Low Carbon Bainitic Steel Research Articles

Understanding the Effect of Austempering Temperature on the Crystallographic Features and Mechanical Properties of Low-Carbon Bainitic Steel

The effect of austempering temperature on crystallographic features and mechanical properties is investigated in low-carbon bainitic steel, focusing on the relationship between microstructure and mechanical properties. After isothermal holding at, above, and below martensite start (MS) temperatures and tempering, a mixed microstructure of martensite/bainite and martensite/austenite (M/A) constituents is obtained. The fraction of M/A constituents increases as the austempering temperature increases, while the density of block boundaries decreases. The instantaneous work hardening rate exhibits continuous decay without a notable transition because of the retained austenite in the M/A constituents. The toughness decreases with increasing austempering temperature, which is related not only to the fraction of M/A constituents but also to the density of block boundaries. Isothermal treatment below the MS temperature enables the formation of structures with fewer M/A constituents and high-density block boundaries, through which excellent toughness can be achieved.

Textural index for martensitic and bainitic steels to assess influence of hot rolling mode on parent austenite structure before quenching

To assess a state of parent austenite before the steel quenching, a scalar textural index for martensite and bainite is introduced in terms of EBSD orientation data. Deformed and recrystallized states of the parent phase are discriminated by the sign of this index, whereas its magnitude in each of the two reflects the texture sharpness depending on the hot rolling mode. Accordingly, in a virtual case of randomly distributed orientations the considered parameter vanishes. Performance of the proposed approach is demonstrated on medium carbon martensitic steel hot rolled at laboratory conditions and on industrial rolled plates of low carbon bainitic steel.

Mechanistic role of vanadium microalloying in improving corrosion resistance of low carbon bainitic steel

We here investigated the effect of vanadium microalloying on the microstructure, mechanical properties, especially corrosion performances of low carbon bainitic steels. The corrosion behaviors of the steels with different vanadium contents in 3.5 wt% NaCl solution were evaluated by electrochemical tests (including polarization curves and electrochemical impedance spectroscopic measurements) and alternating immersion test (including weight loss and rust layer observation). Results show that the mechanical properties and corrosion resistance of low carbon bainitic steels can be synergistically improved with vanadium microalloying. With help of the electron backscatter diffraction characterization and scanning Kelvin probe force microscopy, we first discovered that although the number of micro-galvanic couples increases because of grain refinement, the Volta potential gradient between the matrix and grain boundaries are decreased with vanadium microalloying, which can promote the formation of compact protective rust layers and improve the corrosion resistance of vanadium micro-alloyed low carbon bainitic steels.

Influence of prior austenite grain size on martensite/austenite constituents in low-carbon bainitic steel

We elucidated here the influence of prior austenite grain (PAG) size on bainitic transformation products from the perspective of kinetics. The refinement of PAGs provided more sites for grain boundary nucleation and accelerated bainitic transformation kinetics. M/A constituents were also refined in refined PAGs with a higher carbon concentration, contributing to higher stability and even formation of retained austenite. The smaller M/A constituents exhibit stronger variant selection and localized strain.

Development of Predictive Models for Tempering Behavior in Low-Carbon Bainitic Steel Using Integrated Tempering Parameters

Low-carbon bainitic steels are known for their excellent combination of strength and toughness, making them suitable for various industrial applications. Understanding the tempering behavior of these steels is crucial for optimizing their mechanical properties through heat treatment. This study presents predictive models for tempering behavior based on empirical data, which is fundamental for understanding the thermal stability and transformation kinetics of the steel. Through integrated tempering parameters, we established predictive models that integrate tempering temperature and time, yielding a robust framework for predicting hardness. The equivalent tempering kinetic curves and nomographs plotted in this study allow for the direct determination of hardness under various tempering conditions, facilitating the optimization of tempering parameters. The nomogram approach provides a practical method for adjusting tempering parameters to achieve desired mechanical properties efficiently. The accuracy of the predictive models was validated through statistical tests, demonstrating a high correlation between predicted and experimental values.

Effects of carbon concentration and transformation temperature on incomplete transformation and crystallography of low-carbon bainitic steel

A close relationship exists between the crystallographic characteristics of a bainitic matrix and formation of martensite/austenite (M/A) constituents. In this work, the isothermal transformation kinetics and crystallographic characteristics of low-carbon bainitic (LCB) steel were investigated to elucidate the formation mechanism of M/A constituents and twin-related V1/V2 variant pairs (Σ3 boundaries). The formation of M/A constituents depends on the bainitic transformation kinetics, which are influenced by the carbon concentration and isothermal temperature. An increase in the carbon concentration or transformation temperature promotes the formation of blocky M/A constituents by increasing the activation energy of the grain boundary and autocatalytic nucleation processes. In addition, increasing the carbon concentration or lowering the transformation temperature increases the strength of the austenite phase and promotes autocatalytic nucleation, thereby producing more twin-related V1/V2 variant pairs. Therefore, the increase in the carbon concentration increases the number of twin-related V1/V2 variant pairs while also causing the formation of blocky M/A constituents. To overcome this limitation, a novel heat treatment process design is proposed that can significantly eliminate blocky M/A constituents and generate high-density twin-related V1/V2 variant pairs.

Improving strength-toughness of low carbon bainitic microalloyed steel via tailoring isothermal quenching process and niobium microalloying

This study focuses on improving the strength-toughness properties of low carbon bainitic steel through the optimization of the isothermal quenching process and the adoption of a microalloying strategy. The impact of Nb microalloying on the microstructure and mechanical properties was thoroughly investigated. The optimal isothermal quenching process for achieving a balance between strength and toughness is as follows: the steels were austenitized at 900 °C for 30 min and then subjected to an isothermal treatment at 450 °C for 10 min. The resulting microstructure consisted mainly of granular bainite (GB) and lath bainite (LB), with GB accounting for 60 % of the structure. After the optimized isothermal quenching process, the ultimate tensile strength and yield strength of the Nb microalloyed steel increased to 1210 MPa and 693 MPa, respectively. The impact energy, KU2, was measured at 41.8 J. The study revealed that Nb microalloying refines the bainite structure by reducing activation energy and increasing the nucleation rate. This refinement was observed in the reduced length and thickness of the bainitic laths, as well as the increased dislocation density. Additionally, fine composite precipitated particles (Nb, V)C with an average diameter of 8.2 nm were dispersed throughout the microstructure. The addition of Nb significantly enhanced the effects of fine grain strengthening, precipitation strengthening, and dislocation strengthening in low carbon bainitic microalloyed steel. As a result, the strength was greatly improved without sacrificing toughness, thanks to the combined actions of these three strengthening mechanisms.

Effect of Cooling Process on Microstructure and Properties of Low Carbon Bainite Steel

This article used Mn-Mo-Cr-B low-carbon bainitic steel as the experimental material. The continuous cooling transformation curve of the steel during continuous cooling was determined using a Gleeble-1500D thermal simulation test machine, and a corresponding phase transformation model for bainitic steel during continuous cooling was established. The influence of different cooling rates and final cooling temperatures on the microstructure and mechanical properties of the steel was investigated. Employing metallography, SEM, and EBSD techniques, the microstructure, crystallographic orientation, and grain boundary angle distribution of the low-carbon bainitic steel were explored, and their relationship with the steel's strength and toughness was studied. The research findings reveal that varying cooling rates and final cooling temperatures impact the phase transformation process and microstructure of the steel, consequently affecting its mechanical properties indirectly. With increasing cooling rate, the diffusion and fineness of martensite increase, and the quantity of lath bainite grows while the laths become finer. Elevated final cooling temperatures lead to larger martensitic-austenitic (MA) islands and reduced lath bainite quantity, causing the laths to become wider. Through analysis of the substructure of bainitic steel, it was determined that the bainite organization in the tested steel comprises primary austenite grains, lath packet, and lath block in succession. Lath packets are composed of lath blocks with different orientations, where lath size predominantly controls strength. Finer lath size corresponds to higher strength, and the influence of substructure on toughness is comparatively minor.

Geometry of dimples and its correlation with mechanical properties in high-strength bainitic steel

ABSTRACT The experimental measurements of 2 D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature.

Influence factors of thickness direction temperature of low carbon bainite steel Q690D during laminar cooling

In this paper, the temperature-transformation finite element model of low carbon bainitic steel Q690D was established based on the characteristics of controlled cooling after rolling. In the process of controlled cooling after rolling, there are many factors that affect the temperature in the direction of the thickness of the steel plate, mainly from the thickness of the steel plate, cooling water flow, open cooling temperature and cooling water collection tube opening method analysis. The effect of different factors on the cooling rate is analysed by simulation to obtain the corresponding variation of temperature in the thickness direction. The results show that the greater the thickness, the less effective the energy transfer to the surface layer. This means that thinner steel plates are easier to control in terms of temperature. The temperature drop in the thickness direction is relatively large when using the mode with upper and Upper and lower spray volume ratio of (650/1000) for the DQ section and Upper and lower spray volume ratio of (500/1000) for the ACC section, and the reasonable range is easier to be obtained. On the premise of not being affected by the cooling rate of temperature drop, in the thickness direction, the latent heat value of phase change is different due to the different time periods of phase change, and the heat generated in each microstructure is different, so the temperature distribution shows that the starting cooling temperature of 860 °C is more gentle and uniform in the temperature slope than the starting cooling temperature of 750 °C. Among the four header opening modes, mode (1111111110000011111) makes the temperature of each layer of the steel plate more uniform and reasonable, superior to the continuous opening mode.

Effect of vanadium microalloying on phase transformation and strengthening mechanism of 1000 MPa low carbon bainitic steel

To develop an ultrahigh-strength hot-rolled strip, the yield strength of low-carbon bainitic steel was increased from 804 to 1074 MPa via V microalloying. The effects of V microalloying on the microstructure and mechanical properties of the alloys are discussed in detail. The microstructures of the test steel containing 0.2 wt % V element is finer, and the Degenerate Upper Lath Bainite (DUB) is replaced by Lower Lath Bainite (LLB). This was clearly due to the addition of V, which promoted the formation of LLB. In this study, the effect of V dissolved in the matrix on the phase transformation kinetics was investigated. This is because it is difficult for V to precipitate during quenching and non-isothermal partitioning (QNP) processes, and it mainly exists in the matrix in the form of a solid solution. The main effect of V is to reduce the activation energy Q* of bainite by 1.76 kJ/mol, increase the nucleation rate of lath bainite, and further refine the bainitic lath. However, a high dislocation density exists in the bainite laths, and the comprehensive strengthening effect of lath strengthening and dislocation strengthening can explain the change in the mechanical properties of the 0.2V steel.

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Atomistic simulation of hardening in bcc iron-based alloys caused by nanoprecipitates

The paper presents results of atomistic simulations of hardening in model bcc iron-based alloys due to secondary phase precipitates formation. The simulations are based on shear stress relaxation technique and experimental data on morphology of precipitates. Atomistic methods were developed to reproduce the shear strength of bcc iron-based materials. The large-scale atomistic simulations were carried out for model binary alloys: Fe–Cr imitating low-carbon ferritic and ferritic–martensitic steels and Fe–Cu imitating low-carbon bainitic steel. A representative set of atomic structures is compiled with account for the chemical composition and concentration and size of secondary phase, comparable with those in steels of the considered classes and available experimental data on radiation and thermal induced precipitation. Obtained results on hardening were verified against experimental data and the dispersed barrier hardening model.

Development of a low-carbon carbide-free nanostructured bainitic steel with extremely high strength and toughness

High-carbon carbide-free nano-bainitic steels show extremely high strength but relatively poor fracture toughness and welding prospects. Therefore, an attempt has been made to develop a strong and damage tolerant low-carbon bainitic steel in the current study by taking recourse to two-step austempering at 350 °C for 20 min and 250 °C for 7 h (20min-TS) and the properties have been compared with another block prepared using a single step transformation for 3 h at 350 °C (3hr-SS). Two stage heat-treatment leads to lath thickness refinement in addition to block refinement as compared to a single step transformation. Moreover, a NW misorientation developed in 20min-TS as compared to KS in a single-stage transformation. An average bainitic lath thickness below 100 nm in a steel having 0.26 wt% carbon has been obtained in 20min-TS which in turn results in an ultimate tensile strength of 1.5 GPa, percentage elongation of 18.5, impact energy of 31 J and plane-strain fracture toughness of 82 MPam−1/2. This is an around two and a half times increase in toughness without significantly compromising on the strength when compared to nanostructured bainitic steels produced in a similar duration of austempering for high carbon steels.

Effect of tempering temperature on microstructure and mechanical properties of a low carbon bainitic steel treated by quenching-partitioning-tempering (QPT) process

The quenching-partitioning-tempering (QPT) treatment was carried out on a low carbon bainitic steel, and the effect of tempering temperature on the microstructure and mechanical properties of the QPT processed steel was investigated. The findings demonstrate that as the tempering temperature increases, the yield strength decreases gradually, while the tensile strength decreases first and then increases. However, the elongation exhibits a trend of increasing first and then decreasing with increasing the tempering temperature. When the tempering temperature is 340 °C, the best mechanical properties can be obtained with the yield strength of 1495 MPa, tensile strength of 1806 MPa, elongation of 17.7%, and product of strength and elongation up to 32.92 GPa%. Such excellent mechanical properties are related to the synergistic combination of tempered martensite, low carbon bainite, retained austenite and carbides.

Low carbon bainitic steel processed by ausforming: Heterogeneous microstructure and mechanical properties

Thermodynamic stability of austenite is a primary factor affecting mechanical properties of low-carbon bainitic steels. In this work, ausforming was applied to a low-carbon steel, in which additional dislocations and defects were induced in austenite prior to isothermal transformation of bainite at a relatively low temperature. Hence, the stability of the retained austenite was improved, and the formation of fresh martensite during the secondary stage of cooling was suppressed. Results of microstructure characterizations by electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and dilatometry were examined. The correlations between the process parameters and developed heterogeneous microstructures were established. It was found that the volume fraction of retained austenite was greatly promoted along with the reductions of fresh martensite and bainitic ferrite. Tensile properties of the bainitic steel were enhanced due to the occurred heterogeneous microstructure and fair balance of its phase constituents. The significantly increased strengths were mainly attributed to microstructure refinement as well as the enrichment of geometrically necessary dislocations (GNDs) within the bainitic ferrite and retained austenite. Furthermore, a large fraction of retained austenite showed a significant impact on the enhancement of both uniform elongation and strength, benefitting from the transformation-induced plasticity (TRIP) effect. The presence of microcracks and tiny dimples with thick tearing edges in the vicinity of the inclusions in the fractographs were directly related to the refined microstructure, which certainly provided a remarkable ability to reduce the formation of macro/micro-cracks and prolong the post-necking. • Ausforming improves the thermodynamic stability of the retained austenite. • Ausformed microstructure with high GND densities hinder martensite transformation. • Strength and ductility are enhanced by the developed heterogeneous microstructure. • Microstructural refinement retards macro/microcracks formation up to fracture.

Investigations of Fatigue Damage in a Nitriding Low-Carbon Bainitic Steel for High-Performance Crankshaft

In the automotive environment, the need to increase the performance of materials requires extra engineering efforts. The possibility of developing new materials is strategically important. Indeed, alternative solutions in terms of material choice allow designers to optimise their projects and keep competitive production costs. Traditional quenched and tempered steels are usually used for highly stressed components, and possible alternatives could be important competitive opportunities. One possible substitute is using bainitic steels to exploit their economic advantages while maintaining acceptable mechanical performances. This paper explores the fatigue life behaviour of a new low-carbon bainitic steel for applications requiring case hardening treatment obtained by the nitriding process. A high-cycle fatigue (HCF) strength assessment is conducted through a test campaign to compare treated and untreated material. The improvement in fatigue strength is evaluated as well as the study of fracture surfaces, residual stress, and microhardness profiles to assess in detail the effectiveness of the nitriding process. It is found that the nitriding leads to an improvement in fatigue life but not as much as expected because of the low ductile behaviour of this steel, the high speed of stress application added, and the embrittlement of the nitriding treatment, as confirmed through fracture surface analysis.

The Role of Microstructure Morphology on Fracture Mechanisms of Continuously Cooled Bainitic Steel Designed for Rails Application

The low-carbon bainitic steel after a continuous cooling process was subjected to fracture toughness investigations using the J-integral approach. The research was focused on the determination of microstructural factors influencing the fracture processes considering the crystallographic units, as well as dimensions and morphology of phases. It was found that the fracture surface is characterized by complex fracture mechanisms (quasi-cleavage, transcrystalline cleavage–ductile, and ductile mode). It was found that the main features influencing the cracking processes are bainitic ferrite packets and prior austenite grain boundaries. The changes in the crack path were also related to the changes in the misorientation angles, and it was found that changes in the crack path direction occur mainly for the bainitic ferrite packets (HABs). Also, the fracture process zone induced by the crack tip was identified. At a distance of about 4 to 5 µm from the fracture, the retained blocky austenite transformed into martensite was observed. Due to the high carbon content in the retained austenite, the transformed martensite was brittle and was the site of microcracks nucleation. Another origin of microcracks nucleation were M/A constituents occurred in the initial microstructure. In the crack tip area, the reduced dislocation density in the bainitic ferrite, which was caused by the formation of sub-grains, was also determined. Finally, the prospective improvement of the fracture toughness of bainitic steels was determined.

Microstructure and Properties of a Low Carbon Bainitic Steel Produced by Conventional and Inverted Two-Step Austempering Processes

Microstructure and Properties of a Low Carbon Bainitic Steel Produced by Conventional and Inverted Two-Step Austempering Processes

Microstructure, crystallography, mechanical and damping properties of a low-carbon bainitic steel austempered at various temperatures

Microstructure, crystallography, mechanical and damping properties of a low-carbon bainitic steel austempered at various temperatures