Austenite Reversion Research Articles

Effects of aging temperature on microstructure and mechanical properties of PH 13-8Mo stainless steel

Precipitation hardening stainless steel (PHSS) possesses high strength, impact resistance, and corrosion resistance, making it extensively employed in aerospace, nuclear power equipment, and petrochemical industries. The precipitates and reversed austenite (RA) generated during heat treatment significantly influence the properties of PHSS. This study investigates the impact of aging temperature (480–620 °C) on PH 13-8Mo stainless steel. As the aging temperature increased from 480 °C to 620 °C, the yield strength (YS) and ultimate tensile strength (UTS) initially increased and then decreased, while the impact energy initially decreased and then increased. At an aging temperature of 510 °C, the strength and hardness reach their maximum values, indicating peak aging conditions. The microstructure after aging treatment is lath martensite, RA and NiAl. With increasing aging temperature, the size of NiAl precipitates increases, accompanied by a rise in RA content. The quantitative analysis assessed the contributions of various strengthening mechanisms (precipitation strengthening, solid solution strengthening, grain refinement strengthening, dislocation strengthening, etc.) to YS. Quantitative calculations revealed that the precipitation strengthening effect was most pronounced at an aging temperature of 510 °C.

Tailoring microstructure and mechanical properties of 17-4PH steel fabricated by wire-arc directed energy deposition via a combination of intrinsic and post-processing heat treatments

This paper examines the effect of intrinsic heat treatment, i.e., multiple transient heating from subsequent passes, and post-processing heat treatment on the microstructure and mechanical properties of 17–4PH stainless steel fabricated by wire-arc directed energy deposition. By carefully controlling the interpass temperature to remain below the martensite start (Ms) temperature, intrinsic heat treatment effectively triggers the formation of Cu-rich clusters and the reversion of austenite. This approach results in a concurrent enhancement in the ultimate tensile strength, increasing from 1036 MPa to 1128 MPa, and elongation, increasing from 13.2% to 18.5%, for the as-built part. During post-processing heat treatment, the microstructural response to direct aging treatment hinges on the initial microstructure induced by intrinsic heat treatment. Notably, direct aging treatment significantly enhances the hardness and ultimate tensile strength of the as-built part created at an interpass temperature surpassing the Ms temperature. Conversely, its influence on the mechanical properties of the as-built component, fabricated at an interpass temperature lower than the Ms temperature, remains minimal. Additionally, the application of a solution treatment effectively eradicates the inherent microstructural characteristics, regardless of the initial fabrication conditions. Nonetheless, the grain size of the solution-treated specimen is markedly influenced by the as-built microstructure. Ultimately, a judicious amalgamation of intrinsic and post-processing heat treatments yields an exceptional combination of properties, including high hardness (465 HV), ultimate tensile strength (1437 MPa), and elongation (14.5%). These results underscore the significance of a comprehensive consideration of intrinsic and post-processing heat treatments to tailor the microstructure and mechanical properties of 17–4PH steel fabricated by wire-arc directed energy deposition.

Calphad-assisted design of high strength – ductility martensitic stainless-steels with reverted austenite

Introducing reverted austenite at lath martensite boundaries has been shown to be an effective method to increase the ductility of martensitic steels. However, significant strength reductions can also be introduced during the reversion process in martensitic stainless steels. To overcome the strength-ductility trade-off, we explore the effects of alloying elements in delaying the overaging of the precipitates and accelerating the austenite formation. To this end, we carry multi-objective optimization considering (1) lattice misfit and diffusivity for kinetics of precipitation, and (2) austenite fraction and thermodynamic driving force of austenite reversion for austenite formation. With the insights from calculations, model alloy compositions are then proposed, and mechanical testing and microstructural analysis are applied. The model alloy confirms that a relatively higher volume fraction of austenite can form prior to overaging, which doubles the total elongation without significant strength loss.

Insight into austenite reversion and mechanical behavior of quenching and partitioning steel inherited hierarchical structure of martensite

In this paper, the start microstructure of full martensite and dual phases comprising ferrite and martensite were introduced into quenching and partitioning (Q&P) steel to tailor the microstructural characteristics and mechanical behaviors, which was achieved by means of a pre-quenching treatment with various temperatures. It is revealed that the conventional Q&P sample possessed bimodal microstructure with predominantly blocky feature, while gradually changed from combination of blocky and lath structures into individually lath feature in pre-quenched Q&P samples. The reversed austenite presented different variants, which predominantly reproduced the orientation of parent austenite and occasionally possessed twin relationship, low angle misorientation or extraneous with parent austenite. The twin-related austenite included annealing twin and reversed twin, whilst the latter showed twin relationship with both neighbored annealing twin and parent austenite. The variant type of reversed austenite significantly influenced the characteristics of its orientation relationship with neighbored pre-quenched martensite. The pre-quenching treatment was favorable to improve the carbon content of meta-stable austenite before Q&P treatment, while the final carbon content in retained austenite was mainly controlled by the synergistic carbon partitioning of martensite and bainite on basis of thermodynamic calculation, accompanied with the partitioning of Mn and Si elements. Moreover, the pre-quenched martensite deformed coordinately in a unit of martensite packet and was beneficial to retard necking to improve ductility. The optimal mechanical properties were obtained with pre-quenching temperature near Ac3, possessing yielding strength of 623 MPa, ultimate tensile strength of 861 MPa and product of strength and elongation exceeding 28 GPa%, attributing to the positive TRIP effect originating from lath-like retained austenite with high carbon content and multi-level hierarchical microstructure inherited from pre-quenched martensite.

Dual nanoprecipitation and nanoscale chemical heterogeneity in a secondary hardening steel for ultrahigh strength and large uniform elongation

Nanoprecipitates and nanoscale retained austenite (RA) with suitable stability play crucial roles in determining the yield strength (YS) and ductility of ultrahigh strength steels (UHSSs). However, owing to the kinetics incompatibility between nanoprecipitation and austenite reversion, it is highly challenging to simultaneously introduce high-density nanoprecipitates and optimized RA in UHSSs. In this work, through the combination of austenite reversion treatment (ART) and subsequent flash austenitizing (FA), nanoscale chemical heterogeneity was successfully introduced into a low-cost UHSS prior to the aging process. This chemical heterogeneity involved the enrichment of Mn and Ni in the austenite phase. The resulting UHSS exhibited dual-nanoprecipitation of Ni(Al,Mn) and (Mo,Cr)2C and nanoscale austenite stabilized via Mn and Ni enrichment. The hard martensitic matrix strengthened by high-density dual-nanoprecipitates constrains the plastic deformation of soft RA with a relatively low fraction of ∼15 %, and the presence of relatively stable nanoscale RA with adequate Mn and Ni enrichment leads to a marginal loss in YS but keeps a persistent transformation-induced plasticity (TRIP) effect. As a result, the newly-developed UHSS exhibits an ultrahigh YS of ∼ 1.7 GPa, an ultimate tensile strength (UTS) of ∼1.8 GPa, a large uniform elongation (UE) of ∼8.5 %, and a total elongation (TE) of ∼13 %. The strategy of presetting chemical heterogeneity to introduce proper metastable phases before aging can be extended to other UHSSs and precipitation-hardened alloys.

On the role of cellular microstructure in austenite reversion in selective laser melted maraging steel

Cellular microstructure is a unique feature in alloys fabricated by selective laser melting (SLM). Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances, while its potential role in microstructure architecting during post-heat treatment is rarely explored. In this work, we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300) steel and revealed how this microstructure influences austenite reversion upon aging. Segregation of Ti and Mo is experimentally detected at cell boundaries. It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging, whereas much less austenite is found in conventionally treated 18Ni(300) steels. The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible. Phase-field simulations suggest austenite grows in a fast, partitionless manner along cell boundaries where the chemical driving force for austenite reversion is substantially enhanced by Ti and Mo segregations, but in a sluggish, partitioning manner towards cell interiors. Contrary to conventional views that austenite fraction should be confined to avoid strength reduction, current SLM-fabricated 18Ni(300) steel containing ∼13% cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite. The design of austenite also shows its potential to enhance fracture toughness. The current study demonstrates that cellular structures could substantially alter austenite reversion behavior, providing a new route for microstructure architecting in additively manufactured steels.

Effect of Austenite Reversion and Its Stability on the Tensile and Impact Transition Behavior of High‐Strength Naval Steel

The present investigation correlates the austenite reversion and its stability during intercritical annealing (IA) with the tensile and Charpy impact properties of a low‐C naval grade steel. The as‐received steel is subjected to water quenching after austenitization at 950 °C, followed by IA at 620, 650, and 680 °C for 2 h. DICTRA simulation is used to predict the austenite reversion kinetics during the IA. Incorporation of cementite phase in the simulation correlates well with the experimental dilatometry results on the austenite growth kinetics. DICTRA simulation predicts a continuous rise in the reverted austenite fraction with increasing IA temperature. However, the percentage of reverted austenite, that gets retained after the subsequent quenching, decreases with increasing annealing temperature due to the reduction of austenite stability at elevated IA temperature. In other words, the amount of fresh untempered martensite increases rapidly with increasing IA temperature. Consequently, the sample annealed at an intermediate temperature of 650 °C (IA650) shows the best combination of strength, elongation, and low‐temperature impact properties as compared to the IA620 and IA680 steels due to an optimum balance of mechanically stable austenite and relatively lower untempered martensite fractions.

Laser powder bed fusion of crack-susceptible stainless maraging steel undergoing solid-state phase transformations

For alloys that experience solid-state phase transformation upon cooling, the absence of as-solidified chemical segregation information makes the elimination of hot cracking challenging. One such material is the hot-crack-susceptible high-strength maraging steel C465. Here, we resolve its hot cracking problem under the laser powder bed fusion process through the addition of titanium nitride (TiN) particles. During solidification, TiN promotes grain refinement and reduces the formation of liquid films with low solidus temperatures. Under cooling, the partial dissolution of TiN lowers the martensite start temperature and yields more retained austenite. Upon subsequent annealing, the dissolved titanium slows down the austenite reversion kinetics, while the dissolved nitrogen improves the yield strength. Materials under tensile deformation follow a three-stage work-hardening behavior, indicative of strain-induced martensitic transformations. This work highlights that besides the nucleant's grain refinement potency, the effect of its partial dissolution during processing needs to be critically examined, when addressing the hot cracking problem for alloys prone to phase transformations.

Influence of reverted austenite on fatigue life of 18 % nickel maraging steel

AbstractMaraging steels containing 18 % nickel are based on a low‐carbon iron‐nickel‐cobalt‐molybdenum system. These steels demonstrate a remarkable combination of high strength and high toughness. To investigate the effect of overaging and resulting austenite reversion on the fatigue behavior of C250 maraging steel, specimens in the solution annealed state were overaged at 510 °C for various intervals, resulting in volume fractions of reverted austenite ranging from 2.6 % to 11.4 %. The staircase method was used to calculate the fatigue limit. S−N curves were generated by testing a minimum of four stress levels and at least three samples at each stress level. Basquin's equation was used to fit the experimental stress‐life data, and the Basquin exponent and fatigue strength coefficient were calculated. The presence of 2.6 % reverted austenite in the microstructure improved the fatigue limit and fatigue ratio, without significantly reducing tensile strength. The results can be interpreted in terms of reverted austenite having a crack blunting effect on the propagating cracks. A small amount of reverted austenite in the range of 2 %–3 % vol has a beneficial effect on the fatigue life.

Tailoring the austenite characteristics via dual nanoparticles to synergistically optimize the strength-ductility in cold rolled medium Mn steel

In this work, we proposed a novel Cu/θ dual nanoparticles strategy to tailor the austenite characteristics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties. We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process. Both experiments and numerical simulations verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix. In addition, quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder the α/γ interface migration and the recrystallized grain growth, thereby further refining the final microstructure. With contributions from the effects of dual nanoprecipitates on the austenite reversion, the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation. Therefore, the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33% to 45%. This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.

Near‐Atomic Scale Characterization of Mn‐Gradient in Reverted Austenite in Hot‐Rolled Medium‐Mn Steels

Herein, the Fe–5Mn–2Al–0.1C (wt%) and micro‐alloyed Fe–5Mn–2Al–0.1C–0.1Nb–0.2V (wt%) medium Mn steels are processed by a novel flash‐austenite‐reverse‐transformation (ART) annealing. The core–shell Mn‐gradient in austenite is observed and compositionally characterized at near‐atomic scale by using atom probe tomography. Compared with the conventional ART annealing, flash‐ART process doubled the austenite fraction in both materials, based on electron backscattering diffraction measurement. The rapid austenite reversion in flash‐ART annealed MMnS is attributed to the negligible partitioning local equilibrium controlled austenite kinetics at the flash temperature. The formation of core–shell Mn‐gradient is due to the different austenite growth kinetics and interface mobilities. Microalloying elements are found to form nano‐carbides, which leads to decelerating austenite reversion because of depleted carbon content in the initial martensitic microstructure, and retarding austenite growth according to pinning effect.

Effect of the intercritical annealing in the formability and microstructure of a medium Mn steel sheet

A medium Mn steel (MMS) was cast by employing a vacuum induction furnace. After that, the steel was hot rolled (HR) in order to achieve a final thickness of 1.4 mm. The microstructure of the sheet was found to be composed of martensite and a little amount of austenite, the Ultimate Tensile Strength (UTS in this processing condition was close to 1600 MPa and a negligible elongation was found. An intercritical annealing (IA) heat treatment was applied to the steel to promote the austenite reversion and increase its amount and stability. Previous thermodynamic simulations and experimental results were used to determine the temperature and time parameters of the IA. After the steel was subjected to this heat treatment, it exhibited an elongation close to 35% and an UTS close to 1100 MPa. Bendability testing was carried out in that condition in order to correlate it with the microstructural changes in the sheet. It was found that the critical bending angle was higher in the IA condition in comparison with the HR state.

Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing temperature on the phase‐transformation kinetics and microstructural evolution of a prototype Fe–0.15C–5.56Mn–1.1Si–1.89Al medium‐Mn third‐generation advanced high‐strength steel are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.

Strategy of complex carbides in microalloyed 4Mn steels processed by quenching and austenite reversion

This work describes a strategy to employ complex carbides for high-yield microalloyed 4Mn steels processed by quenching and reversion (Q&R). The nano/microstructural details are investigated by using electron microscopy and atom probe. It is found that complex carbides are classified into two types and contribute to yield strength in different ways. First, coarse carbides (MC1), which are retained after austenization and quenching, provide the Zener pinning effect and contribute to strengthening by microstructure refinement and precipitation hardening. Second, nanoscale carbides (MC2), which precipitate in martensite during austenite reversion, provide precipitation hardening. Furthermore, the precipitation of MC1 accelerates austenite reversion due to the finer martensite microstructure. Hence, the strengthening effects and TRIP effect can be tuned by controlling the proportion of the two types of carbide. This concept is validated in Ti-Mo 4Mn steel, which exhibits a high yield strength of about 850 MPa, a high ultimate tensile strength of about 950 MPa, and excellent elongation of over 20%. The complex carbide strategy is able to increase yield strength without sacrificing much ductility for Q&R medium-Mn steels.

Austenite reversion suppression with deep cryogenic treatment: A novel pathway towards 3rd generation advanced high-strength steels

This study investigates the impact of deep cryogenic treatment (DCT) on microstructure and properties of a martensitic stainless steel AISI 431. The reasonably simplistic microstructure tailoring with DCT delivers a novel pathway towards advanced 3rd generation high-strength steels (AHSS), whilst utilizing existing alloys and chemistry. The resulting microstructural changes deliver improved strength and hardness of the material, whilst still sustaining good elongation properties. DCT reduces the material's toughness but improves its deformation resistance and wear resistance. The study reveals that DCT modifies the microstructural evolution during tempering. During the conventional treatment, AISI 431 develops austenite reversion transformation (ART), which allows the high strength and elongation capabilities formed with transformation-induced plasticity (TRIP). In contrast, DCT progresses the microstructure even further by retransforming the reverted austenite formed through ART into tertiary α-martensite, ε-martensite and carbides that increase the strength and deformation behavior of AISI 431. The novel microstructural modification that we have entitled cryogenic austenite retransformation (CAR), is both experimentally and theoretically assessed. The underlying transformation mechanisms of CAR and individual stages of DCT were evaluated with ex-situ electron backscatter diffraction and transmission electron microscopy. The impact of DCT on the modified transformation capabilities of AISI 431 is discussed in relation to possible implementation in automotive industry as a universally applicable treatment procedure.

Effects of the microstructure and reversed austenite on the hydrogen embrittlement susceptibility of Ni-Cr-Mo-V/Nb high-strength steel

The effects of the microstructure and reversed austenite (RA) on the hydrogen embrittlement (HE) behavior were investigated in NiCrMoV/Nb high-strength steel prepared using three different heat treatments. The quenching and tempering (QT) steel showed a noble HE resistance for the lowest hydrogen diffusion and maximum apparent hydrogen concentration compared to quenching (Q) steel and quenching, lamellarizing, and tempering (QLT) steel. Thermal desorption spectroscopy (TDS) analysis confirmed that RA had an activation energy of 32.5 kJ/mol which indicates a stable trapping site. The QLT steel containing more RA exhibited a higher HE susceptibility, suggesting that RA degrades the HE resistance.

Evading strength-ductility trade-off in directed energy deposited precipitation hardenable stainless steels: A pathway through precipitation kinetics modeling, design of heat treatment, and evolution of clusters

The kinetics of precipitation and austenite reversion phase transformations in an as-built arc-directed energy deposition (arc-DED) PH13–8Mo stainless steel are studied. The material in the as-built condition is analyzed using the differential scanning calorimetry (DSC) technique to determine the fraction of transformed precipitates and reverted austenite at any time/temperature during non-isothermal transformations. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic equation is employed to model the transformed phase volume fraction during precipitation and austenite reversion processes. Using the non-isothermal phase transformation kinetics, an isothermal kinetics model is developed. The isothermal kinetics modeling results are then employed to design and develop direct aging heat treatments to enhance the hardness and strength of arc-DED-PH13–8Mo steel. The developed heat treatment results in the concurrent enhancement of strength and ductility in the material. Such an achievement is a result of the evolution of nano-scaled β-NiAl clusters. It appears that the formation of β-NiAl clusters increases the dislocation storage capability in the material, resulting in the evasion of the strength-ductility trade-off. The results of the current study provide great insight into the crucial role of clusters in the strength-ductility trade-off in PH stainless steels.

Strain partitioning in ferrite and mechanical properties of a cast multiphase stainless steel

Multiphase stainless steels can offer very attractive combinations of high strength, high toughness and good corrosion resistance due to the characteristics of multiphase, metastable, and multi-scale. In this study, the strain partitioning in ferrite of a cast multiphase stainless steel was investigated by EBSD. The deformation occurs first in the softer ferrite and later transfers to the martensite/austenite regions. As the strain increases, subgrains form within the ferrite, effectively accommodating the deformation of different phases. In addition, the effect of transformation-induced plasticity (TRIP) caused by reversed austenite (RA) and the nanoscale G-phase in ferrite can together contribute to the high strength and toughness of the cast multiphase stainless steel.

Breaking strength-ductility trade-off in laser-powder bed fused Fe–Cr–Ni–Al maraging stainless steel: Controlled precipitation and preserved dislocations

In the current study, cylindrical rods of Fe–Cr–Ni–Al maraging steel (with the brand name CX) are fabricated using the laser-powder bed fusion (L-PBF) technique. The material is analyzed using the differential scanning calorimetry (DSC) technique under continuous heating at different heating rates. The DSC results are employed to identify the heat flow peaks associated with the precipitation and austenite reversion phase transformations. The peaks are then analyzed and processed to determine the onset, peak, and end temperatures associated with each phase transformation. These results are used to calculate the degree of phase transformation under non-isothermal conditions and to evaluate the activation energy of transformation through the Kissinger method. In the next step, the modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is employed to model the kinetics of phase transformations during non-isothermal heating. The validated model is then used to predict the kinetics of precipitation and austenite reversion phase transformations under isothermal aging heat treatments. The isothermal modeling results are employed to design a direct aging heat treatment to promote the evolution of nanometric β-NiAl precipitates and meanwhile preserve the pre-existing dislocation networks. Such a heat treatment results in the enhancement of both yield and tensile strengths from 929 MPa and 1032 MPa in the as-built condition to 1659 MPa and 1738 MPa after heat treatment. Meanwhile, ductility slightly changes from 15.3% in the as-built condition to 14.7% after heat treatment. Such an achievement in breaking the strength-ductility trade-off is due to a controlled heat treatment process that preserves the dislocation networks.

Exploring the relationship between the accelerated austenite reversion and two-steps solution treatment in a Cr-Ni-Mo cryogenic maraging stainless steel

Exploring the relationship between the accelerated austenite reversion and two-steps solution treatment in a Cr-Ni-Mo cryogenic maraging stainless steel