Nanostructured Bainitic Steel Research Articles

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Effects of austempering on the microstructure and mechanical properties of high-strength nanostructured bainitic steel containing 3.5 wt% aluminum

This study aims to produce a cost-effective and low-density, high-strength nanostructured bainitic steel by optimizing the austempering process conditions. To do this, a steel containing 0.8C-1.5Mn-0.6 Cr-0.35Mo-3.5Al was selected and the effects of austempering time and temperature on the microstructure and mechanical properties of this steel were studied. Xrd technique was used to recognize the phases present in the microstructure and measure the retained austenite volume fraction and carbon content. Hardness measurements showed that the highest value was obtained from the specimen containing the highest volume fraction of the bainitic ferrite associated with the retained austenite with the highest carbon content, at the completion time of transformation. Such a combination of microstructure and hardness was applied to find the optimum time at different temperatures during the austempering process. The microstructures of the optimal specimens were evaluated through optical microscopy (OM) and field-emission scanning electron microscopy (FESEM), and the average thickness of the bainitic ferrite plates was calculated. Finally, tensile tests were performed on the optimal specimens to assess the fracture surface through FESEM. For high steel with 3.5 wt% Al, the results indicated that it is possible to obtain a microstructure consisting of bainitic ferrite plates with an average thickness of 53 nm surrounded by the carbon-rich retained austenite with a hardness of 561HV, a yield stress of 1614 MPa, an ultimate tensile strength of 1981 MPa, and a total elongation of 9 %.

Thermal stability of microstructure and corresponding hardness variations in two high-carbon nanostructured bainitic steels

The autotempering phenomenon and carbon partitioning from bainitic subunits during the bainite transformation suggest that the tempering behavior of the nanostructured bainite may be different compared to that of martensite. This article aims to investigate the thermal stability of the microstructure in two low alloy nanostructured bainitic steels containing 0.89 wt% carbon content. Nanostructured bainite was obtained after austempering at 250 °C for optimum heat treatment time. Nanostructured bainitic steels were tempered at the temperature range of 300–700 °C for 30 min. Additionally, the effect of prolonged austempering beyond the optimum heat treatment condition at austempering temperature was studied. Microstructural characteristics were evaluated using optical and field emission gun scanning electron microscopes and X-ray diffraction methods. Hardness variations during tempering of nanostructured bainite compared with that of martensite microstructure obtained in the same primary steels after austenitizing and quenching. Results indicated that microstructural characteristics of nanostructure bainite didn’t change significantly at heat treatment times longer than that needed for bainite transformation completion at isothermal bainite heat treatment temperature. Moreover, results approved the higher tempering resistance of nanostructured bainite compared to that of martensite in the same steel. It has been shown that tempering the nanostructured bainite up to 400 °C didn’t change the microstructure and hardness, while the changes were pronounced at higher tempering temperatures. It has been suggested that cementite precipitation was more significant when a higher number of dislocations were present in nanostructured bainite before starting the tempering process.

Mechanical Properties of Step-Austempered Nanostructured Bainitic Steel with Modified Chemical Composition at Low and High Strain Rates

Mechanical Properties of Step-Austempered Nanostructured Bainitic Steel with Modified Chemical Composition at Low and High Strain Rates

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 Austempering Time on Electrochemical and Immersion Corrosion Behaviour of High Carbon, Carbide-Free Nanostructured Bainitic Steel in an Aqueous 3.5% NaCl

Effect of Austempering Time on Electrochemical and Immersion Corrosion Behaviour of High Carbon, Carbide-Free Nanostructured Bainitic Steel in an Aqueous 3.5% NaCl

Ballistic Efficacy of Carbide Free High Strength Nano Structured Bainitic Armour Steels

Carbide free nano-structured bainitic steels typically have strength more than 2.0 GPa and impact toughness of 7 J or less. Most of this class of steels have sluggish kinetics and takes 16-72 h for complete bainitic transformation. The present work discusses key perspectives in developing carbide free nano-structured bainitic steel having strength more than 2.0 GPa and toughness more than 15 J. Further, ballistic evaluation of newly developed carbide free nano-structured bainitic steels having strength more than 2.0 GPa and high toughness of 20 J was carried out to understand the adaptability of these steels in combat vehicle applications. A comparison is made between newly developed high strength and tough carbide free nano-structured bainitic steels with typical martensite based ARMOX 500 class of armour steels. The developed nano-structured bainite showed ballistic performance much superior to ARMOX 500 steel. Monolithic plates of bainite provide complete protection against 7.62 AP projectiles at an areal density of 120 kgm-2. The ballistic efficiency of monolithic plates was further enhanced by using perforated geometrical configurations.

Evaluation of the Impact and Fracture Toughness of a Nanostructured Bainitic Steel with Low Retained Austenite Content

The impact and fracture toughness of a nanostructured, kinetically activated bainitic steel was determined using Standard methods. Prior to testing, the steel was quenched in oil and aged naturally for a period of 10 days in order to obtain a fully bainitic microstructure with a retained austenite content below 1%, resulting in a high hardness of 62HRC. The high hardness originated from the very fine microstructure of bainitic ferrite plates formed at low temperatures. It was determined that the impact toughness of the steel in the fully aged condition improved remarkably, whereas the fracture toughness was in line with expectations based on the extrapolated data available in the literature. This suggests that a very fine microstructure is most beneficial to rapid loading conditions, whereas material flaws such as coarse nitrides and non-metallic inclusions are the major limitation for obtaining a high fracture toughness.

Microstructure evolution and damage mechanism of nanostructured bainitic steels under cyclic high energy impact

• The microstructure evolution under cyclic high-energy impact was studied. • The largest residual stress occurs at a distance of 10 mm beneath the impacted surface. • The volume fraction of austenite decreases with the accumulated stress increasing. • The transformation of retained austenite induces the nucleation of fatigue cracks due to strong stress concentration. The microstructure evolution and damage mechanisms of nanostructured bainitic steels under cyclic high-energy (∼1000J) impact were studied. The characterization results of impacted samples show that several zones of stress layer, deformation zone and transition zone can be divided based on a gradual increase in the volume fraction of retained austenite. Corresponding well to the simulation results, the measured residual stress shows a peak value at a distance of 10 mm beneath the impacted surface, which results in a large amount of transformed martensite and localized high strain in the deformation zone, thus inducing the nucleating of fatigue cracks and macroscopic microstructure failure.

Ductilization of high carbon, high silicon carbide-free nanostructured bainitic steel

In the present study, the influence of the volume fraction of blocky austenite on the microstructure and mechanical properties of novel bainitic steel was investigated. The rolled homogenized sheets were austenitized at 950 °C and austempered at 250 °C for 18h (B15VA-1) and 50 h(B15VA-2). X-ray diffraction study of B15VA-1 & B15VA-2 confirmed the presence of carbide-free nanostructured bainitic ferrite (BF) and retained austenite (RA). Optical, scanning and transmission electron microscopic studies delineate the morphology of retained austenite to filmy and blocky forms. On the tensile deformation, majority of blocky austenite in B15VA-1 transformed to strain-induced martensite of ε type. Hardness and yield strength (YS) were increased with an increasing amount of bainitic ferrite, however, ultimate tensile strength (UTS) was found to be a strong function of the extent of strain-induced martensitic (SIM) transformation. As a result of the high amount of SIM, UTS of B15VA-1 increased to 1889 MPa. Ductility is mainly controlled by the transformation-induced plasticity of blocky austenite. Uniform plastic elongation and total plastic elongation were found to be 25% and 28%, respectively. The work hardening behavior can be described by the Swift model. Deformation under tensile load takes place in three stages which are homogeneous deformation, strain-induced martensitic transformation and multiphase deformation-led interaction of dislocations and their recovery. The alloy with nanostructured bainitic ferrite fails by mixed mode of ductile-brittle fracture.

Effect of prior austenite grain size on the tensile properties and fracture toughness of nano-structured bainite

Prior austenite grain size (PAGS) can significantly impact the bainitic lath thickness formed during austempering of high carbon steels. This study examines the effect of two different PAGS on the microstructure development and in turn the tensile properties and fracture toughness of nano-structured bainitic steels. Two PAGS have been obtained by austenitizing at 900 °C and 1000 °C respectively but the bainite fraction has been maintained to be around the same range by austempering at 250 °C. Less amount of blocky austenite is formed when the PAGS is larger. It is also observed that with an increase in the PAGS, the thickness of bainitic lath and retained austenite (RA) decreases and the tensile strength, tensile ductility and the plane strain fracture toughness is enhanced. The fraction of RA transformed to martensite is less for larger PAGS during a tensile test while showing greater ductility owing to the greater stability of finer RA films. • Higher austenitization temperature results in higher prior austenite grain size (PAGS). • Higher PAGS leads to finer, morphology of bainite and more amount of film type austenite. • Refinement of bainitic lath thickness leads to enhancement of strength, ductility and plane strain fracture toughness. • Bainitic ferrite block size reduces with an increase in PAGS. • Fractography features correspond to bainitic ferrite block size.

Effect of tempering on stability of retained austenite and tensile properties of nanostructured bainitic steel

In this investigation, the evolution of the mechanical stability of retained austenite (RA) and the strength-plasticity of nanostructured bainitic steel tempered at a temperature ranged from 250 °C to 510 °C were studied. Results show that the mechanical stability of RA decreased after tempering mainly due to the weakened shielding effect of the bainite ferrite (BF) on RA. The RA with high mechanical stability generated a little mechanically induced martensite when the tempering temperature was below 380 °C, leading to less contribution to elongation. In addition, the poor deformation ability of the bainitic matrix resulted in nanostructured bainitic steel with low elongation. With increasing tempering temperature to 380 °C and higher, the mechanical stability of RA decreased significantly and mechanically induced martensite transformation occurred easily. However, the contribution of RA to the elongation remained low due to the formation of a substantial quantity of mechanically induced martensite after 380 °C specimen yield. The elongation was improved remarkably with increasing tempering temperature to 450 °C and above. The main reason was the improvement of the coordinated deformation ability of bainite matrix with a large number of carbide particles precipitated in BF and RA. Finally, the specimen tempered at 450 °C presented the optimum tensile mechanical properties. The yield and ultimate tensile strength of nanostructured bainitic steel increased at first and then decreased with increasing tempering temperature, and both of them reached the maximum in the 250 °C specimen.

The Microstructure Transformations and Wear Properties of Nanostructured Bainite Steel with Different Si Content.

Nanostructured bainite (NB) bearing steel has excellent strength and ductility combinations, which can improve the fatigue life and wear resistance of bearing steel in harsh conditions. However, the phase transformations and the correlation between the microstructure and wear properties of NB bearing steel are still unclear. In this study, bearing steels with different Si contents (GCr15SiMo and GCr15Si1Mo) were prepared to have nano-bainitic structures, and their microstructure transformations and wear mechanisms were investigated. The results show that the Si element can inhibit the precipitation of carbides and can then promote the block-like retained austenite formation and refine the bainitic ferrite lamellar structure. The impact energy of GCr15Si1Mo is larger than that of GCr15SiMo because the nanostructured bainite and retained austenite are the main toughness phase in these steels. The wear results indicate that the steels which possess appropriate strength and toughness are helpful for improving wear resistance properties. Finally, the wear resistance performance of the GCr15Si1Mo austempered at 210 °C and GCr15SiMo austempered at 230 °C was good in this work.

Short fatigue crack growth in an artificial-defect-containing nanostructured bainitic steel

For a full exploitation of novel steels in applications under mechanical cyclic loading, it is vital to understand the crack-microstructure interactions. In this work, the fatigue behaviour of a nanostructured bainitic steel in the short crack regime has been studied considering the structural integrity of the material and its polycrystalline nature. For that purpose, a specimen containing an artificial defect has been designed and manufactured, subjected to interrupted fatigue testing, and examined by crystallography-sensitive techniques. Results show that, depending on the loading conditions, different crystallographic boundaries can play an important role in the crack path and crack growth rate.

Influence of austempering temperature on microstructure and mechanical properties of high-carbon nanostructured bainitic steels

We investigated the influence of austempering temperature on microstructure and mechanical properties of high-carbon nanostructured bainitic steels. The austempering temperature changed the volume fraction of bainitic ferrite, martensite, block- and lath-type retained austenite. The mechanical stability of retained austenite is affected by its chemical composition and morphology. For the high-carbon nanostructured bainitic steels, the micro-hardness and strengths are increased with increasing the volume fraction of martensite and decreasing the lath thickness of bainitic ferrite, while the ductility is increased as retained austenite with sufficient mechanical stability increases. In particular, the volume fraction and mechanical stability of retained austenite act as a critical factor for improving the toughness of high-carbon nanostructured bainitic steels. As a result, it has been seen that controlling the mechanical stability of retained austenite with different morphology and fraction is essential to develop advanced high-carbon nanostructured bainitic steels with an excellent combination of hardness, strength, ductility, and toughness for a given chemical composition.

Effect of austempering temperature on high cycle fatigue behaviour in nanostructured bainitic steels

The effect of three different austempering temperatures viz. 250°C (NB250), 300°C (NB300) and 350°C (NB350) on high cycle fatigue behaviour of carbide-free nanostructured bainite was investigated. There was an increase in yield strength with a decrease in austempering temperature due to finer bainitic ferrite (BF) and its higher volume fraction compared to retained austenite (RA). However, NB350 showed significant strain hardening compared to NB250 and NB300 due to transformation induced plasticity (TRIP). This also had implications in enhancing its fatigue life at almost all stress levels compared to NB250 and NB300 inspite of having a lower yield strength and tensile strength.

Effect of Electropulsing on Nanostructured Bainitic Steel

Effect of Electropulsing on Nanostructured Bainitic Steel

Effect of austempering temperature on corrosion evolution during immersion of high carbon nano-structured bainitic steel in aqueous chloride environment

Nano-structured bainitic (NSB) steels have a dual-phase structure comprising carbon-rich retained austenite (RA) and bainitic ferrite (BF). These steels exhibit high strength as well as sufficient ductility and damage tolerance and hence can find potential application in marine areas. However, a systematic study of the corrosion behavior is required to assess the reliability and longevity of these steels for the above applications. The role of austempering temperature (250 and 350 °C: specimens NB250 and NB350, respectively) and in turn the microstructural constituents on the corrosion response in aqueous chloride environment has been studied using immersion tests at different intervals of time (10, 20 and 30 days). Characterization of corrosion products has been done using microscopy and multiple spectroscopy techniques to reveal the attack mechanism and corroded layer composition. The corrosion rate, measured using weight loss experiments, decreases with a reduction in austempering temperature of the specimens and with an increase in immersion duration. Cross-sectional imaging of the corroded regions shows highly non-uniform but deeper penetration of corrosion with an increase in austempering temperature. In contrast, specimen transformed at the lowest temperature showed uniform and shallow penetration of corrosion. The corroded layer formed over NB350 after 10 days mostly comprised of oxyhydroxides but oxides took predominance after 30 days. However, NB250 showed mostly oxyhydroxides on the surface after 10 as well as 30 days. Hence, decreasing the austempering temperature while transforming austenite to nano-bainite not only provides higher strength but also greater corrosion resistance in saline environments.

Effect of tempering temperature on microstructure and mechanical properties of nanostructured bainitic steel

The previous research on tempered nanostructured bainite has been mainly focused on the carbide precipitation at high temperatures with few details on nanostructured bainite tempered at lower temperatures. In this research, the microstructure evolution and the corresponding mechanical properties of the nanostructured bainite tempered at a temperature ranged from 180 °C to 400 °C were studied via X-ray diffraction, transmission electron microscope and high-resolution thermal dilatometer. Results show that retained austenite occurred bainitic transformation when the tempering temperature ranged from 210 °C to 320 °C. The amount of newly formed bainite increased at first and then decreased slightly with increasing tempering temperature. When the tempering temperature reached 380 °C and higher, the cementite precipitated from retained austenite and reduced the stability of over-stable retained austenite. As a result, it can be concluded that stress-induced martensitic transformation would occur easily in retained austenite during deformation process, leading to the optimum increase of retained austenite contribution to toughness.

Development of technology for the production of a lightweight observation and protective container (LOOK) made of nanostructured ultra-strength steels

The article contains results of research and analyses concerning application of nanostructured bainitic steel in the form of plates for manufacturing of armour components. The presented results of examination of microstructure and properties include a wide range of laboratory experiments and industrial tests, which resulted in the achievement of the assumed functional properties. In the period of 2017-2021, a scientific and industrial consortium consisting of Łukasiewicz – Institute of Ferrous Metallurgy (leader); WITPiS, Tarnów Mechanical Works, Alchemia and Heatmasters Poland carried out a project funded by the POIR 04.01.04 programme aimed to develop the design and to manufacture an observation and protective container with a specified resistance to penetration by armour-piercing projectiles and with a lower mass of steel armouring in relation to that currently produced. The aim of the project was achieved by using armour plates made of nanostructured bainitic steel (nanobainitic), which are characterised by high resistance to high-energy impact concentrated in a small area. The technological tests carried out in the project mainly concerned the development of a new container and industrial technology of armour plates production and their application in the armour of this container. Based on the results of investigation of the semi-industrial scale material, the optimum chemical composition for industrial scale melting and casting was determined. An industrial technology for the production of plates of nano-structured bainitic steel was developed, which includes the following processes: smelting and casting, preliminary heat treatment and ingot hot processing, as well as hot rolling, final heat treatment, and surface treatment. A test batch of the material in the form of 1500×2470 mm armoured plates was fabricated under industrial conditions. The final result of the project is a container armoured with bainitic nanostructured steel plates with implementation documentation and a technology for producing armoured plates from this steel under the technical and technological conditions of domestic steel manufacturers.