Mn Austenitic Steel Research Articles

Sequential γ-ε-α′ transformation preserves high elongation in an ultrafine-grained high Mn steel

While grain refinement is a prominent way to improve the yield strength of metastable austenitic steels, it usually leads to a reduction in the uniform elongation as γ-α′ transformation tends to be suppressed. In the current study, it is found that the sequential γ-ε-α′ transformation helps an ultrafine-grained high Mn austenitic steel maintain high uniform elongation while having more than doubled yield strength compared to the coarse-grained counterpart. Although the grain refinement is unfavorable for the γ-ε transformation due to ε variant selection within the refined γ grains, the ε-α′ transformation is not affected as multiple α′ variants can form within one ε grain. The combination of a higher α′ transformation rate and delayed transformation kinetics enables continued strain hardening, leading to high uniform elongation.

The adverse effect of grain refinement on hydrogen embrittlement in a high Mn austenitic steel

This study presents a novel observation wherein the reduction in grain size (GS) from 20 μm to 1 μm in austenitic steel adversely affects hydrogen embrittlement (HE). The findings indicate an increase in total absorbed hydrogen with grain refinement when subjected to cathodic hydrogen charging. The results demonstrate that even with grain refinement to 1 μm, the primary mode of hydrogen damage remains intergranular, with a significant increase in relative elongation loss (REL%). This phenomenon is attributed to the early nucleation of intergranular hydrogen-induced cracks (HICs) and the accelerated propagation rates of HICs, leading to local stress increments and rapid alloy failure. The accelerated development of HICs is ascribed to the impact of grain refinement on intensified normal stress exerted on grain boundary planes, as well as the suppression of ε-martensite and its positive effect on crack arresting. Besides the enhanced degradation rate of the hydrogen-affected area (HAA) due to grain refinement, the higher ratio of HAA to the entire cross-sectional area was identified as a crucial factor contributing to rapid failure and the inverse effect of grain refinement on HE resistance.

Influence of hydrogen on deformation and embrittlement mechanisms in a high Mn austenitic steel: In-Situ neutron diffraction and diffraction line profile analysis

Austenitic steels have relatively high resistance to hydrogen embrittlement and play a critical role in hydrogen service applications. In particular, high Mn austenitic steels are considered economically viable alloy alternatives for these applications. The current study employed in-situ and ex-situ neutron diffraction techniques combined with diffraction line profile analysis (DLPA) to investigate the influence of hydrogen on deformation and embrittlement mechanisms in a high Mn (approximately 30 wt pct) austenitic steel. Investigation using both neutron diffraction and electron backscatter diffraction revealed the presence of extensive deformation twins and stacking faults within the steel microstructure after tensile deformation in the non-charged condition. These microstructural features suggest planar deformation behavior, which is expected from the relatively low stacking fault energy (SFE) of the alloy (approximately 29 mJ/m2). Hydrogen pre-charging resulted in apparent increases in both dislocations and stacking faults, contributing to macroscopic hardening and embrittlement mechanisms. Overall, numerical parameters obtained through neutron DLPA were used to elucidate the underlying mechanisms associated with hydrogen effects on the mechanical behavior, i.e. macroscopic strengthening, strain hardening rate, and embrittlement.

Sequential solution annealing of a CrMnCN austenitic stainless steel

The influence of various solution annealing treatments on the microstructure and mechanical properties of a new Cr–Mn austenitic stainless steel with 1.15 wt% (C + N) and 0.51 C/N ratio has been investigated. A sequential solution annealing (SSA) treatment is designed to sufficiently dissolve and lower numerous precipitates including carbides and nitrides in the austenite matrix. In this process, the hot-forged steel is firstly solution heat treated at 1120 °C for 2 h, then continuously heated to 1180 °C for 0.5 h, and further heated to 1200 °C for 0.5 h, followed by water quenching. The experimental results show that the austenite grain size increases, and number of precipitates decreases, with increasing temperature and time of solution annealing treatments. The highest impact energy is obtained after this SSA treatment due to a more homogeneous grain distribution, a smaller amount of carbides remaining on the grain boundary, and solid solution strengthening of interstitial (C + N) atoms.

Microstructure, mechanical properties and sensitization of ultra-low nickel Cr–Mn austenitic stainless steels

The present study deals with the effect of shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) processes with different welding speeds on microstructural and mechanical properties including intergranular corrosion susceptibility of ultra-low nickel chrome-manganese austenitic stainless steel (ASS). The microstructural analysis was performed by an optical microscope. The Fisher Ferritscope was used for determining ferrite content in the weldment. The microstructural analysis shows that the size of dendrites of δ ferrite and the width of heat affected zone (HAZ) decreases when welding speed increases. It was found more δ-ferrite in SMAW weld compared to GMAW weld. It was also found that when welding speed increases, tensile strength, impact strength and hardness values decrease for both welding processes. From the U-bending test, it was found that the welded joints are strong enough to withstand bend at 1800 without any cracks for both welding processes. It was also found that Cr-Mn ASS weld becomes susceptible to intergranular corrosion on welding. For both welding processes, the degree of sensitization value decreases with an increase in welding speed for both the welding processes as evaluated by the double loop electrochemical potentiodynamic reactivation (DLEPR) test. Also, the weld joint made using the GMAW process is more susceptible to intergranular corrosion than that made using the SMAW process.

Effect of Grain Size on the Local Deformation Behavior Associated with the Discontinuous Yielding in a High Mn Austenitic Steel

Effect of Grain Size on the Local Deformation Behavior Associated with the Discontinuous Yielding in a High Mn Austenitic Steel

Achieving ultra-high flow stress and hardness in a medium Mn stainless steel by the formation of strain-induced martensite and G-phase precipitates

This work demonstrates how the deformation of a medium Mn austenitic stainless steel and its subsequent thermal treatment results in the steel having an ultra-high flow stress, and with a microstructure containing strain-induced martensite (α′) and G-phase precipitates. A 14 wt.%Cr-8 wt.%Mn-1 wt.%Ni austenitic stainless steel was uniaxially compressed at room temperature and quasi-static strain rates to determine the evolution of strain-induced martensite with strain. Uniaxial compression resulted in a sigmoidal stress-strain curve (enhanced work-hardening) with the flow stress reaching ∼2.2 GPa, and the α′ fraction saturating at 0.8 true strain. Atom-probe tomography showed that aging at 400 °C for 72 h formed 2–3 nm sized G-phase precipitates containing Cu-rich clusters and having a nominal composition of Ni8Si2Mn20Cu17Cr8Fe44. Such a microstructure resulted in a steel with an ultra-high tensile strength and a compressive flow stress of around 2 GPa. Though the tensile ductility was low, there was sufficient flowability in compression.

In-situ investigation of the interaction between hydrogen and stacking faults in a bulk austenitic steel

The interaction between hydrogen (H) atoms and various microstructural defects remains a key to understand the H-induced damage and the subsequent premature failure of high-strength metallic materials. Previous studies on this subject are mainly focused on the in-situ probing of dislocations in a thin foil placed in an environmental transmission electron microscopy (TEM) cell. Here, a three-point bending test coupled with electron channeling contrast imaging (ECCI) has been applied to investigate the interaction of H with stacking faults (SFs) in a bulk high Mn austenitic steel. The expansion of some SFs, in terms of one partial dislocation movement within a partial dislocation pair, was observed on the H pre-charged sample when kept at a constant loading (i.e., a continuous H migration and likely build-up close to the pre-prepared notch tip). A temporal-resolved cross-correlation EBSD (CC-EBSD) measurement shows that the migration of H towards the notch tip region has a minor effect on the internal stress evolution. However, the local shear modulus (μ) and stacking fault energy (γSF) can be reduced by a local H segregation. Further theoretical calculation of SF ribbon width indicates that the reduction of μ by H results in the shrinkage of SF, while the H-induced reduction of γSF results in SF expansion at a lower resolved shear stress. The observed expansion of SF ribbons can be interrupted by a combined reduction of both μ and γSF due to H.

Enhancing strength and ductility in a near medium Mn austenitic steel via multiple deformation mechanisms through nanoprecipitation

High-Mn austenitic steels, usually deformed via the TWIP (twinning induced plasticity) mechanism, suffer high cost and other technical issues (e.g., hot-dip galvanizing, welding, etc.) from the high Mn content. Nevertheless, decreasing the Mn content would result in a shift of deformation mechanisms from TWIP to TRIP (transformation induced plasticity), usually causing quasi-cleavage brittle fracture. Herein, we report that by massive nanoprecipitation of coherent disordered particles, the grain sizes of a near medium Mn austenitic steel are successfully refined to 0.9 ± 0.4 μm, leading to a significant stacking fault energy increment of 6.4–7.9 mJ m–2 and accordingly, the transition of deformation mechanism from TRIP to multiple deformation mechanisms, namely, stacking faults, dislocation slip, nanotwinning and ԑ-martensite transformation. Moreover, the high-density of coherent nanoprecipitates effectively refines ԑ martensite and nanotwins from 20–500 nm and 10–50 nm to a few atomic columns and 1–15 nm, respectively. More importantly, these deformation mechanisms were sequentially activated at different deformation stages, resulting in a consistently high work hardening rate. Based on the synergistic refinement effects of grains, twins and martensite and the transition of deformation mechanism, a near medium Mn ultrafine-grained (UFG) austenitic steel with 15 wt.% Mn is developed, which shows a unique combination of high tensile strength (1210 ± 19 MPa) and large elongation (72 ± 6 %). These findings provide a new route to addressing the trade-off between the Mn content and mechanical performance of high-Mn austenitic steels which could facilitate their widespread applications.

Exploring the improvement of strength and cryogenic impact toughness in hot-rolled high Mn austenitic steel for cryogenic application

In general, there is a steep decrease in cryogenic (−196 °C) impact absorbed energy of high Mn austenitic steels after aging at 600–800 °C, which is also the temperature range of aging precipitation. Thus, the precipitation hardening is hardly used in hot-rolled high Mn austenitic steel for cryogenic application and its effect on cryogenic impact toughness is also not understood up to now. In the present work, A high Mn austenitic steel alloyed with Al and V was designed, and the effect of aging temperature on the evolution of microstructure and mechanical properties was investigated by means of electron backscattered diffraction, transmission electron microscopy and X-ray diffraction. The grain size is nearly not affected by aging temperature, exhibiting similar grain size of 6.7–8.1 μm, but the volume fraction and size of precipitated particles greatly depend on aging temperature. The average particle size slightly increases from 5.6 to 6.7 nm with increasing aging temperature from 550 to 650 °C, whereas the precipitated particles are greatly coarsened at 700 °C and the average particle size reaches 16.3 nm. These fine particles can provide a sufficient precipitation hardening of about 200 MPa, leading to a high yield strength of 587 MPa. Meanwhile, this strong precipitation hardening only slight lowers cryogenic impact absorbed energy from 132 to 97 J, also implying that the above-mentioned aging brittle effect should be suppressed or very weak.

Improved Mechanical Properties of Two Twinning‐Induced Plasticity Steels with Novel Grain Morphology

The mechanical properties and microstructures of two high Mn austenitic steels with novel grain morphology prepared by directional solidification and warm forging are investigated and compared with common twinning‐induced plasticity (TWIP) steel with equiaxed grains (EGs). The directional solidified sample (DS) is composed of columnar grains with simplified grain boundaries mainly characterized by low‐angle boundaries (LABs) with a fraction of 0.886. The warm forged sample (WF) contains numerous fibrous‐liked grains and fine nearly equiaxed grains featured with dominate LABs and complicated dislocation substructure. The significantly improved elongation and energy absorption capability of DS compared with EG are ascribed to the larger dislocation mean free path together with better plastic deformation compatibility and synergy between and within grains of DS, respectively. The largest energy absorption efficiency with enhanced yield strength of WF is mainly attributed to numerous fine grains with dominated LABs and complicated dislocation substructure. Inhomogeneous deformation characterized by well‐developed twins structure with dense dislocation pileup occurs in some regions of WF. It will contribute to enhancing work hardening, leading to a relatively high uniform elongation and significant increased strength. On the contrary, it may easily cause severe stress concentration and promote the occurrence of necking followed by plastic instability.

Grain Orientation Spread in Dynamically Recrystallized Austenitic Steel

Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10-3-10-1 s-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.

Activation of a hybrid twinning mechanism in a Cr-Ni-Si-V-N medium manganese austenitic steel containing precipitates

Twinning nucleation in a medium Mn austenitic steel containing precipitates is studied at low strain ~ 0.02, using transmission electron microscopy. In the near vicinity of the precipitates, unfaulting dislocation reaction is favored, while twinning is activated farther from the precipitates. Twin nucleation follows a hybrid mechanism, involving creation of stacking faults through classical dislocation dissociation, while those stacking faults subsequently overlapped following a non-classical alternated stacking fault pair mechanism to create a three-layer twin nucleus. The observed twinning behavior is interpreted from an energy barrier variation viewpoint within the matrix.

Dominant factors of cryogenic toughness of heat-affected zone of welding in high Mn austenitic steel

ABSTRACT High Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperatures by adding a large amount of Mn which are γ-stabilizing elements. In order to clarify the factors governing the cryogenic toughness of heat-affected zone (HAZ) of welds, investigation of relationship between microstructure and Charpy impact toughness of multi-pass welded joint and a simulated thermal cycle test was conducted for high Mn steel containing 0.5%C-25%Mn-5%Cr. In the MAG welding, HAZ near fusion line showed high absorbed energy, whereas it was minimized at 5 mm towards the HAZ side from fusion line reheated up to around 1073 K. Furthermore, in a simulated thermal cycle test of continuous cooling, the absorbed energy was also minimized at 1073 K. On the other hand, in the simulated thermal cycle test of quenching, the inverse dependence of the grain size on a conventional steel was shown, and the toughness increased as the grain size became coarse. Grain growth occurs according to the maximum temperature of HAZ, and M23C6 is formed on the grain boundary in the subsequent continuous cooling process. It was clarified that M23C6 became only initiation site of fracture under Charpy impact loading, and its existence density was the main factor of initiation energy. On the other hand, propagation process is a ductile fracture phenomenon in which γ stability is maintained regardless of the presence of M23C6.

Change in Charpy impact toughness and carbide precipitation after ageing heat treatment of high Mn austenitic steel

ABSTRACT High-Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperature by adding a large amount of C and Mn which are γ-stabilizing elements. In some cases, Cr is added together with C as a solid solution strengthening element to the high-Mn austenitic steel. Depending on the conditions of ageing heat treatment, the formation of Cr carbide may affect the mechanical properties. In this study, the formation behaviour of Cr carbide and the effect on cryogenic Charpy impact toughness in the ageing heat treatment at from 773 to 1273 K for 30 s to 18 ks of a high-Mn austenitic steel with 0.5%C-25%Mn-5%Cr. Regardless of the ageing heat treatment conditions, no cleaved but 100% ductile fracture surface was observed in all samples after the Charpy impact test at 77 K. On the other hand, M23C6 containing Cr was formed on the austenite grain boundaries in the sample aged for a long time at 1073 K, and the Charpy absorbed energy showed lower than those of the other ageing conditions. As the results of both detailed microstructure observation of M23C6 and adjacent austenite matrix and DICTRA simulation, it was clarified that M23C6 mainly act as a fracture initiation point but propagation to the matrix becomes ductile because no sharp alloy deficiency layer is formed at the M23C6/austenite matrix interface. As a result of calculation of stacking fault energy using a thermochemical model, it was confirmed that γ stability at cryogenic temperature could be sufficiently secured even in the alloy-deficient region near M23C6 as well as in the stable matrix.

Corrosion behaviour of Al-added high Mn austenitic steels in molten lead bismuth eutectic with saturated and low oxygen concentrations at 450 ℃

Corrosion tests were performed on newly developed alumina-forming austenitic (AFA) steels in stagnant lead bismuth eutectic (LBE) with saturated and low oxygen concentrations at 450 ℃ for 430 h. The steels exhibited enhanced corrosion resistance to the LBE environments with the increasing of Al content. A continuous and protective Al-rich oxide scale formed on the steel specimens that were exposed to LBE with a low oxygen concentration, whereas a non-protective and stratified oxide scale formed in the oxygen saturated LBE.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stacking fault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategy to address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructural transitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of research on the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels, this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mn transformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steel with improved weld characteristics by presenting many research results on the correlations between the microstructure and mechanical properties of welds and heat-affected zones for these three different steel groups.

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

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.