Stability Of Retained Austenite Research Articles

Insights into the role of retained austenite stability in TRIP-aided steel: ductilizing and toughening

Insights into the role of retained austenite stability in TRIP-aided steel: ductilizing and toughening

Synchrotron X-ray radiation study of retained austenite stability and the effect on fatigue behavior of bainitic railway steels

The effect of retained austenite stability on the fatigue performance was studied in bainitic rail steel subjected to different heat treatments, including normalizing at 900 °C and normalizing at 900 °C combined with tempering at 320 °C. The normalized steel had yield strength of 851 MPa, tensile strength of 1131 MPa, uniform elongation 7.0 %, and total elongation 17.5 %. Tempering the steel at 320 °C resulted in similar strength and plasticity compared to the normalized condition. However, the fatigue performance of steel was improved significantly after tempering. The ultimate fatigue stress (endurance limit) after 1 × 107 cycles was 671 MPa, which was 38 MPa greater than the normalized steel. Microstructural analysis revealed similarities between normalized and tempered matrices at crystallographic interfaces and grain boundaries. Synchrotron X-ray radiation indicated that the initial retained austenite content was ∼15.5 % in both normalized and tempered conditions. However, at tensile stress of 800 MPa, the amount of retained austenite transformed in the normalized steel was ∼4 %, which was twice that of the tempered steel. The enhancement of retained austenite stability was an important aspect in improving the fatigue performance. The analysis of interphase spacing showed that the strain in the normalized matrix synchronized with the retained austenite, and the stress was concentrated in the retained austenite, which promoted transformation of retained austenite. The matrix of tempered steel was strained first, which prevented stress concentration in the retained austenite and rendered the retained austenite stable.

Deconstructing the Retained Austenite Stability: In Situ Observations on the Austenite Stability in One- and Two-Phase Bulk Microstructures During Uniaxial Tensile Tests

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.

Low-Temperature Impact Fracture Behavior of Medium Manganese Steel with Bcc-fcc Duplex Microstructures

Impact fracture behavior at low temperatures was investigated in medium manganese steel with bcc-fcc duplex microstructures. The impact energy was above 150 J (−80~20 °C) and the fractography showed dimples for inter-critical annealing at 630 °C (QHA) because of the high retained austenite stability and low martensite dislocation density. The impact energy was from 180 J (20 °C) to 60 J (−80 °C) and the fractography was intergranular for inter-critical annealing at 610 °C (QLA) because of the low stability of RA and carbides precipitated at the prior austenite grain boundaries. The impact energy was below 60 J (−80~20 °C) and the fractography showed cleavage for direct quenching (DQ) because of the high dislocation density of martensite.

Deconstructing the Retained Austenite Stability: A Comparative Study of Two-Phase and Bulk Microstructures

The stability of retained austenite is a key factor in the design of advanced high-strength steels that exhibit excellent mechanical performance, including high strength and high ductility/toughness. However, the contribution of certain microstructural factors, such as the morphology and size of the austenite, and the surrounding matrix, to this stability is still not fully understood, partly due to the inherent difficulties in separating these factors in multiphase microstructures. Therefore, this study uniquely compared the stabilities of retained austenite in two-phase microstructures with bulk austenitic microstructures of the same composition, across four medium-Mn steels upon quenching. By fixing the austenite chemical composition, we could exclude the influence of composition and examine the influence of other factors, such as morphology, size, and the surrounding matrix, on the stability of austenite. Our experimental results showed that retained austenite in the two-phase microstructures was more stable than the bulk austenitic microstructures of the same composition, regardless of morphology and size. Analysis using thermodynamic calculations revealed that neither the steel composition nor the size alone could explain the high stability of the retained austenite in the two-phase microstructures. Instead, we propose that microstructural factors, including size, morphology, and matrix, have a significant influence on the metastable austenite in two-phase microstructures. While these factors have been studied previously, our study introduces a novel perspective by excluding the influence of the austenite composition, thus contributing to a more comprehensive understanding of retained austenite stability. These findings may guide the design of advanced steels and highlight the importance of considering the contribution of these microstructural factors in tailoring the stability of metastable austenite.

Microstructure and mechanical properties of medium manganese steel in intercritical annealing

A new type of cold-rolled medium manganese steel was investigated. The test steel was treated by Q&P process. With the increase in annealing temperature, the grain size gradually increases, the carbides dissolve progressively and the fine lath grains transform into massive or equiaxed grains. Both the volume fraction and the stability of the retained austenite are continuously decreasing, which together affect the change of mechanical properties. At the intercritical annealing temperature of 680°C, the specimens have higher retained austenite content and transformation rate. At the same time, due to the discontinuous TRIP effect caused by the retained austenite stability gradient inside the specimen, it exhibits excellent comprehensive mechanical properties, and the product of strength and elongation is 43.5GPa·%.

Stability of retained austenite and work hardening behavior in ultra-fine medium carbon bainitic steel

The effect of retained austenite stability on plastic deformation was investigated by stepwise tensile test of medium carbon steel material at two different bainite isothermal temperatures below or above the martensite start (Ms) point. The results show that higher retained austenite transformation leads to higher elongation. Compared with the above-Ms bainitic steel (320 °C-12 h), the below-Ms bainitic steel of retained austenite (240 °C-48 h) has higher mechanical stability. This is because the transformation induced plasticity effect (i.e., the TRIP effect) of retained austenite plays a significant role in the process of tensile strain. The block retained austenite distributed between the bainite sheaf deforms rapidly in the initial stage, and the film retained austenite between the bainitic-ferrite lath transformation is slow in the later stage of tensile deformation. This is attributed to the low stability of block retained austenite, which easily transforms into hard and brittle martensite under external stress, while the film retained austenite has high stability and cannot easily undergo phase transformation under the action of external force. Meanwhile, the work hardening ability of the below-Ms bainitic steel is higher than that of the above-Ms bainitic steel at the initial stage of tension, but the opposite occurs in the later stage of tension.

Quasi in-situ observations of microstructure evolution during low cycle fatigue in carbide-free bainitic steel

Microstructure evolution during fatigue is crucial to the initiation and propagation of fatigue cracks in carbide-free bainitic steels. Utilizing quasi in-situ electron back-scattered diffraction method, the morphology of retained austenite and the accumulated plastic deformation within the bainitic laths at different fatigue stages were characterized. The observations disclosed that the retained austenite in various locations has different mechanical stability, and the dislocations in the bainitic laths will evolve to low-energy substructures continually throughout the fatigue process. Furthermore, combined with hysteresis curves analysis, a microstructural evolution model that comprehensively considered the retained austenite stability and the evolution of dislocations was proposed.

Effect of Intercritical Deformation on Microstructure and Mechanical Properties of Quenching and Partitioning Low Carbon Multiphase High-Strength Steel

Low carbon multiphase high strength steel is widely used in the automobile industry. In this work, the effect of intercritical deformation on the partitioning of alloying elements and the evolution of microstructure, as well as the effect of retained austenite stability on mechanical properties, were studied in a low carbon steel. The results demonstrate that the intercritical deformation enhances the C, Mn partition from ferrite to austenite during annealing at 770 ℃, and the volume fraction of the retained austenite increased from 8.8% to 12.3%. The DIQ&PB sample shows good balance between strength (1226.5 MPa) and ductility (24.4%), whose product of strength and elongation reached a larger value of 29926.6 MPa·% due to the intercritical deformation. This research provides theoretical guidance for the process design of automobile high-strength steels, considering the integration between rolling and heat cycles.

An alternative quenching and partitioning (Q&P) process via spheroidization

A hot-rolled Fe-CMnSiMo alloy was subjected to quenching followed by a spheroidization treatment, which resulted in a microstructure consisting of Mn enriched cementite particles dispersed in the ferrite matrix. The as hot-rolled and spheroidized material was subjected to short-duration intercritical annealing (IA) treatment followed by the full quenching and Q&P treatments. The IA treatment was such that the austenite formed as a result of dissolution of cementite particles retains its higher Mn content. The influence of this chemical inhomogeneity on martensite transformation and retained austenite stability was investigated using a dilatometer and interrupted compression experiments, respectively. The results were compared with the microstructure obtained in the absence of chemical heterogeneity by subjecting the hot-rolled material to similar heat treatment directly without the quenching and spheroidization treatment. After the IA treatment, the Ms temperature of the spheroidized material was 36 °C higher than the Ms temperature of the as hot-rolled material. Nevertheless, a higher fraction of retained austenite was stabilized in the spheroidized material after the IA treatment followed by full quenching. The SEM, TEM and DICTRA modelling results showed that cementite particles dissolve during the IA treatment of the spheroidized material and the product austenite retains its higher Mn content. Interrupted compression test results show that the retained austenite in the spheroidized material has greater stability because of its higher Mn content and small size.

Significance of Retained Austenite Stability on Yield Point Elongation Phenomenon in a Hot‐Rolled and Intercritically Annealed Medium‐Mn Steel

The effect of retained austenite stability on yield point elongation (YPE) phenomenon in a hot‐rolled and intercritically annealed medium‐Mn steel is elucidated here. Furthermore, the influencing factors of austenite reversion are studied. The results indicate that the shear phase transition is suppressed by the strain hardening of prior austenite grain introduced by low‐temperature rolling, resulting in refined martensite laths with high dislocation density. The nucleation and growth of intercritical austenite are accelerated by high‐density dislocation substructures. Different characteristics of retained austenite (RA) are obtained through tailoring the rolling temperature and austenite reversion transformation (ART) process. YPE phenomenon is a result of the equilibrium between work hardening (via martensitic transformation and dislocation multiplication) and softening (by stress relaxation and dislocation glide). In the initial deformation stage, large amounts of geometrically necessary dislocations (GND) are “stored” in the RA with higher stability, and the local stress can be effectively relaxed and transferred, leading to the occurrence of YPE. The optimum properties are obtained in R760‐A630‐0.3h samples with a combination of yield strength of 918 MPa, tensile strength of 1019 MPa, and total elongation of 25.3%, which is mainly attributed to the finer ferrite matrix and sustained transformation‐induced plasticity effect.

Improving mechanical properties and corrosion resistance of 0.5 wt.% C TRIP steel by adjusting retained austenite stability and microstructural constituents

Here the effect of retained austenite stability and martensite-austenite island/fresh martensite on mechanical properties, fracture behavior, and corrosion resistance of 0.5 wt.% C TRIP steel was studied. The volume fraction of retained austenite is distinctly high at ∼ 30% in 0.5 wt.% C TRIP steel compared to the conventional TRIP steels (10–20%) while the morphologies of retained austenite are mainly film-like, mixed film-like and small blocky (0.3–0.7 μm), and large blocky (0.7–1.5 μm), respectively, in the steels after bainitic holding at 360 °C, 400 °C and 450 °C. In addition, the volume fraction of bainite decreases from 53 vol.% to 44 vol.% and 32 vol.%, and that of martensite-austenite island/fresh martensite content increases from 2 vol.% to 9 vol.% and 21 vol.% with increasing temperatures. The two-stage mechanical stability of retained austenite with large volume fraction and mixed film-like and blocky morphologies ensures stable and continuous TRIP effect. As a result, after bainitic holding at 400 °C for 30 min, the superiorly comprehensive mechanical properties are obtained for the steel, showing ultrahigh tensile strength of 1220 MPa and retaining good ductility of 44%. Moreover, the effects of multiphase on strength and ductility were discussed and the presences of martensite-austenite island/fresh martensite and blocky retained austenite lead to premature crack initiation. Crack initiation and propagation mainly happen along the interface between deformation-induced martensite and the ferritic matrix and that between M-A island/FM and the ferritic matrix. The decreasing carbon content in RA and the occurrence as well as increase of M-A island/FM can deteriorate corrosion resistance of 0.5 wt.% C TRIP steels.

Effect of retained austenite on the microstructure and mechanical properties of cold-rolled medium-manganese Q&P steel

ABSTRACT The effect of retained austenite stability on microstructure evolution and mechanical properties of a new cold-rolled medium-manganese steel (0.12C, 7.69Mn, 1.45Si, 2.76Al wt-%) under intercritical annealing Q&P process at different partitioning temperatures was investigated. The evolution of retained austenite was characterized and statistically analysed using SEM, TEM, EBSD, and XRD techniques. The results show that the stability of retained austenite depends to a large extent on the applied heat treatment and is mainly attributed to the retained austenite content and morphology, whose carbon content is not the main influence. More retained austenite content and the formation of heterogeneous austenite structures (granular, massive, lamellar) are observed at a partitioning temperature of 150°C. The retained austenite of different morphologies has different stabilities and can sustain the TRIP effect at different strain stages, obtaining the best comprehensive mechanical properties with 34.5% elongation and the product of tensile strength and elongation is up to 43.5 GPa·%.

Effect of tempering on the stability of retained austenite in carbide-free bainitic steel

In this study, a C–Mn–Si alloyed bainitic steel was treated by bainitic austempering (BAT) process to form a multiphase of bainitic ferrite, filmy retained austenite (RA), and blocky martensite/austenite. Tempering was employed to optimize the microstructure and mechanical response of steels after BAT treatment. Microstructural alterations because of tempering that affects the mechanical stability of RA was discussed via X-ray diffractometer, high resolution transmission electron microscopy, and auger electron spectroscopy. Results showed that an excellent combination of strength and ductility was achieved after tempering (ultimate tensile strength: 1489 MPa; yield strength: 1014 MPa; total elongation: 33.2%), which was attributed to improved overall stability of RA and high fracture damage resistance. Interestingly, compared to normal BAT steel, the transformation fraction and rate of RA exhibit three stages with increase of strain in tempered BAT steel, leading to abnormally high stability of RA at strain of 2%–10%. It is suggested that the nano-sized η carbides precipitated after tempering could promote the dislocation multiplication in BCC matrix (i.e., bainitic ferrite and martensite) and obstruct the movement of dislocation from BCC matrix to RA, which results in a ductile but strong tempered matrix that provides a “shielding effect” on RA and improves RA's stability.

Relationship between Microstructure and Properties of 1380 MPa Grade Bainitic Rail Steel Treated by Online Bainite-Based Quenching and Partitioning Concept

According to the concept of the bainite-based quenching and partitioning (BQ&P) process, we designed the online heat treatment routes of bainitic rail steel for heavy haul railway. The new heat treatment process reduced the fraction and size of the blocky martensite/austenite (M/A) islands formed during the conventional air-cooling process. The M/A islands are coarse and undesirable for mechanical properties. A new kind of 1380 MPa grade bainitic rail steel with more uniform microstructure and better mechanical properties was produced by the online BQ&P process. We characterized the multiphase microstructures containing bainite, martensite, and retained austenite of 1380 MPa grade bainitic rail steels via optical microscope, scanning electron microscopy, transmission electron microscopy, and X-ray diffractometer. We investigated in-depth the relationship between the microstructure, retained austenite stability, and mechanical properties, particularly the resistance to wear and rolling contact fatigue, of the new 1380 MPa grade bainitic rail steels. Meanwhile, the conventional air-cooling bainitic rail steel was studied as a comparison.

Improving mechanical properties and retained-austenite stability of a medium carbon Q&P steel by adjusting phase ratio

In this study we explored mechanisms for improving the mechanical properties and retained-austenite (RA) stability of a Q&P steel by adjusting the phase ratio. With increasing annealing temperature, the yield strength of the steel increased and the ductility firstly increased and then decreased. These observations are closely related to the specific phase constituent and retained austenite stability produced by the Q&P process. The hard phase (tempered martensite, bainite, martensite-austenite island/secondary martensite) fraction increased with increasing annealing temperature, and the RA size and morphology changed from large, blocky to small, blocky and lath-like. At 800 °C, the phase ratio of RA, bainite, ferrite, and initial martensite is 2:1:2.2:2, which produced the superior combinations of yield strength (σy), tensile strength (σTS), and ductility (δ) (e.g. σy = 960 MPa, σTS = 1450 MPa, δ = 28%; or σy = 1320 MPa, σTS = 1540 MPa, δ = 14%). The superior mechanical properties are attributed to the coordinated deformation between hard phase and soft phase and TRIP effect.

Austenite Stability and Deformation Behavior in Medium Mn Steel Processed by Cyclic Quenching ART Heat Treatment.

This study investigated the austenite stability and deformation behavior of cyclic quenching-austenite reverse transformation processed Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb medium Mn steel. A number of findings were obtained. Most importantly, the extent of the TRIP effect was mainly determined by an appropriately retained austenite stability rather than its content. Simultaneously, chemical elements were the key factors affecting austenite stability, of which Mn had the greatest impact, while the difference of retained austenite grain size and Mn content resulted in different degrees of retained austenite stability. Additionally, there were still large amounts of strip and granular-retained austenite shown in the microstructure of the CQ3-ART sample after tensile fracture, revealing that the excessively stable, retained austenite inhibited the generation of an extensive TRIP effect.

Effect of martensite pre-quenching on bainite transformation kinetics, martensite/bainite duplex microstructures, mechanical properties and retained austenite stability of GCr15 bearing steel

In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

Retained austenite stability on rolling contact fatigue performance of 8620 case‐carburized steel

AbstractVarying levels of retained austenite (RA) were achieved through varying undercooling severity in uniformly treated case carburized 8620 steel. Specimens were characterized via XRD and EBSD techniques to determine RA volume fraction and material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to improvement in RCF lives. XRD measurements after RCF testing indicated that little RA decomposition had occurred during RCF. A continuum damage mechanics (CDM) finite element model (FEM) was then developed to investigate the effects of RA stability on RCF. The results obtained from the CDM FEM captured similar behavior observed in the experimental results. Utilizing the CDM FEM, a parametric study was undertaken to examine the effects of RA quantity, RA stability, and applied pressure on RCF performance. The study demonstrates that the energy requirements to transform the RA phase are critical to RCF performance.

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

Advanced 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.