Stability Of Austenite Research Articles

Ultrastrong and ductile additively-manufactured medium-carbon steel via modulating austenite stability

Selective laser melting (SLM) is a very promising near net shape approach to manufacture metallic complex-shaped parts. However, it is highly challenging to manufacture ultrahigh strength medium-carbon steels via SLM due to the cracking and low ductility issues. In this contribution, the mechanisms of crack and limited ductility in the conventional medium-carbon H13 steels by SLM were investigated, which was likely attributed to the poor stability of retained austenite (RA) and the resultant formation of brittle fresh martensite. A modified crack-free H13 steel for SLM was successfully additively manufactured via in-situ minor alloying of 316 L powders, leading to the substantially enhanced austenite stability with heterogeneity. The modified SLM-fabricated H13 steel exhibited an ultrahigh tensile strength (UTS) of ∼2.0 GPa with a fractured elongation of ∼19%. This work demonstrates the critical role of austenite stability in cracking behavior and ductility of the SLM-fabricated medium-carbon steels.

The significant role of bimodal lamellar heterostructure for Lüders deformation and TRIP effect in 18Cr–8Ni austenitic stainless steel

A series of 18Cr–8Ni ASSs with homogeneous or heterogeneous structures were prepared to reveal the unique deformation mechanism of hetero-deformation-induced (HDI) hardening in lamellar heterostructure. By adjusting the annealing temperature, the nano-grained (NG) and coarse-grained (CG) unimodal homogeneous structures were obtained by annealing at 600 °C and 1000 °C, respectively, while the nano-grained/ultrafine-grained (NG/UFG) and ultrafine-grained/fine-grained (UFG/FG) bimodal heterogeneous structures were achieved by annealing at 700 °C and 800 °C. Compared to the conventional bimodal homogeneous structure (UFG/FG), the hetero-zone boundaries (HBs) of bimodal lamellar heterostructure (NG/UFG) can not only provide extra HDI hardening effect, but also coordinate the inhomogeneous deformation for Lüders deformation. The HDI hardening and Lüders deformation greatly facilitate the transformation induced plasticity (TRIP) effect to achieve. This is due to the lower austenite stability in soft region (coarser grain), leading to the formation of deformation-induced martensite (DIM) under lower stress, on the other hand, the Lüders deformation achieved a dynamic balance between both sides of hetero-boundary, avoiding the stress concentration and promoting the TRIP effect. Consequently, the DIM content of the NG/UFG specimen is considerably higher than that of the UFG/FG specimen (78 vol% to 27 vol%).

Fundamental aspects of phase formation in WC-based cemented carbides containing FeMn-based binders

The hardmetal industry is continuously seeking for (Co, Ni)-free binder alternatives. Several attempts have been made to use FeMn binders, Mn being a very effective austenite stabilizer. Substitution of Co binders for FeMn based binders comes along with many challenges such as: narrow carbon windows, evaporation of Mn during sintering or the formation of stable Mn-oxides. This work presents an overview on the phase formation in hardmetals with different carbon contents prepared from FeMn, FeMnSi and FeMnNi binders. Due to the efficient role of Mn as cementite stabilizer, it was not possible to avoid the presence of eta-carbides and cementite under the conditions studied, and samples with intermediate carbon contents presented both eta-carbides and cementite in the microstructure. Only after the addition of 25 wt% of Ni to the binder it was possible to observe materials with two phase microstructures. The challenges related to Mn evaporation and formation of stable oxides can be overcome by a proper selection of the starting materials and the sintering conditions. It is thus proved that the main challenge posed by these materials is the difficulty to obtain microstructures without detrimental phases.

In situ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

The yield stress of Fe-24Ni-0.3C (wt%) metastable austenitic steel increased 3.5 times (158 → 551 MPa) when the average grain size decreased from 35 μm (coarse-grained [CG]) to 0.5 μm (ultrafine-grained [UFG]), whereas the tensile elongation was kept large (0.87 → 0.82). In situ neutron diffraction measurements of the CG and UFG Fe-24Ni-0.3C steels were performed during tensile deformation at room temperature to quantitatively elucidate the influence of grain size on the mechanical properties and deformation mechanisms. The initial stages of plastic deformation in the CG and UFG specimens were dominated by dislocation slip, with deformation-induced martensitic transformation (DIMT) also occurring in the later stage of deformation. Results show that grain refinement increases the initiation stress of DIMT largely and suppresses the rate of DIMT concerning the strain, which is attributed to the following effects. (i) Grain refinement increased the stabilization of austenite and considerably delayed the initiation of DIMT in the 〈111〉//LD (LD: loading direction) austenite grains, which were the most stable grains for DIMT. As a result, most of the 〈111〉//LD austenite grains in the UFG specimen failed to transform into martensite. (ii) Grain refinement also suppressed the autocatalytic effect of the martensitic transformation. Nevertheless, the DIMT with the low transformation rate in the UFG specimen was more efficient in increasing the flow stress and more appropriate to maintain uniform deformation than that in the CG specimen during deformation. The above phenomena mutually contributed to the excellent combination of strength and ductility of the UFG metastable austenitic steel.

Role of Ni content on microstructural and mechanical responses of Nb-stabilized metastable austenitic stainless steel weld metals

Three Nb-stabilized metastable austenitic stainless steel (15Cr–9Ni–Nb steel) weld metals with Ni contents of 8.4 wt%, 9.2 wt%, and 10.2 wt% were designed and prepared by gas tungsten arc welding. The effects of Ni content on the solidification behavior, microstructure, and the mechanism of deformation-induced martensitic transformation in the tensile process were investigated via scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, and high-resolution transmission electron microscopy, and their effects on mechanical properties were also discussed. As the Ni content increased, the solidification mode of weld metals changed from ferrite-austenite mode to austenite-ferrite mode, which made the segregation of Nb and Si at the interdendritic region more obvious and increased the total volume and the average size of primary NbC significantly. The large amount of coarse primary NbC in the AF weld metal damaged the impact toughness and reduction of area by inducing intergranular cracks and intergranular fractures. Moreover, both the three weld metals had poor mechanical stability and the deformation-induced martensitic transformation (DIMT) (γ→α′) occurred during the tensile process, which contributed to increased tensile strength. As the Ni content increased, the mechanical stability of austenite increased, and the strengthing effect of DIMT decreased, while the uniform elongation of weld metals increased.

Tailoring microstructure evolution and austenite stability of TRIP steels by Rare-Earth micro-alloying

The microstructure evolution and austenite stability of Rare Earth (RE)-alloyed Transformation Induced Plasticity (TRIP) steels subjected to intercritical annealing and isothermal bainitic transformation have been investigated for superior strength-ductility performance. Structure morphology, austenitizing temperature and phase proportion during thermomechanical process were carefully compared, confirming that minor RE additions indeed refined the microstructure, and affected the thermodynamics of austenite and bainite transformation. Austenitizing ranges of TRIP steels were lifted with increasing RE content then stabilized when exceeding 360 ppm; the intercritical austenite reduced but dissolved more CMn atoms, hence the bainite transformation in RE-alloyed TRIP steels was significantly inhibited, which elevated the retained austenite quantity and thermal stability simultaneously. Therein, more austenite nucleated from the structure fragments with RE-refined lamellar morphologies and strictly grew into acicular counterparts or aggregated into clusters. The aggregated austenite, composed of several subgrains with [011¯] coaxial relations or Σ3 60°〈111〉 misorientation but coherent interfaces, were engulfed or being swallowed up by coalescing ferrites in sluggish growth, guaranteeing the higher mechanical stability than intergranular austenite. In this study, the product of strength and elongation of TRIP steels after optimized RE-alloying was increased by 38% as compared to that without RE additions. The micro-alloying effect of RE elements in TRIP steels highlights the innovations of new-generation automotive steels.

Fe-14Ni-14Cr-2.5Al steel showing excellent corrosion-resistance in flowing LBE at 550 °C and high temperature strength

We investigated the corrosion behavior, microstructural evolutions, and mechanical properties in an alumina forming austenitic (AFA) Fe-14Ni-14Cr-2.5Al steel after exposure in the flowing lead-bismuth eutectic (LBE) at 550 °C for 4008 h and aging in air for 5000 h. The results showed that the AFA steel exhibited not only excellent corrosion-resistance in the flowing LBE but also a strong stability of austenite and high yield strength (589 MPa) at 550 °C. The excellent corrosion-resistance was ascribed to the rapid formation of a thin Al2O3 scale. For the first time, the precipitation of γ'-Ni3Al phase was observed in the AFA steel after long-term aging at 550 °C.

Micromechanics-based understanding of the stability of film-like austenite in steels

Based on the carbon content and processing technique, the austenite morphology in steels can be either film-like or blocky. Experiments have shown that a film-like morphology of austenite shows a higher resistance to strain-induced martensitic transformation than its blocky counterpart. In the present work, using aspect ratio to distinguish austenite morphology, a micromechanical model is developed to show that the presence of an unfavourable stress field discourages the austenite to martensite transformation in film-like morphology, thereby lending its stable nature compared to the blocky version.

On the Temperature-Dependence of Deformation-Induced Martensite Formation in AISI 304L Type Steel

In this study, the deformation-induced α-martensite formation in AISI 304L steel was investigated in the temperature range between 75 °C and − 196 °C in the light of the temperature-dependence of hydrogen embrittlement phenomena. For this purpose, tensile tests with in-situ and ex-situ magnetic measurement of the α-martensite volume content as a function of plastic strain were carried out. In addition, a theoretical assessment of the temperature-dependence of the austenite stability was undertaken, evaluating chemical and non-chemical driving force contributions to the martensitic γ → α transformation as proposed by Ghosh and Olson. The experimental results clearly show an increase in the α-martensite volume content and a shifting of the phase transformation to lower strain levels upon reducing the temperature to − 75 °C. A further reduction of the temperature to − 196 °C revealed to have no significant impact on the α-martensite formation. The theoretical assessments indicate a similar temperature-dependence of the austenite stability as observed experimentally and suggest contributions of the thermal friction work at the fcc/bcc interface to be responsible for a constant austenite stability in low-temperature regimes. Additional investigations of deformed microstructures showed that element segregation effects on the local austenite stability are pronounced around room temperature, but become less relevant at low temperatures.

Effect of precipitation-induced element partitioning during tempering on mechanical properties of hot-rolled 3Mn steel after intercritical annealing

This work explores the carbide evolution behavior and mechanical property evolution mechanisms of intercritical-annealed 3Mn steel during the tempering process by comparing different tempering times (5/10/20 min). The results showed that the elongation of 3Mn steel increased from 19.5% to 26.5% after tempering while maintaining a tensile strength above 1250 MPa. It is also found that carbide precipitation during tempering is first followed by dissolution. The mechanical properties exhibited abnormal degradation at tempering for 10 min, with a tensile strength of 1265 MPa and a total elongation of 16.0%. However, the optimal mechanical properties were achieved at tempering for 20 min, with a tensile strength of 1286 MPa and a total elongation of 26.5%. During the initial stage of tempering, a transformation occurs from M3C to stable M7C3, with the presence of numerous dislocations and grain boundaries promoting the precipitation and growth of abundant carbides. This leads to a reduction in the C and Mn content in the matrix, consequently decreasing the austenite content and stability, resulting in the transformation of austenite to the BCC phase during tempering instead of cooling. The decreased austenite content and stability lead to reduced ductility. Subsequently, dislocations were annihilated during tempering, suppressing carbide precipitation, and the over-precipitated carbides dissolved under thermodynamic driving force, leading to an increase in C and Mn content in the remaining austenite, thus enhancing its stability. The precipitation and dissolution of carbides during tempering, along with the partitioning of elements between carbides and austenite, strengthen austenite stability and improve mechanical properties.

Hydrogen Embrittlement Behavior of a Commercial QP980 Steel

The hydrogen embrittlement (HE) behavior of a commercial QP980 steel is studied in this work. The HE susceptibility results indicate that QP980 suffers from a severe HE, and the fracture mode transforms from ductile dimpling to brittle quasi-cleavage under the attack of hydrogen. The EBSD results show that strain-induced martensite transformation can rarely occur at a strain close to the HE fracture strain, which is mainly attributed to the high mechanical stability of austenite. The TKD-KAM analysis results indicate that hydrogen-induced strain localization in martensite can be mitigated by the hydrogen-trapping effect of surrounding austenite, while it is most pronounced in martensite adjacent to ferrite. Correspondingly, HE cracking is considered to initiate in martensite adjacent to ferrite under the synergistic action of HELP and HEDE mechanisms, and then cracks can propagate through ferrite or along phase interfaces. Our findings suggest that to further improve the HE resistance of QP steel with stable austenite, it is necessary to consider introducing effective hydrogen-trapping sites (such as carbides, film austenite) into martensite, which is deemed to be beneficial for increasing the resistance against hydrogen-induced cracking initiation in martensite.

Manipulation of magnetocaloric and elastocaloric effects in Ni–Mn–In alloys by lattice volume and magnetic variation: Effect of Co and Fe co-doping

The effects of Co and Fe co-doping Ni–Mn–In alloy on the phase stability, lattice parameters, magnetic properties, and electronic structures are systematically investigated by using the first-principles calculations. Results indicate that Fe atoms replace the excess Mn2 atoms by direct and indirect coexistence (Fe→Mn2 and Fe→In→Mn2); Co substitutes the Ni atoms by direct substitution (Co→Ni) for the Ni–Mn–In alloy. The austenites all exhibit the ferromagnetic (FM) state for the studied compositions. The NM martensites are in the ferrimagnetic (FIM-1) state for the Ni2Mn1.5In0.5, Ni2Mn1.25In0.5Fe0.25, Ni1.75Mn1.5In0.5Co0.25, and Ni1.75Mn1.25In0.5Co0.25Fe0.25 alloys, while the other compositions are in the FM state. The phase stability of austenite and martensite decreases with increasing Co and Fe co-doping. A magnetic-structural coupling transition occurs at x < 0.25 and y < 0.25. The Ni1.91Mn1.5In0.5Co0.08 and Ni1.91Mn1.42In0.5Co0.08Fe0.08 alloys exhibit an A→6M→NM transformation, accompanied by a magnetic transition. When Co and Fe are co-doped, the hybridization strength between Co and Fe is greater than that between Co/Fe and Mn. The enhancement of magnetocaloric and elastocaloric effects is favored by larger magnetization difference (∆M) and lattice volume change (∆V/V0). Based on the calculated phase stability, magneto-structure coupling, ∆V/V0 and c/a ratio, one can predict that the Ni2-xMn1.5-yIn0.5CoxFey alloy with Co content 0 ≤ x ≤ 0.25 and Fe content 0 ≤ y ≤ 0.05 is predicted to have good magneto-controlled functional behavior.

Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV_{10}) of 320 J cm^{-2} compared to 66 J cm^{-2} in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

Realizing strength–ductility synergy in a lean duplex stainless steel through enhanced TRIP effect via pulsed electric current treatment

Optimization of the strength–ductility compromise in lean duplex stainless steels (LDSSs) often involves controlling the stability of austenite by chemically adjusting it to be metastable and enabling phase transformation through a transformation-induced plasticity (TRIP) effect during plastic deformation. This study investigated the effects of pulsed electric current (PEC) treatment on the spatial distribution of alloying elements within the microstructure and enhancement of the mechanical properties of a new type of LDSS material, NSSC2120. The results showed that PEC treatment had a significant positive impact on the mechanical properties of NSSC2120. Specifically, after applying 20 pulses of PEC treatment at a current density of 700 A/mm2 for 8 ms, the material exhibited a 27% increase in ductility and 8% increase in strength. The microstructural evolution induced by the PEC treatments was investigated using quasi-in-situ electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The PEC treatment facilitated the diffusion of alloying elements, which played a crucial role in triggering the TRIP effect during plastic deformation owing to the decreased stability of austenite with a lower content of austenite-stabilizing elements, thereby resulting in simultaneous improvement in both strength and ductility. Significantly, the rapid thermal process employed in the PEC treatment did not give rise to detrimental precipitates within the microstructure of the material, rendering PEC treatment a promising alternative to conventional heat treatment. The simplicity, speed, and low energy consumption associated with the PEC treatment make it an appealing choice for processing similar materials and achieving both microstructural modification and optimization of mechanical properties.

Effect of element segregation on deformation mechanisms of nano/ultrafine-grained 304 stainless steel

The effect of Cr and Ni element segregation on austenite stability and deformation mechanisms of nano/ultrafine-grained 304 stainless steel obtained by cryogenic rolling plus annealing treatment was systematically studied. The results showed that the nano/ultrafine-grained steel had a good combination of strength and ductility (yield strength ∼896 MPa, tensile strength ∼977 MPa, and elongation ∼39.6%). According to the findings, the formation of Cr segregation in the body-centered cubic structure at the intersection of austenite grain boundaries and localised austenite grains with low Ni content reduced the austenite stability of the nano/ultrafine-grained steel and promoted the generation of strain-induced martensite during plastic deformation, resulting in the deformation mechanism dominated by the transformation-induced plasticity effect.

Improved Mn–segregation bands and its influence on deformation behavior in a severely warm–rolled 10Mn lightweight steel

The hot–rolled microstructures of medium Mn lightweight steels led to the formation of Mn–segregation bands due to a relatively high Mn content of 9–12 wt%, ultimately deteriorating their overall mechanical properties. In this study, a multi–pass warm rolling of 95% reduction was proposed to minimize the Mn–segregation bands in the 10 wt% Mn lightweight steel. This was followed by the detailed microstructural examinations and ex–situ tensile testing to evaluate the mechanical behavior response and deformation mechanism operating upon straining. Severe warm rolling accelerated the dynamic ferrite transformation and recrystallization in the Mn–rich austenitic bands, thus improving the microstructural homogeneity. The austenite stability was further improved so as to enable continuous strain hardening by transformation/twinning–induced plasticity (TRIP/TWIP) effects. This, thereby, led to obtain high tensile strength of 985–1201 ± 15 MPa together with large elongation of 34.3–76.0 ± 1.1%. In addition, the strain compatibility between ferrite and austenite also proves to give rise to an alleviation of strain localization at the ferrite/austenite interface, which also postponed the plastic instability and contributed to the large ductility by the coordinated TRIP/TWIP effects.

Experimental Analysis of the Stability of Retained Austenite in a Low‐Alloy 42CrSi Steel after Different Quenching and Partitioning Heat Treatments

Quenching and partitioning (Q&amp;P) steels are characterized by an excellent combination of strength and ductility, opening up great potentials for advanced lightweight components. The Q&amp;P treatment results in microstructures with a martensitic matrix being responsible for increased strength whereas interstitially enriched metastable retained austenite (RA) contributes to excellent ductility. Herein, a comprehensive experimental characterization of microstructure evolution and austenite stability is carried out on a 42CrSi steel being subjected to different Q&amp;P treatments. The microstructure of both conditions is characterized by scanning electron microscopy as well as X‐ray diffraction (XRD) phase analysis. Besides macroscopic standard tensile tests, RA evolution under tensile loading is investigated by in situ XRD using synchrotron and laboratory methods. As a result of different quenching temperatures, the two conditions considered are characterized by different RA contents and morphologies, resulting in different strain hardening behaviors as well as strength and ductility values under tensile loading. In situ synchrotron measurements show differences in the transformation kinetics being rationalized by the different morphologies of the RA. Eventually, the evolution of the phase specific stresses can be explained by the well‐known Masing model.

Comparative study on effects of annealing temperature on microstructures and mechanical properties of high-Al low-Si steel

We elucidate here profound effects of quenching and partitioning process after partial (IQP) or full austenitization (FQP) on the microstructure and mechanical properties of high strength hot-dip galvanized steel with high Al and low Si content. The results showed that the FQP steel contains 3.38% vol.% retained austenite with mainly film-like morphology, while the IQP steel mainly consists of 6.44–8.13 vol.% retained austenite with block shape. Additionally, retained austenite in the IQP sample transforms into martensite more easily duo to higher nucleation rate, which means lower austenite stability. A combination of various austenite grain size and morphology make the TRIP effect throughout the whole deformation process and increased the elongation for IQP sample. When the steel was treated by annealing temperature of 967 °C, partitioning temperature and time of 460 °C and 100 s respectively, the steel exhibited excellent mechanical properties with tensile strength of 754 MPa, total elongation of 27% and the product of strength and ductility exceeds 20 GPa%. The proposed content in the present study demonstrates compatibility with the continuous hot-dip galvanizing process.

Synergistic improvement of strength and ductility via doping cerium into PH13–8Mo stainless steel by laser powder bed fusion

In this study, the PH13–8Mo stainless steel parts doped without and with cerium (Ce) were fabricated via laser powder bed fusion followed by post-heat treatment, and systematically compared in terms of microstructure, phase constituent, and tensile properties. The comparative results show that doping Ce-modified grains with the equiaxed morphology and finer size, increased the mechanical stability of austenite, and enhanced the sphericity of oxide inclusion in the resultant PH13–8Mo. Additionally, the coherency between the newly-formed CeAlO3 inclusion and matrix was effectively improved after doping Ce, as compared to the original aluminum oxide inclusion without doping Ce. The resultant PH13–8Mo parts doped with Ce yielded an ultimate tensile strength of 1446 ± 20 MPa with a fracture elongation of 16.0% ± 1.5%, for the first time meeting the AMS 5629E H1000 standard for the PH13–8Mo made by additive manufacturing (AM). The enhanced strength results from the strengthening effects of nanoscale precipitation (inclusion) and grain refining. Meanwhile, the ductilizing mechanism can be attributed to the enhanced inclusion sphericity and ameliorative coherency between the inclusion and matrix, and improved misorientation angle of grain boundary owing to the modification by Ce, which efficiently reduced the stress concentration and enhanced cracking resistance during deformation. Therefore, doping rare earth elements presents a promising pathway to synergistically improve the strength and ductility of stainless steels by AM.

The role of nickel in forming a structure providing increased service properties of reactor structural materials

Nickel is an essential alloying element in steels used as structural materials in the most common nuclear power reactors of the VVER type. The paper considers reviews the results of structural studies of traditional and advanced materials of the vessels and internals of VVER-type reactors with high nickel contents in their compositions. It is shown that an increased nickel content (up to 5 wt.%) in the steels of VVER pressure vessels contributes to the formation of a more dispersed structure with a smaller size of substructural elements and an increased density of dislocations, as well as a higher volume density of carbide phases. The revealed features of the structure of the reactor pressure vessel steel with high nickel content have the prerequisites for improving the strength and viscoplastic properties due to the increased number of barriers both for the dislocation motion and brittle crack propagation. Using the example of materials for VVER internals, it is shown that the nickel content increased in them up to 25 wt.% contributes to an increase in the volume density of radiation defects (dislocation loops of various types) and radiation-induced phase precipitates (G-phase). As nickel increases from 10 to 25 wt.%, there is a tendency to reduce swelling, which contributes to less shape change of the components of the reactor vessel internals. At the same time, in the steel with the highest nickel content, the highest nickel content was found in the near-boundary regions of the matrix, which contributes to greater austenite stability and a lower probability of the formation of an embrittling α-phase. The data obtained in the work on the effect of nickel alloying on the steel structural phase state and service characteristics were used in the development of new materials for the vessels and internals of advanced reactors.