Showing papers on "Ferrite (iron) published in 2022"

Tensile strength improvement of martensitic ODS steels with Zr and Hf additions

Abstract: Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M23C6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y2O3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y2O3, ZrO2 and Y4Zr3O12 were identified in 9Cr–Zr steel, and YxHfyOz and Y2H2O7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance.

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

Abstract: In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND// preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

Austenite transformation and deformation behavior of a cold-rolled medium-Mn steel under different annealing temperatures

Abstract: In this study, the austenite transformation in a cold-rolled medium-Mn steel (MMnS; 7 wt% Mn) was adjusted by inter-critical annealing (IA) at 680 °C (above the Ac1 temperature) and by partition annealing (PA) at 650 °C (below the Ac1 temperature). The deformation behavior associated with the microstructural evolution, and crystallographic changes were investigated using in situ synchrotron X-ray diffraction during tensile deformation. Electron backscatter diffraction was used to characterize the microstructure. A considerable amount of austenite (approximately 30 and 20 vol%) was promoted by reversion transformation during the IA and PA treatments, respectively. The difference in deformation behaviors between the IA and PA specimens was attributed to the different mechanical stabilities of the reverted austenite. The relatively low mechanical stability of retained austenite (RA), due to less Mn enrichment during IA, led to a pronounced activation of the transformation-induced plasticity effect, which improved the strain hardening capacity, the ultimate tensile strength, and total elongation. However, the low recovered/recrystallized fraction of the ferrite phase resulting from PA contributed to a significant increase in the yield strength. The current understanding of the characteristics and mechanical stability of RA induced by annealing at different temperatures below and above the Ac1 temperature will help in further optimizing annealing parameters to achieve better mechanical properties for MMnS.

Influence of Si content on the microstructure and mechanical properties of silicon stainless steel

Abstract: The influence of Si addition on the microstructure and mechanical behavior of cast silicon stainless steel alloys was investigated. It was recognized that after incorporation of Si in the range of ∼ 2–6 wt%, microstructure changes significantly. It transforms from single-phase austenitic to duplex microstructure containing ferrite and austenite. In addition, TiC precipitates with angular and spherical morphology were observed in all the sample conditions. Uniaxial tensile tests suggested that the incorporation of Si improved the strength and ductility as long as the microstructure remains primarily austenitic. However, at much higher silicon content (∼6 wt%), microstructure transformed to duplex structure, followed by the drastic reduction in ductility and toughness. Overall, the alloy with ∼4 wt% Si exhibited comparatively better strength, ductility as well as toughness. The mechanical properties of the individual phases, characterized using nanoindentation, indicated that the hardness and modulus of the austenite phase improved significantly after Si incorporation up to ∼6 wt%. Concomitantly correlation between the mechanical properties and crystallographic orientation of grains was also established by performing post-nanoindentation EBSD analysis. The results indicated that the average hardness and modulus of each phase are significantly related to the crystallographic orientation of grains, i.e., it is highly anisotropic. Finally, the nanomechanical properties of the individual phases were utilized to predict and compare the bulk mechanical properties using the ECM model. A reliable quantitative comparison between nanomechanical and bulk mechanical properties of alloys with single-phase microstructure was obtained, whereas a more refined ECMs are required for envisaging the bulk characteristics of the materials with duplex microstructure.

Hydrogen embrittlement fracture mechanism of 430 ferritic stainless steel: The significant role of carbides and dislocations

Abstract: The effect of electrochemical hydrogen charging on the mechanical properties and the fracture mechanism of AISI430 ferritic stainless steel were studied by tensile test. Electrochemical hydrogenation experiments of 2, 4, 8 and 12 h on the tensile specimen with a current density of 50 mA/cm2. Hydrogen has a significant impact on the mechanical properties of the material: the elongation of the material is reduced from 27.8% to 13.0%, the yield strength is increased from 325 MPa to 380 MPa, and the tensile strength is increased from 500 MPa to 575 MPa. The obvious increase in tensile strength is attributed to two aspects: (1) the hydrogen-induced dislocations pinning; (2) the influence of the second phase carbide particles on the critical shear stress of dislocation glide. As the hydrogen outgassing time increases, the hardness measurement proves the softening effect of hydrogen. Since the reduction of the binding energy of the carbide and ferrite matrix interface is attributed to the HEDE mechanism, the crack nucleation in the vicinity of the carbide leads to the generation of the cleavage (C) region. Under low diffusible hydrogen content, the samples are mainly C mode and F-MVC mode guided by the synergistic effect of hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced vacancies (HESIV) mechanisms and under high diffusible hydrogen content, the transgranular (TG) mode replaces the F-MVC mode, and the HEDE mechanism is more dominant than the HELP mechanism.

Excellent cryogenic strength-ductility synergy in duplex stainless steel with heterogeneous lamella structure

Abstract: Ferrite and austenite duplex stainless steel with heterogeneous lamella structure was fabricated by severe cold-rolling and short-time annealing. The processed steel exhibited superior cryogenic mechanical properties with a tensile strength of ∼1650 MPa and an ultimate tensile strain of ∼30%. The excellent combination of cryogenic strength and ductility was attributed to fine grain size, nanotwins, back-stress hardening, and deformation-induced martensitic transformation.

Investigation of austenite decomposition behavior and relationship to mechanical properties in continuously cooled medium-Mn steel

Abstract: The austenite decomposition behavior in medium-Mn steel was investigated here by combining thermo-mechanical controlled processing (TMCP) and continuous cooling processes. Microstructures were characterized by means of optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. Furthermore, the effect of rolling reduction and cooling path on microstructural evolution and mechanical properties were studied. The results indicated that the austenite region was expanded by manganese, and the ferrite, pearlite, and bainite were not nucleated before martensite transformation even when the cooling rate was decreased to 0.07 °C/min. The approach to tailor yield ratio without compromising tensile strength depends on the control of martensite lath morphology and internal dislocation density. The resistance of lattice to shear transformation increases with the increasing strain hardening of prior austenite, which leads to decrease martensite-start (Ms) temperature. The martensite formed at lower transformation temperatures exhibited relatively high dislocation density and refined laths, resulting in high work-hardening capacity during the early stage of plastic deformation. The crack propagation can be effectively deviated or even terminated by high angle grain boundaries (HAGBs) and the nonparallel structures of martensite packets. These studies on the kinetics and thermodynamics of martensitic transformation confirmed the viability of processing continuously cooled medium-Mn steel in the hot rolling production line.

Manganese controlled transformation and twinning of the nanoscale austenite in low-carbon-medium-Mn steel

Abstract: A Fe-10.5Mn-0.06C steel consisting of austenite and ferrite dual phases with a volume fraction ratio of 7:3 was fabricated via the quenching and partitioning (Q&P) processing. Tensile tests at various temperatures and strain rates were performed to reveal the mechanical properties of the steel, and the deformation microstructures were characterized by using TEM, t-EBSD and APT techniques. At a constant strain rate of 0.1 s−1, yield strength, tensile strength increase as the temperature decreases from 100 °C to −50 °C, and reach the highest values of 560 MPa and 1390 MPa at −50 °C, respectively. The specimens deformed at low temperatures (below 200 °C) exhibit a characteristic of three-stage work-hardening behavior, while those deformed at higher temperatures (200 and 300 °C) only show two-stage work-hardening curves. The extra hardening stage found at low temperatures is associated with the concurrent twinning and transformation in austenite with varying Mn contents as well as the interaction between twins and dislocations. With increasing temperature, the diffusion of Mn from austenite to ferrite is depressed, inhibiting the martensitic transformation. Consequently, the TRIP effect is weakened and the steel shows the reduced work-hardening rates at higher temperatures. In addition, a few nanoscale austenite residues (

Spall strength dependence on peak stress and deformation history in Lean Duplex Stainless Steel 2101

Abstract: The influence of peak compressive stress on the dyamic tensile fracture (spall) in a multi-phase steel, austenite, ferrite and martensite phases, was investigated in as-received and pre-strained conditions. Plate impact experiments were done at ⁓3.0 GPa and ⁓6.0 GPa compressive peak stresses. Results showed that peak stress increase results in about 20% increase in the spall strength for samples tested in the as-received condition, and about 15% increase for the pre-strained samples. These increases could be related to the effects of peak stress on the tensile stress that causes spallation. However, introducing plastic deformations in the pre-strained samples had negligible effects on the spall strength. Microstructural examinations revealed that incipient spall damage was parallel to the phase boundaries and mainly accommodated by ferrite grains as quasi-cleavage transgranular fracture. Samples shocked in the as-received condition with full spallation experienced a quasi-cleavage fracture within the ferrite compared with a cleavage fracture within the ferrite for samples shocked in the pre-strained condition. Deformation within the austenite phase due to plate-impact testing was dependent on the initial sample condition, the pre-strained samples showed more deformation than the as-received samples.

Effect of microstructure on hydrogen embrittlement susceptibility in quenching-partitioning-tempering steel

Abstract: Hydrogen embrittlement (HE) restricts the application of high strength steel in sustainable energy productions. As one type of efficient hydrogen trap sites, the NbC carbide precipitation is a superior approach to mitigate the HE susceptibility. The microstructure, mechanical properties and HE susceptibility were investigated in different high strength steels treated by quenching and partitioning (Q&P), quenching-partitioning-tempering (Q-P-T) and intercritical annealing quenching and partitioning (IAQP) processes in this study. The results show that the NbC particles can significantly improve the HE resistance of high strength steel treated by Q-P-T process. The NbC carbide precipitation has four different morphologies with varied sizes, which can effectively trap a large amount of diffusible hydrogen atoms. The retained austenite phases with different morphologies and locations have different hydrogen trap ability, but ferrite phase does not show strong hydrogen trap ability. Meanwhile, the NbC carbide precipitation can enhance the yield strength of steel effectively through precipitation strengthening, but has little effect on its ductility.

Microstructure and coordination mechanism of interface of stainless steel/carbon steel cladding plate prepared by vacuum diffusion bonding

Abstract: Vacuum diffusion bonding (VDB)-prepared 304 austenite stainless steel (SS)/Q235 carbon steel (CS) clad plates were the focus of this study. Specifically, the metallurgically bonded interfaces, which are known to have good mechanical properties, were investigated. The microstructures of the interfaces were extensively investigated using transmission electron microscopy (TEM). The characterization results revealed the existence of annealing twins and lath martensite precipitates at the interface; the high strength and plasticity at the interface were determined to be attributed to these features. In addition, the distribution and Burgers vector of dislocations at the ferrite–austenite interfaces were investigated by using two-beam method of TEM electron diffraction contrast image (EDCI) technology and high-resolution TEM imaging. As a result, the coordination mechanisms at the ferrite–austenite interfaces during the deformation of the SS/CS clad plates were clarified; the observations include: (1) the perfect dislocation with the Burgers vector a 2 ⟨ 111 ⟩ in the ferrite moved across the interface and formed a perfect dislocation with the Burgers vector a 2 ⟨ 110 ⟩ in the austenite; (2) the ferrite lattice was joined to the perfect dislocation with the Burgers vector a 2 ⟨ 110 ⟩ in the austenite; and (3) the ferrite lattice continuously extended across the interface, and was connected to the austenite lattice. According to the results on the Burgers vector of dislocations, two types of dislocation reactions occurred at the ferrite–austenite interfaces during the deformation. Lastly, the transfer matrix of the orientation relationship (OR) between austenite and ferrite was calculated. The results revealed that the OR differed from that of the conventional OR between body-centered cubic and face-centered cubic structures; thus, it can be concluded that the preparation of SS/CS clad plates via VDB technology may lead to the formation of a unique OR between ferrite and austenite.

Effect of a two-step annealing process on deformation-induced transformation mechanisms in cold-rolled medium manganese steel

Abstract: In this work, the relationship between the mechanical properties and deformation-induced transformation mechanisms in transformation-induced plasticity (TRIP) medium manganese steel subjected to a two-step annealing process is comprehensively examined. Two-step annealed samples have bimodal microstructures, which are reversely transformed from partially recrystallized thermal martensite into lath- and globular-shaped grains of austenite (γ)–ferrite (α). This process widens the distribution of austenite stability, increasing the product of strength and elongation (PSE) from 35.5 GPa% to 46.9 GPa% at 600 °C annealing. When the annealing temperature is further increased to 640 °C, the difference of partitioning Mn content in austenite transforms the strain-induced TRIP effect into the stress-assisted TRIP effect. The yield strength and PSE are significantly reduced to 596 MPa and 30.5 GPa%, respectively. In addition, different degrees of stability in austenite occur in different martensite transformations (i.e., γ → e martensite, γ → e martensite → α′ martensite) at the same deformation stage, and the deformation mechanism of γ is generally based on the cooperation of the TRIP effect and twinning. For the coarse metastable austenite grains annealed at 640 °C, the α’ martensite transformation is assisted by the applied stress. An intergranular fracture occurs because of a high stress concentration mismatch between the deformed martensite and ferrite, leading to premature fracture.

The effect of cold-rolling prior to the inter-critical heat treatment on microstructure and mechanical properties of 4340 steel with ferrite – Martensite microstructure

Abstract: This study investigates the effect of cold rolling prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850 °C for 1 h followed by oil quenching, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness. The inter-critical annealing treatment (750 °C, 120min) was performed to generate a ferrite-martensite microstructure. Microstructural studies showed that increasing the applied cold rolling, leading to increase in volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were measured by tensile, impact and hardness tests. Results showed that ultimate tensile strength, yield strength, micro-hardness and toughness (e.g. total elongation, uniform elongation and impact energy) increased with increasing the applied cold rolling. Improvement of mechanical properties were related to increase in martensite volume fraction and ferrite grain refinement. Analysis of strain hardening behavior of DP steels, by Hollomon analysis, showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (nI) increased with increasing volume fraction of martensite. The energy absorption capacity (UE × UTS) increased with increasing cold rolling deformation.

Intermediate stacking fault and twinning induced cooperative strain evolution of dual phase in lean duplex stainless steels with excellent cryogenic strength-ductility combinations

Abstract: The effects of intermediate substructures on the strain evolution in dual phase and the correlated mechanical properties at different temperatures were investigated in two lean duplex stainless steels (LDSSs) with different stacking fault energy (SFE). At room temperature (RT), the 1#-LDSS shows a lower yield strength (YS) but high tensile elongation (TEL) values than those of the 2#-LDSS due to a more significant TRIP effect, resulting from a lower SFE and mechanical stability of austenite (γ). Meanwhile, the plasticity of the ferrite (α) strip is also accommodated by the grain rotation process and texture transformation from component ⟨ 12 3 ‾ ⟩ { 111 } to ⟨ 11 2 ‾ ⟩ { 111 } , preventing a drastic dislocation generation in the α/γ interfaces. However at liquid nitrogen temperature (LNT), although a strong dislocation hardening of dual phase accompanied with a two-stage γ→e→α’ transformation sequence occurs at low strains of 1#-LDSS, the severe dislocation accumulation in ferrite becomes the potential site of crack nucleation, leading to a high ductility loss compared to the RT. In contrast, a simultaneous enhancement of strength and ductility can be obtained in the 2#-LDSS at LNT. The introduction of intermediate stacking faults (SFs) and twins due to a lower critical resolved shear stress (CRSS) can not only provide additional work hardening capacity in the early deformation stage, but also lead to a gentle dislocation increment in ferrite. The cooperative co-deformation contributes to a sustainable high strain hardening until large strains. Therefore, a high YS over 1.3 GPa and a large TEL of about 50% can be obtained in the 2#-LDSS