Austenite Phase Fraction Research Articles

Unveiling the Correlation among Microstructure, Texture, Slip System Activity, and Tensile Property in a Hot Rolled Ni Containing Medium‐Mn Steel

The microstructure–texture–tensile property relationships in a Ni‐containing medium‐Mn steel (Ni‐MMS), subjected to limited thermomechanical processing steps (i.e., hot forging and hot rolling), have been established in this study. The microstructure of the specimens hot rolled (HR) at 1173 (HR‐1173K) and 1373 K (HR‐1373K) exhibits ferrite and austenite phases. The austenite phase fraction in the HR‐1173K specimen is found to be higher than the HR‐1373K condition. The volume fraction of austenite to martensite transformation during tensile testing is also observed to be higher in the HR‐1173K specimen (≈13%) than the HR‐1373K condition (≈2%). This suggests the occurrence of more efficient transformation‐induced plasticity effect in the HR‐1173K specimen in comparison to the HR‐1373K condition, resulting in improved strength–ductility synergy in the former specimen. The smaller grain size of both the phases and higher fraction of twin boundaries as well the evolution of higher intensity γ‐fiber <111>//ND in the HR‐1173K specimen leads to better tensile properties. In addition, the overall activation of the primary slip systems (combination of face‐centered cubic and body‐centered cubic phases slip system), estimated through visco‐plastic self‐consistent simulation, is higher in HR‐1173K specimens, resulting in improved strain hardening response as compared to HR‐1373K one.

The Influence of Microstructure and Process Design on the Plastic Stability of 4 wt% Medium‐Manganese Steels

The influence of different microstructures on the plastic stability of an air‐hardened industrially produced medium‐manganese steel is presented. For this matter, heat treatment parameters before and during intercritical annealing (IA) are varied, to achieve different microstructures. The resulting duplex microstructure is consecutively tested by tensile tests, which are monitored by digital image correlation (DIC) to obtain information on the local plastic deformation. The tests are accompanied by microstructure investigations using optical, scanning electron, and transmission electron microscopy. Finally, X‐ray and electron backscatter diffraction experiments are performed before and after deformation, to describe the altering phase fractions. It is demonstrated that the effect of the deformation temperature prior to IA treatment has a significant influence on the duplex microstructure, as it changes the austenite morphology from lamellar to globular and increases the phase fraction. The change in austenite phase fraction and morphology results in a higher yield strength (≈100 MPa), as well as higher uniform and total elongations (+2% and +5%, respectively). The DIC and tensile tests reveal that these differences in the austenite phase lead to a complete change in the strain hardening behavior, from continuous flow to discontinuous serrated flow, with clearly visible deformation bands during plastic deformation.

Behavior of Retained Austenite and Carbide Phases in AISI 440C Martensitic Stainless Steel under Cavitation

In this work emphasis was given to determine the evolution of the retained austenite phase fraction via X-ray diffractometry technique in the as-hardened AISI 440C martensitic stainless steel surface subjected to cavitation for increasing test times. Scanning electron microscopy results confirmed the preferential carbide phase removal along the prior/parent austenite grain boundaries for the first cavitation test times on the polished sample surface during the incubation period. Results suggest that the strain-induced martensitic transformation of the retained austenite would be assisted by the elastic deformation and intermittent relaxation action of the harder martensitic matrix on the austenite crystals through the interfaces between both phases. In addition, an estimation of the stacking fault energy value on the order of 15 mJ m−2 for the retained austenite phase made it possible to infer that mechanical twinning and strain-induced martensite formation mechanisms could be effectively presented in the studied case. Finally, incubation period, maximum erosion rate, and erosion resistance on the order of 7.0 h, 0.30 mg h−1, and 4.8 h μm−1, respectively, were determined for the as-hardened AISI 440C MSS samples investigated here.

Phenomenological sintering model and experimental validation of gravity-induced distortions in binder-jetted stainless steel components

Sintering is an important consolidation step employed in sinter-based metal additive manufacturing processes. Binder Jetting (BJT) starts with green components with low green density (40–60 %) that results in large sintering shrinkages and geometrical shape distortions caused by external forces (e.g. gravity). Consequently, the prediction of the final sintered geometry is crucial during the design process. In this work, a novel sintering simulation framework for gravity-affected sintering of stainless-steel components is presented, including the Rios-Olevsky-Hryha sintering model and the methodology for the identification of the required material parameters. The constitutive law includes material constants to account for the powder packing effects and the delta-ferrite transformation occurring at high temperatures. The material shear viscosity was explicitly related to the equilibrium phase fraction of austenite and delta-ferrite during sintering temperatures. Dilatometry experiments were conducted and followed by the data postprocessing for the model calibration. The calibrated model was incorporated in a FEM code and validated against experimental data from BJT sintered components, showing the remarkably accuracy of the numerical simulations, with small geometric deviations (0.56 mm) related to the assumption of isotropic shrinkage in the model proposed. In parallel, other alternative models were implemented based on different normalized bulk viscosity formulations, which underestimate/overestimate the sintered distortions.

A Comprehensive Material Model for the Super-Duplex Stainless Steel SAF2507 in a Welding Environment

The aim of this work is to describe a reliable methodology for determining parameters of a material model suitable for implementation in a welding simulation using the finite element method (FEM). The adopted methodology employs a multi-scale approach integrating a microstructure evolution model, a representative volume element (RVE) calibrated through experimental methods, including a thermal–mechanical simulator, and electron backscatter diffraction (EBSD) experiments. The result is a complete material model, which covers thermal, mechanical and metallurgical material models for SAF2507 (EN 1.4410), that shows promising results and was successfully implemented in finite element (FE) code. A direct comparison of experimental and calculated results shows a deviation of up to 12% for the phase fraction of austenite and 25% for the mean grain diameter of ferrite.

Improvement of shape recovery by controlling the microstructure and mechanical properties in a thermomechanically-treated Fe-based alloy

In this research, the effects of thermomechanical treatment including heavy cold rolling and post-deformation annealing on the improvement of shape recovery in an Fe–9Ni–7Mn (at.%) martensitic alloy were investigated. Experimental results showed that the 80 % cold rolling produced a little fraction of strain-induced austenite phase in the martensitic matrix due to the strain concentration in the microstructure and the temperature rise under deformation. During post-deformation annealing at 590 °C in a dual phase (α+γ) region, a greater fraction of the austenite was introduced in the martensitic matrix such that its amount first increased by increasing the annealing time to 1.8 ks and then decreased at a more time of reversion. The microstructural evolution revealed that the austenite reversion under post-deformation annealing initially occurred by a shear nature at the early steps of reversion and then continued by diffusion. Tensile testing represented a good mixture of a relatively high ultimate tensile strength of ∼780 MPa and proper ductility of ∼8.5 % in the post-deformation annealed (PDA) sample for 0.9 ks. In addition, the shape memory properties were examined under the conditions of cold rolling and post-deformation annealing. The results showed a maximum shape recovery rate of ∼23 % for the cold rolled specimen which increased to ∼50 % after post-deformation annealing for 0.9 ks. The presence of a relatively high amount of retained austenite with proper stability and strength in the microstructure of the PDA sample for 0.9 ks was the reason for achieving the highest value of shape recovery.

Characterization and modelling of microstructure evolution during partitioning in Q&P steels

Development of a simulation methodology and model which combines the analytical estimation of the microstructure at different points of the Q&P route with the numerical modelling of the main physical phenomena during partitioning was done. Detailed characterization of microstructure was performed at different stages of the Q&P process: at the quenching temperature (QT), at different interrupted conditions of partitioning and after final cooling down to room temperature. In addition, some investigations during the heating step from quenching to partitioning temperature was done by measuring the RA fraction. Then, the evolution of carbon free bainite and retained austenite phase fraction during the partitioning and different processing conditions was analyzed. The austenite grain size distribution produced by the first quenching was experimentally characterized and implemented in the model. Modelling approach to describe the main phenomena taking place during the partitioning step, i.e. carbon diffusion and bainite formation, was proposed. The built one-dimensional model predicts the microstructure evolution during partitioning step in the Q&P processing: starting from quenching temperature and going to the final cooling to room temperature. Finally, model performance was evaluated using two experimental datasets: one for testing and another for the validation.

In-situ XRD investigation of [formula omitted] phase precipitation kinetics during isothermal holding in a hyper duplex stainless steel

The precipitation kinetics and formation mechanisms of σ phase during isothermal holding of a hyper duplex stainless steel in the temperature range from 750 °C to 1025 °C were studied by means of in-situ high energy X-ray diffraction experiments. Rietveld analysis of the data determined the nose temperature as 900 °C where a σ phase fraction of 1 wt% is attained after 15 s. The measurements revealed a distinct change from initial interface controlled nucleation of σ phase to a later stage of diffusion controlled growth during holding at 1000 °C after the σ phase fraction reaches half its final value. At lower holding temperatures the change from interface control to diffusion control happens more gradual and at later stages of the transformation. At these lower temperatures σ phase precipitation is accompanied by the formation of secondary austenite. Changes in σ phase formation mechanisms and kinetics were correlated with variations in the morphology of the precipitates through SEM analysis of quenched samples. Isothermal holding at 1000∘C and 1025 °C as well as above the solvus temperature of σ phase at 1120 °C leads to local fluctuations of austenite and ferrite phase fractions and the concomitant movement of austenite/ferrite phase boundaries even when no overall change in the phase fractions occurs.

Changes in Microstructure and Mechanical Properties of 17-4PH Stainless Steel according to the Application of Laser Rotation in the Powder Bed Fusion(PBF) Process

17-4 precipitation hardened stainless steel (17-4PH SS) has been reported to have excellent mechanical properties and excellent corrosion resistance, and is one of the materials used and studied with the powder bed fusion (PBF) method. Powder bed fusion (PBF) is a new manufacturing technology that has recently attracted attention in automotive, aerospace and other industries because of its ability to produce complex geometries for high-strength and lightweight applications. In the PBF process, each layer has a different laser scan length resulting from the application of laser rotation. The laser rotation could affect the laser scan length, which causes a difference in the peak temperature and cooling rate of the deposited layer, resulting in microstructure changes. This work aims to investigate how varying the laser scan pattern in the PBF process affects the microstructure and mechanical properties of 17-4PH SS. A decrease in cooling rate was observed after applying laser scan rotation, resulting in a higher austenite phase fraction. It was confirmed that a transformation induced plasticity (TRIP) phenomenon affects mechanical characteristics. These results could be suggested for fabricating thin wall shaped such as tire blow mold parts in the powder bed fusion process using the 17-4PH SS.

Study of multi-layer deposition in L-DED process for 15–5 PH stainless steel: A numerical and experimental approach

The present work investigates the multilayer deposition of 15–5 Precipitation Hardening (PH) stainless steels via Laser based Direct Energy Deposition (L-DED). The laser power and scanning speed are kept constant at 450 W and 5 mm/s with energy density of 75 J/mm2. At present, very limited literatures are available on the thermal behavior of 15–5 PH multi-layer deposition in relation with its micro structural characteristics. The complex thermal phenomena include repetitive heating and cooling cycles were simulated using 3-D multi-physics model. Through this numerical investigation temperature data of the specific region was captured and correlated with the micro structural morphology. The 15–5 PH multilayer deposition showed clear columnar grains and ferrite transformation. The XRD result revealed that austenite (ɣ) phase fraction increased with the increase in layer deposition. The IPF phase fraction map shows 1.5 % increase in retained austenite phase fraction with each layer addition. In 15–5 PH multilayer deposition the grains are primarily oriented along the 〈101〉/〈001〉 direction and with addition of layer High angle grain boundary (HAGB) fraction increases. The detailed texture analysis shows that as the layer increases, there is formation of brass, copper, shear and rotated goss structure. Similarly, it exhibited proportional rise in hardness due to its δ-ferrite dominating phases and higher HAGB percentage.

Microstructure engineering of fusion zone in resistance spot welding of martensitic stainless steels: The role of Ni interlayer thickness

Martensitic stainless steel (MSS) spot welds show low fracture toughness due to the formation of a brittle microstructure at the joint area. To realize the full advantage of this class of steels in industrial applications, the strategy of altering the microstructure of the weld nugget through introduction of a Ni interlayer was investigated. For this purpose, Ni interlayers with variable thicknesses of 50 to 400 μm were utilized. The microstructural evolutions were thoroughly scrutinized using Scheil-Gulliver solidification modeling and electron backscatter diffraction (EBSD) analysis. Conventional quasi-static mechanical tests, namely, tensile-shear (TS) and cross-tension (CT) were used to correlate microstructure-mechanical properties relationships. In the absence of Ni interlayer, the weld nugget was predominantly composed martensite-M and presumably residual delta ferrite-δ (M+ δ-95%) phases with minor amounts of austenite-γ (5%), failing to meet the minimum peak load requirements under TS and CT loading conditions. By applying Ni interlayers of 50 and 75 μm, a noticeable increase in the fraction of austenite phase (up to 31%) in the weld nugget was detected; nevertheless, a slight to moderate improvement in the mechanical properties was observed since the solidification mode was still primary δ, and M+δ were still the predominant phases in the microstructure. A significant improvement in mechanical properties was achieved in 100 μm Ni interlayer owing to change in the solidification mode from primary δ to γ along with the formation of a mainly austenitic microstructure. However, due to the incomplete mixing of melted base metals and Ni interlayer, a different microstructure consisting of M, δ and γ was also occasionally perceptible in the weld nugget of this sample. The best mechanical properties in terms of strength and ductility occurred with increasing thickness to 200 μm and above, which coincided with the formation of a fully austenitic microstructure in the entire weld nugget.

Control of microstructure and shape memory properties of a Fe-Mn-Si-based shape memory alloy during laser powder bed fusion

• Microstructure modification by varying the laser scan velocity is demonstrated for a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. • Pronounced Mn evaporation affects the primary solidification phase and, thus, the phase fraction in the samples. • Phase-transformation bcc-δ → fcc-γ induced by the intrinsic heat treatment refines the microstructure and reduces the crystallographic texture. • Microstructural changes result in variation of the alloy's shape memory and mechanical properties. Fe-Mn-Si shape memory alloys are materials, whose functional properties strongly depend on microstructural factors such as grain size and orientation, phase fraction and chemical composition. The present study demonstrates the possibility of microstructure modification via process parameter variation on a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. By varying the scan speed, samples characterized by coarse elongated grains with strong <001> orientation along the build direction or by finer equiaxed grains without preferential orientation can be fabricated. Changes in the volume phase fraction of bcc-δ ferrite and fcc-γ austenite are also introduced by selective Mn evaporation. A direct correlation between the generated microstructure and achieved mechanical and shape memory properties is found.

Grain size effect on the temperature-dependence of elastic modulus of nanocrystalline NiTi

The effect of grain size on temperature-dependent elastic modulus (TDEM) of polycrystalline NiTi over a temperature span of 200 °C was investigated by dynamic thermomechanical analysis (DMA). It is found that the TDEM rapidly reduced with grain size down to nanoscale. In-situ neutron diffraction and molecular dynamics (MD) simulation reveal that the volume fraction and thermal stability of austenite phase at low temperature gradually increased with the reduction of grain size due to the mechanical constraint of grain boundary, which results in the two-phase coexistence during cooling. The compensation of the intrinsic opposite TDEM of austenite and martensite leads to the overall reduced TDEM in nanocrystalline NiTi. This study strongly implies that the TDEM of shape memory alloy can be tailored by grain size.

Influence of chemical segregation on bainitic microstructures in a carburized bearing steel

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (zcase) and 0.16 wt% C (zcore). The microstructure of zcase is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For zcore, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of zcore are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.

In-situ study of tensile deformation behaviour of medium Mn TWIP/TRIP steel using synchrotron radiation

This investigation proposes a pathway for generating optimum microstructure for a medium Mn containing twinning-induced plasticity/transformation-induced plasticity (TWIP/TRIP) steels. The steel contains a two-phase microstructure consisting of austenite and ferrite phase arranged in a lamellar fashion. The microstructure was developed following a specially designed thermo-mechanical processing schedule that involved quenching and partitioning. The feedback for the design of thermo-mechanical processing was obtained by carrying out in-situ deformation in high energy synchrotron radiation under tensile loading. Diffraction patterns were analysed for estimating the retained austenite phase fraction and other aspects of microstructural evolution. X-ray line profile analysis was performed to determine crystallite size, dislocation density, and twin density at individual stages of deformation. In the early stages, deformation occurs by dislocation slip in both phases, while in the later stages, deformation in the austenite phase occurs via twinning and martensitic transformation, and in the ferrite phase by dislocation slip. Strain hardening behaviour has been analysed and correlated with microstructural parameters.

Austenite transformation during deformation of additively manufactured H13 tool steel

This investigates the austenite transformation in additively manufactured, H13 tool steel, which exhibits a combination of high hardness, toughness, and resistance to high operating temperatures. The alloy was fabricated using the laser powder bed fusion (LPBF) technique. As-printed and heat-treated specimens in their undeformed and tensile tested conditions were analyzed using electron backscatter diffraction (EBSD) techniques. It was identified that in the as-printed specimen, the retained austenite phase fraction was 26.9%, which dropped to 1.66% following heat treatment. The mechanical test results revealed that the austenite fraction, as well as carbide precipitation, affects the strength and ductility of the LPBF-fabricated H13 tool steel. In order to further investigate the strengthening mechanism, a detailed microstructural analysis of the deformed sample was also carried out. As expected, it was observed that the metastable austenite phase dynamically transforms to martensite. To explain this phenomenon, the Gibbs energy associated with dislocations generated by tensile testing at room temperature was calculated. This stored energy was then compared to the theoretical Gibbs energy needed for the phase transformation to occur. The results show that the Gibbs energy due to dislocation density can explain the observed diffusionless dynamic phase transformation.

The effect of microstructure and mechanical properties on the slurry erosion behavior of carbide-free bainitic and martensitic steel

The effect of microstructure and mechanical properties on the slurry erosion behavior of carbide-free bainitic (CFB) steel, austempered at 350, 400 and 450 °C and quenched and tempered martensitic (QT) steel, tempered at 300, 400 and 500 °C has been investigated using slurry pot tester and silica sand erodent particles. The samples were tested for 15 h with a rotating speed of 1000 rpm. The result shows that the QT steel has better erosion resistance than the CFB steel. The retained austenite (RA) phase fraction (17.75%) in 350 °C austempered sample, which provides erosion resistance by transformation-induced plasticity effect, is sufficient for outstanding erosion performance among the CFB steels. However, this RA phase fraction in CFB steel is inadequate to provide better erosion resistance than the QT steel. Among all the samples tested, the sample which is tempered at 300 °C shows the best slurry erosion resistance. The analysis of the results suggests that the hardness and transformation-induced plasticity associated with RA phase fraction, are the deciding factors for erosion resistance in QT and CFB steel, respectively. The material removal mechanism in the CFB steel is the crater formation leading to fatigue failure of lips and coalescence of micro-craters, whereas in QT steel it is only fatigue failure of lips.

Melt flowing behaviors and microstructure evolution during laser offset welding of dissimilar metals between AH36 and 304 steels

The study aimed to reveal the mechanism of microstructure evolution in the fusion zone of dissimilar AH36 and 304 steel laser welds and provide a feasible method for controlling austenite/martensite phase balance via adjusting laser offset (defined 304 side (-) and AH36 side (+)). The results suggested that asymmetrical fusion line boundaries were observed when centerline of dissimilar joints was irradiated by laser beam (L = 0 mm). Besides, fraction of austenite phase in weld center was decreased from 10.8% to 0.01% and broader inherited lathy martensite grain was obtained when laser offset shifted from 304 stainless steel side of 0.5 mm (L = −0.5 mm) to AH36 steel side of 0.5 mm (L = 0.5 mm). Microstructural difference of fusion zone under varying laser offset was determined by Gibbs free energy of the martensite transformation and growth time of matrix-austenite grain, which were related with element distribution and cooling rate from austenitic temperature to Ms temperature. The inhomogeneous element distribution was observed due to inadequate mixing of two liquid steels when L = −0.5 mm while strong mixing was obtained at L = 0 mm and L = 0.5 mm. The phenomenon was attributed to different dilution of base metal irradiated by laser beam and driving force for melt flowing. Therefore, more martensite grains resulted in increasing average hardness with laser offset shifting from −0.5 to 0.5 mm while impact absorbed energy was lower, due to weaker deformation capacity of coarse martensite grain.

Investigating the Microstructural Properties and Phase Distributions of TIG Welded AISI/SAE 304L Stainless Steel Runners in TNT Filled Ammunutions

Among the stainless steels, the most widely used group is austenitic stainless steels. The prominent reason why austenitic stainless steels are preferred in welded stainless steel fabrications is their good weldability. AISI/SAE 304L austenitic stainless steels exhibit adequate corrosion resistance in oxidizing environments. AISI/SAE 304L austenitic stainless steels are also used in filling interfaces of runners between the Tri-Nitro-Toluen filling vessel and ammunitions in defense industry. For the manufacture of the runners, the stainless steel plates and the stainless steel pipes are mainly joined by fusion welding. These runners are cleaned with hot tapping water at 70°C after each filling of ammunitions by TNT so that cleaning operation can deteriorate the corrosion properties of runners especially weld zones. Austenitic stainless steel weldments include basically delta-ferrite and austenite phases on weld regions. Delta ferrite phase exhibits resistance to chloride bearing medias while austenite phase is more resistive to high temperature oxidizing ambients In this study; 3 mm thick 304L austenitic stainless steel plates and rings are welded to each other with two different filler weld wires of ER316L and ER2209 by Tungsten Inert Gas welding method under pure argon shielding gas. Weld metals and heat-affected zones of welded joints were examined using optical metallurgical microscope. It has been determined that the additional filler weld wires have changed the weld metal and the heat affected zone microstructures and phase distributions dominantly. Microstructures of all welded joints have been compared. Besides, phase distributions of weld metals and heat effected zones are made by image analysis software and Schaefflers diagram. ER2209 welding filler metal increased the phase fraction of delta-ferrite in weld metals while it decreased the austenite phase fraction as compared to ER316L filler metal.

Processing and properties of yttria and lanthana dispersed ODS duplex stainless steels

In the present study, yttria (alloy B) and lanthana (alloy C) based oxide dispersion strengthened (ODS) duplex stainless steels (Fe–21Cr–8Ni–1Ti) have been synthesized using mechanical alloying followed by spark plasma sintering (SPS). For comparison, duplex steel without dispersion (alloy A) has also been synthesized. The variation of sintering temperature (900, 1000 and 1050 °C) on the density, microstructure, and mechanical properties of the steels have been reported. The austenite phase fraction increased with the sintering temperature as well as by the addition of oxides. TEM analysis confirmed the presence of nano-clusters (size in the range of 2–8 nm) of Y–Ti–Cr–O and La–Ti–Cr–O in the alloy B and alloy C, respectively. Alloy C, sintered at 1000 °C, exhibited the highest micro-hardness and compressive strength of 739 ± 9.1 HV and 2835 ± 87 MPa, respectively. At all sintering temperatures, the lower hardness values of the alloy B as compared to alloy A were ascribed to the significantly higher austenite phase fraction in the former, which resulted in overcoming the dispersion strengthening effect. Overall, La2O3-based ODS duplex stainless steels exhibited the better room temperature mechanical properties than Y2O3-based ODS steels.