Austenite Reversion Research Articles

Multiphase-field simulation of austenite reversion in medium-Mn steels

Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility. This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite. Such a unique microstructure is processed by intercritical annealing, where austenite reversion occurs in a fine martensitic matrix. In the present study, austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS® coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2. In particular, a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite. The simulated microstructural morphology and phase transformation kinetics (indicated by the amount of phase) concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction, respectively.

Corrosion resistance of UNS S41426 stainless steel in acid media and its relation to chemical partition and austenite fraction

The present work evaluates the impact of tempering treatment on austenite reversion and corrosion resistance of UNS S41426 in 0.5 M H2SO4 solution. Higher austenite fraction was related to higher tempering temperature, engendering lower hardness degrees. Despite a reduction in corrosion rates in immersion tests, Ni-rich austenite decreased corrosion resistance in more oxidizing environments. Cr-depleted regions created a secondary anodic peak in polarization tests. Chemical partitioning is presented to be the main feature that influences corrosion in supermartensitic stainless steel. Finally, tempering at 650 °C presented the best results as it had ten times less corrosion rate than as-quenched state.

Effect of aging temperature on the heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel for cryogenic applications

The evolution of heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel was investigated at different aging temperatures. As the aging temperature increases, more reversed austenite forms with the recovery of martensite matrix. When aging temperature is up to 560 °C, more lath-like reversed austenite coalesces together and leads to the formation of martensite and austenite dual-phase microstructure. Fine η-Ni3(Ti, Al) particles initially precipitate at 440 °C and grow up with increased aging temperature. Notably, it was found that η-precipitates can be encompassed in reversed austenite at 500 °C. Thermodynamic calculations further verify the austenite reversion, and nanoprecipitation can occur independently of each other though there is competition of Ni element. However, it leads to the coarsening and dissolution of η-precipitates as the aging temperature further increases. The desirable heterogeneous microstructure of the aged martensite matrix, soft reversed austenite and stable η-precipitates at 500 °C contributes to a high yield strength (~ 1 GPa, 25 °C; ~ 1.4 GPa, -196 °C) and a reasonable cryogenic impact toughness (~ 60 J, − 196 °C), which is mainly ascribed to the precipitation strengthening of η-precipitates, transformation-induced plasticity (TRIP) toughening effect from the lath-like reversed austenite and the increased misorientation toward high-angle grain boundary.

Modeling of Reversed Austenite Formation and Its Effect on Performance of Stainless Steel Components

Abstract The kinetics of reversed austenite formation in 301 stainless steel and its effect on the deformation of an automobile front bumper beam are studied by using modeling approaches at different length scales. The diffusion-controlled reversed austenite formation is studied by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, based on the experimental data. The model can be used to predict the volume fraction of reversed austenite in a temperature range of 650–750 °C. A three-dimensional elastoplastic phase-field model is used to study the diffusionless shear-type reversed austenite formation in 301 steel at 760 °C. The phase-field simulations show that reversion initiates at martensite lath boundaries and proceeds inwards of laths due to the high driving force at such high temperature. The effect of reversed austenite (RA) and martensite on the deformation of a bumper beam subjected to front and side impacts is studied by using finite element (FE) analysis. The FE simulations show that the presence of reversed austenite and martensite increased the critical speed at which the beam yielded and failed. RA fraction also affects the performance of the bumper beam.

Flash annealing yields a strong and ductile medium Mn steel with heterogeneous microstructure

Austenite reversion treatment (ART) is usually used to obtain retained austenite (RA) in medium Mn steel (MMS). In this contribution, we proposed a flash-annealing strategy to produce MMS, which allows us to efficiently obtain a large amount of RA in MMS via the “explosive” nucleation of austenite and the fast C partitioning. Furthermore, flash-annealing can also render a heterogeneous ferritic matrix consisting of recrystallized/non-recrystallized ferrite grains, leading to heterogeneous deformation-induced hardening. As compared to the conventional ART processed MMS, the unique heterogeneous microstructure in the flash-annealed MMS leads to a higher yield and ultimate tensile strength but a marginal ductility loss. Flash-annealing provides an alternative route to tailor microstructure and optimize the mechanical performance of steels.

Role of cementite and retained austenite on austenite reversion from martensite and bainite in Fe-2Mn-1.5Si-0.3C alloy

The effects of cementite (θ) and retained austenite (RA) on the kinetics and microstructure evolution in austenite (γ) reversion of an Fe-2Mn-1.5Si-0.3C (mass%) alloy have been studied by investigating the reversion from as-quenched and tempered martensite, and bainite. Pre-tempering of martensite or prolonged austempering, both resulting in θ precipitation, enhanced the formation of globular γ at high reversion temperature, while the presence of RA in carbide-free bainite structure obtained by short time austempering suppressed its formation. High-temperature pre-tempering or prolonged austempering retarded the reversion kinetics, while low-temperature pre-tempering or the presence of RA have negligible influences on the kinetics. Nucleation of γ at θ / ferrite interface results in the formation of globular γ. Theoretical analysis reveals that coarser θ particle has a higher nucleation potency for globular γ, and thus coarsening of θ particles by pre-tempering or austempering promotes its formation. On the other hand, the precipitation of θ particles before γ reversion is suppressed when a large amount of RA is present, thus globular γ formation is suppressed. It was concluded that the retardation on reversion kinetics by high-temperature pre-tempering or austempering is mainly owing to the suppression on the growth of reverted γ because of enhanced Mn enrichment into θ and coarsened θ particle.

Effect of Combined Tempering and Aging in the Austenite Reversion, Precipitation, and Tensile Properties of an Additively Manufactured Maraging 300 Steel

Maraging 300 is an ultrahigh strength steel with significant alloy addition, resulting in a martensitic matrix hardened by precipitation through aging treatment. In these steels, intercritical tempering can provide reverted austenite and precipitation of intermetallic products, increasing the ductility of additively manufactured parts due to austenite presence. Studies deal with postprocessing of additive manufactured parts of maraging steel; however, few focused on phases evolution during the heat treatments and their mechanical response. In the present work, a maraging 300 steel processed by laser-based powder-bed fusion was studied with a focus on microstructural and mechanical properties after applying several postprocessing heat treatments. Tensile tests assessed the mechanical properties, and the microstructure was analyzed by scanning and transmission electron microscopy. A synchrotron beamline with x-ray diffraction was used to conduct in situ measurements of martensite and austenite evolution. The in situ phase evolution revealed that isothermal heat treatments were efficient in promoting martensite-to-austenite reversion. Likewise, the presence of austenite significantly enhanced the ductility, however, at some mechanical strength expense.

Microstructural Impact of Si and Ni During High Temperature Quenching and Partitioning Process in Medium-Mn Steels

Austenite stabilization through carbon partitioning from martensite into austenite is an essential aspect of the quenching and partitioning (Q&P) process. Substitutional alloying elements are often included in the chemical composition of Q&P steels to further control the microstructure development by inhibiting carbide precipitation (silicon) and further stabilize austenite (manganese and nickel). However, these elements can interfere in the microstructure development, especially when high partitioning temperatures are considered. In this study, the microstructural development during the Q&P process of four low-carbon, medium-manganese steels with varying contents of silicon and nickel is investigated. During partitioning at 400 °C, silicon hinders cementite precipitation in primary martensite thereby assisting carbon partitioning from martensite to austenite. During partitioning at temperatures of 500 °C and 600 °C, presence of nickel inhibits pearlite formation and promotes austenite reversion, respectively. It is observed that the stabilization of austenite is significantly enhanced through the addition of nickel by slowing down the kinetics of competitive reactions that are stimulated during the partitioning stage. Results of this study provide an understanding of the interplay among carbon, silicon and nickel during Q&P processing that will allow the development of new design strategies to tailor the microstructure of this family of alloys.

Microstructural characterization of grade 300 and grade 350 maraging steels and electrochemical study in hydrofluoric solution

The electrochemical behavior of solution-annealed grade 300 and grade 350 maraging steels subjected to two different aging temperatures in an aqueous hydrofluoric acid (HF) medium was evaluated. The microstructural and hardness studies showed the effects of the chemical composition and heat treatments on the material properties. Although it is not possible to observe them, the fine intermetallic composites deposited during aging allow a significant increase in the hardness of materials, which was reduced when aged at 560 °C, where there was formation of reverted austenite. The results showed that, in both steels, solution-annealed samples that were not aged showed a better corrosion behavior than the aged samples. The reversion of austenite during aging at temperatures above 560 °C increases the phase limits and its susceptibility to corrosion. The greater amount of cobalt and titanium in grade 350 maraging steel increases the amount of fine intermetallic precipitate rich in molybdenum and titanium during aging, therefore improving its mechanical properties, but also reducing its corrosion resistance.

Role of Intercritical Annealing in Enhancing the Cross-Tension Property of Resistance Spot-Welded Medium Mn Steel

Among the third-generation advanced high-strength steels, medium Mn steels are one of the most promising candidates in the modern automotive industry. However, their application is challenged by the weldability and corresponding poor mechanical property. In this study, the resistance spot welding (RSW) technique was used to join 7Mn steels. A post-weld heat treatment containing intercritical annealing was followed to tune the nugget microstructure. In this case, the trapped C and Mn during RSW can fully diffuse, diminishing the grain boundary embrittlement. Meanwhile, the austenite reversion transformation occurs in the joint, facilitating the formation of a mixed microstructure consisting of ferrite and austenite. With these efforts, the cross-tension strength (CTS) of the RSW 7Mn steel can reach as high as 11.6 kN with outstanding plastic deformation, giving rise to a button pullout fracture mode. This study provides an accessible approach to tailor the microstructure and final CTS of RSW joints of the medium Mn steel, which can guide the optimization of the RSW process.

Lamellar Pearlite as an Initial Microstructure for Austenite Reversion Treatment

Through carbon and manganese partitioning to austenite from polygonal ferrite, bainite and martensite, retained austenite (RA) could be stabilized at room temperature in advanced high-strength steels. Alternatively, the present study utilized lamellar pearlite as an initial microstructure for austenite reversion treatment at 750 °C and successfully produced the microstructure consisting of film RA and lath martensite. This heat treatment is named as pearlitic reversed austenitization. The austenite formed from cementite was enriched in manganese and, in turn, was retained at room temperature; whereas, the austenite formed from ferrite was depleted in manganese and, in turn, transformed to martensite during cooling to room temperature. Different holding times at 750 °C led to different microstructures and RA fractions. After tempering 1 min at 300 °C, a high ultimate tensile strength of 1791 MPa and a decent total elongation of 8.1% were achieved due to tempered martensite matrix and transformation-induced plasticity effect. These tensile properties are comparable to C250 maraging steel. This investigation opens a new avenue to produce high-strength and good ductility steels based on pearlite.

Effect of aging temperature on the austenite reversion and mechanical properties of a Fe–10Cr–10Ni cryogenic maraging steel

Fe–10Cr–10Ni cryogenic maraging steel is one of the key candidate materials applied in harsh cryogenic condition due to its high strength and good cryogenic toughness. In this study, the effect of aging temperature on its microstructure and mechanical properties was systematically studied based on austenite reversion and nano-precipitation. It shows that a desirable combination of high strength (834 MPa, 25 °C) and excellent cryogenic impact toughness (164 J, −196 °C) can be obtained by aging treatment at 500 °C. Multi-scale characterizations were conducted to reveal the microstructure characteristics of the steel. It was found that obvious film-like reversed austenite nucleate and grow at the high-angle grain boundaries of martensite matrix in the steel aged at 500 °C, whereas the higher aging temperature resulted in a larger content of blocky reversed austenite in martensite blocks. Austenite reversion mechanism was proposed based on the double-spherical-cap model and diffusion kinetics of Ni element. Besides, it was found that the precipitation sites of Ti-rich particles are not only distributed in matrix but also located at the dislocations, and they were identified as the clusters of Ni3Ti precipitates. Finally, the origin of the above cryogenic toughness includes the transformation-induced plasticity (TRIP) effect from the soft film-like austenite, higher density of high angle grain boundaries of martensite and fine nanoscale precipitates. Moreover, the precipitation strengthening from the clusters of Ni3Ti precipitates contributes to the high strength of the steel aged at 500 °C.

Extraordinary warm ductility of a Mn-rich high-strength steel achieved at temperature below 0.5 Tm

Abstract This work employed austenite reversion during deformation to achieve large ductility at a temperature below 0.5 Tm (melting temperature) in Mn-rich high strength steels. This goal was achieved in a Mn-rich steel without Al, in which a higher austenite reversion rate below 0.5 Tm was obtained. The tensile results evidenced the extraordinary ductility of over 90% at 550 °C, which is about 0.48 Tm of the steel. Thermodynamic calculation was applied to elucidate the effects of Al and strain energy. Moreover, electron backscatter diffraction was used to investigate microstructure evolution under various conditions. It was found that rate of austenite reversion is more critical than ferrite recrystallization in enhancing ductility during warm deformation.

High pressure torsion processing of maraging steel 250: Microstructure and mechanical behaviour evolution

Maraging steels are precipitation strengthened martensitic steels with an unusual combination of strength and ductility. High Pressure Torsion (HPT) has been used in this study to produce maraging steel 250 grade (AMS 6512) with finer laths and higher dislocation density, both of which act as nucleation sites for precipitation, and reverted austenite formation. This study focusses on the effect of such a processing on the evolution of the microstructure, including kinetics of precipitation, recrystallisation and austenite reversion as well as the stability of the precipitates thus formed. It was found that the aging kinetics accelerated substantially in the HPT processed samples, by achieving peak aging conditions at considerably shorter temperature/time durations and also a peak hardness higher than the as-received sample by 41%. Detailed microstructural characterisation revealed a change in the precipitate morphology from spherical to plate like form in the overaged conditions. The impact of this on mechanical response of these steels was quantified using tensile tests. A 70% increase in ultimate tensile strength was achieved in HPT processed samples after peak aging. Changes in strength and ductility were correlated to the changes in the microstructure and their impending contributions to different strengthening mechanisms at play to enable better design of maraging steels.

Austenite reversion through subzero transformation and tempering of a boron-doped strong and ductile medium-Mn lightweight steel

Effects of subzero treatment and B doping on austenite reversion are investigated in quenched and tempered Fe–9Mn–5Al-0.3C and Fe–9Mn–5Al-0.3C-0.005B (wt.%) lightweight steels. In the as-quenched condition, the steel microstructure consist of a triplex structure of austenite, ferrite, and martensite. B doping leads to a reduction in the prior austenite grain size by grain boundary segregation and precipitation of boro-carbides, which increases the stability of austenite against the athermal martensitic transformation. After tempering at 200 °C for 2 h, nano-lath reverted austenite is formed by the C-enrichment instead of the carbide precipitation owing to the high Al content. This reversion effect is promoted further by the subzero treatment at −196 °C for 0.5 h prior to tempering, which enables the remaining austenite in the as-quenched state to transform and, thus, provides additional sites for austenite reversion. In addition, the subzero treatment and B doping result in the synergistic effect of the delay of crack initiation through the transformation of retained austenite in contact with ferrite and the improvement of bonding strength. Thus, the B-doped steel subjected to quenching, subzero treatment, and tempering exhibits a very high yield strength of approximately 1 GPa, the tensile strength of over 1.3 GPa, and an excellent elongation of 29.4%, which outperform the tensile properties of conventional austenitic or (austenite + ferrite) duplex lightweight steels.

Mechanical behavior of multi-stage heat-treated HSLA steel based on examinations of microstructural evolution

Abstract In the current work, multi-stage heat treatment involving quenching (Q), intercritical treatment or lamellarization (L), and tempering (T) is applied in high-strength low-alloy (HSLA) steel to study the microstructural evolution by multi-scale characterizations, kinetics of austenite reversion and corresponding mechanical behavior are further investigated. The as-quenched (AQ) and QT specimens possess high strength but low ductility and especially low toughness, whereas the QL and QLT specimens exhibit a significantly improved toughness. Introducing L-treatment leads to austenite reversion with the formation of globular γG grains on high angle grain boundaries (HAGBs) and acicular γA grains on lath boundaries during L-annealing, both of which exhibit low stability and further transform to the corresponding globular marteniste (GM) and acicular martensite (AM) upon L-quenching. The Averami exponent n for austenite reversion based on JMAK analysis varies in the range of 0.285 and 0.329 with an activation energy of 293 kJ/mol, indicating the austenite reversion corresponds to a diffusional process that controlled by competition between solute drag and solute diffusion. Mechanical properties are studied in terms of microstructural characteristics involving volume fraction of martensite, dimension and morphology of intercritical ferrite, interactions of dislocation and nano-scale carbides, which are responsible for the variation in the strain hardening behavior. The L-treatment process can produce a significant grain refinement effect via microstructural modification, refined QL-martensite particles with high density HAGBs are obtained, thus improve the comprehensive performance especially impact toughness of the steel.

Leveraging Solute Segregation in Laser Powder Bed Fusion to Achieve Superior Strength and Ductility Via Single-Step Heat Treatment in Ti-Free Grade 300 Maraging Steel

Maraging steels are Fe-Ni precipitation-strengthened steels that can be heat treated for achieving high strength with an accompanying loss in ductility, a trend common to most metals. We show that the segregation of Ni during laser powder bed fusion, commonly considered a nuisance that is dealt with via solutionizing heat treatments, can be leveraged via direct aging to achieve higher strength and ductility simultaneously, compared to a solutionized and aged condition. The high dislocation density in combination with solute segregation in the as-fabricated material promotes the precipitation of Mo-rich intermetallic phases that are responsible for the high strength, and the Ni segregated regions facilitate austenite reversion during direct aging. The reverted austenite enhances the elongation via transformation-induced plasticity. While overcoming the strength–ductility conundrum, direct aging can significantly reduce the energy and emissions associated with multistep heat treatments, especially in the global tool and die industry, while laser powder bed fusion (LPBF) can enable tools with integrated cooling channels for increased downstream production efficiency.

Insights into the Transformation-Induced Plasticity (TRIP) Effect in Ti-Free Grade 300 Maraging Steel Manufactured by Laser Powder Bed Fusion (LPBF)

The near-net fabrication capabilities of laser powder-bed fusion (LPBF) offer the potential to manufacture tools with complex cooling channels that can improve the production efficiency of the tools with reduced greenhouse gas emissions. High strength, toughness, and wear resistance are some of the key properties required by the tooling industries to qualify tools. While strength can be increased through appropriate heat treatments, the increase in strength comes with an associated ductility loss. We show that the non-equilibrium conditions inherent to LPBF can suppress the transformation-induced plasticity effect to room temperature in a Ti-free version of grade 300 maraging steel (G300MS), which is commonly not observed at room temperature in G300MS manufactured using conventional methods. The presence of retained austenite along with Ni-rich regions was found to increase the kinetics for austenite reversion during aging, thereby enabling the transformation of austenite into e martensite during tensile deformation at room temperature, in turn increasing the strength with minimal ductility loss.

Effect of Post-Weld Aging Temperature on Microstructure and Mechanical Properties of Weld Metal of 15-5 PH Stainless Steel

This study is an attempt to correlate the microstructure and mechanical properties of weld metal of commercial 15Cr-5Ni precipitation-hardening stainless steel at different post-weld treatments. The weld metal is conducted by gas tungsten arc welding, and post-weld aging is performed at three different temperatures, viz. 580 °C, 600 °C and 620 °C. After post-weld aging treatment, the microstructure and mechanical properties of weld metals are investigated by OM, SEM, EDS, EBSD and impact tester. The results indicate that microstructure of weld metals by post-weld aging treatment consists predominately of martensite, reversion austenite and copper-rich phase. The amount of austenite and the size of copper-rich phase increase with increase in the post-weld aging temperature. The tensile strength of weld metals first increases and then decreases with the increasing of post-weld aging temperature. The former is due to the size and distribution of copper-rich phase, and the latter is due to the increasing of amount of reversion austenite.

Contribution of grain boundary misorientation to intragranular globular austenite reversion and resultant in grain refinement in a high-strength low-alloy steel

This paper elucidates the determining role of grain boundary types/misorientations on formation of intragranular globular austenite, which has random orientation or large deviation from the parent austenite, in a high-strength low-alloy bainitic steel during reaustenitization. Results revealed that within the parent austenite with lath bainite, reverted austenite with random orientation or large deviation from the parent austenite can nucleate at the packet and block boundaries with high misorientations, and rapidly evolve to become globular austenite with increase of temperature, making a significant contribution in final austenite grain refinement. While in granular bainite, the low misoriented boundaries belonging to the same Bain group are the preferred nucleation sites for the reverted austenite, which is detrimental to the formation of intragranular globular austenite. In addition, the precipitation mode of cementite is different in the reverse process, that is, cementite is precipitated from the matrix of lath bainite, but it is precipitated from retained austenite or martensite/austenite (M/A) particles in granular bainite, which will also lead to the different or consistent variant orientation of reverted austenite and matrix. Therefore, the final austenite grain refinement in initial granular bainite was mainly due to the formation of intergranular globular reverted austenite, and the refining effect is much lower than that of lath bainite.