Volume Fraction Of Austenite Research Articles

Effect of Intercritical Austenitization and Starting Matrix on Martensite Start Temperature and Austenite Carbon Concentration in Ductile Iron

AbstractIn this work, the effect of the intercritical austenitization temperature and time, Ni + Cu concentration and starting matrix (fully ferritic or fully pearlitic) on the volume fraction of austenite (fγ), martensite start temperature (Ms) and carbon concentration of the parent austenite (C0) in ductile iron was investigated using standard metallographic techniques and high-resolution dilatometry. The results showed that a fully pearlitic starting matrix gives higher fγ and C0 and lower Ms than a fully ferritic starting matrix at the same austenitization temperature under continuous heating. On the other hand, the austenitization time has an effect on the parameters under study only during the first minutes of intercritical austenitization. Finally, at the same intercritical austenitization temperature, the addition of Ni + Cu increases the volume fraction of austenite.

Rolling Contact Fatigue Behaviors of 20CrNi2Mo Steel by a New Carbon and Nitrogen Composite Infiltration Process

ABSTRACTIn this paper, a new type of composite infiltration process was adopted for 20CrNi2Mo steel. Rolling contact fatigue (RCF) tests were carried out on the specimens treated with carburizing (C) and composite infiltration with carburizing and nitriding (C‐N). The results showed that after C and C‐N treatments were performed, the surface microhardness was increased by 78% and 114%, respectively, and the maximum CRS were −220 and −530 MPa. Moreover, the residual austenite volume fraction was controlled to approximately 10% for each treated sample. The fatigue limit of the C‐N sample was 11.3% higher than that of the C sample. The fatigue failure mechanisms are caused by the maximum shear stress distribution and surface roughness. The surface layer of the C‐N sample with higher hardness and more compressive residual stress inhibited the initiation of fatigue cracks, and the appropriate residual austenite in the carbon‐nitrogen infiltrated layer inhibited the propagation of fatigue cracks.

High-silicon spring steel transformation to carbide-free bainitic (CFB) steel: a mechanical and tribological approach under rolling/sliding conditions

The present study concentrates on the development of carbide-free bainite steel and explores its tribological and mechanical characteristics using a newly devised disk-on-disk tribometer. The microstructure of the treated samples was examined through heat treatment at various holding temperatures (300, 350, and 400 °C) and durations (10, 20, and 30 min), along with isothermal transformation. The wear rate demonstrated significant sensitivity to holding duration, transformation temperature, phase fraction, and the stability of retained austenite. The microstructural analysis encompassed atomic force microscopy (AFM), FE-SEM, x-ray diffraction (XRD), and energy-dispersive x-ray spectroscopy (EDS). The study extensively investigated the robust correlation among mechanical properties, microstructure, wear behavior, and the presence of retained austenite (RA) and bainite volume fraction. The findings indicate that carbide-free bainite steel holds promise for replacing conventional railway wheel track steel in the steel industry. The structural property correlations have developed of high silicon steel, hardness reduced with increasing temperature and isothermal preserving time; resulting in a gradual increase in the wear rate of CFB steel. The salt bath soaking temperature of 350 °C with a period of 30 min was found to be the preeminent wear properties condition for CBF steel.

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.

Quantitative analysis of the micromechanical behavior and work hardening in Fe-0.1C–10Mn steel via in-situ high-energy X-ray diffraction

In the current work, the micromechanical behavior and work hardening behavior of Fe-0.1C–10Mn (in wt.%) steel deformed at 100, 63, 25 and −50 °C were investigated via in-situ high-energy X-ray diffraction (HE-XRD) technique. As the deformation temperature decreased, the yield strength (YS) and ultimate tensile strength (UTS) increased, while the total elongation (TE) reached a maximum value at 25 °C. The transformation kinetics of retained austenite (RA) was fitted by the Olson and Cohen (OC) model. The phase stress and flow stress contributed by the constituent phases were obtained based on the lattice strain and the volume fraction of the corresponding phase. The work hardening rate was decomposed into four contributors related to the TRIP effect and load partitioning, ie., the austenite phase stress, load partitioning between austenite and martensite, martensitic formation rate and load partitioning between ferrite and austenite. The influence of each contributor on the work hardening behavior was quantitatively evaluated and stacked, the stacked results agreed reasonably well with the experimental work hardening rate obtained from the true stress-strain curve. Finally, the volume fraction of austenite to martensite transformation promoted by the Lüders band (LB) and the stacking fault energy (SFE) of RA were found to be highly temperature-dependent. A linear relationship was revealed between the volume fraction of austenite to martensite transformation during the LB propagation and the SFE of RA. These findings offer insights into the TRIP effect and the LB propagation in medium-Mn steels.

Measurements of Carbon Content and Retained Austenite Volume Fraction in Steels by Neutron Bragg-edge Transmission Analysis at a Compact Neutron Source, AISTANS

Measurements of Carbon Content and Retained Austenite Volume Fraction in Steels by Neutron Bragg-edge Transmission Analysis at a Compact Neutron Source, AISTANS

Study of Effects of Post-Weld Heat Treatment Time on Corrosion Behavior and Manufacturing Processes of Super Duplex Stainless SAF 2507 for Advanced Li-Ion Battery Cases.

Lithium-ion batteries are superior energy storage devices that are widely utilized in various fields, from electric cars to small portable electric devices. However, their susceptibility to thermal runaway necessitates improvements in battery case materials to improve their safety. This study used electrochemical analyses, including open-circuit potential (OCP), potentiodynamic polarization, and critical pitting temperature (CPT) analyses, to investigate the corrosion resistance of super duplex stainless steel (SAF 2507) applied to battery cases in relation to post-weld heat treatment (PWHT) time. The microstructure during the manufacture, laser welding, and PWHT was analyzed using field-emission scanning electron microscopy, X-ray diffraction, and electron backscatter diffraction, and the chemical composition was analyzed using dispersive X-ray spectroscopy and electron probe micro-analysis. The PWHT increased the volume fraction of austenite from 5% to 50% over 3 min at 1200 °C; this increased the OCP from -0.21 V to +0.03 V, and increased the CPT from 56 °C to 73 °C. The PWHT effectively improved the corrosion resistance, laying the groundwork for utilizing SAF 2507 in battery case materials. But the alloy segregation and heterogeneous grain morphology after PWHT needs improvement.

Dry sliding wear behavior of the high-strength nanostructured bainitic steel containing 3.5 wt% aluminum

The acceptable wear behavior of nanostructured bainitic steels has enhanced their potential to be industrialized, and their applications in the automotive industry. This study focused on the wear behavior of the high-strength nanostructured bainitic steel containing 3.5 wt% aluminum (low-cost and low-density) to assess the dominant sliding wear mechanism and the correlation between the microstructure and applied load on the sliding wear resistance. A pin-on-disk tribometer was used to evaluate dry sliding wear behavior, and the SWR was computed by weighing the specimens. To identify the wear mechanism, the worn surfaces and cross-sections were analyzed using XRD, FESEM, and EDS. The results showed that the retained austenite volume fraction and carbon content of austenite (the stabilizer of the retained austenite) of high-strength nanostructured bainitic steels are not the only determinants of wear behavior. Indeed, the thickness of bainitic ferrite plates (strength source) is the major parameter affecting wear resistance. An increase in the Vαb/tαb ratio due to a decreased austempering temperature reduced the SWR and enhanced wear resistance. Moreover, the change from a normal load of 50 N to a load of 150 N changed the dominant wear mechanism from oxidative to adhesive. Also, increasing the isothermal transformation temperature and applied load by increasing the thickness of the tribological-transition-zone (TTZ) causes a decrease in sliding wear resistance. In summary, the formation of a thin TTZ due to the increase in the Vαb/tαb ratio is the main reason for the improvement of the wear resistance of the austempered sample at the lowest temperature (250 °C).

Study on Strain Hardening and Cryogenic Toughness of Marine 10Ni5CrMoV Steel during Tempering

The tempering process is applied in marine 10Ni5CrMoV steel to study microstructure, and mechanical properties by multi‐scale characterizations, strain hardening behavior, and cryogenic toughening mechanism are further investigated. As the tempering temperature increases from 590 to 630 °C, the dislocation density decreases by up to 25% and the austenite volume fraction decreases by up to 35%. Additionally, the morphology of austenite changes from strip to bulk, which weakened the pinning effect on the laths, leading to a coarsened martensite and an increase in the equivalent grain size. Based on the modified Crussard–Jaoul analysis, the specimens at different tempering temperatures exhibit a single‐stage strain hardening behavior during plastic deformation. The increase in strength is attributed to the continuous transformation‐induced plasticity effect and the increasing dislocation density. Furthermore, high austenite volume fraction and the proportion of grain boundary misorientation above 45° promote the release of stress concentration and hinder the propagation of cracks. This results in an increase in the ductile–brittle transition temperature (DBTT) of the specimens from −105 to −135 °C. At a tempering temperature of 610 °C, the specimen demonstrates an outstanding balance between yield strength (868 MPa) and cryogenic toughness (DBTT of −135 °C).

Synergistic improvement of strength and ductility by high magnetic field assisted intercritical annealing in lightweight medium Mn steel

The microstructures and mechanical properties of lightweight medium Mn steels were studied by high magnetic field (HMF) assisted intercritical annealing process (HMF-IA). The results show that the volume fraction of retained austenite increased and reached the maximum value of 56.2 % when the magnetic flux density (MFD) was 10 T at first and then decreased with the increase of MFD. The application of 10 T HMF promoted the diffusion ability of carbon atoms into austenite by magnetic stress-driven dislocation movement and increased the volume fraction and grain size of austenite. HMF also played a critical role in dislocation movement and element partitioning during the IA process. Due to the wide range of bimodal size distribution of austenite after the HMF-IA process, stronger transformation induced plasticity effect and twin induced plasticity effect happened in the process of plastic deformation. For the sample annealed under 10 T, comparing with the sample annealed without HMF, the product of strength and elongation value increased from 45.7 GPa·% to 55.1 GPa·%. Additionally, it also kept a persistent high strain hardening. The present study provides a new understanding of the role of HMF in adjusting the relationship between microstructure and mechanical properties during the annealing process and opens up a new path for designing industrial medium Mn steel with low cost, high performance and simple process.

Intercritically Annealed Medium-Manganese Steel: Insights into Microstructural and Microtextural Evolution, Strain Distribution, and Grain Boundary Characteristics.

Aluminum-incorporated medium-manganese steel (MMnS) has potential for lightweight transport applications owing to its impressive mechanical properties. Increasing the austenite volume fraction and making microstructural changes are key to manufacturing MMnS. However, the grain boundary character and strain distribution of intercritically annealed low-density MMnS have not been extensively scrutinized, and the effects of crystallographic texture orientation on tensile properties remain ambiguous. Therefore, in this study, the microstructure, microtexture, strain distribution, and grain boundary characteristics of a hot-rolled medium-Mn steel (Fe-0.2 C-4.3 Al-9.4 Mn (wt%)) were investigated after intercritical annealing (IA) at 750, 800, or 850 °C for 1 h. The results show that the 800 °C annealed sample exhibited the highest austenite volume fraction among the specimens (60%). The duplex microstructure comprised lath-type γ-austenite, fine α-ferrite, and coarse δ-ferrite. As the IA temperature increased, the body-centered cubic phase orientation shifted from <001> to <111>. At higher temperatures, the face-centered cubic phase was oriented in directions ranging from <101> to <111>, and the sums of the fractions of high-angle grain boundaries and coincidence-site-lattice special boundaries were significantly increased. The 800 °C annealed sample with a high austenite content and strong γ-fiber {111}//RD orientation demonstrated a noteworthy tensile strength (1095 MPa) and tensile elongation (30%).

Effect of Scanning Electron Beam Pretreatment on Gas Carburization of 22CrMoH Gear Steel

22CrMoH was selected for the gear steel material in this work, and the temperature field change in the scanning electron beam was analyzed to determine the optimal scanning parameters and explored the effect of scanning electron beam pretreatment (Abbreviated as: SEBP) on gas-carburizing (GC) efficiency and organizational properties of gear steel. The results showed that the scanning electron beam caused the material to form a thermally deformed layer 110 μm thick, and it promoted the adsorption of carbon atoms on the surface and their inward diffusion. Under the same gas-carburizing conditions, the carburizing efficiency was improved, and the thickness of the carburized layer increased from 0.78 to 1.09 mm. Furthermore, the hardness of the GC specimens with the SEBP increased from 615 to 638 HV0.05 at 0.1 mm of the sample surface, whereas the hardness of the cross-sectional region decreased gradually, indicating that the scanning electron beam enhanced the adhesion between the carburized layer and matrix zone. A comparative analysis of the microstructures of the GC specimens with and without the SEBP showed that the carbide particles in the surface layer of the samples become smaller and that of volume fraction of residual austenite reduced in size. In terms of the mechanical properties, the surface friction coefficient decreased from 0.87 to 0.46 μ and the GC specimen with the SEBP had a higher cross-sectional hardness gradient. Its friction coefficient was reduced from approximately 0.8 to almost 0.45 μ, and the wear amount of the specimens with SEBP was 47.7% of that of the matrix specimens.

Anomalous magnetization induced by local chemistry fluctuations in Mn-containing α’-martensite

An anomalous increase in magnetization was investigated in a cold-rolled high-Mn TRIP steel (16.8 wt% Mn) annealed at 350°C for times varying from 5 to 120 min, before the start of austenite reversion (α’ → γ). Besides in-situ and ex-situ magnetic measurements, the full characterization of this phenomenon was also performed with the aid of X-ray diffraction (XRD), Mössbauer spectroscopy, and microstructural characterization using electron backscatter diffraction (EBSD) and atom probe tomography (APT). Based on XRD measurements, the volume fraction of austenite and α’-martensite was estimated as a function of annealing time, as well as their lattice strain. Phase quantification confirmed the absence of newly-fresh α’-martensite in the material during annealing at 350°C. Short annealing up to 15 min promoted the increase of Ms (saturation magnetization) due to stress relief in α’-martensite (Villari effect). Further annealing to 30 min promotes the decrease in Ms driven by short-range solute reorganization within the lattice. After 60 min annealing, the creation of long-range solute-depleted zones (i.e. confined zones highly enriched in Fe) causes a new increase in Ms. In comparison, for 120 min of annealing time, Ms tends to remain unaltered. These findings revealed that short- and long-range chemical fluctuations strongly affect the saturation magnetization of the steel and brought new insights on the use of magnetic probing as a tool for phase quantification in Mn-bearing steels.

Numerical Simulation of Reversed Austenite Evolution during Intercritical Tempering of Low-Carbon Martensitic Stainless Steel.

Since the formation of reversed austenite during critical tempering treatment is an important factor affecting the mechanical properties of 13Cr4Ni martensitic stainless steel, a detailed study of the content and morphology of reversed austenite in heat treatment is needed. In this study, the variation curves of a reversed austenite volume fraction with holding time at different tempering temperatures were measured by in situ X-ray diffraction (XRD), and the reversed austenite and carbides of each process were evaluated by transmission electron microscopy (TEM). The austenite content shows a parabolic change with the increase in the tempering temperature; the maximum can reach a peak of about 6.8% at 610 °C, and drops to 0% at 660 °C. It also shows a parabolic change with the extension of the holding time, reaching a maximum of about 9.2% at 5 h of holding time, and a decreasing trend at 10 h of holding time, about 6.8%. The results show that the precipitation of carbides in the microstructure causes elemental segregation at grain boundaries and inside, which is one of the main factors affecting the thermal stability of reversed austenite formation. The kinetic process of reversed austenite during the tempering process was simulated using the JMAK model and the KM model, which can describe the trend of reversed austenite content during the tempering process. Combining the two models, a mathematical model for the room-temperature reversed austenite content under different processes was obtained, and this can predict the room-temperature austenite content.

Correlation of microstructure, mechanical properties, and residual stress of 17-4 PH stainless steel fabricated by laser powder bed fusion

17-4 precipitation hardening (PH) stainless steel is a multi-purpose engineering alloy offering an excellent trade-off between strength, toughness, and corrosion properties. It is commonly employed in additive manufacturing via laser powder bed fusion owing to its good weldability. However, there are remaining gaps in the processing-structure-property relationships for AM 17-4 PH that need to be addressed. For instance, discrepancies in literature regarding the as-built microstructure, subsequent development of the matrix phase upon heat treatment, as well as the as-built residual stress should be addressed to enable reproducible printing of 17-4 builds with superior properties. As such, this work applies a comprehensive characterisation and testing approach to 17-4 PH builds fabricated with different processing parameters, both in the as-built state and after standard heat treatments. Tensile properties in as-built samples both along and normal to the build direction were benchmarked against standard wrought samples in the solution annealed and quenched condition (CA). When testing along the build direction, higher ductility was observed for samples produced with a higher laser power (energy density) due to the promotion of interlayer cohesion and, hence, reduction of interlayer defects. Following the CA heat treatment, the austenite volume fraction increased to ∼35 %, resulting in a lower yield stress and greater work hardening capacity than the as-built specimens due to the transformation induced plasticity effect. Neutron diffraction revealed a slight reduction in the magnitude of residual stress with laser power. A concentric scanning strategy led to a higher magnitude of residual stress than a bidirectional raster pattern.

Interfacial Segregation of Sn during the Continuous Annealing and Selective Oxidation of Fe-Mn-Sn Alloys.

The effect of Mn on interfacial Sn segregation during the selective oxidation of Fe-(0-10)Mn-0.03Sn (at.%) alloys was determined for annealing conditions compatible with continuous galvanizing. Significant Sn enrichment was observed at the substrate free surface and metal/oxide interface for all annealing conditions and Mn levels. Sn enrichment at the free surface was insensitive to the Mn alloy concentration, which was partially attributed to the opposing effects of Mn on segregation thermodynamics and kinetics: Mn increases the driving force for Sn segregation via reducing Sn solubility in Fe but also reduces the effective Sn diffusivity by increasing the austenite volume fraction. This insensitivity was exacerbated by the depletion of solute Mn near the surface due to the selective oxidation of Mn. Thus, Sn segregation occurred in regions with a local Mn concentration lower than the nominal bulk composition of the alloys suggested. Sn enrichment at the metal/external oxide interface was reduced compared to the free surface and decreased with increasing bulk Mn content, which was attributed to changes in the external oxide morphology and metal/internal oxide interfaces acting as Sn sinks.

Cyclic plasticity behavior and deformation mechanism of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings

Fatigue performance and durability application have always been the key issues impeding the understanding of aero-engine pressure pipe due to its non-standardized shape, making tests more challenging. This paper constructs an optimized testing system to elucidate the plastic shakedown and ratcheting behavior of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings. Moreover, microstructural evolution's role in regulating the cyclic plasticity behavior is qualitatively established. Experimental results indicate that the pipe deformation exhibits compressive ratcheting behavior in plastic shakedown state, and the initial cyclic hardening followed by cyclic saturation occurs during the whole cyclic response. This corresponds to a decreasing ratcheting displacement rate followed by a steady state appears. Except for the first two similar deformation stages mentioned above, the abrupt cyclic softening and the resultant increasing ratcheting displacement rate are obtained at the final fracture stage of the pipe deformation in ratcheting state. A series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and acoustic emission (AE) are combined to reveal the underlying microstructural mechanisms during cyclic deformation. The volume fraction of austenite and ferrite and the average grain size change during different stages of plastic shakedown and ratcheting deformation. The synergistic effects of dislocation structure multiplication, destruction, and dissolution at different stages of plastic shakedown and ratcheting deformation dominate the cyclic plasticity behavior.

Effect of compositional variations on the heat treatment response in 17-4 PH stainless steel fabricated by laser powder bed fusion

17–4 precipitate hardening (PH) stainless steel is used in various applications including in the aerospace, marine, and chemical industries, largely due to its unique combination of corrosion resistance and high strength, which is achieved by the formation of nanoscale Cu-rich precipitates during aging. 17–4 PH has been widely researched for its applicability for laser powder bed fusion (LPBF). However, there are discrepancies in the literature on its heat treatment response, which seem to be linked to compositional variations. Systematic studies of the interplay between these variations and nanoscale precipitation are currently missing. Using atom probe tomography, we present a systematic study of the heat treatment responses of two variants of LPBF 17–4 PH builds fabricated from different powder feedstocks, with significant differences in N contents (High vs Low N 17–4). Both variants formed predominantly δ-ferritic as-built microstructures. The as-built High N 17–4 variant showed a higher volume fraction of austenite which further increased upon solution annealing and quenching. The consequence was no appreciable hardening effect due to the absence of Cu precipitation in either austenite or martensite after aging, degrading the alloy's desirable property profile. Conversely, Low N 17–4 showed no austenite in the as-built condition and a fully martensitic matrix after solution annealing. This variant had the desired aging response; a ∼ 140 HV 5 increase in hardness due to nanoscale Cu precipitation. Our findings describe the deleterious effects of compositional variations incurred during the LPBF process flow and how they can be overcome in 17–4 PH and similar steels.

Effects of austempering on the microstructure and mechanical properties of high-strength nanostructured bainitic steel containing 3.5 wt% aluminum

This study aims to produce a cost-effective and low-density, high-strength nanostructured bainitic steel by optimizing the austempering process conditions. To do this, a steel containing 0.8C-1.5Mn-0.6 Cr-0.35Mo-3.5Al was selected and the effects of austempering time and temperature on the microstructure and mechanical properties of this steel were studied. Xrd technique was used to recognize the phases present in the microstructure and measure the retained austenite volume fraction and carbon content. Hardness measurements showed that the highest value was obtained from the specimen containing the highest volume fraction of the bainitic ferrite associated with the retained austenite with the highest carbon content, at the completion time of transformation. Such a combination of microstructure and hardness was applied to find the optimum time at different temperatures during the austempering process. The microstructures of the optimal specimens were evaluated through optical microscopy (OM) and field-emission scanning electron microscopy (FESEM), and the average thickness of the bainitic ferrite plates was calculated. Finally, tensile tests were performed on the optimal specimens to assess the fracture surface through FESEM. For high steel with 3.5 wt% Al, the results indicated that it is possible to obtain a microstructure consisting of bainitic ferrite plates with an average thickness of 53 nm surrounded by the carbon-rich retained austenite with a hardness of 561HV, a yield stress of 1614 MPa, an ultimate tensile strength of 1981 MPa, and a total elongation of 9 %.

Calphad-assisted design of high strength – ductility martensitic stainless-steels with reverted austenite

Introducing reverted austenite at lath martensite boundaries has been shown to be an effective method to increase the ductility of martensitic steels. However, significant strength reductions can also be introduced during the reversion process in martensitic stainless steels. To overcome the strength-ductility trade-off, we explore the effects of alloying elements in delaying the overaging of the precipitates and accelerating the austenite formation. To this end, we carry multi-objective optimization considering (1) lattice misfit and diffusivity for kinetics of precipitation, and (2) austenite fraction and thermodynamic driving force of austenite reversion for austenite formation. With the insights from calculations, model alloy compositions are then proposed, and mechanical testing and microstructural analysis are applied. The model alloy confirms that a relatively higher volume fraction of austenite can form prior to overaging, which doubles the total elongation without significant strength loss.