Advanced Bainitic Steels Research Articles

Recent Progress in Laser Powder Bed Fusions Processes of Advanced High-Strength Steels

This review is focused on the perspectives of the application of Advanced High Strength Steels (AHSSs) in the field of additive technologies directed at the laser powder bed fusion/selective laser melting processes. In principle, AHSSs require significant attention due to their promising mechanical properties for usage in the automotive industry towards reducing the weight of vehicles. Although additive manufacturing represents a promising perspective towards expanding the industrialization of AHSSs in a wider area of their applications, they have not been sufficiently investigated concerning their usage in LPBF/SLM processes. AM techniques enable the fabrication of complex machine parts, including those with a cellular structure, which can contribute to further reducing the weight of vehicles or structures. Maraging steels have recently attracted the attention of researchers, and today are a common grade of steel produced by LPBF techniques. The other group of AHSSs are high-Mn steels with an austenitic matrix characterized by the TRIP and TWIP effects. Less published research has been conducted on medium-Mn steels, which require additional intercritical annealing and preheating during printing. Moreover, the advanced bainitic steels and low-density, high-strength steels represent a new window for further research into the use of the LPBF processes for their fabrication.

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

The potential of mechanical twinning in ultrafine retained austenite to enhance high cycle fatigue property of advanced bainitic steels

We have studied here the high cycle fatigue properties and sub-surface fatigue crack initiation mechanism in advanced bainitic steel subjected to conventional bainitic austempering (BAT) and novel “disturbed” bainitic austempering (DBAT) processes. The retained austenite in DBAT steel mainly exhibited ultrafine film-like morphology, whereas a considerable amount of coarse blocky retained austenite was present in the BAT steel. The study indicated that the DBAT steel had higher fatigue strength and endurance, compared to the BAT steel. The enhanced fatigue properties are attributed to mechanical twinning that occurred in film-like retained austenite at the sub-surface fatigue crack initiation area.

Role of retained austenite with different morphologies on sub-surface fatigue crack initiation in advanced bainitic steels

We describe here the distinct high cycle fatigue properties of advanced bainitic steels to elucidate the significant role of retained austenite with different morphologies on sub-surface fatigue crack initiation process. The inter-plate film-like retained austenite at the straight front of small crack tip transformed to martensite, leading to the arrest of small crack growth and altered the active slip systems. However, the blocky retained austenite or martensite/austenite islands located at prior austenite grain boundaries induced intergranular fatigue cracking because of ease of activation of slip in retained austenite.

Formation of “carbide-free zones” resulting from the interplay of C redistribution and carbide precipitation during bainitic transformation

Advanced bainitic steels exploit their constitution of a phase mixture of bainitic ferrite and retained austenite. The desired microstructures are usually adjusted via isothermal or continuous cooling treatments below the bainite-start-temperature. In the present work, we analyze the microstructural evolution during bainite formation in detail and we report on a novel mechanism for the formation of “carbide-free zones” in the close proximity of austenite. Carbides are formed in the interior of bainitic ferrite, which is isolated from surrounding austenite, while the depletion of C in the vicinity of austenite suppresses their formation.

Friction stir welding of advanced high strength (bainitic) steels for automotive applications

Friction stir welding has been emerging to be the most prominent and sought after metal joining technology as it leaves the signatures of minimal phase transformation during welding and precise weld joint. On the other hand, Bainitic steel has been the candidate steel for automotive applications as it fuses and balances the characteristics mechanical strength and deformability. It can also be tuned playing with various strengthening mechanisms. Most importantly, in automotive sector, the ductility of cut-edges (stretch-formability), fatigue, and welding (about 300 spot welds in a car) are quite essential. In this connection, bainite can help in optimizing these key contents. Advanced bainitic steels (microalloyed bainitic steels, and carbon-free bainitic steels) are also being developed to improve the formability performance, sensitivity to damaging systems, and toughness compared to traditional bainitic steels. The present paper emphasizes the feasibility of friction stir welding on bainitic steel through depth of penetration at different stir speeds. The optimization of stir speed was done through profiling temperature changes (thermography) that occur during welding. The microstructural analysis was done to analyze the resulting phases during welding. To support the microstructural analysis, microhardness evaluation was also undertaken at all weld conditions.

An assessment of the contributing factors to the nanoscale structural refinement of advanced bainitic steels

Abstract A new generation of steels has been designed, which on transformation at low temperature (200–350 °C), leads to a nano-scale microstructure, known as NANOBAIN. The microstructure consists of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes (20–40 nm). These advanced steels present the highest strength/toughness combinations ever recorded in bainitic steels. Their properties are mainly a consequence of the formation of nanoscale bainitic ferrite plates at very low temperatures. Transmission electron microscopy observations have shown that plastic relaxation in the austenite adjacent to the bainite plates may control the final size of the bainitic ferrite plates. The dislocation debris generated in this process resists the advance of the bainitic ferrite–austenite interface, the resistance being greatest for strong austenite. The yield strength of the austenite must then feature in any assessment of plate size. In this scenario, the plates are expected to become thicker at high temperatures because the yield strength of the austenite will then be lower. The goal of this study is to evaluate the influence of yield strength of austenite to the nanoscale structural refinement of advanced bainitic steels. In this sense, in situ measurements of austenite strength before bainite formation using a deformation dilatometer Bahr 805D have been performed in a medium carbon high silicon steel transforming at intermediate temperatures (325–400 °C) to a submicron structure of bainite and in a high carbon high silicon steel transforming at low temperatures (200–350 °C) to nanostructured bainite. The role of the transformation driving force on the bainite plate thickness will be also discussed.

Toughness deterioration in advanced high strength bainitic steels

Carbide free bainitic steels alloyed with manganese have achieved the highest strength and toughness combinations to date for bainitic steels. Ultimate tensile strengths ranging from 1600 to 1800 MPa were achieved while keeping a total elongation higher than 10%. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. This improvement in toughness is achieved suppressing the precipitation of cementite during bainite formation by alloying the steel with about 1.5 wt% of silicon. However, it has been observed that strongly orientated martensite bands, associated to inhomogeneous manganese redistribution during solidification, lead to a remarkable deterioration in toughness in these advanced bainitic steels. The stress concentration associated with highly heterogeneous hardness distribution in the microstructure contributes to the premature crack nucleation.

Theoretical design and advanced microstructure in super high strength steels

A theoretical design procedure based on phase transformation theory alone has been successfully applied to design steels with a microstructure consisting of a mixture of bainitic ferrite and retained austenite. Using thermodynamics and kinetics models, a set of four carbide free bainitic steels with a 0.3 wt.% carbon content were designed and manufactured following a thermomechanical treatment consisting of hot rolling and two-step cooling. The designed steels present significant combinations of strength and ductility, with tensile strengths ranging from 1500 to 1800 MPa and total elongations over 15%. However, a carbon content of 0.3 wt.% is still high for in-use properties such as weldability. In this sense, a reduction in the average carbon content of advanced bainitic steels was proposed. Improved bainitic steels with a carbon content of 0.2 wt.% reached combinations of strength and ductility comparable to those in TRIP assisted steels.

Effects of Morphology and Stability of Retained Austenite on the Ductility of TRIP-aided Bainitic Steels

In order to improve the ductility of carbide free bainitic microstructures, consisting of a bainitic ferrite matrix and a mixture of austenite and martensite, the TRIP effect i.e. the strain induced transformation of retained austenite to martensite, should be controlled. In this sense, the effect of the chemical composition on the mechanical stability of the retained austenite and the morphology, size, and distribution of this phase has been studied to determine the role that plays on the ductility behaviour of advanced bainitic steels. Results suggest that apart of the retained austenite, the morphology of the bainitic matrix is an important factor controlling ductility. Bainitic microstructures formed by coiling with coarse and blocky bainite morphology have shown higher uniform deformation values than those obtained by air cooling with the typical thin bainite platelets.

Design of Advanced Bainitic Steels by Optimisation of TTT Diagrams and T0 Curves

Cementite is responsible of the limited application of conventional bainitic steels, however it has been proof that cementite precipitation during bainite formation can be suppressed by the judicious use of silicon in medium carbon steels. In this work, thermodynamic and kinetic models were used to design steels with an optimum bainitic microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite. Using these models, a set of seven carbide free bainitic steels with a 0.3 wt% carbon content were proposed for manufacturing. The work presented here is concerned with the microstructural and mechanical characterisation of the steels manufactured. Except for the steel with the highest content of alloying elements, all the grades present the same microstructure composed of carbide-free upper bainite and retained austenite after hot rolling and a two-steps cooling. Theirs tensile strengths range from 1600 to 1950 MPa while keeping a uniform elongation equal to 4% and a total elongation over 10%. Regarding toughness at room temperature, they match quenched and tempered martensitic steels.

A Model for the Microstructure of Some Advanced Bainitic Steels

Some applications of phase transformation theory towards the exploitation of bainitic microstructures are discussed, with particular emphasis on the quantitative aspects of alloying element effects. Examples are used to illustrate the principles involved in the design of advanced bainitic alloys of the type currently under investigation in the steel industry