Ultra-high-strength (over 1 GPa) hot-rolled steel sheets have been actively developed to protect passengers in cases of vehicle crashes, and their applications have been expanded to cold-rolled steel ...sheets. A major alloying element for forming meta-stable austenite is Mn in (austenite + martensite) duplex microstructures, which is readily obtained at medium-Mn level ((3–10) wt.%). However, these medium-Mn hot-rolled duplex microstructures inevitably include Mn-segregated bands, which often lead to anisotropic mechanical properties and deteriorate the strength or uniform elongation. However, in this study, we show favorable effects of the Mn-segregated band, by carefully controlling the composition, size, and shape of austenite in Mn-rich and Mn-lean bands in medium-Mn duplex steels (composition; Fe-0.1C-10Mn-1Si-0.3Mo-0.5 V (wt.%)). The austenite grown coarsely in the Mn-rich band provoked transformation-induced plasticity (TRIP) more efficiently than the austenite finely transformed from the martensite in the Mn-lean band. The Mn composition acted more dominantly on the austenite stability than the austenite size, resulting in continuous TRIP in the austenite of the Mn-rich band. This austenite enables continuous strain hardening, thereby leading to high yield and tensile strengths of (1.0–1.6) GPa together with large ductility of 20%, which offers promise for new applications to ultra-high-strength automotive hot-rolled steel sheets.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A fine-grained duplex steel with a composition of Fe–5Mn–2Al-0.6C (wt.%) was fabricated using a powder metallurgy (PM) technology of thermomechanical consolidation by hot extrusion of powder compact. ...The microstructure consisted of α′-martensite, residual austenite (RA), and a small amount of Al2O3 particles. Subsequent heat treatment was performed to adjust the microstructure and mechanical properties. This study focuses on the pre-and post-heat treatment microstructure of the material, elucidates the reasons for its formation, and explains how microstructural changes influence tensile deformation behavior. The results show that the heat treatment caused the precipitation of nanoscale carbide within the martensite, which significantly increased the yield strength from 792 MPa to 1078 MPa. While heat treatment altered the size and distribution of RA regions in the microstructure, the amount of RA transformed during deformation remained proportional to the flow stress in both materials, indicating that the flow stress dictates the amount of phase transformation of ultrafine grained RA. Therefore, in the heat-treated material with a higher yield strength, a larger amount of RA transformed during the initial stage of plastic deformation, leaving less RA to sustain a work-hardening rate in subsequent deformation, resulting in a decrease in ductility from 5.5 % to 3.6 % and fracture stress from 1674 to 1453 MPa. In addition, the in-situ formed Al2O3 particles facilitated grain refinement of the extruded rod, but had less effect on the microstructure and properties of the heat-treated sample. This study serves as a valuable reference for comprehending the influence of microstructural changes in PM manganese steel on tensile deformation behavior.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The mechanical and microstructural evolution of Alloy 2205 during severe plastic deformation is examined in this study. A combination of accumulative roll bonding (ARB) and cold rolling results in ...the successful formation of a nanograined dual-phase microstructure of austenite and ferrite with some transformed martensite. Severe deformation to cumulative reductions of 80.5, 92.5, 95, and 97 pct were performed. Microscopy indicates that grain dimensions in the sheet normal direction is less than 100 nm for reductions ≥ 92.5 pct. Shear banding is observed at reductions ≥ 95 pct while twinning is only observed at reductions < 92.5 pct. Neutron diffraction measurements indicated the presence of martensite for reductions ≥ 95 pct at ~ 8 pct volume fraction. Taken in conjunction, it appears that during initial ARB processing, both slip and twinning are active plastic mechanisms. As twinning becomes exhausted, martensitic transformation, slip, and intermittent shear banding account for the active plasticity mechanisms. Material hardness saturates at 92.5 pct reduction, with a maximum hardness of 45 HRC. Sub-sized tensile testing confirms this approximate hardness with measurements indicating a UTS of ~ 1440 MPa. Texture analysis of crystal orientation distributions in the plate normal direction suggest an approximate Kurdjumov–Sachs orientation relationship at all reductions above 80 pct indicating stability of the orientation relationship at high strains. The intragranular structure develops a fine scale sub-grain content with increasing deformation, resulting in a continual evolution of texture up to and including 97 pct reduction. The final structure presents strong components of Goss and rotated cube texture in both the austenite and ferrite. In this body of work we aim to compare ARB of an industrially relevant FCC/BCC system (Alloy 2205) to historical model FCC/BCC systems such as Cu/Nb.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This work explains the occurrence of transformation-induced plasticity via stress-assisted martensite, when designing ultrafine-grained duplex steels. It is found that, when the austenite is reduced ...to a fine scale of about 300nm, the initial deformation-induced microstructure can be dominated by parallel lamellae of ε martensite or mechanical twinning, which cannot efficiently provide nucleation sites for strain-induced martensite. Hence, α′ martensite nucleation occurs independently by a stress-assisted process that enhances transformation-induced plasticity in ultrafine-grained austenite. This metallurgical principle was validated experimentally by using a combination of transmission Kikuchi diffraction mapping, transmission electron microscopy and atom probe microscopy, and demonstrated theoretically by the thermodynamics model of stress-assisted martensite.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Despite their good corrosion resistance and optimal mechanical properties, duplex stainless steels are affected by hydrogen embrittlement. Therefore, understanding the hydrogen-defect interactions in ...these steels is crucial. This study uses internal friction to evaluate the interactions of hydrogen with microstructural defects. Analysis of the internal friction spectra of the steels subjected to straining and hydrogen charging, together with thorough microstructural characterization, gives new insights in hydrogen interactions with defects present in the different phases, i.e. ferrite (body centered cubic) and austenite (face centered cubic).
While no significant effect of tensile deformation can be observed by thermal desorption spectroscopy, the internal friction spectra show a clear influence of the presence of defects. Detailed analysis of these spectra reveals the interactions in the austenite as dominant, while no indications for hydrogen-dislocation interactions in ferrite are observed. This can be related to limited trapping in ferrite due to the austenite sink action or to limited dislocation formation in ferrite. Indications for hydrogen interactions with dislocations in the austenite are found, possibly suggesting enhanced dislocation mobility when surrounded by hydrogen. Moreover, a pronounced influence of hydrogen charging on the vacancy cluster related peaks is observed, indicating strong interactions between hydrogen and vacancy clusters in austenite. This can be put in contrast to behavior of pure ferritic steels, where dislocations provided the strongest hydrogen interactions. As these specific defects are of primary interest in the hydrogen embrittlement mechanisms, internal friction is concluded to provide important unique insights in hydrogen-defect interactions, even for complicated multiphase microstructures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The experimental quantification of retention factors related to the post-fire strength as well as the post-fire ductility of intentionally selected stainless steel grades applied in construction is ...the objective of the research presented here. These steel grades are characterized by a two-phase austenitic-ferritic microstructure of the duplex type. In this context, two mutually corresponding chromium-nickel-molybdenum steel grades are subjected to analysis, namely
steel belonging to the standard duplex group (DSS 22% Cr) and
steel belonging to the lean duplex group (LDSS). The similarities and differences in the mechanical properties exhibited by these steel grades after effective cooling, following more or less prolonged simulated fire action conforming to several development scenarios, are identified and indicated. The resistance of a given steel grade to permanent structural changes induced by the heating program proved to be the critical factor determining these properties and resulting in many cases in increased susceptibility to brittle fracture. The results obtained experimentally seem to confirm the quantitative estimates of post-fire retention factors forecast by Molkens and his team, specified for the steels exhibiting a duplex-type structure and tested by us. However, several of these estimates might be considered somewhat risky. Nevertheless, our results do not confirm the significant post-fire strengthening of steel grades belonging to the LDSS group following prior heating at a sufficiently high temperature, as reported earlier by Huang Yuner and B. Young.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of ...low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.
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•Major changes in microstructure are resulted from the electron wind energy.•Electropulsing (EP) dissolves ordered L20 (B2) phase at lower than solvus temperature.•EP deforms, fragments and refines existing B2 and accelerates precipitation of it.•EP partially recrystallizes austenite and recovers as well as spheroidizes B2.•Improvement in tensile properties by the refinement, and redistribution of phases.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The article presents the influence of heat treatment on the kinetics of transformations in lean duplex LDX2101 steel and a weld made of standard duplex 2209 material, which was welded by manual metal ...arc welding. Changes in the microstructure, hardness, and magnetic phase content were analyzed after heat treatment was conducted at a temperature of 800 °C for a period ranging from 15 to 1440 min. Light and scanning microscopy, Vickers hardness measurements, and magnetic phase content measurements using a ferritoscope were used for the research. In the LDX2101 steel, the presence of δ-ferrite and γ austenite was identified and additional Cr
N nitrides were observed in the heat-affected zone. After heat treatment, the decomposition of δ ferrite into γ
austenite and Cr
N nitrides was observed in both areas. In the case of weld made by the coated electrode in 2209 grade, a ferritic-austenitic microstructure with allotriomorphic austenite (γ
), Widmanstätten austenite (γ
), and idiomorphic austenite (γ
) and δ-ferrite area with "bee swarms" of fine precipitations of chromium nitrides Cr
N and non-metallic inclusions (NMIs) of slag, formed during the welding process, are observed in the as-welded state. After heat treatment, the presence of the χ phase (after 15 min of annealing) and the σ phase (after 120 min of annealing) was additionally identified. The kinetics of intermetallic phase evolution in welds made from 2209 material were presented. The obtained results of hardness measurements and metallographic tests were correlated, which allowed for a quick check of the precipitation processes on the used element.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The present work aims to investigate the microstructure and mechanical behavior of the dissimilar welded joint (DWJ) of martensitic A 335-grade P91 steel and high alloy ferritic austenitic A 182 F69 ...steel produced by the gas tungsten arc welding (GTAW) process. The microstructural studies showed the variety of the microstructure across the DWJ, which formed mainly due to the solidification and solid-state phase transformation as a result of the weld thermal cycle. The microstructural studies were performed in as-welded as well as post-weld heat treatment (PWHT) conditions. PWHT has been conducted to improve the mechanical properties of the P91 heat-affected zone (HAZ) at 760 °C (PW 1) and 810 °C (PW 2) for 2 h. For the mechanical properties of the DWJ, tensile tests, and microhardness tests were conducted. The fractured tensile specimen has been characterized using the scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The centre region of the weld metal showed the equiaxed dendrites while the interface near the boat fusion line, on both the P91 and F69 sides, showed the cellular and columnar dendrites. PWHT has observed a negligible effect on the microstructure and hardness of WFZ and F69 HAZ, while a significant change was observed for P91 HAZ. The tensile strength of the DWJ was measured maximum (735 MPa) for as-welded condition and reduced after the PWHT and for PW 2, it was measured lower than that of P91 and F69 base metal. The failure of the tensile specimen was also noticed in the soft inter-critical HAZ (ICHAZ) region for PW 2 condition.
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•Study on mechanical properties of dissimilar welded joint of P91 and F69 steel.•Study on the role of filler and heat treatment on the mechanical performance of the DWJ.•Detail characterization of the weld metal and HAZ interface for the different conditions of the DWJ.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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