high-Mn Austenitic Steel Research Articles

Dominant Factors of Cryogenic Toughness of Heat Affected Zone of Welding in High Mn Austenitic Steel

Dominant Factors of Cryogenic Toughness of Heat Affected Zone of Welding in High Mn Austenitic Steel

Change in Charpy Impact Toughness and Carbide Precipitation after Aging Heat Treatment of High Mn Austenitic Steel

Change in Charpy Impact Toughness and Carbide Precipitation after Aging Heat Treatment of High Mn Austenitic Steel

On the activation of alternated stacking fault pair twinning mechanism in a very large-grained Fe–29Mn–2.4Al steel

An alternated stacking fault (SF) pair twinning mechanism involving local fcc → hcp transformation in a very large-grained high-Mn austenitic steel is reported, which hitherto was valid for nano crystalline metals. It was identified using high-resolution transmission electron microscopy that a pre-twinned ε-martensite like phase, containing local hcp structure is converted to a stable three-layer twin lamella through nucleation of new SF in-between the pre-existing SFs. Activation of observed mechanism is deciphered in terms of the low intrinsic and unstable stacking fault energies ratio of the steel (~ 0.13), predicted using a model based on density functional theory.

Direct observation of dislocation plasticity in high-Mn lightweight steel by in-situ TEM

To gain the fundamental understanding of deformation mechanisms in an aluminum-containing austenitic high-Mn steel (Fe-32Mn-8.9Al-0.78 C (wt.%)), in-situ straining transmission electron microscopy (TEM) analysis is conducted. The in-situ observation during the deformation demonstrates that the plastic deformation is accommodated by the pronounced planar dislocation gliding followed by the formation of slip bands (SBs) and highly dense dislocation walls (HDDWs). Experimental evidences of the glide plane softening can be obtained from the interaction between the gliding perfect dislocations and the L’12 ordered precipitates in the austenite matrix. Furthermore, the observation of the localized cross-slip of dislocations at the slip band intersections enables to understand why slip bands are extensively developed without mutual obstructions between the slip bands. The enhanced strain hardening rate of the aluminum-containing austenitic high-Mn steels can be attributed to the pronounced planar dislocation glides followed by formation of extensive slip band which prevent premature failure by suppressing strain localization.

Effects of Ni and Cu addition on cryogenic-temperature tensile and Charpy impact properties in austenitic 22Mn-0.45C–1Al steels

Austenitic high-Mn steels present a dominant deformation mechanism of TWinning Induced Plasticity (TWIP) or TRansformation Induced Plasticity (TRIP), which effectively enables be used for various cryogenic applications. This mechanism affects significantly tensile or impact properties, and can be carefully tuned by adjusting alloying compositions; however, the alloying effects of Ni and Cu have not been studied yet. In the present study, the steels were fabricated by adding Ni or Cu and their microstructural evolutions were examined for the quasi-statically-tensioned and dynamically-Charpy-impacted specimens. In the room-temperature tensile deformation, many twins were populated without any martensite. At cryogenic temperature, however, ε- and α′-martensite were formed together with twins in the 22Mn-0.45C–1Al (Base) and 1-wt.%-Ni-added (1Ni) steels, whereas they were not in the 1-wt.%-Cu-added (1Cu) steel. This TWIP or TRIP amount showed a good correlation with the stacking fault energy (SFE) calculated by considering Mn-segregation bands, and affected the tensile ductility. At cryogenic temperature, the TRIP occurred in the Base and 1Ni steels as SFEs of low-Mn bands decreased down to the TRIP range, whereas it did not in the 1Cu steel, and thus the Base and 1Ni steels showed the lower ductility than the 1Cu steel. For the cryogenic-temperature Charpy impact test, the martensite was not formed even at the heavily-deformed area near the notch tip as the time was not enough for inducing the martensitic transformation, while twins were populated. Since the twin fraction near the notch-tip area increased in the order of the 1Cu, 1Ni, and Base steels, it was expected that the Charpy impact energy increased in this order, but the Base steel showed the lowest energy because of the precipitation of fine M23C6-type carbides on grain boundaries.

The Effects of Recrystallization on Strength and Impact Toughness of Cold-Worked High-Mn Austenitic Steels

High-Mn austenitic steels have been recently developed for a storage or transportation application of liquefied natural gas (LNG) in cryogenic fields. Since the structural materials are subjected to extremely low temperature, it requires excellent mechanical properties such as high toughness strength. In case of high-Mn steels, twinning deformation during the cold-working process is known to increase strength yet may cause embrittlement of heavy deformed twin and anisotropic properties. In this study, a recrystallization process through appropriate annealing heat treatments after cold-working was applied to improve the impact toughness for high-Mn austenitic steels. Microstructure and mechanical properties were performed to evaluate the influence of cold-worked and annealed high-Mn austenitic steels. Mechanical properties, such as strength and impact toughness, were investigated by tensile and Charpy impact tests. The relationship between strength and impact toughness was determined by microstructure analysis such as the degree of recrystallization and grain refinement. Consequently, both elongation and toughness were significantly increased after cold-working and subsequent annealing at 1000 °C as compared to the as-received (hot-rolled) specimen. The cold-worked high-Mn steel was completely recrystallized at 1000 °C and showed a homogeneous micro-structure with high-angle grain boundaries.

Effect of nanoscale V2C precipitates on the three-body abrasive wear behavior of high-Mn austenitic steel

High-Mn austenitic steel reinforced with carbide particles can significantly enhance the wear resistance of steel. However, micron-sized particles may be detached from the surface of the matrix during repeated sliding wear, resulting in severe deterioration of the wear resistance of steel. In this study, different amounts of nanoscale V2C particles were precipitated from alloyed high-Mn austenitic steel by tempering at 400, 450 and 500 °C. With the increased tempering temperature, the strain hardening rate, nanohardness, modulus, and yield strength of the experimental steel all were significantly enhanced. A higher hardness and modulus enhanced the wear resistance of steel during the initial stage of three-body abrasive sliding wear. The higher strain hardening rate and yield strength improved the wear resistance of steel by enhancing the supporting effect of the austenitic matrix on the micron-sized particles. The wear mechanism of the tested steel was transformed from deep grooves to microploughing, pits, and fatigues with increased tempering temperature.

Microstructures and Impact Wear Behavior of Al-Alloyed High-Mn Austenitic Cast Steel After Aging Treatment

The microstructural evolution and impact wear behavior of Al-alloyed ultra-high-Mn austenitic cast steel were investigated after different heat treatments, including quenching (Q) and quenching + aging (Q + A). Another ultra-high-Mn austenitic cast steel without Al was compared under the same experimental condition. After Q + A, Al addition contributed to the precipitation of κ-carbides which improves the yield strength and wear resistance of the austenite matrix. However, the mechanical properties and wear resistance of the Al-free steel were strongly deteriorated due to the precipitation of plate-like M3C-type carbides, as well as the worn surface of the Al-free steel had the feature of micro-scratches. After Q + A (2 h), the Al-alloyed steel obtained optimal wear resistance and the main wear feature was strain fatigue. But, excessive aging time (4 h) would lead to a large fragile peeling from the worn surface and the wear resistance was decreased. Also, the worn subsurface microstructures with different heat treatments were observed by transmission electron microscopy (TEM). These results show that the deformation microstructures of the Al-free steel contained stacking faults, dislocation tangle and deformation twins. The deformation character of Al-alloyed steel is dominated by planar glide, and with increase in the aging time, planar dislocation substructures gradually evolved, relating to dislocation pileups, Taylor lattices and micro-bands.

Simultaneous enhancement of strength and ductility in a finegrained Fe-30Mn-0.11C steel at 77 K

High Mn austenitic steels are a class of materials for cryogenic applications. Previous studies on Fe-34.5Mn-0.04C austenitic steels have shown that refining grain size can suppress the formation of deformation-induced ε and α’ martensite and improve the tensile strength and ductility at temperatures down to 123 K. Here we report a new observation of simultaneous increase of strength and ductility at liquid nitrogen temperature (77 K) in a fine grained Fe-30Mn-0.11C (wt. %) steel where the addition of 0.11%C stabilizes the austenite. This improvement is attributed to complete suppression of martensitic transformation and enhanced occurrence of deformation twinning.

High-strain-rate deformation behaviour of new high-Mn austenitic steel during impact shock-loading

ABSTRACTThe influence of impact shock-loading was investigated on the mechanical behaviour, microstructure evolution, and texture of a new high-Mn steel. The mechanical behaviour of the shock-loaded specimen was compared with quasi-static compression condition. A Hopkinson pressure bar setup was used to perform the high-strain-rate deformation. Continuous yielding was observed under both low and high-strain-rate deformation. The yield stress also illustrated a positive strain-rate sensitivity with increasing strain-rate. An obvious increase in the flow stress of the steel was detected at higher strain-rate. A strain hardening followed by a strain softening was detected in stress–strain curve after shock-loading. Electron backscatter diffraction also displayed the activation of strain-induced ϵ-martensite and ά-martensite while deformation twinning was absent at high-strain-rate.

Cu addition effects on TRIP to TWIP transition and tensile property improvement of ultra-high-strength austenitic high-Mn steels

Austenitic high-Mn steels have been nominated as desirable ultra-high-strength cold-rolled steels whose mechanical properties are greatly improved by powerful deformation mechanisms of transformation- and twinning-induced plasticity (TRIP and TWIP). In this study, an austenitic high-Mn TRIP steel was suggested to achieve a good strength-ductility balance, and 1–2 wt.% Cu was added as an element for increasing stacking fault energy (SFE) as well as an austenite stabilizer to exploit a transition from TRIP to TWIP. The non-Cu-added steel showed the highest yield and tensile strengths (502 MPa and 1137 MPa, respectively) and the lowest elongation (34.6%) with a serrated flow. Yield and tensile strengths decreased with increasing Cu content, while the elongation was the highest in the 1%-Cu-added steel. TRIP and TWIP mechanisms showed good agreements with calculated SFEs in consideration of (Mn,Cu)-segregated bands. In the non-Cu-added steel, the TRIP occurred step by step as localized deformation bands passed through the specimen gage section to activate the serrated flow, which were reduced (or improved) by the transition from TRIP to TWIP with increasing Cu content. In the 1%-Cu-added steel, overall tensile properties were improved (yield strength; 461 MPa, tensile strength; 1093 MPa, elongation; 65.1%) as both TRIP and TWIP were well homogenized to produce synergic effects.

Grain Size-Dependent Mechanical Properties of a High-Manganese Austenitic Steel

The effect of grain size on the mechanical properties of a high-manganese (Mn) austenitic steel was investigated via electron-backscattered diffraction, transmission electron microscope, X-ray diffraction, and tensile and impact tests at 25 °C and − 196 °C. The Hall–Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm0.5 at 25 °C, which increased to 14 MPa mm0.5 at − 196 °C. The effect that the grain boundary strengthening had on improving the yield strength at − 196 °C was better than that at 25 °C. The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25 °C, while they remained nearly unchanged at − 196 °C. The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at − 196 °C, which promoted the cleavage fracture in the steels with large-sized grains. Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.

Exceptional combination of ultra-high strength and excellent ductility by inevitably generated Mn-segregation in austenitic steel

Austenitic high-Mn steels have been nominated as potential alloys for ultra-high strength and excellent ductility, but the high-Mn content inevitably generates band-shaped Mn-segregation due to dendritic solidification from the steel melt. This segregation band often causes unintended deformation mechanisms and anisotropic mechanical properties, and deteriorates formability. Here, we effectively utilize the Mn-segregated bands for the improvement of tensile properties by actively producing TWinning- and TRansformation-induced plasticity (TWIP and TRIP) mechanisms in high- and low-Mn-segregated bands. We also adopt a mixed microstructure of non-recrystallized and recrystallized austenite mainly formed in high- and low-Mn-segregated bands, respectively. The TWIP+TRIP mechanisms generating highly-sustained strain hardening and high strain hardening, coupled with partial recrystallization and precipitation hardening, are working successfully for overcoming low-yield-strength characteristics of austenite to reach 1 GPa and for achieving the excellent tensile strength of 1.5 GPa and ductility of 44%. Our results demonstrate how the Mn-segregation-induced TWIP+TRIP mechanisms can be a novel idea in ultra-high-strength steel design.

On Primary Recrystallization of High-Mn Austenitic Steels

The grain refinement is an effective approach to strengthen high-Mn TWIP/TRIP steels. The development of recrystallized microstructure with a grain size of about one micron increases the yield strength of high-Mn steels above 500 MPa. The fine grained microstructures can be easily developed by cold rolling followed by primary recrystallization. The recrystallized grain size can be expressed by a power law function of the strain hardening during the previous cold rolling with an exponent of -2. Taking the dislocation density as the main strengthener, the grain size is an inverse proportion to the dislocation density. Then, the number density of recrystallized grains can be expressed by a power law function of dislocation density evolved during cold rolling with an exponent of about 2.

Design of Fully Austenitic Medium Manganese Steels

Due to their higher ferrite potential compared to high Mn twinning-induced plasticity (TWIP) steels, medium Mn steels usually exhibit austenitic-ferritic microstructures, which makes them suitable for third-generation advanced high-strength steel applications. Nevertheless, the strain hardening characteristics of medium Mn steels are inferior to those of fully austenitic high Mn steels. The present work introduces alloy design strategies to obtain fully austenitic medium Mn steels capable of the TWIP effect. To achieve a fully austenitic microstructure, the martensite start temperature is reduced by raising the C concentration to above 1 mass-%, which in turn facilitates the formation of cementite. The formation of cementite during cooling from austenitization temperature is counteracted by alloying with Al. Microstructural examination of slowly-cooled Fe−Mn−Al−C and Fe−Mn−C steels indicated that Al changes the morphology of intergranular cementite from plate-shaped to equiaxed.

Heating rate effect on liquid Zn-assisted embrittlement of high Mn austenitic steel

The effect of heating rate on liquid metal embrittlement (LME) of a galvanized twinning induced plasticity (TWIP) steel was investigated by hot tensile testing. The specimens were heated with different heating rates to 600, 700 and 800 °C, and elongated to 40%. During the hot tensile tests, various Fe-Zn intermetallic compounds were formed depending on the heating rate and the deformation temperature, and they were distinguished by X-ray diffraction and energy dispersive spectroscopy. δ and Γ were observed in the coating after 600 °C deformation regardless of heating rate. Since both phases did not liquefy at 600 °C, the steel was not embrittled. Heating to 700 °C sometimes produced δ which would be liquid at 700 °C. It was only produced at the highest heating rate and embrittled the steel. Γ was produced at 800 °C regardless of heating rate. Hence, the steel suffered from LME in all heating rate condition. Based on the results, the importance of selecting temperature and heating rate to study LME by hot deformation testing is discussed.

The sequential twinning-transformation induced plasticity effects in a thermomechanically processed high Mn austenitic steel

Different initial microstructures with various bimodal grain size distributions (BGSD) were produced in a high Mn austenitic steel through applying a predetermined set of thermomechanical processing cycles. The corresponding room temperature mechanical properties and the related strain hardening behaviors were assessed using tensile testing method. The results indicated that in the microstructure with high grain size bimodality, the length and amplitude of rapid hardening region was well higher than the others. This was attributed to its higher capability to α'-martensite formation. In addition, the threshold strain to initiate martensitic transformation was shifted to the lower one in the microstructure with higher bimodal grain size distribution. The latter was related to the lower arisen back stresses in the interior regions of the coarser grains. Furthermore, different transformation paths were identified as the BGSD changed. The austenite could directly transform to α'-martensite (γ→α') in the microstructure with lower BGSD; in this case the α'-martensite mainly appeared at the intersections of deformation twins. In contrast, in microstructures with higher BGSD, the nucleation occurred at the intersections of ε-martensite platelets. The co-existence of these transformation paths provided an extended transformation induced plasticity effect ending to a higher elongation to fracture in the course of deformation. In order to summarize the contribution of various strain hardening mechanisms, a deformation map was also constructed. Accordingly, the enhanced ductility/strength properties were attributed to the sequential operation of extended transformation induced plasticity and twinning induced plasticity effects.

Effect of Hot Deformation on Texture and Microstructure in Fe-Mn Austenitic Steel During Compression Loading

Hot compression tests of a new high-Mn austenitic steel were carried out at deformation temperatures of 700, 800, 900, and 1000 °C under strain rate of 0.01 s−1. The hot deformation behavior was investigated by the analyses of flow curves, texture, and deformed microstructures. Microstructures of the deformed specimens and macrotexture were examined using electron backscatter diffraction and x-ray diffraction methods, respectively. The results showed that the flow stress depended strongly on the deformation temperature and decreased by increasing deformation temperature. The microstructural evidence indicated that the dynamic recrystallization (DRX) process of experimental steel was initiated at 800 °C with necklace structure. The volume fraction of DRX grains was considerably increased by increasing deformation temperature to 1000 °C. Texture of the DRX grains tended to become a weak texture and was associated with the formation of Goss and R-Cube components. Meanwhile, martensitic transformation was detected in the hot-deformed austenite. The martensitic transformation was the most difficult in the DRX grains because of the effect of small grain size. The tendency of transformation was decreased after compression at 1000 °C.

Deformation and recrystallization microtextures of an austenitic steel during asymmetrical hot rolling process

Deformation and recrystallization microtextures during the asymmetrical hot rolling (ASHR) process were investigated for a high-Mn austenitic steel by means of electron backscattered diffraction. The effects of velocity ratio and rolling reduction on the texture evolution were also analyzed. It was found that with the increase in velocity ratio and rolling reduction, the surface layer of 60% rolled ASHR plate develops a shear texture containing the γ-fiber, rotated cube and rotated copper components, but in the central layer is still a fcc rolling texture with a minor rotation around the transverse direction. The formation of surface shear texture could be understood as a consequence of the increased shear strain level, and it followed some certain crystallographic rotational relationships with the fcc rolling texture components. Additionally, the change of deformation texture under ASHR condition had an influence on the recrystallization texture, in which the proportion of {112} and {113} orientations was slightly improved, due to their rapid growth advantage and the orientation heredity of rotated copper shear texture.

Effect of the amount and temperature of prestrain on tensile and low-cycle fatigue properties of Fe-17Mn-0.5C TRIP/TWIP steel

A high-Mn austenitic steel represents excellent combination of tensile strength and ductility, but shows very low yield strength. To increase the yield strength, pre-deformation is applied before use. However, the other properties such as fatigue resistance should also remain high. In this study the influence of pre-strain on tensile and low cycle fatigue (LCF) resistance were investigated. The amount of pre-strain (ε=0, 0.2 and 0.4) and pre-straining temperature (203K∼490K) were varied, which resulted in the variation of deformation-induced twin boundaries and fraction of martensite. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate-type samples. As the amount of pre-strain increased, yield and tensile strength were increased, but ductility and LCF life were decreased. The LCF properties of samples at a fixed amount of pre-strain were analyzed in the context of the fractions of martensite and mechanical twin boundaries. The presence of a small amount of ε-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of ε-martensite degraded the mechanical properties. In contrast, the presence of a high fraction of mechanical twin boundaries enhanced both tensile and fatigue properties under the conditions used in this work. The sizes of dislocation cells decreased and spacing of fatigue striations narrowed as the fraction of mechanical twin boundaries increased. Results suggest that a microstructure composed of ~20% ε-martensite and numerous mechanical twin boundaries can represent superior tensile and fatigue properties; this microstructure was obtained by step pre-straining (SP) process, where pre-strain was imposed twice at different temperatures.