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The present study has focused on the detailed dilatometric and electron microscopic analysis of the formation of austenite and its decomposition in two Cerium (Ce) modified steels containing 0.6 and 0.03 wt pct Ce. Despite a long incubation time, the austenite formation is very fast in the high-Ce steel (0.6 wt pct Ce) during heating. Whereas, the low-Ce steel (0.03 wt pct Ce) promotes early nucleation of austenite but shows a significant delay in the completion of the transformation. Similar trend has been observed for the decomposition of austenite during cooling; the low-Ce steel shows early start of transformation with a sluggish kinetics. The role of Ce on the overall transformation kinetics of austenite during the heating–cooling cycle has been investigated and discussed from thermodynamic viewpoint and nucleation probability. Engineering stress–strain curves have shown a better combination of strength and ductility in the low-Ce steel compared to the high-Ce one. Coarse and spherical Ce2O3 particles (average dia. ~ 4 µm) in grain interior along with the brittle and grain boundary elongated Ce-C/Ce-Fe intermetallic phases (length ~ 8 µm and width ~ 3 µm) are responsible for the lower strain hardening as well as an early failure of the high-Ce steel. Finally, the microstructure-tensile property correlation has been established using chemical composition and fractographic analysis. © 2021, The Minerals, Metals & Materials Society and ASM International.

World University root node

Department of Metallurgical & Materials Engineering

Metallurgical and Materials Transactions A

Role of Cerium on Transformation Kinetics and Mechanical Properties of Low Carbon Steels

The relation between microstructure and mechanical properties of carbide-free bainitic steel 0.2% C – 1.53% Mn – 1.1% Si – 1% Cr – 0.3% Mo – 0.5% N under thermomechanical treatment is studied under isothermal conditions and under continuous air cooling. Impact tests of Charpy specimens, metallographic and x-ray diffraction analyses of the steel deformed by different variants are conducted. The size of the initial austenite grains and the Vickers hardness of the steel are determined. The hot plastic deformation is shown to affect positively the impact toughness of the steel treated for bainite. © 2021, Springer Science+Business Media, LLC, part of Springer Nature.

Metal Science and Heat Treatment 63 (7-8)

Thermomechanical Treatment of Carbide-Free Bainitic Steel

In the present work, attempts have been made to design and develop low-carbon, low-alloy, high strength carbide-free bainitic steels for application in heavy-haul rail tracks. The study starts with an analytical approach for understanding the effects of different alloying elements on the thermodynamic and kinetic aspects of bainite transformation and the resultant microstructure and mechanical properties. Based on the analysis, two steel compositions were designed for further investigation. The alloys were prepared by melting and casting and processed by hot-rolling and air cooling, which is the usual route for the industrial production of rail sections. Microstructures of the developed steels primarily consisted of plates of carbide-free bainitic ferrite, interspersed with fine films of retained austenite. These continuously cooled steels offer ultra-high strength levels with very good ductility which are far superior to existing pearlitic grade rail steels. Detailed characterization and quantification of microstructural constituents have been carried out and they are correlated with the tensile proprieties using semi-empirical formulations available in literature. © 2019 Elsevier B.V.

Materials Science and Engineering: A

Development of continuously cooled low-carbon, low-alloy, high strength carbide-free bainitic rail steels

IN Patent C22C38/

Ultra-high strength and wear resistant carbide-free bainitic rail steel and method of manufacturing the same

The present study investigates the orientation dependence of deformation-induced martensite (DIM) transformation in carbide-free bainitic steel using experimental macrotexture analysis and crystal plasticity-based modelling. The results indicate that the P {110}〈122〉 component of retained austenite preferentially undergoes DIM transformation during uniaxial tensile deformation at room temperature. It is also found that the P component of retained austenite possesses the lowest Schmid factor value compared with other possible texture components. The lowest Schmid factor value of the P component results in the highest resolved shear stress value for its slipping. As a result, during plastic deformation when most of the initial components of the retained austenite reorient themselves to other relatively stable orientations, the P component remains almost unaffected. Accumulation of stress concentration thus increases along this component, which eventually leads to its deformation-induced transformation to martensite. © 2020, The Minerals, Metals & Materials Society and ASM International.

Orientation Dependence of Deformation-Induced Martensite Transformation During Uniaxial Tensile Deformation of Carbide-Free Bainitic Steel

The isothermal bainite transformation in low-carbon low-alloy steel under direct and step-quenching conditions have been investigated in the present work. Varying quantities of prior bainite and prior martensite were formed separately by quenching above and below Ms temperature to understand their effects on the extent and kinetics of subsequent bainite transformation in Bs − Ms regime. Formation of prior bainite at higher temperature has been found to stabilise part of remaining austenite against subsequent bainite transformation at lower temperature showing thermal stabilisation of austenite. Hence, the total amount of bainite in two-step treatment is less than that obtained in single-step isothermal bainite transformation at the latter temperature, indicating the inapplicability of ‘additivity rule’ for continuously cooled bainite. A decrease in the bainitic ferrite plate volume due to higher strength of parent austenite and higher nucleation rate, together with limited number of bainite nucleation sites at the latter temperature are believed to be responsible for the thermal stabilisation of austenite. Prior martensite also stabilises part of remaining austenite against further bainite transformation through fragmentation of austenite grains and increased defect density in austenite. The kinetics of bainite transformation in each of the above-mentioned conditions have been analysed quantitatively using an established model and the results are correlated with the microstructures. © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group.

Philosophical Magazine

Understanding the effect of prior bainite/martensite on the formation of carbide-free bainite

Inconsistencies exist in literature regarding the effect of prior austenite grain size (PAGS) on the extent and kinetics of bainite transformation. Attempts have been made in the present work to address these issues in a low-alloy carbide-free bainitic steel using dilatometry, over a range of PAGS. The bainite transformation kinetics in the above-mentioned conditions have been analysed quantitatively using established kinetic model to extract information related to the transformation mechanisms in such conditions. Greater obstruction from grain boundaries in fine-grained austenite restricts sheaves of bainite to develop completely and thereby reduces the volume fraction of bainite in comparison with coarse-grained austenite. Initial nucleation rate of bainite transformation increases with decreasing PAGS due to an increase in the nucleation site density. However, the maximum nucleation rate decreases consistently with decreasing PAGS due to gradual reduction in the autocatalytic factor. © 2020 Informa UK Limited, trading as Taylor & Francis Group.

Effect of prior austenite grain size on the formation of carbide-free bainite in low-alloy steel

IIT Kharagpur

Development of High Strength Bainitic Steels

The study has examined the work hardening characteristics and damage mechanisms of a newly developed carbide-free bainitic steel. These are correlated with the microstructural constituents using the established empirical equations and the experimental findings. Carbide-free bainitic steel has shown primarily three stages of deformation in modified Crussard-Jaoul analysis, which best describes the work hardening behaviour. Plastic deformation of retained austenite precedes the deformation of bainitic ferrite. The blocks of retained austenite transform to strain induced martensite at lower strain levels than the films of retained austenite. With increasing strain, cross slip assisted dynamic recovery, and sub-grain formation become predominant in bainitic ferrite. Although early void initiation occurs at the non-metallic inclusions, these do not play a significant role in the final fracture. Void initiation and splitting of phase boundaries due to strain incompatibility, decohesion of prior austenite grain boundaries and cracking of martensitic-austenitic constituents are the main damaging mechanisms. © 2019 Elsevier B.V.

Work hardening behaviour and damage mechanisms in carbide-free bainitic steel during uni-axial tensile deformation

Rolling/sliding wear behaviour of two newly designed low-carbon, continuously cooled, carbide-free bainitic rail steels has been studied and compared with pearlitic rail steel. Bainitic rail steels showed higher wear resistance than pearlitic rail steel and wear resistance increases with increasing starting hardness. Detailed microstructural characteristics of the steels undergoing wear were investigated. Wear resistance of carbide-free bainitic rail steels has been found to increase with increasing retained austenite content and decreasing bainitic ferrite lath thickness. © 2018 Elsevier B.V.

Wear

Dry rolling/sliding wear behaviour of pearlitic rail and newly developed carbide-free bainitic rail steels

The development of ferrite-martensite dual-phase microstructures by cold-rolling and intercritical annealing of 0.06 wt pct carbon steel was systematically studied using a dilatometer for two different heating rates (1 and 10 K/s). A step quenching treatment has been designed to develop dual-phase structures having a similar martensite fraction for two different heating rates. An increase in heating rate seemed to refine the ferrite grain size, but it increased the size and spacing of the martensitic regions. As a result, the strength of the steel increased with heating rate; however, the formability was affected. It has been concluded that the distribution of C during the annealing treatment of cold-rolled steel determines the size, distribution, and morphology of martensite, which ultimately influences the mechanical properties. Experimental detection of carbon distribution in austenite is difficult during annealing of the cold-rolled steel as the phase transformation occurs at a high temperature and C is an interstitial solute, which diffuses fast at that temperature. Therefore, a cellular automata (CA)-based phase transformation model is proposed in the present study for the prediction of C distribution in austenite during annealing of steel as the function of C content and heating rate. The CA model predicts that the carbon distribution in austenite becomes more inhomogeneous when the heating rate increases. In the CA model, the extent of carbon inhomogeneity is measured using a kernel averaging method for different orders of neighbors, which accounts for the different physical space during calculation. The obtained results reveal that the 10th order (covering 10-µm physical spaces around the cell of interest) is showing the maximum inhomogeneity of carbon and the same effect has been investigated and confirmed using auger electron spectroscopy (AES) for 0.06 wt pct carbon steel. Furthermore, the optimization of carbon homogeneity with respect to heating rate has been performed using a bi-objective genetic programming (BioGP) strategy for the steel composition varying from 0.06 to 0.12 wt pct carbon along with other parameters like average austenite grain size and time of heating as the input variables. The analysis of the results obtained from BioGP suggests that the homogeneity of carbon increases with the increasing carbon concentration of steel. This is corroborated by analyzing the AES results obtained for 0.28 wt pct carbon steel using the same technique as that used for 0.06 wt pct carbon steel. © 2016, The Minerals, Metals & Materials Society and ASM International.

Effect of carbon distribution during the microstructure evolution of dual-phase steels studied using cellular automata, genetic algorithms and experimental strategies

In order to understand the effect of grain size bimodality on the tensile properties, low-carbon steel samples having bimodal ferrite grain size distributions, with different coarse-grain sizes (15-54μm), fine-grain sizes (2.4-13.0μm) and coarse-grain fractions (0.10-0.45) have been tensile tested. Samples having bimodal grain structures showed lower strength (UTS: 395-744MPa), better tensile ductility (e u: 8.5-19.4%) and lower YS to UTS ratio (0.63-0.84) than the ultra-fined grained steel (UTS=900MPa, e u=5%, and YS:UTS=0.93). In case of mixed grain structures, effective grain size (D eff), numerical average grain size (D avg) and the average grain size in the fine-grain regions (D f) showed better correlation with strength (i.e. strength ∝ D -1/2) than the weighted average grain size (D wt). Coarse-grain sizes in the range of 15-35μm and fraction of the coarse-grain regions in the range of 15-35% can be recommended for achieving the best tensile ductility. Measurement of micro-hardness and the change in grain aspect ratios from the coarse- and fine-grain regions indicated that the bimodal grain structure can be considered as a composite structure, where stress and strain vary between the coarse- and fine-grain regions. Lower nucleation and higher growth of the voids in the carbide-depleted, coarse-grain regions (than carbide-rich, fine-grain regions) allow higher plastic deformation till fracture and improve the total-elongation of the steels. © 2012 Elsevier B.V.

Effect of bimodal distribution in ferrite grain sizes on the tensile properties of low-carbon steels

The effects of ferrite-pearlite and ferrite-martensite starting microstructures on ferrite grain sizes and carbide particle sizes after cold rolling and prolonged annealing treatment have been investigated. Ferrite-martensite starting microstructures showed finer grain and particle sizes and improved tensile properties after cold rolling annealing cycle compared to ferrite-pearlite starting microstructures. A 'fibrous' martensite morphology developed by intermediate quenching treatment is more beneficial in that respect compared to the 'blocky' martensite morphology obtained from the step quenching treatment. © 2012 Institute of Materials, Minerals and Mining.