scholarly journals Retained Austenite Destabilization during Tempering of Low-Temperature Bainite

2020 ◽  
Vol 10 (24) ◽  
pp. 8901
Author(s):  
Victor Ruiz-Jimenez ◽  
Matthias Kuntz ◽  
Thomas Sourmail ◽  
Francisca G. Caballero ◽  
Jose A. Jimenez ◽  
...  
Keyword(s):  
Retained Austenite ◽  
Bainitic Ferrite ◽  
Microstructural Characterization ◽  
Thermal Treatments ◽  
Low Carbon ◽  
X Ray Diffraction ◽  
Microscope Examination ◽  
Volume Changes ◽  
Fresh Martensite ◽  
Thermal Stability Of

The thermal stability of nanostructured microstructures consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite has been studied in two steels containing 0.6 C (wt %) by tempering cycles of 1 h at temperatures ranging from 450 to 650 °C. Volume changes due to microstructural transformations during thermal treatments were measured by high-resolution dilatometry. The correlation of these results with the detailed microstructural characterization performed by X-ray diffraction and scanning electron microscope examination showed a sequence of different decomposition events beginning with the precipitation of very fine cementite particles. This precipitation, which starts in the austenite thin films and then continues in retained austenite blocks, decreases the carbon content in this phase so that fresh martensite can form from the low-carbon austenite on cooling to room temperature. In a subsequent tempering stage, the remaining austenite decomposes into ferrite and cementite, and due to carbide precipitation, the bainitic ferrite loses its tetragonality, its dislocation density is reduced, and the bainitic laths coarsen.

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.

In situ x-ray observation of bainitic transformation of austempered silicon alloyed steel

2009 ◽  
Vol 24 (4) ◽  
pp. 1559-1566 ◽  
Author(s):  
Xiang Chen ◽  
Esa Vuorinen
Keyword(s):  
Volume Fraction ◽  
Early Stage ◽  
Bainitic Ferrite ◽  
Bainitic Transformation ◽  
Low Carbon ◽  
X Ray Diffraction ◽  
Heterogeneous Distribution ◽  
X Ray ◽  
Austempering Temperature

The in situ x-ray diffraction observations of the bainitic transformation were conducted by using the high-temperature x-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The d{110} value of bainitic ferrite decreases with increasing austempering temperature, which is related to the decrease of carbon concentration in bainitic ferrite. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation at any austempering temperatures. This asymmetry diffraction peak after the formation of bainitic ferrite could be attributed to a heterogeneous distribution of carbon in different regions of austenite and show that two types of austenite with different carbon contents, low-carbon austenite (γLC) and the high-carbon austenite (γHC), exist during the transformation. The microstructure after cooling down to room temperature is presented to show the effectiveness of the x-ray diffraction analysis.

scholarly journals Thermal stability of retained austenite in bainitic steel: an  in situ study

2011 ◽  
Vol 467 (2135) ◽  
pp. 3141-3156 ◽  
Author(s):  
A. Saha Podder ◽  
I. Lonardelli ◽  
A. Molinari ◽  
H. K. D. H. Bhadeshia
Keyword(s):  
Martensitic Transformation ◽  
Retained Austenite ◽  
Elevated Temperatures ◽  
Bainitic Ferrite ◽  
Ambient Conditions ◽  
Two Phase ◽  
Tempering Temperature ◽  
Thermal Stability Of ◽  
Lower Carbon

The tempering of two-phase mixtures of bainitic ferrite and carbon-enriched retained austenite has been investigated in an effort to separate the reactions that occur at elevated temperatures from any transformation during cooling to ambient conditions. It is demonstrated using synchrotron X-radiation measurements that the residue of austenite left at the tempering temperature partly decomposes by martensitic transformation when the sample is cooled. It is well established in the published literature that films of retained austenite are better able to resist stress or strain-induced martensitic transformation than any coarser particles of austenite. In contrast, the coarser austenite is more resistant to the precipitation of cementite during tempering than the film form because of its lower carbon concentration.

scholarly journals Thermal Stability of Nanocrystalline Structure In X37CrMoV5-l Steel

2015 ◽  
Vol 60 (1) ◽  
pp. 511-516 ◽  
Author(s):  
E. Skołek ◽  
S. Marciniak ◽  
W.A. Świątnicki
Keyword(s):  
Thermal Stability ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Bainitic Ferrite ◽  
Nanocrystalline Structure ◽  
Thin Layers ◽  
Isothermal Quenching ◽  
Initial Stage ◽  
Increasing Temperature ◽  
Thermal Stability Of

AbstractThe aim of the study was to investigate the thermal stability of the nanostructure produced in X37CrMoV5-1 tool steel by austempering heat treatment consisted of austenitization and isothermal quenching at the range of the bainitic transformation. The nanostructure was composed of bainitic ferrite plates of nanometric thickness separated by thin layers of retained austenite. It was revealed, that the annealing at the temperature higher than temperature of austempering led to formation of cementite precipitations. At the initial stage of annealing cementite precipitations occurred in the interfaces between ferritic bainite and austenite. With increasing temperature of annealing, the volume fraction and size of cementite precipitations also increased. Simultaneously fine spherical Fe7C3carbides appeared. At the highest annealing temperature the large, spherical Fe7C3carbides as well as cementite precipitates inside the ferrite grains were observed. Moreover the volume fraction of bainitic ferrite and of freshly formed martensite increased in steel as a result of retained austenite transformation during cooling down to room temperature.

Determination of low levels of retained austenite in low-carbon high-manganese steel using X-ray diffraction

2015 ◽  
Vol 628 ◽  
pp. 110-115 ◽  
Author(s):  
Helder Carvalho Ferreira ◽  
Francisco Jose Martins Boratto ◽  
Vicente Tadeu Lopes Buono
Keyword(s):  
Retained Austenite ◽  
Manganese Steel ◽  
Low Carbon ◽  
X Ray Diffraction ◽  
High Manganese ◽  
High Manganese Steel ◽  
X Ray ◽  
Low Levels

Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling

2005 ◽  
Vol 53 (20) ◽  
pp. 5439-5447 ◽  
Author(s):  
N VANDIJK ◽  
A BUTT ◽  
L ZHAO ◽  
J SIETSMA ◽  
S OFFERMAN ◽  
...  
Keyword(s):  
Thermal Stability ◽  
Retained Austenite ◽  
X Ray Diffraction ◽  
X Ray ◽  
Trip Steels ◽  
Thermal Stability Of

Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

2021 ◽  
Vol 1016 ◽  
pp. 429-434
Author(s):  
Eman El-Shenawy ◽  
Hoda Refaiy ◽  
Hoda Nasr El-Din
Keyword(s):  
Mechanical Properties ◽  
Thermal Stability ◽  
Retained Austenite ◽  
Bainitic Ferrite ◽  
Residual Austenite ◽  
Ferrite Matrix ◽  
Sheet Steels ◽  
Coiling Process ◽  
Hot Rolled ◽  
Thermal Stability Of

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Evolution of Microstructures in a Low Carbon Bainitic Steel during Reheating

2005 ◽  
Vol 475-479 ◽  
pp. 121-124 ◽  
Author(s):  
Hui Bin Wu ◽  
Shan Wu Yang ◽  
S.Q. Yuan ◽  
Cheng Jia Shang ◽  
Xue Min Wang ◽  
...  
Keyword(s):  
Thermal Stability ◽  
Bainitic Ferrite ◽  
Hardness Measurement ◽  
Granular Bainite ◽  
Low Carbon ◽  
Time Curve ◽  
History Of ◽  
Two Peaks ◽  
Thermal Stability Of ◽  
Synthetic Microstructures

Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibited synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650°C or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones, accompanied by obvious change of hardness. The rate of microstructures evolution was closely related to relaxation time of deformed austenite. The sample relaxed for 60s displayed the highest thermal stability, while microstructure evolution was quickest in the sample relaxed for 1000s even though it was softest before reheating. By hardness measurement, it was found that softening was not only process occurring during reheating, in which hardness fluctuated with time. There were two peaks in hardness-time curve of each sample having undergone relaxation, while single peak occurred in the curve of the sample not being relaxed. These results indicate that thermal stability of microstructures is determined by their history of formation.

Application of Advanced Experimental Techniques for the Microstructural Characterization of Nanobainitic Steels

2011 ◽  
Vol 172-174 ◽  
pp. 1249-1254 ◽  
Author(s):  
Ilana Timokhina ◽  
Hossein Beladi ◽  
Xiang Yuan Xiong ◽  
Yoshitaka Adachi ◽  
Peter D. Hodgson
Keyword(s):  
Dislocation Density ◽  
Carbon Content ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Accurate Determination ◽  
Bainitic Ferrite ◽  
Microstructural Characterization ◽  
Atom Probe ◽  
Isothermal Heat Treatment ◽  
Local Composition

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 350°C bainitic transformation temperature for 1 day to form fully bainitic structure with nano-layers of bainitic ferrite and retained austenite, while a 0.26C-1.96Si-2Mn-0.31Mo (wt%) steel was subjected to a successive isothermal heat treatment at 700°C for 300 min followed by 350°C for 120 min to form a hybrid microstructure consisting of ductile ferrite and fine scale bainite. The dislocation density and morphology of bainitic ferrite, and retained austenite characteristics such as size, and volume fraction were studied using Transmission Electron Microscopy. It was found that bainitic ferrite has high dislocation density for both steels. The retained austenite characteristics and bainite morphology were affected by composition of steels. Atom Probe Tomography (APT) has the high spatial resolution required for accurate determination of the carbon content of the bainitic ferrite and retained austenite, the solute distribution between these phases and calculation of the local composition of fine clusters and particles that allows to provide detailed insight into the bainite transformation of the steels. The carbon content of bainitic ferrite in both steels was found to be higher compared to the para-equilibrium level of carbon in ferrite. APT also revealed the presence of fine C-rich clusters and Fe-C carbides in bainitic ferrite of both steels.

scholarly journals Thermal Stability of Shear-Induced Precursors and Their Effect on the Crystalline Structure of β-Nucleated Isotactic Polypropylene

2019 ◽  
Vol 2019 ◽  
pp. 1-7
Author(s):  
Yefei Tian
Keyword(s):  
Thermal Stability ◽  
Isotactic Polypropylene ◽  
Early Stage ◽  
Thermal Treatments ◽  
Crystalline Modification ◽  
X Ray Diffraction ◽  
Nucleation Agent ◽  
X Ray ◽  
Thermal Stability Of ◽  
Crystal Content

The thermal stability and lifetime of shear-induced precursors under various annealing temperatures, as well as the influence of their relaxation on the crystalline modification in β-nucleated isotactic polypropylene (iPP), are investigated using an ARES rheometer. The wide-angle X-ray diffraction results show that the β-crystal content of sheared β-nucleated iPP samples gradually increases with thermal treatments. The relaxation of shear-induced precursors during annealing which caused the decrease of shear nuclei may restrain the counteraction effect between the shear flow and β-nucleation agent as well as result in the increase of β-crystal content. At the early stage of relaxation, the relaxation degree is closely related to the increase of β-crystals, for which the deeper relaxed shear-induced precursors result in the more restoration of β-crystals. However, when the relaxation degree exceeded a certain limitation, where the β-crystals reached the maximum, the relaxation of shear-induced precursors will no longer influence the crystal structure of β-nucleated iPP.

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