Nanostructured Bainitic Steel Research Articles

Enhanced thermal stability in nanostructured bainitic steel

We report an attempt at increasing the thermal stability of nanocrystalline bainite to tempering heat treatments by enhancing the silicon concentration of the alloy. Validation experiments have been conducted using synchrotron X-irradiation during tempering heat treatment. It is found that the change in alloying successfully stabilizes the austenite at elevated temperatures by retarding cementite formation to temperatures as high as 500 °C. Other changes reflected in the lattice parameters of the major phases have revealed further information about the mechanisms involved.

Developing very hard nanostructured bainitic steel

Novel nanostructured high carbon high silicon, carbide-free bainitic steels with very high strength and good ductility have been developed in the recent decade. In this work, an alloy with a high carbon content and no manganese was designed and cast. The prepared samples were heat treated through an austempering process in the range 200–350°C. Optical and scanning electron microscopes and XRD were used to analyze the microstructures precisely. Bainitic ferrite plates of just a few tens of nanometer thickness were obtained with the hardness of 697±6HV. It is reasonable to say that the unprecedented hardness values obtained in this work are mostly caused by the extraordinary carbon content of the alloy.

Tensile behaviour of a nanocrystalline bainitic steel containing 3wt% silicon

Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125–350 °C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the BS–MS temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2–1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ∼3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

Toughness variations in nanostructured bainitic steels

► The volume fraction and morphology of high carbon retained austenite together, are important factors affecting the ductility. ► Higher amount of the lathlike austenite is essential in order to improve fracture toughness. ► We showed that controlling the chemical composition can further influence fracture toughness of nanostructured bainitic steels without strength deterioration. ► We could reach higher toughness without strength deterioration. Outstanding combination of strength and ductility in nanostructured bainitic steels makes them prone to be used as an important engineering material. However, there is still a necessity to investigate their toughness and effective parameters. This work aims to study the fracture toughness of these group of high strength carbide free bainitic steels with respect to chemical composition, transformation temperature, volume fraction and morphology of the present phases in the microstructure. Results indicate that the volume fraction and morphology of high carbon retained austenite together, are important factors affecting the ductility. In addition, higher amount of the lathlike austenite is essential in order to improve fracture toughness. Moreover, it has been shown that controlling the mechanical stability of retained austenite and the primary austenite grain size by Ni and V addition can further influence fracture toughness of nanostructured bainitic steels.

Sliding wear resistance and worn surface microstructure of nanostructured bainitic steel

This research studied the effect of austempering temperature on the sliding wear resistance of high-Si–Al steel, with a nanostructured bainitic microstructure obtained by austempering, and compared it with a tempered martensitic microstructure obtained by tempering. Results show that the sliding wear resistance of the nanostructured bainitic microstructure increases as the austempering temperature is decreased from 260 to 220°C. The wear resistance of these austempered samples is 21–51%, greater than that of tempered ones, although austempered samples have a lower hardness than tempered ones. In addition, the worn surface microstructure of samples tempered and austempered at 240°C was also examined, indicating that the single-phase α nanograins were formed in the worn surface of both samples. The mean size of nanograins, in the worn surface of samples austempered at 240°C, is less than that of the tempered ones.

CHARACTERIZATION OF NANO-STRUCTURED BAINITIC STEEL

A 0.79 C -1.5 Si -1.98 Mn -0.98 Cr -0.24 Mo -1.06 Al -1.58 Co ( wt %) steel was isothermally heat treated at 200°C for 10 days to produce a nano-structured bainitic steel. The microstructure consisted of nanobainitic ferrite laths with a high dislocation density and retained austenite films having extensive twins. The crystallographic analysis using TEM and EBSD revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with their parent austenite. There was only one type of packet identified in a given transformed austenite grain. Each packet consisted of two different blocks having variants with the same habit plane, but different crystallographic orientations. Atom Probe Tomography (APT) revealed that the carbon content of nanobainitic ferrite laths was much higher than expected from the para-equilibrium level. This was explained due to the long heat treatment time, which led to the formation of fine Fe - C clusters on areas with high dislocation densities in bainitic ferrite laths.

High-cycle bending fatigue behaviour of nanostructured bainitic steel

The high-cycle bending fatigue behaviour of high-carbon Si–Al-rich steel with a nanostructured bainitic microstructure was studied. A fatigue limit for no failure in 107 cycles of the nanostructured bainitic steel is determined not only by hardness or strength, which determines the fatigue limit of conventional high- and ultrahigh-strength low-alloy steels, but also by microstructure. Retained austenite and secondary cracks in fatigue fractures of the nanostructured bainitic steel have positive effects on the fatigue property.

On measurement of carbon content in retained austenite in a nanostructured bainitic steel

In this study, the carbon content of retained austenite in a nanostructured bainitic steel was measured by atom probe tomography and compared with data derived from the austenite lattice parameter determined by X-ray diffraction. The results provide new evidence about the heterogeneous distribution of carbon in austenite, a fundamental issue controlling ductility in this type of microstructure.

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

The key evidence for understanding the mechanical behavior of advanced high strength steels was provided by atom probe tomography (APT). Chemical overstabilization of retained austenite (RA) leading to the limited transformation-induced plasticity (TRIP) effect was deemed to be the main factor responsible for the low ductility of nanostructured bainitic steel. Appearance of the yield point on the stress-strain curve of prestrained and bake-hardened transformation-induced plasticity steel is due to the unlocking from weak carbon atmospheres of newly formed during prestraining dislocations.

Understanding of the Bainite Transformation in a Nano-Structured Bainitic Steel

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 200°C for 10 days to form a nano-scale bainitic microstructure consisting of nanobainitic ferrite laths with high dislocation density and retained austenite films. The crystallographic analysis using TEM and EBSD revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with the parent austenite. There was only one type of packet identified in a given transformed austenite grain. Each packet consisted of two different blocks having variants with the same habit plane, but different crystallographic orientations. The presence of fine C-rich clusters and Fe-C carbides with a wide range of compositions in bainitic ferrite was revealed by Three-dimensional Atom Probe Tomography (APT). The high carbon content of bainitic ferrite compared to the para-equilibrium level of carbon in ferrite, absence of segregation of carbon to the austenite/bainitic ferrite interface and absence of partitioning of substitutional elements between the retained austenite and bainitic ferrite were also found using APT.

Atom Probe Tomography Analysis of Precipitation during Tempering of a Nanostructured Bainitic Steel

Carbon distribution during tempering of a nanostructured bainitic steel was analyzed by atom probe tomography (APT). Three different types of particles are detected on samples tempered at 673 K (400 °C) for 30 minutes: lower bainite cementite with a carbon content of ~25 at. pct, e-carbides with a carbon content close to 30 at. pct, and carbon clusters, small features with a carbon content of ~14 at. pct indicative of a stage of tempering prior to precipitation of e-carbide. After tempering at 773 K (500 °C) for 30 minutes, the e-carbide-to-cementite transition was observed. Solute concentration profiles across carbide/ferrite interfaces showed the distribution of substitutional elements in e-carbide and cementite for all the tempering conditions.