hardening. However, it is often difficult to assess microstructure-mechanical property relationships, since particle inoculation promotes heterogeneous nucleation of equiaxed grains during rapid solidification. In this work, two 316 L samples were manufactured by laser powder bed fusion, where the powder of one of them was inoculated with TiC nanoparticles. The effect of inoculants on the microstructure and its high temperature behavior was assessed. Electron Probe Micro Analyzer proved that inoculants did not get dissolved during the printing process and they predominately lay in the intercellular regions, which were solute enriched. Advanced characterization proved that inoculation did not affect the solidification structure, which remained cellular and with a similar size, or the grain size, although it did modify the bulk texture. Finally, the effect of dispersion hardening on the behavior at high temperature of a 316 L steel was evaluated by small punch tests, which proved that the addition of TiC improves all, ductility, yield strength and ultimate tensile strength at high temperature. Moreover, samples processed by LPBF showed high temperature behavior and superior strength and ductility, as compared to the ones obtained in a reference annealed steel, even though the grain size obtained in the former case was at least 50 times larger than the one obtained for the reference condition

Carbon Clustering in Low-Temperature Bainite

The bainitic ferrite phase formed at temperatures below 573 K (300 °C) in high-carbon high-silicon steels holds an amount of carbon well above that expected from the thermodynamic paraequilibrium with austenite. Diffraction experiments have shown that the ferrite lattice is sufficiently Zener-ordered to possess a tetragonal symmetry, which allows the structures to be supersaturated in carbon. It could be expected that carbon undergoes ordering beyond that indicated by the Zener-ordering temperature as in the early stages of tempering of Fe-based martensites. This study examines the formation of cluster arrangements of carbon within bainitic ferrite and their relationship to the tetragonal distortion.This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of two Coordinate Projects (ENE2015-70300-C3-2-R and MAT2016-80875-C3-1-R); and the Research Fund for Coal and Steel under the Contract RFSR-CT- 2014-00019. APT was conducted as a part of a user proposal at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy Office of Science User Facility. R.R. heartfully thanks R.E. Hackenberg, who provided a good piece of history and insight that greatly assisted the course of this research.Peer Reviewe