Partitioning Of Mn Research Articles

The Effect of Heating Rate on the Microstructure Evolution and Hardness of Heterogeneous Manganese Steel.

The use of a rapid heating method to achieve heterogeneity of Mn in medium-manganese steel and improve its comprehensive performance has been widely studied and these techniques have been widely applied. However, the heating rate (from α to γ) has not received sufficient attention with respect to its microstructure-evolution mechanism. In this study, the effect of heating rate on the microstructure evolution and hardness of heterogeneous medium-manganese steel was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and DICTRA simulation. The results showed that the Mn distribution was heterogeneous in the initial microstructure of pearlite due to strong partitioning of Mn between ferrite and cementite. At low heating rates (<10 °C/s), the heterogeneity of Mn distribution was diminished to some extent due to the long-distance diffusion of Mn in high-temperature austenite. Contrastingly, at high heating rates, the initial heterogeneity of the Mn element could be largely preserved due to insufficient diffusion of Mn, which resulted in more ghost pearlite (GP: pearlite-like microstructure with film martensite/RA). Moreover, the high heating rate not only refines the prior austenite grain but also increases the total RA content, which is mainly composed of additional film RA. As the heating rate increases, the hardness gradually increases from 628.1 HV to 663.3 HV, due to grain refinement and increased dislocation density. Dynamic simulations have also demonstrated a strong correlation between this interesting microstructure and the non-equilibrium diffusion of Mn.

Microstructure Evolution and Tensile Properties of Medium Manganese Steel Heat Treated by Two-Step Annealing

In this paper, the nucleation and growth of austenite are controlled through a two-step annealing process to achieve multi-scale distribution and content increase of retained austenite in low manganese series medium-Mn steel. Combining SEM, EBSD, AES, and other experimental equipment, the evolution rules of the microstructure, properties, and element distribution behavior of the test steel during the annealing process are studied. Compared with one-step annealing, the two-step annealing significantly broadens the size distribution range of retained austenite. In the first step, after annealing at a higher intercritical temperature (760 °C), the ferrite and the M/A island are obtained, completing the initial partition of Mn and the refinement of microstructures. During the second step of annealing (720 °C), the primary Mn-rich martensite region provides higher nucleation driving force and finer dispersed nucleation sites, promoting the nucleation and growth of reverse transformation austenite. At the same time, the metastable-retained austenite formed after the first step of annealing continues to grow through interface movement. Furthermore, a high proportion (23.4%) of retained austenite with multi-scale distribution is formed in the final microstructure, and the product of strength and elongation increased from 21.8 GPa·% by the one-step annealing process to 30.1 GPa·%.

Role of cementite on tensile properties in auto-tempered 0.15C–5Mn martensitic steel

The effects of austenitizing temperature and cooling rate on microstructure and subsequent tensile properties were investigated in 0.15C–5Mn martensitic steels. Tensile strength and elongation increased with decreasing austenitizing temperature (1000 °C to 800 °C) in both air-cooled and water-quenched steels. Improvement of tensile properties originated from the promoted transformation-induced plasticity (TRIP) effect, as lower austenitizing temperature limited Mn diffusion to make cementite preferred site for Mn partitioning, thereby increasing the fraction of retained austenite after heat treatment. Cooling rate also affected the mechanical properties, as air-cooled samples showed lower strength and higher elongation compared to water-quenched samples. During the air-cooling, supersaturated carbon in martensite is redistributed to form additional carbides by auto-tempering effect. The formation of carbides softened the martensite to decrease strength while increased tensile elongation. Fraction of retained austenite influenced by the austenitizing temperature and formation of carbide affected by the cooling rate acted as competing mechanisms affecting the tensile properties of the martensitic steels, indicating that heat treatment should be controlled carefully to obtain the desirable mechanical properties.

Pearlite Growth Kinetics in Fe-C-Mn Eutectoid Steels: Quantitative Evaluation of Energy Dissipation at Pearlite Growth Front Via Experimental Approaches

Essential understanding of the pearlite growth kinetics is of great significance to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, a series of eutectoid steels with Mn addition up to 2 mass pct were isothermally transformed at various temperatures from 873 K to 973 K to clarify the pearlite growth kinetics and the underlying thermodynamics at its growth front. The microscopic observation indicates the acceleration in pearlite growth rate and refinement in lamellar spacing by decreasing the transformation temperature or the amount of Mn addition. After analyzing the solute distribution at pearlite growth front via three-dimensional atom probe, no macroscopic Mn partitioning across pearlite/austenite interface is detected, whereas Mn segregation is only observed at ferrite/austenite interface. Furthermore, in-situ neutron diffraction measurements performed at elevated temperatures reveal that the magnitude of elastic strain generated during pearlite transformation is very small. Based on the thermodynamic model, these experimental results are used to estimate the contribution of various factors to the total energy dissipation. Compared with the Mn-free alloy, the retardation effect of Mn addition on pearlite growth kinetics, which is partly due to the reduced driving force for pearlite growth, can be well explained by further considering the solute drag effect of Mn.

Improving laser welded joint performance of medium-Mn steel through C and Mn partition by intercritical annealing process

In this paper, the fracture failure mechanism of laser welded (LW) joints of medium-Mn steel is studied. And the mechanical properties of the joints were greatly improved by designated intercritical annealing (IA) process. Experimental results showed that the elongation of the as-welded joint of medium-Mn steel is very low, and the fracture shows typical brittle fracture characteristics. Electron probe micro-analyzer (EPMA) test showed that there were many Mn segregation bands in the joint, and electron backscatter diffraction (EBSD) surface scanning test further demonstrated that many Mn23C6 carbides were produced at the martensite grain boundary (GB) of the fusion zone (FZ), which was the main reason for the occurrence of brittle fracture in the FZ. The IA process improves the segregation of Mn element at the GB of the joint and reduces the intergranular brittleness. At the same time, more austenite was distributed by C and Mn elements, and more Fe3C phases were found near austenite in the EBSD diagram, which provided nucleation centers for the formation of austenite. The product of strength and elongation (PSE) of the IA treated joint was dramatically increased by 2693.65%.

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.

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.

High strength-high ductility combination in a low-density medium Mn steel prepared by a highly efficient route

Medium Mn steels are the primary candidates in the fields of automobile, construction machinery et al. due to the advantages of high strength-ductility combination, and low costs. By now, the balance of good performance and high preparation efficiency is still a critical issue to be unsolved, which restricts their further applications. This work designs a novel Al-contained medium Mn low-density steel and explores the effect of a short intercritical-annealing process. The microstructure evolution of multi-scales, especially the austenite characteristics and Mn partitioning behaviors is analyzed, and the relationship between austenite and tensile properties is discussed. The results indicate Al can broaden the annealing processing window and improve the annealing temperature. When annealed at 780 °C, the austenite reverted transformation happens quickly and after annealing for 3 min, the austenite volume fraction maintains stable in the range of 52.4–56.7 vol%, but the morphology and Mn concentration are very different. A large amount of blocky austenite with a coarse size of more than 1 μm and a low Mn concentration of 8–9 wt% forms at the longer annealing time of 20 min, while the austenite almost presents a lath-like shape with a high Mn concentration exceeding 10 wt% at annealing time of 4 min. The tensile properties are closely related to the volume fraction and mechanical stability of austenite, and the sample annealed for 4 min exhibited the best performance with tensile strength of 1192 MPa and total elongation of 33 %, due to its slow TRIP effect.

Promoting ductility and formability in a carbide free bainitic steel via pre-annealing treatment

This study aimed to overcome the existing trade-off in strength–ductility in carbide free bainitic steel and improve ductility without compromising strength. Herein, we proposed a novel and simple method to obtain Mn-rich cementite via a pre-annealing (PA) treatment. PA promoted the partitioning of Mn into cementite particles. The Mn-rich cementite increased the Mn content in the retained austenite (RA) and enhanced the RA stability. Moreover, the initial number of Mn-rich cementite microstructures is positively correlated with RA. The effect of PA on the bainitic ferrite (BF) content was evaluated via comparison with the non-PA sample, revealing that the PA treatment promoted BF nucleation and increased the BF content. The PA-treated samples exhibited the optimal transformation induced plasticity (TRIP) effect and mechanical properties; in particular, the PA-3 sample (with 3 h treatment period) improved ductility and formability, with the total elongation increased from 11.9% in the non-PA sample to 18.0% in the PA-3 sample, corresponding to an increase of ∼51.2%. Additionally, the Erichsen cupping value increased from 7.52 mm in the non-PA sample to 9.78 mm in the PA-3 sample, corresponding to an increase of 30.1%. The overall applicability of PA for the improvement of formability of the carbide free bainitic steel is evident in all PA-treated samples.

On the role of Manganese Partitioning in Metastable Austenite by Quenching and Partitioning Treatment

With the aim to study the role of “frozen” concentration gradient of manganese (Mn) element in stability of retained austenite (RA) with multiple‐stage martensite transformation, a series of intercritical annealing (IA) temperatures is conducted before quenching and partitioning (Q&amp;P) treatment. Morphology and distribution of RA are observed by field emission gun scanning electron microscope and electron back‐scatter diffraction. The volume fraction (7%–16%) and stability of metastable RA is found to be affected profoundly by IA temperature. Thermodynamic and kinetic analysis are conducted to elucidate the evolution of RA in process of IAQP treatment. The predicted levels of RA are in good accordance with measurements. It is found that the inhomogeneous partitioning of Mn in period of IA, combining with the incomplete partitioning of carbon during Q&amp;P, radically regulated the Q&amp;P microstructure. The incomplete partitioning of carbon in RA, with excess carbon segregation at dislocations and boundaries, lead to partition‐less bainite transformation owing to the average carbon content in RA lower than the “To” threshold.

Critical role of dynamic precipitation on enhanced creep resistance in Mg–Zn alloy by Mn substitution for Zr

Mg alloys with excellent creep resistance are able to withstand fatigue and damage under high temperature and complex load. Adding alloying elements rather than rare earth elements, such as Mn, can further reduce the cost and thus facilities the widespread applications of Mg alloys. In this paper, the individual effect of Mn and Zr elements on the dynamic microstructural evolution of Mg–Zn alloys during creep is evaluated, on the basis of identical initial microstructure before creep. And the essential origin of Mn substitution for Zr improving the creep resistance of Mg–Zn alloys by one order of magnitude is revealed. Experimental investigations and first principle calculations reveal that prior-precipitation of α-Mn particles serve as the heterogeneous nucleation sites for the precipitation of β1′ phases, and Mn partitioning in MgZn2 phases promote the nucleation and thermal stability of β1′ precipitates during creep. High number density of fine precipitates can effectively inhibit the occurrence of continuous dynamical recrystallization (CDRX), pin dislocations and impede the activation of cross-slip, which are the main reasons for the enhanced creep resistance in ZM60 alloy. As Zr does not participate in the formation of precipitates in ZK60 alloy, the MgZn2 phases with low thermal stability are prone to growth and coarsening during long-term creep. Large number of fine CDRXed grains and sparse coarse precipitates in ZK60 alloy cannot effectively impede the dislocation movement, which contributes to the occurrence of cross-slip and deteriorate the creep resistance. This work provides new insights for designing and fabricating outstanding heat-resistance Mg alloys.

Effect of tempering and partitioning (T&P) treatment on microstructure and mechanical properties of a low-carbon low-alloy quenched and dynamically partitioned (Q-DP) steel

We performed a direct quenching and dynamical partitioning (Q-DP) process, following intercritical annealing, to a low-carbon low-alloy steel. Performing an additional tempering treatment on the Q-DP steel, we investigated the carbon partitioning behavior upon tempering. We emphatically analyzed the effect of tempering and partitioning (T&P) treatment on the microstructure evolution and mechanical properties of the Q-DP steel, especially on carbon partition, retained austenite stabilization, TRIP effect and work hardening. Results show that the synergetic partition of carbon and Mn from ferrite to austenite during intercritical annealing and the dynamical partition of carbon upon quenching jointly achieve the stabilization of retained austenite in the Q-DP sample. The actual volume fraction of retained austenite is larger than the theoretical calculating value, which is attributed to the dynamical partition of carbon. The T&P treatment at 300 °C for 15 min does not result in obvious decomposition of retained austenite, but leads to apparent carbon partition from martensite to retained austenite. The carbon partition upon tempering enhances the mechanical stability of retained austenite, thus weakening the TRIP effect during tensile deformation and promoting the TRIP effect to occur at higher true strain level. The much higher work hardening rate at the late deformation stage near necking, for the quenched tempered and partitioned (Q-T&P) sample, leads to a higher uniform elongation, compared with the Q-DP sample. The existence of carbon-enriched stable retained austenite after necking happens finally enhances the post necking elongation of the steel.

Deformation‐Inhibited Cementite Precipitation Effect and Its Influence on Residual Stress

High‐strength steels with low carbon content have low stress relaxation during tempering and are prone to distortion during processing. To improve tempering stress relaxation, 10% compression predeformation is introduced. Carbide precipitation is characterized by dislocation density and dilatometry curves, and the evolution of residual stress during tempering is investigated. The results show that predeformation can inhibit cementite precipitation, increase the effective carbon concentration in the ferrite matrix, and induce the diffuse precipitation of alloy carbides. The dispersed alloy carbides provide more interfaces for Mn partitioning, strengthen the transformation plasticity, and enhance the stress relaxation. After tempering at 700 °C for 30 min, the average residual stress and the elastic strain energy of undeformed/predeformed specimens are 39.18/13.25 MPa and 25.40%/90.92%, respectively.

Experimental and computational study on the C15 phase structure stability of YzNi2-yMny system

Y0.95Ni2 intermetallic is a promising candidate for hydrogen storage applications, but currently suffers from hydrogen-induced amorphization (HIA) mainly caused by its low stability. The structure stability of AB2 Laves phase is mainly controlled by the geometric factor rA/rB. The present work is focused on the ternary Mn-Ni-Y system, as Mn addition helps achieving a close-to-ideal rA/rB (≤1.37) to avoid HIA or HID. Through a combination of X-ray diffraction, neutron diffraction, electron probe micro-analysis and first-principles calculations, new insight on the physicochemical properties and phase equilibrium of this ternary system was gained. Mn substitution is found to suppress the formation of a super-structure with ordered vacancies, in favor of a C15 Laves structure with a disordered distribution of Y vacancies. At low concentration Mn is accommodated only on the Ni sites, compensated by vacancies on the Y site (without long-range order). At high concentration, Mn is accommodated on both Ni and Y sites, with reduced Y vacancy concentration. The partitioning of Mn across the two sites allows to form a single-phase ternary intermetallic across a wide compositional range and suggests increased stability of the phase.

Abnormal Trend of Ferrite Hardening in a Medium-Si Ferrite-Martensite Dual Phase Steel

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

Enhancing mechanical properties of hot-rolled Al-alloyed medium-Mn steel by novel double-step intercritical annealing

The novel heat treatment strategy for advanced medium-Mn steel is presented in the work. Conventional intercritical annealing (IA) was followed by short IA to improve the strength-ductility product of Al-alloyed medium-Mn steel without significant prolonging the duration of heat treatment process. The proper design of the new heat treatment schedules required both experimental and modelling approaches to optimize the microstructure and mechanical properties of investigated steel. The secondary IA step was performed in a temperature range from 750 °C to 850 °C. The obtained microstructures are composed of ferrite and fine lath-type austenite in different proportions. Specimens annealed at 800 °C and 850 °C display also some fraction of martensite. No martensite was observed for the specimen held at 750 °C. An increase of second IA temperature resulted in a gradual increase of tensile strength due to a higher martensite fraction and a reduced fraction of ferrite in the microstructure. The optimal martensite-austenite ratio resulted in combination of high strength and moderate plasticity for the specimen heat treated at 800 °C. The strength-ductility product was about 20.5 GPa%. The C and Mn partitioning from ferrite to austenite and dissolution of cementite are main factors affecting the stability of retained austenite during second IA step and work hardening behavior. The tensile behavior of specimens after two-step IA was affected by serrations resulting from the Portevin-Le Chatelier (PLC) effect.

Precipitation behavior of Cu/Ni(Mn,Al) co-precipitates in the medium-Mn dual-phase steel

• High Mn content promotes the partition of Mn to NiAl by occupying Al sublattice. • Cu/Ni(Mn, Al) co-precipitates are formed just in the martensite of the DP steel. • The growth of precipitates doesn’t follow volumetric diffusion-controlled process. The precipitation behavior of Cu/Ni(Mn,Al) co-precipitates in the medium-Mn dual-phase (DP) steels was investigated through a combination of atom probe tomography (APT) and small-angle neutron scattering (SANS). The results reveal that high Mn content of the DP steel promotes the partition of Mn to NiAl nanoprecipitates by occupying the Al sublattice, thereby forming a Cu/Ni(Mn,Al) co-precipitates in the martensite. The growth of the co-precipitates doesn’t follow the volumetric diffusion-controlled process, which is controlled by the coupled diffusion of multiple chemical elements induced by the synergistic phase transformation. The coarsening coefficient of the co-precipitates in the DP steel are ten times more than it in the previous martensitic steel, leading to the change of precipitation strengthening effect. The mechanism for the co-precipitates’ coarsening is also addressed and discussed.

Microstructure and Mechanical Properties of the 6 wt% Mn-Doped Martensitic Steel Strengthened by Cu/NiAl Nanoparticles

The microstructure and mechanical properties of 6 wt.% Mn-doped martensitic steel have been investigated through a combination of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and small-angle neutron scattering (SANS). The 6 wt.% Mn-doped steel exhibits a yield strength of ~1.83 GPa and an elongation-to-failure of ~7% under peak aging, and the ~853 MPa of precipitation strengthening is much higher than that observed in the 1.5 wt.% and 3 wt.% Mn-doped steels. The steel is composed of α'-martensite and slightly equiaxed α-ferrite together with a high proportion (~62.3%) of low-angle grain boundaries, and 6 wt.% Mn doping and the aging treatment have an effect on the matrix's microstructure. However, 6 wt.% Mn doping can obviously increase the mean size of the Cu/NiAl nanoparticles by enhancing the chemical driving force of the Mn partitioning on the NiAl nanoparticles, which differs from the refining effect on the nanoparticles in 3 wt.% Mn-doped steels. Furthermore, larger Cu/NiAl nanoparticles can significantly improve the yield strength of martensitic steel through precipitation-strengthening mechanisms.

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&amp;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.

Stabilizing austenite via intercritical Mn partitioning in a medium Mn steel

In this study, a novel processing route of intercritical Mn partitioning is proposed to stabilize the reverted austenite in a medium Mn steel. The Fe-5Mn-0.2C-1.5Al (wt.%) steel is annealed unceasingly at two intercritical temperatures to produce a compositional core-shell structured reverted austenite which consists of a core in low Mn content and a shell in relatively high Mn content. This distinctive microstructure can not only allow much more retained austenite to be obtained compared with the conventional processing route, but also produce active transformation induced plasticity effect over a broad strain range, leading to excellent mechanical properties of high strength and enhanced ductility.