Austenite Reverse Transformation Research Articles

Impact Toughness Dependent on Annealing Temperatures in 0.16C-6.5Mn Forged Steel for Flywheel Rotors

For the application of forged medium-Mn steels on flywheel rotors, the effect of annealing temperatures from 300 °C to 650 °C on the impact toughness of 0.16C-6.5Mn forged steel was investigated to demonstrate the microstructural characteristics and austenite reverse transformation determining the impact toughness. The results obtained through standard Charpy V-notch impact tests at ambient temperature show that the impact absorbed energy holds at lower than 10 J almost constantly at annealing temperatures of 300 °C to 500 °C, and a representative intergranular fracture is presented. At an annealing temperature of 600 °C, the impact absorbed energy increases to 147 J, with the ductile fracture characteristics showing plenty of fine dimples, and the high impact toughness is attributed to the high volume fraction above 30% and the moderate stability of reverted austenite. Subsequently, the annealing temperature rises higher than 600 °C, the impact absorbed energy decreases, and the fracture morphology shows brittleness characterized by more flat facets of intergranular fractures and small quasi-cleavage facets, presumably corresponding to the insufficient transformation and twinning-induced plasticity effect due to weakening the Mn partitioning from quenched martensite to reverted austenite, which results in lower austenitic stability. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the 0.16C-6.5Mn forged steel annealed at 600 °C, which holds the highest impact absorbed energy, and is explored for the possibility of flywheel rotor application in a service environment. The DBTT reaches −21 °C, obtained through the Boltzmann function, and the impact absorbed energy is approximately 72 J.

Effects of soaking temperature on the microstructure and mechanical properties of a medium Mn steel for hot/warm forming

The increasing demands for better crashworthiness of vehicles require structural components with excellent energy absorption performance and anti-intrusion resistance. Here, we explored the potential of using a medium Mn steel with a 10 wt.% Mn content for hot/warm forming with the aim to produce automotive components with improved strength–ductility combination. The soaking temperature was varied from 670 to 870 °C for tuning the microstructure and the resultant mechanical properties. It is found that a duplex microstructure consisting of ferrite with a high dislocation density and retained austenite with a volume fraction of 53.5% is formed at 720 °C, leading to an exceptional strength–ductility combination (yield strength: 1016 MPa; ultimate tensile strength: 1407 MPa; total elongation: 32.7%). The energy absorbed before fracture is 39.8 GPa·%, which is four times more than that of the widely-used 22MnB5 boron steel. On basis of the experimental results, the effects of soaking temperature on austenite reverse transformation, ferrite recrystallization and martensitic transformation are discussed in depth to provide guidance for optimizing the strength–ductility combination of medium Mn steels processed by hot/warm forming.

A Novel Process for Hot‐Galvanized Quenching and Partitioning Steel with Excellent Yield–Ductility Synergy by Prior Treatment

The present work is aimed at a process to obtain a hot‐galvanized quenching and partitioning (Q&P) steel with excellent yield–ductility synergy by prior treatment. For this purpose, the cold‐rolled sheets with ferrite–pearlite structure are applied to “quenching (Q),” “quenching and tempering (Q&T),” “quenching and austempering (QAT),” and “quenching and austempering followed by tempering (QAT&T)” before the continuous galvanizing line (CGL)‐compatible Q&P treatment. Compared with Q&P sample, Q + Q&P, Q&T + Q&P, QAT + Q&P, and QAT&T + Q&P samples can obtain more homogeneous filmy/lath retained austenite (RA) with an increased C concentration, which can provide the transformation‐induced plasticity (TRIP) effect and improve elongation significantly. The maximum austenite reverse transformation rates are obtained in Q&T and QAT&T samples because of the increased nucleation sites, such as grain boundaries and cementite particles. QAT&T + Q&P sample has the longest plateau region before break and obtains the attractive mechanical properties with a yield strength of 694 MPa, tensile strength of 927 MPa, total elongation of 31.3%, and product of strength and elongation above 29 GPa%.

An in situ synchrotron X-ray study of reverse austenitic transformation in a metastable FeMnCo alloy

This study concerns reverse austenitic transformation of plastic strain-induced hexagonal close-packed martensite. With the aid of in situ synchrotron X-ray diffractometry, the kinetic features of the transformation and the defect content evolution in a metastable (Fe60Mn40)85Co15 alloy are quantitatively examined using 5, 20, and 100 °C/min heating rates. It is found that the reverse austenitic transformation can be activated below 200 °C and completes within a short time scale. Through a Kissinger-style kinetic analysis, the activation energy of the reverse austenitic transformation is determined as 171.38 kJ/mol, confirming its displacive nature. Although exponential attenuation is observed in both stacking fault probability and dislocation density upon the initiation of the transformation, the resulting microstructure (single-phase face-centered cubic structure) remains highly defected, exhibiting high Vickers hardness, but still preserving somewhat strain hardenability. Atomistic mechanisms for the reverse austenitic transformation are further conceived according to the crystallographic theory of martensitic transformation.Graphical abstract

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

To meet the requirement of fuel efficiency and CO2 emission, tremendous efforts have been made in the last decade to improve the mechanical properties of medium Mn steels. In article 2200257, Guo, Hu, and Luo focus on the mechanism of the reverse austenitic transformation kinetics and several processing strategies that utilize these mechanisms for improving mechanical performance.

Influence of Ni and Process Parameters in Medium Mn Steels Heat Treated by High Partitioning Temperature Q&P Cycles

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

In this contribution, the nucleation and growth mechanism of austenite during the intercritical annealing (IA) of medium Mn steels (MMSs) are reviewed. The nucleation of austenite can be enhanced due to the introduction of (sub)grain boundaries, dislocations, and carbides before austenitization. Although martensite may transform to austenite in a displacive way during flash heating, austenitic nuclei usually grow with the diffusion and partitioning of solute C/Mn atoms. The latter, however, is still difficult to be modeled because the combined influence of interface dissipation energy, defects, and carbide particles is not well understood and quantified. Next, four recent emerging processing technologies, including prior deformation for generating dislocations, additional tempering for the precipitation of carbides, additional annealing before IA, and flash heating, are also analyzed according to the above‐stated nucleation and growth mechanism. Finally, it is proposed that the targeted austenite grains for improving the mechanical properties of MMS shall be produced by enhancing both nucleation sites and growth kinetics of austenite via the deliberate design of processing routes.

Microstructures and mechanical properties of three medium-Mn steels processed via quenching and partitioning as well as austenite reversion heat treatments

In order to establish the processing conditions to achieve the target of tensile strength of ∼1500 MPa with minimum retained austenite (RA) fraction of 10% in hot-rolled 3–4%Mn medium Mn steels, three hot-deformed steels with the base composition of Fe-0.3C–1Si-0.5Al (concentrations in wt.%) and variable contents of Mn (3 or 4 wt%), Ni (1 or 2 wt%), Mo (0.2 or 0.4 wt%) and V (0 or 0.2 wt%) were processed using both the quenching and partitioning (QP) and austenite reverse transformation (ART) treatments. Physical simulation experiments were carried out using small cylinders in a thermomechanical simulator and the differences in the amounts, morphology and stability of RA were established by using X-ray diffraction and electron probe microanalysis, besides standard material characterization techniques. Both processes provided relatively high fractions of austenite; up to 26 vol% using the QP route and up to 49 vol% by the ART treatment under specific conditions. These amounts were comparable to those predicted for the conditions of thermodynamic equilibrium using the ThermoCalc software. The stability of RA for various conditions was estimated in terms of the average carbon contents of the RA. In QP process RA had mostly blocky-like appearance while in ART process the majority of RA was lath-like shaped. The hardness measurements revealed that the level of hardness in the QP structures was higher than that of the ART samples, possibly affected by RA fraction but more obviously as a result of martensite tempered at relatively low temperatures in the QP and at high temperatures in the ART treated structures. Preliminary tensile properties were measured on selected laboratory hot-rolled samples and results analyzed based on RA characteristics. The preliminary results indicated that the target is achievable in the laboratory scale through both the processing routes.

Characterization of a Medium Mn-Ni Steel Q&P Treated by a High Partitioning Temperature Cycle

In this work, a medium Mn-Ni steel was treated through Quenching and Partitioning (Q&P) with a partitioning temperature (PT) of 650 °C, which corresponded to the start of the austenite reverse transformation (ART) phenomenon. The influence of the quenching temperature (QT) and partitioning time (Pt) on austenite stabilization and mechanical properties was investigated. A strong influence of the quenching temperature was observed. Results were compared with those obtained after a Q&P treatment with 400 °C partitioning temperature. The Q&P cycle with quenching to room temperature and a high partitioning temperature produced a steel with a high retained austenite (RA) volume fraction and exceptional strength–ductility balance. The analysis of the mechanical stability of the retained austenite revealed a significant stress-induced transformation. Nevertheless, the austenite, which was stable at stresses above the yield stress, provided significant TRIP-assisted ductility. Bending, hole expansion and post-stamping properties were also evaluated for the most promising conditions.

Phase transition and defect relaxation behavior in a medium manganese steel

Phase transition and defect relaxation behavior of a Fe-7Mn-0.1C medium manganese steel have been systematically studied mainly by using internal friction (IF) technique. From temperature-dependent IF curves in both heating and cooling processes, two relaxation IF peaks (the P1 located at ~ 200 °C on heating and the P2 at ~ 550 °C on cooling) and two phase transition IF peaks (the Ptr1 located at ~ 670 °C on heating and the Ptr2 at ~ 300 °C on cooling) were observed. The P1 peak disappears when the cementite (θ) particles and/or retained austenite (γR) exist in the microstructure, and is associated with the Snoek-Kê-Köster relaxation (SKK relaxation). The P2 peak is suggested to be originated from grain boundary relaxation in austenite (γ). The formation mechanisms of the Ptr1 and the Ptr2 come from reverse austenitic transformation (RAT) and martensitic transformation (MT), respectively. In addition, the apparent asymmetry of the Ptr1, the variation of peak height, the difference between the Ptr1 and Ptr2 peaks, and the effect of IF studies on the mechanical properties are discussed in detail in the present work. These results provide a valuable reference for the design of high-performance medium Mn steels.

Influence of softening annealing on microstructural heredity and mechanical properties of medium-Mn steel

Softening annealing (SA) is often required for producing medium-Mn steels (MMS) as it lowers hardness so that they can be cold rolled to reduce thickness. The influences of different SA processes on the microstructural heredity during the processing route and the final tensile properties were studied. It was found that the SA process could either intensify or weaken the influence of the Mn segregation resulting from solidification on the subsequent microstructural evolution during the process, i.e., microstructural heredity. In the case when no SA was employed, both recrystallization and rapid growth of ferrite grains preceded the reverse austenitic transformation during the intercritical annealing (IA) in the Mn-lean regions, where very coarse ferrite grains were formed. This deteriorated ductility was due to the propagation of cracking along the boundary of the coarse-grained and fine-grained regions. In contrast, SA at a sufficiently high temperature could dissolve cementite, producing uniformly distributed austenite grains. They transformed to martensite during cold rolling but were reborn during IA. As a result, ultrafine austenite and ferrite grains were uniformly distributed, which improved ductility significantly. This study hints at a new approach to altering the microstructural heredity resulting from the heterogeneous Mn distribution in MMS.

Strengthening Mechanism and Carbide Precipitation Behavior of Nb-Mo Microalloy Medium Mn Steel.

This study investigates the strengthening mechanism and carbide precipitation behavior of medium Mn steel with Nb-Mo microalloy after cyclic quenching and austenite reverse transformation treatment. The results show that the Nb/Mo element not only precipitates (Nb,Mo)C in the grains, hindering the movement of dislocations and increases the strength, but also segregates at the austenite/ferrite grain boundary, thus delaying the transformation from austenite to ferrite. In addition, a large amount of nano-scale cementite is retained after cyclic quenching and austenite reverse transformation, which has a positive effect on the proportion of retained austenite in medium Mn steel. Moreover, the carbides with small size and low Mn content are dissolved, and the decomposed C and Mn content are beneficial to the nucleation of austenite during the intercritical annealing process at a temperature of 690 °C.

Ultra-high-strength multi-alloyed steel with enhanced cryogenic toughness using thermally stable retained austenite

In low-carbon steel, obtaining ultra-high-strength inevitably deteriorates ductility and cryogenic toughness. Therefore, in this study, we investigated the role of “lamellarization” on the fraction and distribution of retained austenite on a low-carbon multi-alloyed steel subjected to a novel quench-lamellarization-intercritical tempering (QLIT) treatment and elucidated its effects on mechanical properties and fracture mechanisms. The performance of the proposed method was compared with that of quench-intercritical tempering (QIT) treatment. The results showed that compared with QIT (800 °C + 620 °C) treatment, adding an L-step in QL680IT and QL710IT significantly increased the fraction of retained austenite and stimulated VC precipitates inside tempered martensite. Lamellarization temperature modified the alloying element enrichment and the first martensitic formation, which further influenced the nucleation and kinetics of reverse austenitic transformation during the subsequent IT process. Ni-, Mn-, and Cr-enriched retained austenite had high thermal stability after holding at −196 °C. Nearly all the retained austenite transformed to martensite under constant tensile and fast impact loads. The transformation-induced plasticity of the high fraction retained austenite significantly improved the strain hardening rate and delayed necking, increasing the elongation, and maintaining tensile strength of 1 GPa. Moreover, the transformation-induced toughness of retained austenite with homogeneous distribution increased the impact energy from 27 to 84 J at −196 °C. The filmy retained austenite at the tempered martensite packet and block boundaries delayed the crack initiation and arrested its propagation via crack deflections, crack-tip passivation, and crack closure, which improved the crack initiation and propagation energies and changed the impact fracture from cleavage facets to large dimples. In addition, the dispersion of nanoscale vanadium carbides in multi-step heat treatment effectively improved the strength without reducing the low-temperature toughness.

Influence of Mn and Ni on Austenite Stabilization during a High Temperature Q&P Treatment

The aim of this work was to study the influence of quenching and partitioning temperatures combined with various levels of Mn and Ni contents on the austenite stabilization along the quenching and partitioning (Q&P) cycle. Three steels with 2 wt.%, 4 wt.% and 6 wt.% manganese and one steel with 2 wt.% nickel content were investigated. Phase transformation temperatures and critical cooling rates were obtained experimentally using dilatometer for each alloy. Q&P cycles with different quenching and partitioning temperatures were also done in dilatometer, thus, allowing monitoring of the expansion/contraction during the whole Q&P cycle. Microstructure characterization was performed by means of a Scanning Electron Microscope and X-Ray Diffraction to measure retained austenite content. It was found that, strongly depending on the Q&P conditions, austenite stabilization or decomposition occurs during partitioning and final cooling. In case of high partitioning temperature cycles, austenite reverse transformation was observed. Certain cycles resulted in a very effective austenite stabilization and interesting microstructure.

Residual Ferrite Control of 9Cr ODS Steels by Tailoring Reverse Austenite Transformation

By tailoring the reverse austenite transformation behavior of 9Cr oxide dispersion strengthened (ODS) ferritic/martensitic steels, the residual ferrite in ODS steels can be controlled. The reverse austenite transformation behavior of ODS steels is closely related to the initial microstructure conditions prior to austenite transformation. For the spark plasma sintered steels, both the amount and size of residual ferrite decrease with increasing heating rate. Nevertheless, high heating rate will increase the amount and size of residual ferrite in annealed ODS steels. As an isothermal treatment is performed at temperatures above Ac1, lower isothermal temperature has a more evident effect on the ferrite distribution in spark plasma sintered steels than that in annealed ones.

Effect of Starting Microstructures on the Reverse Transformation Kinetics in Low-Carbon Steel

The effect of the initial starting microstructures on the austenite reverse transformation kinetics is thoroughly studied in low-carbon steel. The different initial starting microstructures including the ferrite + pearlite, bainite, and martensite are obtained through varied forward transformation. It is found that the bainite phase demonstrates highest reverse transformation rate while the ferrite + pearlite shows the lowest transformation rate. The above observation can be explained through the different grain size of the initial starting microstructures as the grain boundaries could act as the nucleation sites for austenite reverse transformation. The explanation is further substantiated based on the consideration of the reverse transformation kinetics from the martensite microstructure with different grain size.

Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength (UTS) up to 1365 MPa and total strain to failure (StF) to 15.5 % has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase (DP) steel, which was manufactured by two-stage annealing of cold rolled steel strip. The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differentiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure. Complex coherent orientation relationships between ferrite, reverse austenite, martensitic phase and vanadium carbide (VC) do exist, leading to a new martensitic transformation mechanism and resultant dual-phase microstructure. Besides, a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation, shows that, all the essential processing including recrystallization, reverse austenitic and martensitic transformation, in debt to the particular effects of VC, can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing, which actually changes the microstructure and, indirectly leads to higher strength and higher ductility. This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.

Effect of the Deformation Degree on the Microstructure Evolution of an Austenite Reverted Transformation-Annealed Medium Manganese Steel

An Fe-0.15C-1.2Si-5Mn-0.09Nb-0.08V-0.07Mo (wt.%) medium manganese steel that was subjected to a novel austenite reverted transformation (ART) annealing process. This paper is based on the conventional ART annealing process, and a deformation and ART annealing process is proposed. The influence of the deformation degree on the microstructure and grain size of the medium manganese steel was determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The results show that the deformation had a great effect on the microstructure evolution and grain size. The microstructure of the medium manganese steel after the deformation and ART annealing process was consistent with the theory of austenite reverse transformation, i.e., the martensite reverse transformation into austenite occurred during the deformation and ART annealing process. The final microstructure was a mixture of martensite and austenite. As the deformation degree increases, the martensite gradually refines, and carbides precipitate in the lath martensite. The retained austenite is gradually transformed from very large to small and is distributed between the martensite laths. The results show that when deformation occurs in the austenite region, a small deformation can obtain more retained austenite.

Austenite Reverse Transformation in a Q&P Route of Mn and Ni Added Steels

In this work, four low carbon steels with different contents of Mn and Ni were heat treated by quenching and partitioning (Q&P) cycles where high partitioning temperatures, in the range of 550 °C–650 °C, were applied. In order to elucidate the effect of applying these high partitioning temperatures with respect to more common Q&P cycles, the materials were also heat treated considering a partitioning temperature of 400 °C. The microstructure evolution during the Q&P cycles was studied by means of dilatometry tests. The microstructural characterization of the treated materials revealed that austenite retention strongly depended on the alloy content and partitioning conditions. It was shown that the occurrence of austenite reverse transformation (ART) in the partitioning stage in some of the alloys and conditions was a very effective mechanism to increase the austenite content in the final microstructure. However, the enhancement of tensile properties achieved by the application of high partitioning temperature cycles was not significant.

A Novel Cyclic-Quenching-ART for Stabilizing Austenite in Nb–Mo Micro-Alloyed Medium-Mn Steel

In the context of obtaining an excellent elongation and tensile-strength combination in the third generation of advanced high strength steel, we emphasized the practical significance of adjusting the retained austenite fraction and stability in medium-Mn steel to obtain better mechanical properties. A novel cyclic quenching and austenite reverse transformation (CQ-ART) was used to obtain a large retained austenite content in Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb (wt.%) Nb–Mo micro-alloyed medium-Mn steel. The results show that after twice cyclic quenching and ART, the alloy exhibited optimum comprehensive properties, characterized by an ultimate tensile strength of 838 MPa, a total elongation of 90.8%, a product of strength and elongation of 76.1 GPa%, and the volume fraction of austenite of approximately 62 vol.%. The stability of retained austenite was significantly improved with the increasing of the number of cyclic quenching. Moreover, the effects of CQ-ART on the microstructure evolution, mechanical properties, C/Mn partitioning behavior, and austenite stability were investigated. Further, the strengthening effect of microalloying elements Nb–Mo was also discussed.