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.

scholarly journals Influence of Prior Martensite on Bainite Transformation, Microstructures, and Mechanical Properties in Ultra-Fine Bainitic Steel

Materials ◽  
2019 ◽  
Vol 12 (3) ◽  
pp. 527 ◽  
Author(s):  
Hui Guo ◽  
Xianying Feng ◽  
Aimin Zhao ◽  
Qiang Li ◽  
Jun Ma
Keyword(s):  
Mechanical Properties ◽  
Retained Austenite ◽  
Bainitic Ferrite ◽  
Growth Direction ◽  
Bainitic Transformation ◽  
Bainitic Steel ◽  
Bainite Transformation ◽  
Multiphase Microstructure ◽  
Microstructures And Mechanical Properties ◽  
The Impact

A multiphase microstructure comprising of different volume fractions of prior martensite and ultra-fine bainite (bainitic ferrite and retained austenite) was obtained by quenching to certain temperatures, followed by isothermal bainitic transformation. The effect of the prior martensite transformation on the bainitic transformation behavior, microstructures, and mechanical properties were discussed. The results showed that the prior martensite accelerated the subsequent low-temperature bainite transformation, and the incubation period and completion time of the bainite reaction were significantly shortened. This phenomenon was attributed to the enhanced nucleation ratio caused by the introduced strain in austenite, due to the formation of prior martensite and a carbon partitioning between the prior martensite and retained austenite. Moreover, the prior martensite could influence the crystal growth direction of bainite ferrite, refine bainitic ferrite plates, and reduce the dimension of blocky retained austenite, all of which were responsible for improving the mechanical properties of the ultra-fine bainitic steel. When the content of the prior martensite reached 15%, the investigated steels had the best performance, which were 1800 MPa and 21% for the tensile strength and elongation, respectively. Unfortunately, the increased content of the prior martensite could lead to a worsening of the impact toughness.

scholarly journals The Effect of Heat Treatment on Microstructure and Mechanical Properties of Cast Bainitic Steel Used for Frogs in Railway Crossovers

2017 ◽  
Vol 62 (4) ◽  
pp. 2147-2151 ◽  
Author(s):  
S. Parzych
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Cast Steel ◽  
Pearlitic Steel ◽  
Strength Properties ◽  
Tensile Yield Strength ◽  
Bainitic Steel ◽  
Microstructure And Mechanical Properties

Abstract This work deals with the effect of heat treatment on a microstructure and mechanical properties of a selected cast steel assigned as a material used for frogs in railway crossovers. Materials used nowadays in the railway industry for frogs e.g. Hadfield cast steel (GX120Mn13) or wrougth pearlitic steel (eg. R260) do not fulfil all exploitation requirements indicated in the UIC (International Union of Railways) Decision No. 1692/96 in terms of train speed that should be reached on railways. One of the possible solution is using a cast steel with bainitic or bainitic-martensitic microstructure that allows to gain high strength properties the ultimate tensile strength (UTS) of 1400 MPa, the tensile yield strength (TYS) of 900 MPa and the hardness of up to 400 BHN. The tested material is considered as an alternative to Hadfield cast steel that is currently used for railway frogs. Results of an experimental analysis of the effect of conducted heat treatment on a microstructure, the volume fraction of retained austenite and mechanical properties of bainitic steel, are shown in this paper. It was found that, the heat treatment leads to a stabilization of retained austenite in grain boundaries area of the primary austenite. Additionally, the heat treatment according to the variant #3 results with an almost 3-times higher impact toughness to that exhibited by material subjected to the other treatments.

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.

Microstructure and mechanical properties of a high-carbon bainitic steel containing Si/Al by different heat treatment processes

2020 ◽  
Vol 10 (11) ◽  
pp. 1932-1940
Author(s):  
Sufyan Naseem ◽  
Enzuo Liu ◽  
Xuefei Huang ◽  
Weigang Huang
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Electron Microscope ◽  
Retained Austenite ◽  
Bainitic Ferrite ◽  
Bainitic Steel ◽  
Boundary Misorientation ◽  
Treatment Processes ◽  
Microstructure And Mechanical Properties ◽  
The Impact

The present study aims to investigate the microstructure and mechanical properties of 0.79 C wt% bainitic steel containing Si and Al by three heat treatment processes: austempering and tempering (B-T), two-step austempering (2S-A) and the austempering-quenching-partitioning (AQP). The optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) were employed to analyze the microstructure of samples. The results demonstrate that the sample subjected to the AQP process exhibited a multiphase microstructure with martensite, filmy retained austenite (RA) and fine bainitic laths. The AQP sample evidenced a high tensile strength of 1705 MPa, yield strength of 1254 MPa, a better total elongation of 16.6%, product of strength and elongation (PSE) of 28 GPa% and the impact toughness of 33 J among all heat treatment processes. The higher strength and toughness could be ascribed to the fine bainitic ferrite as well as an appropriate amount of filmy retained austenite. A fraction of martensite that was formed during the quenching step at 110 °C possibly divided the untransformed austenite into small areas, which could refine the microstructure. EBSD analysis showed that the AQP sample exhibited a higher proportion (64%) of boundary misorientation angle greater than 15° than that of the 2S-A. These high angle boundaries can improve the toughness of steel.

scholarly journals Microstructure and mechanical properties of CuNiMo austempered ductile iron

2004 ◽  
Vol 40 (1) ◽  
pp. 11-19 ◽  
Author(s):  
Olivera Eric ◽  
Marina Jovanovic ◽  
Leposava Sidjanin ◽  
Dragan Rajnovic
Keyword(s):  
Mechanical Properties ◽  
Ductile Iron ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Bainitic Ferrite ◽  
Austempered Ductile Iron ◽  
X Ray Diffraction ◽  
Microstructure And Mechanical Properties ◽  
Cast Ductile Iron

Microstructure and mechanical properties of Cu, Ni and Mo alloyed cast ductile iron have been investigated after austempering. Samples were austenitised at 860oC for 1h and then austempered at 320oC and 400oC in the interval from 0,5 to 5h. The X-ray diffraction technique and the light microscopy were utilized to investigate the bainitic transformation, while tensile and impact tests were performed for characterization of mechanical properties. By austempering at 320oC in the range between 2 and 5h, a microstructure typical for austempered ductile iron was produced, i.e. a mixture of free bainitic ferrite and highly carbon enriched retained austenite. The characteristic of the whole range of austempering at 400oC is the appearance of martensitic structure. The maximum impact energy (133 J) coincides with the maximum value of volume fraction of retained austenite that was obtained after 2,5h of austempering at 320oC. The appearance of martensite during austempering at 400oC is the main cause for much lower tensile properties than at 320oC.

scholarly journals Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

Metals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 2055
Author(s):  
Mattia Franceschi ◽  
Alvise Miotti Bettanini ◽  
Luca Pezzato ◽  
Manuele Dabalà ◽  
Pascal J. Jacques
Keyword(s):  
Mechanical Properties ◽  
Silicon Carbide ◽  
Mechanical Response ◽  
Bainitic Ferrite ◽  
Single Step ◽  
Isothermal Treatment ◽  
Bainitic Steel ◽  
Microstructure And Mechanical Properties ◽  
Single Temperature ◽  
High Silicon

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350 °C and 370 °C, respectively), and three kinds of two-step routes (370 °C + 300 °C, 370 °C + 250 °C, and 350 °C + 250 °C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature.

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 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.

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