scholarly journals Novel Approach of Nanostructured Bainitic Steels’ Production with Improved Toughness and Strength

Materials ◽  
2020 ◽  
Vol 13 (5) ◽  
pp. 1220
Author(s):  
Peter Kirbiš ◽  
Ivan Anžel ◽  
Rebeka Rudolf ◽  
Mihael Brunčko
Keyword(s):  
Electron Microscopy ◽  
Mechanical Properties ◽  
Fine Structure ◽  
Retained Austenite ◽  
Bainitic Ferrite ◽  
High Hardness ◽  
Tensile Tests ◽  
Isothermal Transformation ◽  
Impact Testing ◽  
Bainitic Steels

The tendencies of development within the field of engineering materials show a persistent trend towards the increase of strength and toughness. This pressure is particularly pronounced in the field of steels, since they compete with light alloys and composite materials in many applications. The improvement of steels’ mechanical properties is sought to be achieved with the formation of exceptionally fine microstructures ranging well into the nanoscale, which enable a substantial increase in strength without being detrimental to toughness. The preferred route by which such a structure can be produced is not by applying the external plastic deformation, but by controlling the phase transformation from austenite into ferrite at low temperatures. The formation of bainite in steels at temperatures lower than about 200 °C enables the obtainment of the bulk nanostructured materials purely by heat treatment. This offers the advantages of high productivity, as well as few constraints in regard to the shape and size of the workpiece when compared with other methods for the production of nanostructured metals. The development of novel bainitic steels was based on high Si or high Al alloys. These groups of steels distinguish a very fine microstructure, comprised predominantly of bainitic ferrite plates, and a small fraction of retained austenite, as well as carbides. The very fine structure, within which the thickness of individual bainitic ferrite plates can be as thin as 5 nm, is obtained purely by quenching and natural ageing, without the use of isothermal transformation, which is characteristic for most bainitic steels. By virtue of their fine structure and low retained austenite content, this group of steels can develop a very high hardness of up to 65 HRC, while retaining a considerable level of impact toughness. The mechanical properties were evaluated by hardness measurements, impact testing of notched and unnotched specimens, as well as compression and tensile tests. Additionally, the steels’ microstructures were characterised using light microscopy, field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The obtained results confirmed that the strong refinement of the microstructural elements in the steels results in a combination of extremely high strength and very good toughness.

scholarly journals Microstructural Features of Strain-Induced Martensitic Transformation in Medium-Mn Steels with Metastable Retained Austenite/ Cechy Mikrostrukturalne Indukowanej Odkształceniem Przemiany Martenzytycznej W Stalach Sredniomanganowych Z Metastabilnym Austenitem Szczątkowym

2014 ◽  
Vol 59 (4) ◽  
pp. 1673-1678 ◽  
Author(s):  
A. Grajcar ◽  
A. Kilarski ◽  
K. Radwanski ◽  
R. Swadzba
Keyword(s):  
Electron Microscopy ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Electron Backscatter Diffraction ◽  
Bainitic Ferrite ◽  
Tensile Tests ◽  
Bainitic Steels ◽  
Transmission Electron ◽  
Strain Induced Martensite ◽  
Backscatter Diffraction

Abstract The work addresses relationships between the microstructure evolution and mechanical properties of two thermomechanically processed bainitic steels containing 3 and 5% Mn. The steels contain blocky-type and interlath metastable retained austenite embeded between laths of bainitic ferrite. To monitor the transformation behaviour of retained austenite into strain-induced martensite tensile tests were interrupted at 5%, 10%, and rupture strain. The identification of retained austenite and strain-induced martensite was carried out using light microscopy (LM), scanning electron microscopy (SEM) equipped with EBSD (Electron Backscatter Diffraction) and transmission electron microscopy (TEM). The amount of retained austenite was determined by XRD. It was found that the increase of Mn addition from 3 to 5% detrimentally decreases a volume fraction of retained austenite, its carbon content, and ductility.

Isothermal Transformation, Microstructure and Mechanical Properties of Ausformed Low-Carbon Carbide-Free Bainitic Steel

2018 ◽  
Vol 941 ◽  
pp. 329-333 ◽  
Author(s):  
Jiang Ying Meng ◽  
Lei Jie Zhao ◽  
Fan Huang ◽  
Fu Cheng Zhang ◽  
Li He Qian
Keyword(s):  
Mechanical Properties ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Bainitic Ferrite ◽  
Isothermal Transformation ◽  
Bainitic Transformation ◽  
Isothermal Treatment ◽  
Bainitic Steel ◽  
Low Carbon ◽  
Microstructure And Mechanical Properties

In the present study, the effects of ausforming on the bainitic transformation, microstructure and mechanical properties of a low-carbon rich-silicon carbide-free bainitic steel have been investigated. Results show that prior ausforming shortens both the incubation period and finishing time of bainitic transformation during isothermal treatment at a temperature slightly above the Mspoint. The thicknesses of bainitic ferrite laths are reduced appreciably by ausforming; however, ausforming increases the amount of large blocks of retained austenite/martenisite and decreases the volume fraction of retained austenite. And accordingly, ausforming gives rise to significant increases in both yield and tensile strengths, but causes noticeable decreases in ductility and impact toughness.

Designing, Processing and Isothermal Transformation of Al-Si Medium Carbon Ultrafine High Strength Bainitic Steel

2017 ◽  
Vol 380 ◽  
pp. 1-11
Author(s):  
Sherif Ali Abd El Rahman ◽  
Ahmed Shash ◽  
Mohamed K. El-Fawkhry ◽  
Ahmed Zaki Farahat ◽  
Taha Mattar
Keyword(s):  
Mechanical Properties ◽  
Carbon Steel ◽  
Retained Austenite ◽  
High Carbon Steel ◽  
Bainitic Ferrite ◽  
High Strength ◽  
Isothermal Transformation ◽  
High Carbon ◽  
Medium Carbon ◽  
The Stability

Medium-carbon, silicon-rich steels are commonly suggested to obtain a very fine bainitic microstructure at a low temperature slightly above Ms. Thereby, the resulted microstructure consists of slender bainitic-ferritic plates interwoven with retained austenite. The advanced strength and ductility package of this steel is much dependent on the fineness of bainitic ferrite, as well as the retained austenite phase. In this article, the aluminum to silicon ratio, and the isothermal transformation temperature have been adopted to obtain ultra-high strength high carbon steel. Optical and SEM investigation of the produced steels have been performed. XRD has been used to track the retained austenite development as a result of the change in the chemical composition of developed steels and heat treatment process. Mechanical properties in terms of hardness and microhardness of obtained phases and structure were investigated. Results show that the increment of aluminum to silicon ratio has a great effect in promoting the bainitic transformation, in tandem with improving the stability and the fineness of retained austenite. Such an advanced structure leads to enhancement in the whole mechanical properties of the high carbon steel.

scholarly journals The Impact of Retained Austenite on the Mechanical Properties of Bainitic and Dual Phase Steels

2022 ◽  
Author(s):  
Bogusława Adamczyk-Cieślak ◽  
Milena Koralnik ◽  
Roman Kuziak ◽  
Kamil Majchrowicz ◽  
Tomasz Zygmunt ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Transformation Temperature ◽  
Retained Austenite ◽  
Tensile Tests ◽  
Total Elongation ◽  
Isothermal Transformation ◽  
Bainitic Transformation ◽  
Bainitic Steel ◽  
Continuous Cooling

AbstractThis paper presents the microstructural changes and mechanical properties of carbide-free bainitic steel subjected to various heat treatment processes and compares these results with similarly treated ferritic–pearlitic steel. A key feature of the investigated steel, which is common among others described in the literature, is that the Si content in the developed steel was >1 wt.% to avoid carbide precipitation in the retained austenite during the bainitic transformation. The phase identification before and after various heat treatment conditions was carried out based on microstructural observations and x-ray diffraction. Hardness measurements and tensile tests were conducted to determine the mechanical properties of the investigated materials. In addition, following the tensile tests, the fracture surfaces of both types of steels were analyzed. Changing the bainitic transformation temperature generated distinct volume fractions of retained austenite and different values of mechanical strength properties. The mechanical properties of the examined steels were strongly influenced by the volume fractions and morphological features of the microstructural constituents. It is worth noting that the bainitic steel was characterized by a high ultimate tensile strength (1250 MPa) combined with a total elongation of 18% after austenitizing and continuous cooling. The chemical composition of the bainitic steel was designed to obtain the optimal microstructure and mechanical properties after hot deformation followed by natural cooling in still air. Extensive tests using isothermal transformation to bainite were conducted to understand the relationships between transformation temperature and the resulting microstructures, mechanical properties, and fracture characteristics. The isothermal transformation tests indicated that the optimal relationship between the sample strength and total elongation was obtained after bainitic treatment at 400 °C. However, it should be noted that the mechanical properties and total elongation of the bainitic steel after continuous cooling differed little from the condition after isothermal transformation at 400 °C.

scholarly journals Enhanced Strength and Ductility by Introducing Nanobainitic Ferrite in Bainitic Steel Used in Sports Equipment

Metals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 2007
Author(s):  
Han Meng ◽  
Zhenjun Hong ◽  
Yu Li ◽  
Xiaoshuai Jia ◽  
Zhihua Yin
Keyword(s):  
Mechanical Properties ◽  
Retained Austenite ◽  
Bainitic Ferrite ◽  
Total Elongation ◽  
Bainitic Steel ◽  
Bainitic Steels ◽  
Cottrell Atmosphere ◽  
The Stability ◽  
Strength And Ductility ◽  
Enhanced Stability

The mechanical properties of carbide-free bainitic steels used in sports equipment were investigated. The nanobainitic ferrite was introduced in bainitic steel to enhance the stability of blocky retained austenite (RA). The blocky RA formed in bainitic austempering process was coarse and led to poor mechanical properties. By introducing the nanobainitic ferrite into blocky RA, the yield strength was improved remarkably, which was increased from 706 to 1180 MPa. Furthermore, the total elongation was almost twice the value compared to the traditional bainitic treatment. The improved mechanical properties were attributed to the enhanced stability of blocky RA. Furthermore, the increased carbon content in RA derived from the carbon dissolved in bainitic ferrite and the carbon trapped in dislocation or Cottrell atmosphere.

scholarly journals Microstructure Evolution and Mechanical Stability of Retained Austenite in Thermomechanically Processed Medium-Mn Steel

Materials ◽  
2019 ◽  
Vol 12 (3) ◽  
pp. 501 ◽  
Author(s):  
Adam Grajcar ◽  
Andrzej Kilarski ◽  
Aleksandra Kozłowska ◽  
Krzysztof Radwański
Keyword(s):  
Electron Microscopy ◽  
Martensitic Transformation ◽  
Microstructure Evolution ◽  
Retained Austenite ◽  
Mechanical Stability ◽  
Electron Backscatter Diffraction ◽  
Bainitic Ferrite ◽  
Tensile Tests ◽  
Central Regions ◽  
Deformation Level

A microstructure evolution of the thermomechanically processed 3Mn-1.5Al type steel and mechanical stability of retained austenite were investigated during interrupted tensile tests. The microstructural details were revealed using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) techniques. It was found that the strain-induced martensitic transformation began in central regions of the largest blocky-type grains of retained austenite and propagated to outer areas of the grains as the deformation level increased. At rupture, the mechanical stability showed only boundaries of fine blocky grains of γ phase and austenitic layers located between bainitic ferrite laths. The effects of various carbon enrichment, grain size, and location in the microstructure were considered. The martensitic transformation progress was the highest at the initial stage of deformation and gradually decreased as the deformation level increased.

Influence of the Austempering Temperature on the Microstructure and Crystallography of a Carbide-Free Bainitic Steel

2011 ◽  
Vol 172-174 ◽  
pp. 797-802 ◽  
Author(s):  
Jean Christophe Hell ◽  
Moukrane Dehmas ◽  
Guillaume Geandier ◽  
Nathalie Gey ◽  
Sebastien Allain ◽  
...  
Keyword(s):  
Retained Austenite ◽  
Volume Fraction ◽  
Bainitic Ferrite ◽  
High Energy ◽  
Isothermal Transformation ◽  
Bainitic Transformation ◽  
Bainitic Steels ◽  
Austempering Temperature ◽  
Kinetics Of

We elaborated two carbide-free bainitic steels with different microstructures through specific heat treatments and alloy design. EBSD analysis was used to point out major differences in these microstructures. In-situ characterizations of the bainitic transformation were performed by high energy synchrotron diffraction to go further into the study of each phase characteristics. The elaborated microstructures exhibited various phase fractions of bainitic ferrite, retained austenite and blocks of martensite and retained austenite. Moreover, the volume fraction of retained austenite increased with higher austempering temperatures. On the other hand, the austempering temperatures showed a strong influence on the kinetics of the bainitic transformation. Isothermal transformation under Ms showed a two stage transformation which led first to the formation of self-tempered martensite and then to bainitic ferrite. Furthermore, the evolution of the austenitic cell parameter showed enrichment in carbon ruled by diffusional mechanisms.

scholarly journals Mechanical and thermal stability of retained austenite in plastically deformed bainite-based TRIP-aided medium-Mn steels

2021 ◽  
Vol 21 (3) ◽  
Author(s):  
Aleksandra Kozłowska ◽  
Adam Grajcar ◽  
Aleksandra Janik ◽  
Krzysztof Radwański ◽  
Ulrich Krupp ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Thermal Stability ◽  
Retained Austenite ◽  
Elevated Temperatures ◽  
Mechanical Performance ◽  
Mechanical Stability ◽  
Electron Backscatter Diffraction ◽  
Cost Effective ◽  
Tensile Tests ◽  
Thermal Stability Of

AbstractAdvanced medium-Mn sheet steels show an opportunity for the development of cost-effective and light-weight automotive parts with improved safety and optimized environmental performance. These steels utilize the strain-induced martensitic transformation of metastable retained austenite to improve the strength–ductility balance. The improvement of mechanical performance is related to the tailored thermal and mechanical stabilities of retained austenite. The mechanical stability of retained austenite was estimated in static tensile tests over a wide temperature range from 20 °C to 200 °C. The thermal stability of retained austenite during heating at elevated temperatures was assessed by means of dilatometry. The phase composition and microstructure evolution were investigated by means of scanning electron microscopy, electron backscatter diffraction, X-ray diffraction and transmission electron microscopy techniques. It was shown that the retained austenite stability shows a pronounced temperature dependence and is also stimulated by the manganese addition in a 3–5% range.

Effect of Si Content on Microstructure and Properties of 60Si2CrVA Spring Steel

2014 ◽  
Vol 968 ◽  
pp. 63-66 ◽  
Author(s):  
Fei Zhao ◽  
Zhan Ling Zhang ◽  
Jun Shuai Li ◽  
Cui Ye ◽  
Ni Li
Keyword(s):  
Mechanical Properties ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Testing Machine ◽  
Spring Steel ◽  
Impact Testing ◽  
X Ray ◽  
Comprehensive Properties ◽  
Spring Steels ◽  
Si Content

The microstructure and mechanical properties of the four spring steels with different Si content treated by Q-I-Q-T process were studied by metallographic microscope, MTS, impact testing machine and X-ray stress analyzer. The results show that the tensile strength and yield strength is first increased and then decreased with the increase of Si content, the volume fraction of retained austenite and elongation are fist decreased and then increased when the Si content is less than 2.1%, and the microstructure become finer and homogeneous. When Si content reaches 2.1%, the comprehensive properties of 60Si2CrVA spring steel is the best.

Microstructure and Properties of a Non-Quenched Prehardened Steel at Different Cooling Rate

2012 ◽  
Vol 524-527 ◽  
pp. 1976-1979
Author(s):  
Yi Luo ◽  
Jin Ming Peng
Keyword(s):  
Mechanical Properties ◽  
Fine Structure ◽  
Optical Microscopy ◽  
Cooling Rate ◽  
Retained Austenite ◽  
Transformation Products ◽  
Electronic Microscopy ◽  
Transmission Electronic Microscopy ◽  
Microstructure And Properties ◽  
Cooling Conditions

Mechanical properties of non-quenched prehardened (NQP) steel air cooled and sand cooled after forged were tested and their microstructure was investigated by optical microscopy and transmission electronic microscopy(TEM). The results show that mechanical properties of the NQP steel are similar at both cooling conditions, and their microstructure is bainite, whose fine structure is main bainite ferrite laths, retained austenite films, retained austenite islands and their transformation products. Bainite ferrite laths of the NQP steel air cooled are narrower than that sand cooled, while more retained austenite islands exist in latter.

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