Austenite Interface Research Articles

Effect of high-temperature annealing on elemental distribution in CSP-produced automotive steel billets

The distribution of C, Si, Mn, P and S in Q&P 980 automotive steel billets produced via the Compact Strip Production (CSP) process was investigated using Electron Probe Microanalysis (EPMA). The segregation ratio of each element was calculated. A diffusion kinetics model was established using the finite element method, and the chemical potential equations of different elements were used as the driving conditions for diffusion. Fick's law was applied to numerically calculate the diffusion behaviour and final concentration distribution of C, Si and Mn during the annealing process at 1100 °C. Results indicate that element segregation in the as-cast billet before annealing decreases gradually from the surface to the core. After annealing, the segregation ratio of elements is reduced, leading to a more homogeneous elemental distribution. It was calculated that the diffusion of C at 1100 °C completes in about 10 s, with micrometer-level long-range diffusion occurring in a shorter period. However, the concentration of Mn does not reach equilibrium at the austenite interface even after 20 h of diffusion at 1100 °C.

Atomic-scale mechanism of effect of co-separation of elements at interface of ferrite and austenite on Cr-depleted zone of duplex stainless steel

Three-dimensional atom-probe tomography and first-principles calculation combined with density functional theory were used to study the effect of the co-segregation of different elements formed during the solidification process of S32205 duplex stainless steel on the Cr-depleted zone at the interface between ferrite and austenite. It was found that the co-segregation of different elements formed during the solidification process of duplex stainless steel can also form Cr-depleted zone at the interface between ferrite and austenite. Moreover, Mo, Si, B, C and P atoms promote co-segregation with Cr atoms, which promotes the formation of Cr-depleted zone at the interface between ferrite and austenite in duplex stainless steel. Mo and Si strongly promote the segregation of Cr at the interface between ferrite and austenite, thereby promoting the formation of Cr-depleted zone. B, C and P elements also promote the segregation of Cr element at the interface between ferrite and austenite and the formation of Cr-depleted zone, but their effect is weaker than that of Mo and Si elements. These conclusions provide a new theoretical basis for improving the intergranular corrosion performance of duplex stainless steel.

Low-Potential Zone at the Interface of Precipitate and Austenite Affecting Intergranular Corrosion Sensitivity in UNS N08028 Nickel–Iron–Chromium Alloy

The precipitates and the intergranular corrosion behavior of a UNS N08028 nickel–iron–chromium alloy sensitized at different temperatures were studied by employing transmission electron microscopy, Kelvin probe force microscopy and other methods. It was found that sigma precipitates appeared at the grain boundaries of the alloy being sensitized. There was a Cr-depleted zone and a low-potential zone around these precipitates. The potential difference between the sigma precipitates and the low-potential zone was 102 mV, and this increased with the growth of the sigma precipitates. At this potential difference, the migration of the vacancies in the passive film accelerated significantly, and then the protectiveness of the passive film decreased. The intergranular corrosion mechanism of the steel has also be discussed.

Effect of ferrite and grain boundary characteristics on corrosion properties of thermal simulated 316 L heat affected zone

The microstructure and corrosion resistance evolution of 316 L heat-affected zone were studied by simulated welding thermal technology. The different thermal simulation parameters affected the ferrite content and grain boundary characteristics. The ferrite content and grain boundary characteristics in thermal simulation samples were studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Combining the electrochemical test, it was observed that the Cr-depleted zone existed at the austenite/ferrite interface and the high angle grain boundary (HAGB) of the austenite interface usually became the nucleation point of pitting corrosion.

Deformation mechanisms of a NiTi alloy under high cycle fatigue

In this paper, high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy (SMA) under different mean strains are investigated. The uni-axial cyclic tensile tests with small amplitude of 0.25% are operated at three different states, i.e. austenite/elastic region, austenite and martensite coexisted region and martensite/elastic region with a pre-strain. Mechanical results indicate that the cyclic deformation strain is mainly in the inhomogeneous deformation band when the sample cycling in the B2 + B19′ phase coexist status. Cyclic deformation in the B2 + B19’ phase showed cyclic hardening effect. With increasing of the mean strain, the saturation stress is decreased, and saturation rate is reduced. The mean strain effect on the high-cycle fatigue life of NiTi is attributed to the inhomogeneous transformation of martensite. High resolution electron microscopy (HREM) observations and atomic simulations showed that partial dislocations with the Burgers vectors of 1/2<110> are induced on (001)M twin planes, the interaction of dislocations and edge dislocations in martensite results in cyclic hardening in martensite. While cyclic softening in martensite at the upper plateau of stress–strain (S–S) curve is attributed to the grain reorientation/rotation phenomenon. It suggests that newly generated stress-induced martensite at the interface of martensite and austenite could increase the high-cycle fatigue of NiTi SMA.

Effect of Microstructure Morphology of Q&amp;P Steel on Carbon and Manganese Partitioning and Stability of Retained Austenite

In this study, we used 0.2C-1.7Si-1.9Mn wt% cold-rolled sheet as the experimental material to prepare the Q&amp;P sample with blocky microstructures and the QQ&amp;P sample with lath-shaped microstructures through the Q&amp;P and QQ&amp;P processes, respectively. The partitioning behavior of carbon and manganese in the two samples after intercritical annealing and partitioning were studied. During the intercritical annealing, the partitioning of carbon and manganese in the Q&amp;P and the QQ&amp;P samples occurred, resulting in the contents of carbon and manganese being significantly higher than those in the ferrite. Meanwhile, due to the migration of the ferrite–austenite interface during the formation of the austenite, the distributions of carbon and manganese in the lath-shaped and blocky austenite were both homogenous. The morphology of the microstructures had little influence on the distribution of carbon and manganese in metastable austenite during intercritical annealing. In the partitioning, the migration of the ferrite–austenite interface and diffusion of manganese can be ignored. Carbon first diffuses from the ferrite grains to the ferrite–austenite interface and then diffuses in the austenite grains. The morphology of the microstructures has a great effect on the homogenization of carbon in austenite grains. Compared with coarse blocky austenite, lath-shaped austenite can shorten the diffusion path of carbon in austenite grains and increase the homogeneity of carbon in austenite grains, thereby improving the thermal stability of lath-shaped austenite. Compared with the Q&amp;P sample, the QQ&amp;P sample has higher content of retained austenite (14.74% vs. 13.96%), better elongation (25.9% vs. 19.2%), and higher product of strength and elongation (27.5 GPa% vs. 24.4 GPa%).

Accelerated Isothermal Phase Transformation and Enhanced Mechanical Properties of Railway Wheel Steel: The Significant Role of Pre‐Existing Bainite

Exploring possible ways to accelerate bainite transformation kinetics and enhance mechanical properties, especially for large‐scale parts, attracted much attention both in industrial manufacturing and fundamental research. Herein, prior continuous cooling transformed bainite is introduced to promote isothermal transformation and improve strength toughness matching of bainite railway wheel steel. Phase transformation study by dilatometer indicates that the time–temperature–transformation curves of tested steel swing back at lower isothermal temperature. This acceleration effect is due to the formation of leaf‐shaped bainite at initial isothermal transformation stage. By tailoring the cooling rate, leaf‐shaped bainite can also be obtained during continuous cooling. It is confirmed that the formation of leaf‐shaped bainite during continuous cooling stage accelerates the subsequently isothermal bainite transformation. In situ microstructure evolution observation demonstrates that prior bainite/ austenite interface provides favorable nucleation site for initial isothermal bainite transformation. Moreover, pre‐existing bainite formed during continuous cooling stage can effectively divide the original austenite grain size. Concurrent enhancement of tensile strength and impact toughness in bainite wheel steel can be achieved by controlling the cooling rate.

Microstructure Evolution and Mechanical Properties of a Wire-Arc Additive Manufactured Austenitic Stainless Steel: Effect of Processing Parameter.

Two single track multi-layer walls with linear energy inputs (LEIs) of 219 and 590 J/mm were deposited by cold metal transfer-based wire arc additive manufacturing system. Combined with the X-ray diffraction technique, scanning electron microscope and uniaxial tensile tests, the influences of LEI and cooling rate (CR) on the microstructure evolution, mechanical properties and fracture mechanisms of the studied steel are analyzed. It is observed that the microstructures of the studied steel are mainly composed of δ-ferrite and austenite dendrites. σ phase is formed on the δferrite–austenite interface under low CR. Meanwhile, the primary dendrites’ spacing decreases with the decrease in LEI or the increase in CR, and the maximal primary dendrites’ spacing is 32 μm. The values of elongation to fracture roughly decline with the decrease in LEI or the increase in CR, but the variations of ultimate tensile strength and yield stress show an opposite trend. In addition, the mesoscopic damages in the studied steel under low LEI are mainly caused by the coalescence of pores. While under high LEI, the cracks are induced by the dislocations piling up around δ-ferrite.

Strain Rate Effect on Microstructural Evolution and Deformation Behavior of Medium‐Mn Transformation‐Induced Plasticity Steels

Herein, the effect of strain rate (10−4–10−1 s−1) on the microstructural evolution and deformation behavior of medium‐Mn transformation‐induced plasticity (TRIP) steels is investigated. The steel after intercritical annealing exhibits an austenite–martensite–ferrite triplex microstructure with a relatively high austenite fraction (66.1%). A negative strain rate dependence of ultimate tensile strength (UTS) and significant serrated behavior are found during the room temperature tensile test. The total elongation (TE) attains a maximum value of 32.0% at the strain rate of 10−2 s−1 and then decreases with increasing strain rate. The evolution of strain‐induced martensitic transformation (SIMT) as a function of strain rate is analyzed considering the adiabatic heating during deformation. The different degrees of SIMT under different strain rates lead to a fracture transition between quasicleavage fracture and ductile fracture. The delamination cracks originating from the interface of martensite and austenite are closely related with the quantity and the local strain of the martensite–austenite interface.

Influence of stress-free transformation strain on the autocatalytic growth of bainite: A multiphase-field analysis

Analytical treatments, formulated to predict the rate of the bainite transformation, define autocatalysis as the growth of the subunits at the bainite–austenite interface. Furthermore, the role of the stress-free transformation strain is often translated to a thermodynamic criterion that needs to be fulfilled for the growth of the subunits. In the present work, an elastic phase-field model, which elegantly recovers the sharp-interface relations, is employed to comprehensively explicate the effect of the elastic energy on the evolution of the subunits. The primary finding of the current analysis is that the role of eigenstrains in the bainite transformation is apparently complicated to be directly quantified as the thermodynamic constraint. It is realized that the inhomogeneous stress state, induced by the growth of the primary subunit, renders a spatially dependent ill- and well-favored condition for the growth of the secondary subunits. A favorability contour, which encloses the sections that facilitate the elastically preferred growth, is postulated based on the elastic interaction. Through the numerical analyses, the enhanced growth of the subunits within the favorability-contour is verified. Current investigations show that the morphology and size of the elastically preferred region respectively changes and increases with the progressive growth of the subunits.

Effect of Plastic Deformation on Passivation Characteristics of Type 304 Stainless Steel

Electrochemical response of Type 304 stainless steel, with different levels of plastic deformation, was investigated in solutions containing different amounts of H2SO4 and KSCN at room temperature. Potentiodynamic polarization and double-loop electrochemical potentiokinetic reactivation (DL-EPR) techniques were used to characterize different levels of plastic deformation. The results have indicated that DL-EPR values and passivation current density increased with increase in plastic deformation. The DL-EPR values were correlated well with hardness and fraction of strain-induced martensite. Attack during DL-EPR tests occurred on strain regions within the matrix such as dislocations in austenite and martensite and/or interface between austenite and martensite. No well-defined correlation was observed between the levels of plastic deformation and potentiodynamic polarization behavior.

Effect of film-like retained austenite on low temperature toughness of high strength offshore steel

Film-like retained austenite was prepared by a quenching + intercritical annealing + tempering (QIAT) heat treatment to study its effect on the low temperature toughness of high strength offshore steel. Based on the Purdy model, the flat growth mechanism of film-like retained austenite was studied and the toughening mechanism of this kind of retained austenite was revealed. The results showed that the microstructure after the QIAT heat treatment was a multi-phase structure containing tempered bainite/martensite, intercritical ferrite and film-like retained austenite. The thickness of the film-like retained austenite was only 0.1 μm. The yield strength was 716 MPa, the tensile strength was 812 MPa and the low temperature toughness at −40 °C reached 150 J. The low temperature toughness was significantly improved compared with the quenching + tempering (QT) treatment. Enrichment of Mn in the retained austenite formation region caused a surge in dissipated free energy induced by austenite interface migration, making it a one-dimensional film-like flat growth. The excellent stability of the film-like retained austenite and the large number of low-angle grain boundaries were the reasons for the excellent low temperature toughness of this offshore steel.

Dendrite fragmentation induced by massive-like \u03b4\u2013\u03b3 transformation in Fe\u2013C alloys

Dendrite arm fragmentation is considered in solidification structure tailoring. Time-resolved and in situ imaging using synchrotron radiation X-rays allows the observation of dendrite arm fragmentation in Fe–C alloys. Here we report a dendrite arm fragmentation mechanism. A massive-like transformation from ferrite to austenite rather than the peritectic reaction occurs during or after ferrite solidification. The transformation produces refined austenite grains and ferrite–austenite boundaries in dendrite arms. The austenite grains are fragmented by the liquid phase that is produced at the grain boundary. In unidirectional solidification, a slight increase in temperature moves the ferrite–austenite interface backwards and promotes detachment of the primary and secondary arms at the δ–γ interface via a reverse peritectic reaction. The results show a massive-like transformation inducing the dendrite arm fragmentation has a role in formation of the solidification structure and the austenite grain structures in the Fe–C alloys.

Friction stir welding of dissimilar stainless steels: evaluation of flow pattern, microstructure and mechanical properties

The friction stir welded joints of austenitic and ferritic stainless steels were implemented at different rotational speeds 700∼900 rpm. The results indicated that the heat input and the material flow in welded zone were improved with increasing rotational speed, and the nugget zone presented blocky, laminated and vortex structure of austenite and ferrite. The joining mechanism transformed from simple mechanical mixing to combination of mechanical and metallurgical connections, proved by the Ni element diffusion at the interface of austenite and ferrite matrix. A reliable joint with the maximum tensile strength was achieved by two-phase laminated structure performed at rotational speed 800 rpm. The fracture occurred in the thermo-mechanically affected zone on the advancing side. The grains of the stir zone were refined from 20 μm to 2 μm by dynamic recrystallization.

Influence of retained austenite and Cu precipitates on the mechanical properties of a cold-rolled and intercritically annealed medium Mn steel

A low carbon Cu-bearing medium-Mn steel (Fe-0.05C-5.7Mn-1.9Cu-2.0Ni-0.4Si) was cold rolled and then intercritically annealed (IA) between 640 °C and 680 °C for 180 s. Ultrafine multi-phase microstructure (∼300 nm grain size) was obtained in this single short time annealing, composed of recovered martensite, fresh martensite, more than 20% retained austenite and copious (1022 m−3) nanosized (∼6 nm in radius) Cu precipitates. A good combination of strength (YS (yield strength) of 1037 MPa and UTS (ultimate tensile strength) of 1210 MPa) and ductility (19% UEL (uniform elongation) and 23% TEL (total elongation)) was obtained at an annealing temperature of 640 °C, with the former enhanced by Cu precipitates, ∼200 MPa, and the latter, improved by significant fraction of stable retained austenite. The microstructure was studied by SEM (scanning electron microscope), TEM (transmission electron microscopy) and XRD (X-ray diffraction). It is observed that Cu precipitates distribute heterogeneously in the amount of Cu precipitates adjacent to martensite/retained austenite interface obviously higher than that predicted based on the average bulk composition. According to thermodynamic analysis through Thermo-calc, DICTRA and Kampmann–Wagner Numerical (KWN-) simulation, the influence of Mn, Ni and Si on the bcc (body-centered cubic) miscibility gap shows that the precipitation of Cu particles could be facilitated by Si, a ferrite stabilizer, and inhibited by the presence of Mn and Ni, both austenite stabilizers. As a result, Cu precipitation is promoted in the diffusion zone formed during IA adjacent to martensite/retained austenite interface.

Effect of the Austenite Interface on Pearlite Transformation Behavior

The effect of the characteristics of austenite interface with ferrite on the pearlite transformation behaviour after intercritical annealing was investigated. Most austenite grains were situated mainly on ferrite grain boundaries and had the Kurdjumv-Sachs (K-S) or near K-S relationship to one of the neighbor ferrite grains before pearlite transformation. The pearlite transformation started mainly from the austenite grain boundary faced to ferrite. The pearlite transformation showed stasis. This indicates that some austenite is stabilized thermally against the pearlite transformation. The fraction of austenite having only the K-S or near K-S interface to neighbor ferrite grains was correspond to the fraction of austenite grains which does not include pearlite. The pearlite transformation was difficult to start from austenite interface having the K-S relationship to ferrite since the interface between austenite grains and ferrite grains was stabilized energetically in the case of their interface having the K-S relationship.

Evolution of Microstructure and Carbon Distribution During Heat Treatments of a Dual-Phase Steel: Modeling and Atom-Probe Tomography Experiments

The temporal evolution of microstructures and carbon distributions in a Fe-0.323C-1.231Mn-0.849Si (mol pct) dual-phase steel during heat treatments are simulated using a two-dimensional cellular automaton model. The model involves austenite nucleation, phase transformations controlled by ferrite (α)/austenite (γ) interface mobility and the local carbon concentration, and long-range carbon diffusion. It is also coupled with a solute drag model to account for the effect of substitutional elements on the interface migration. The results show that after holding at 800 °C for 300 seconds the transformed γ-volume fraction is lower than the paraequilibrium prediction. During subsequent cooling at 6 °C s−1, the γ → α transformation takes place after a stagnant stage; the carbon concentrations in both the α- and γ-phases increase and become non-uniform. When cooled below 450 °C, the γ-volume fraction is nearly unchanged. A small amount of carbon enriched martensite, transformed from the remaining γ-phase, exists in the room temperature microstructure. The simulated microstructures and carbon concentrations in martensite compare reasonably well with the experimental micrographs and atom-probe tomographic measurements. During tempering at 400 °C, martensite decomposes and the carbon concentration in the α-matrix increases. The simulation results are used to understand the mechanisms of yield strength variations after different heat treatments.

Corrosion and passive behaviour of duplex stainless steel 2205 at different cooling rates in a simulated marine-environment solution

The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates (1, 5, 10, 15, 20 °C s−1 and water quenched) in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott–Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 °C s−1, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1:1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite–austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20 °C s−1.

Dynamic Strain Aging Phenomena and Tensile Response of Medium-Mn TRIP Steel

Dynamic strain aging (DSA) and rapid work hardening are typical behaviors observed in medium-Mn transformation-induced plasticity (TRIP) steel. Three alloys with manganese ranging from 10.2 to 13.8 wt pct with calculated room temperature stacking fault energies varying from − 2.1 to 0.7 mJ/m2 were investigated. Significant serrations were observed in the stress-strain behavior for two of the steels and the addition of 4.6 wt pct chromium was effective in significantly reducing the occurrence of DSA. Addition of chromium to the alloy reduced DSA by precipitation of M23(C,N)6 during batch annealing at 873 K (600 °C) for 20 hours. Three distinct DSA mechanisms were identified: one related to manganese ordering in stacking faults associated with e-martensite and austenite interface, with activation energies for the onset and termination of DSA being 145 and 277 kJ/mol. A second mechanism was associated with carbon diffusion in γ-austenite where Mn-C bonding added to the total binding energy, and activation energies of 88 and 155 kJ/mol were measured for the onset and termination of DSA. A third mechanism was attributed to dislocation pinning and unpinning by nitrogen in α-ferrite with activation energies of 64 and 123 kJ/mol being identified. Tensile behaviors of the three medium manganese steels were studied in both the hot band and batch annealed after cold working conditions. Ultimate tensile strengths ranged from 1310 to 1404 MPa with total elongation of 24.1 to 34.1 pct. X-ray diffraction (XRD) was used to determine the transformation response of the steels using interrupted tensile tests at room temperature. All three of the processed steels showed evidence of two-stage TRIP where γ-austenite first transformed to e-martensite, and subsequently transformed to α-martensite.

A dilatometric analysis of inverse bainite transformation

The unique two-stage dilatation curve observed during the inverse bainite transformation of a hypereutectoid low alloy steel is analyzed to understand the transformation kinetics. A new algorithm is proposed to extract the bainitic phase fractions from the raw dilatometry data. The proposed data extraction algorithm is generic that relies only on the density of phases involved in the transformation. To verify the extracted phase fraction, a kinetics model is developed using the principles of diffusion and Johnson–Mehl–Avrami–Kolmogorov kinetics. The predicted phase fractions by the kinetics model agree fairly well with the experimental phase fraction results from dilatometry, metallography, and XRD. The two-stage transformation can be explained by the kinetics of inverse bainite as a diffusion-controlled transformation product. The transformation proceeds in a para-equilibrium mode, involving only the diffusion of carbon at the inverse bainite/parent austenite interface