Transformation Of Austenite Research Articles

Shear Transformation of Austenite in Steels Considering Stresses’ Effects

Shear Transformation of Austenite in Steels Considering Stresses’ Effects

Dynamic Ferrite Formation and Evolution above the Ae3 Temperature during Plate Rolling Simulation of an API X80 Steel

Thermo-mechanically controlled rolling is a technique used to produce steel strips and plates. One of the steels widely used in the production of heavy plates for application in oil and gas pipelines is API X80. The hot rolling process of this family of steels consists of applying deformation passes at high temperatures, mainly above Ae3, inside the austenite phase field. It has been shown that during deformation, the phenomenon of dynamic transformation (DT) of austenite into ferrite leads to lower hot deformation resistance within the stable austenite region. In this investigation, hot torsion simulations of an industrial rolling process under continuous cooling conditions were used to monitor the formation of ferrite by DT. Stress–strain flow curves and equivalent mean flow stresses followed by sample characterization via optical and electron microscopy showed the inevitable formation of ferrite above the Ae3. The employed 10-pass deformation schedule was divided into 5 roughing and 5 finishing passes, thereby promoting an increased volume fraction of ferrite and decreased critical strain for the onset of DT and dynamic recrystallization (DRX). A microstructural analysis confirmed the formation of ferrite from the first roughing strain until the last finishing pass. The volume fraction of DT ferrite increased due to strain accumulation, an increased number of deformation passes and as the temperature approached the Ae3, leading to a characteristic torsion texture at the end of the simulation.

Nanosized Lamellar Structures and Tensile Properties of Intercritical‐Annealed Medium Mn Steels Containing Multiphases

The effect of extended intercritical annealing time on the changes of the microstructure and tensile properties is studied in a medium Mn steel. Nanosized lamellar structures are formed after intercritical annealing at 550 °C for 8 and 16 h; they consist of mainly tempered‐α′ martensite with retained austenite. Many cementite precipitates are observed in the tempered‐α′ martensite matrix and at the high‐angle boundaries. The orientation relationship between θ‐cementite and tempered‐α′ martensite is identified to be ()θ//()tempered‐α′, [011]θ//[011]tempered‐α′, and []θ//[]tempered‐α′. Fresh α′ martensite is detected between the tempered‐α′ martensite and austenite lamellae in the quenched specimens. The fresh α′ martensite is formed due to partial martensitic transformation of austenite upon cooling. The fractions of cementite and fresh α′ martensite increase with extended annealing time, leading to reduced austenite fraction after quenching. Additional baking treatment at 200 °C increases yield stress and elongation due to the stabilization of the austenite. This study provides a detailed understanding of these complex nanosized microstructures and discusses the key factors governing their yield stresses and work hardening behaviors.

Effect of tempering temperature on the microstructure, mechanical properties and abrasive wear behavior of a new C–Cr–Mo–V alloy steel used in TBM cutter ring

Tunnel boring machine (TBM) cutter ring material is a type of C–Cr–Mo–V alloy steel which possesses high hardness and wear resistance. To systematically study the effect of tempering temperature on the microstructure, mechanical properties and abrasive wear behavior of TBM cutter ring, a new C–Cr–Mo–V alloy steel (DB3) was subjected to tempering treatment and abrasive wear test. The results indicate that the tempering temperature controlled the hardness and toughness by adjusting the amount of precipitates and austenite transformation. Outstanding mechanical properties were obtained upon tempering at 520 °C. The wear resistance of DB3 steel was mainly controlled by hardness under non-impact abrasive wear condition. However, under the condition of impact abrasive wear, the wear resistance of DB3 steel was affected by hardness and toughness, and only in a certain range of toughness, the wear resistance increased with the increase of hardness. The experimental results can provide some reference for the heat treatment process of DB3 steel used in TBM cutter ring.

A thermomechanical finite strain shape memory alloy model and its application to bistable actuators

This work presents a thermomechanical finite strain shape memory alloy model that utilizes a projection method to deal with the incompressibility constraint on inelastic strains. Due to its finite strain formulation, it is able to accurately predict the behavior of shape memory alloys with high transformation strains. The key feature of this model is the thermomechanical modeling of the shape memory effect and superelastic behavior by optimizing a global, incremental mixed thermomechanical potential, the variation of which yields the linear momentum balance, the energy balance, the evolution equations of the internal variables as well as boundary conditions of Neumann- and Robin-type. The proposed thermal strain model allows to properly capture transformation induced volume changes, which occur in some shape memory alloys. A finite strain dissipation potential is formulated, which incorporates the disappearance of inelastic strains upon austenite transformation. This important property is consistently transferred to the time-discrete potential using a logarithmic strain formulation. Yield and transformation criteria are derived from the dual dissipation potential. The implementation based on an active set search and the algorithmically consistent linearization are discussed in detail. The model is applied in three-dimensional simulations of a bistable actuator design to explore its capabilities.

Experimental Investigation on the Effect of Deep Cryogenic Treatment on 100Cr6 Bearing Steel

Cryogenic treatment is the process of cooling a material to extremely low temperatures to generate enhanced mechanical and physical properties. The present investigation examines the effect of deep cryogenic treatment on the enhancement of mechanical properties, such as wear resistance, corrosion resistance, tensile strength and impact strength of the plunger material 100Cr6 bearing steel. The improvement in the wear resistance, corrosion resistance, tensile strength and impact strength of the deep cryogenically treated samples over the conventionally heat-treated ones, is 50%, 26%, 13% and 27% respectively. This study suggests that the formation of very small carbides dispersed in the tempered martensite structure, can be the main reason for the enhancement of certain mechanical properties, along with the retained austenite transformations.

Dilatometric study of continuous cooling transformation of intercritical austenite in cold rolled AHSS-DP steels

The austenite phase transformations under continuous cooling from intercritical austenitization at 800 °C and full austenitization at 910 °C were investigated in a cold rolled advanced high strength dual phase steel (AHSS-DP) strip. Experimental continuous cooling transformation (CCT) diagrams were obtained using induction-quenching dilatometry and the corresponding microstructures were characterized by scanning electron microscopy (SEM) analysis and Vickers microhardness measurements. In general, it was shown that the experimental behavior of phase transformations in current DP steels depends on the dynamic changes that occur in the hardenability of austenite during continuous cooling at different rates. The main results reveal that the martensite-start transformation temperature (Ms) depends on the cooling rate, particularly when an intercritical austenitization and slow cooling rates conditions coincide. The unusual behavior of Ms temperature is explained in terms of solute distribution, which changes the chemical composition of metastable austenite (local enrichment in carbon content) during cooling, causes changes in hardenability, lowers both the Ms temperature and the critical cooling rate at which martensite is formed. In general terms, the behavior of Ms at slow cooling rates is relevant information that has not been reported yet in the specific literature because it is generally known that Ms does not depend on the cooling rate when quenching is performed from the austenite phase field.

Structure and Properties of Forged Steel with Regulated Austenite Transformation

Structure and Properties of Forged Steel with Regulated Austenite Transformation

Formation of structural features of powder materials during cooling after heat treatment

Introduction . Recently, sintered materials and products made of them have been increasingly used in powder metallurgy. In this regard, the issue of obtaining sintered products with high performance properties is acute. To achieve such properties, the materials are subjected to heat treatment. This procedure significantly affects their structure and mechanical properties. In production, sintered materials are most often subjected to subsequent hardening and tempering, as a result of which their equilibrium structure is established, grain growth stops, and strength characteristics improve. The article discusses the problems that arise in the formation of the qualitative structure of dispersed-hardened alloys as a result of their heat treatment. Problem Statement . The objective of this work is to study the phase changes in the process of cooling of powder steels and alloys in order to determine the modes of their heat treatment in order to form optimal conditions for the martensitic transformation of austenite. Theoretical Part . Phase transformations in powder steels occur in the temperature range at which their structures are rearranged, and as a result, the properties of the material change. The main factors affecting the phase transformations are the chemical composition of the alloy, the structure imperfection and the size of the grains. Changes in the structure and properties of alloys are considered in comparison with compact materials. Heat treatment significantly affects the phase and structural characteristics of powder materials, which are related to the mechanical characteristics of the alloys themselves. Conclusions . The conducted studies have shown that with an increase in the heterogeneity of the solid solution of steels, the temperature of the beginning of the martensitic transformation increased. A decrease in the temperature of the martensitic transformation with an increase in the degree of homogeneity of the solid solution occurs due to its enrichment with carbon and other alloying elements (chromium, molybdenum). With an increase in the percentage of carbon, an increase in the porosity of samples, the starting point of martensitic transformation also decreases. The temperature of the beginning of the martensitic transformation is not affected by carbides that are with austenite. These conclusions will help us to evaluate the mechanical properties of materials, as well as to develop recommendations for the practical application of heat treatment in the manufacture of products of complex shape.

Effect of Uniaxial Compressive Stress on Phase Transformation Kinetics of Low-Carbon Steel.

To attain microstructure transformation and the kinetics of phase transformation under stress during the annealing process, dilatometric curves of phase transformation for Q235 steel were tested using a Gleeble-3500 thermal-mechanical simulator under different uniaxial compressive stresses. The Johnson–Mehl–Avrami (JMA) model considering impingement correction was applied to study the phase-transformation kinetics during annealing. The results showed that the grain size increased with increasing uniaxial compressive stresses because it provided additional energy for grain growth. Furthermore, the interfacial migration velocity decreased with increasing stress owing to grain coarsening and a decrease in the density of the α/γ boundary. Meanwhile, the stress reduces the sum of the misfit accommodation energy and interface energy caused by the transformation, and the driving force required for the transformation of austenite to ferrite decreases. Hence, it was concluded that uniaxial compressive stress plays a complex role in the phase transformation, which inhibits interfacial migration and the transformation rate while providing additional energy for the transformation.

Mechanism and Application of Reverse Austenitic Transformation in Medium Mn Steels: A Systematic Review

In this contribution, the nucleation and growth mechanism of austenite during the intercritical annealing (IA) of medium Mn steels (MMSs) are reviewed. The nucleation of austenite can be enhanced due to the introduction of (sub)grain boundaries, dislocations, and carbides before austenitization. Although martensite may transform to austenite in a displacive way during flash heating, austenitic nuclei usually grow with the diffusion and partitioning of solute C/Mn atoms. The latter, however, is still difficult to be modeled because the combined influence of interface dissipation energy, defects, and carbide particles is not well understood and quantified. Next, four recent emerging processing technologies, including prior deformation for generating dislocations, additional tempering for the precipitation of carbides, additional annealing before IA, and flash heating, are also analyzed according to the above‐stated nucleation and growth mechanism. Finally, it is proposed that the targeted austenite grains for improving the mechanical properties of MMS shall be produced by enhancing both nucleation sites and growth kinetics of austenite via the deliberate design of processing routes.

Torsional Fracture Resistance of Niti Rotary Glide Path Files under Flexural Stress

This study aimed to evaluate the effects of flexural stress on the torsional fracture resistance of NiTi glide path files. PathFile #16/02, RaCe #15/04, RaCe Evo #15/04, HyFlex EDM #15/03, TruNatomy Glider #17/02, and V Taper 2H #17/04 were examined by scanning electron microscopy (SEM) (n = 3/brand) and subjected to differential scanning calorimetry (n = 2/brand). Torsional fracture resistance testing was performed in straight (ISO 3630-1) and flexural modes (n = 15/brand/mode). Flexural mode testing involved instruments rotating within a stainless-steel artificial double-curved canal. Ultimate strength and distortion angle until failure were recorded, and fractured instruments were examined by SEM. Statistical analyses involved independent sample t-test and one-way analysis of variance with Games–Howell pots hoc test. Austenitic transformation- finishing temperatures of heat-treated files were above body temperature. For RaCe Evo, HyFlex EDM, TruNatomy Glider, and V Taper 2H, the flexural mode resulted in a significantly higher distortion angle compared to the straight mode (p < 0.05). The maximum torque of RaCe Evo, HyFlex EDM increased with the flexural stress (p < 0.05). V taper 2H showed the highest distortion angle and ultimate strength. SEM showed typical patterns of torsional fracture for all tested files. The flexural stress positively affected distortion angle of heat-treated NiTi glide path files.

In situ laser-ultrasonic monitoring of Poisson’s ratio and bulk sound velocities of steel plates during thermal processes

We demonstrate the in situ measurement of zero-group-velocity Lamb waves and thickness-resonances in steel sheet samples using laser-based ultrasound, while changing the sample temperature from ambient to 1000 ∘C. The ratio of resonance frequencies yields the temperature dependent Poisson’s ratio of its material without requiring knowledge on the sheets’ thickness. Changes in the temperature derivative of the determined Poisson’s ratio reflect changes in the microstructural constituents. We show that additional resonances present in the response spectrum can be used to cross-check the evaluation. With additional knowledge on the thickness, longitudinal and transverse sound velocities are determined as well. We find good qualitative agreement between the temperature dependence of the measured quantities and reference data obtained from dilatometry and thermodynamic material simulations. The determination of the Poisson’s ratio and transverse sound velocity complements previous investigations limited to the longitudinal sound velocity. This is particularly evident above the Curie temperature during the ferrite to austenite transformation and could be beneficial to disentangle transformations with more than two microstructural constituents involved. Finally, the limitations posed by elastic anisotropy introduced by rolling texture, and experimental uncertainties due to temperature inhomgeneities are discussed. We conclude that the laser-ultrasonic measurement of plate resonances provides a robust, contact-free, non-destructive, and in situ determination of Poisson’s ratio.

Effect of quench-tempering and laser quenching on the microstructure and properties of high-chromium cast iron

In this paper, quench-tempering (QT) and laser quenching (LQ) treatments were performed on the high-chromium cast iron to obtain better microstructure and mechanical properties. The changes in microhardness, microstructure, retained austenite (RA) content, grain size, phase and elemental content of the QT and LQ samples were compared and analyzed. The results showed that the RA content of the untreated samples was 77.02% and the was 26.8 nm. The QT samples showed an increase in carbide dissolution, gradual rounding from sharp angles, transformation of austenite into martensite, reduction of RA content to 28.48%, average grain size (AGS) of 35.8 nm, increase in surface microhardness by 16.6%, and the friction coefficient was reduced by 6.9%. The LQ samples showed a large amount of fine martensite, daisy-like eutectic carbides and diffusely distributed secondary carbides, and the RA content was reduced to 17.72%, with an AGS of 29.9 nm. Compared with the QT samples, the LQ samples showed a 25.1% reduction in AGS, a 16.5% increase in surface microhardness, and a 9.7% decrease in friction coefficient, which was more wear resistant. However, the hard and brittle surface layer of the LQ samples tended to be a source of cracks during impact, resulting in a final reduction of 8.4% in impact toughness compared to the QT samples.

Early Martensitic Transformation in a 0.74C–1.15Mn–1.08Cr High Carbon Steel

The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the variant pairing of the early-formed martensite. A mixed microstructure with tempered early-formed martensite and fresh later-formed martensite was achieved by a heat treatment with an isothermal hold below the martensite start temperature. In-situ high-energy X-ray diffraction showed no further transformation of austenite to ferrite/martensite during the isothermal hold. The tempered early-formed martensite was characterized with a combination of light optical microscopy and local tetragonality determination by electron backscatter diffraction. The characterization allowed qualitative as well as quantitative analysis of the tempered early-formed martensite with regard to the prior austenite grain boundaries (PAGB) and variant pairing. The early-formed martensite was shown to grow predominantly along the PAGBs and clustering was observed indicating an autocatalytic nucleation mechanism. The variant pairing of the early-formed martensite had a stronger plate character compared to the later-formed martensite.

DEVELOPMENT AND IMPLEMENTATION OF A NEW METHOD FOR MODELING PHASE-STRUCTURAL TRANSFORMATIONS DURING COOLING OF ALLOY STEELS

Formulation of the problem. Modern research on the phase transformations modeling in low-alloy steels allow solving the problem of phase transformations quantitative determination for a given chemical composition of steel and different cooling rates. However, the possibilities of available universal software products for the complex alloy steels analysis are limited. The impossibility for users to integrate their own subroutines according to the phase transformation diagrams is their main disadvantage. Purpose of research. Modeling phase-structural transformations during cooling of complex-alloy steels taking into account the formation of all structural components, in particular residual austenite. To research, steels 25Cr2Mo1V and 38CrNi3MoV and existing analytical models were used, which were adapted to carry out the relevant calculations. Results. A new method for modeling phase-structural transformations during cooling of alloy steels is developed. Structural diagrams depending on the rate of continuous cooling are constructed for the investigated steels. For the first time, the amount of residual austenite is taken into account according to the developed method. Under developed method thermokinetic diagrams of investigated steels austenite transformation are constructed. According to the diagrams, the decay of austenite steel 38CrNi3MoV begins at lower temperatures compared to steel 25Cr2Mo1V. Steel 25Cr2Mo1V, with continuous cooling at a rate of 1.0 °C/s (conditions close to natural air cooling), consists of 18 % ferrite, 1 % pearlite, 80 % bainite and 1 % residual austenite. Steel 38CrNi3MoV cooled at a rate of 1,0 °C/s consists of 2 % ferrite, 47,5 % bainite, 50 % martensite and 0,5 % residual austenite. It is shown that the calculated data correlate well with practical results at the conditions of natural air cooling.

Carbides precipitation and kinetics of H13 steel subjected to deep cryogenic treatment

The influence of deep cryogenic treatment (DCT) on carbides precipitation and isothermal softening behaviour as a function of both temperature and time during tempering of H13 steel was investigated. The results show that the retained austenite transformed into martensite leading to an increase in hardness. The carbon content of martensite decreased after DCT and tempering according to the calculation of the X-ray diffraction spectrum. This indicates that more fine carbides precipitated from the martensitic matrix after DCT. The morphology of microstructure and distribution of carbides have verified the properties improvement mainly depending on the retained austenite transformation and more fine carbides precipitated after tempering. In addition, the carbides precipitating and coarsening are the main causes of thermal softening.

Microstructure Investigations of Phase Transformation in Cold Working AISI 316L Austenitic Stainless Steel

Development of high temperature and corrosion-resistant materials is one of the key issues for the deployment of advanced nuclear reactors and also to accommodate the problem that occurred in the conventional reactor as the lesson-learned from Fukushima Daiichi nuclear reactor power plant accident. One of the high performance materials for that purpose is austenitic stainless steel such as AISI 316L that widely used for power plant. In this study we investigate the characteristics of AISI 316L austenitic phase transformation if cold working is applied. In general, during the cold working process the mechanical characteristic and the phase of the austenitic steel will change. It is expected that the characteristic of AISI 316L austenitic steel will be improved by optimum cold working mechanism. Cold working of AISI 316L austenitic steel at various percentage reduction of 5%, 15%, 25% and 38% have been done. Afterward, the sample was characterized using X-Ray Diffraction and Optical Microscope to analyze the microstructure characteristics and phase transformation. The results showed that the phase transformation in AISI 316L austenitic steel occurred from austenite – gamma (FCC: Face-Centered Cubic lattice) to martensite – alpha prime (BCC: Body Centered Cubic lattice). The percentage of martensite phase was increasingly growth related to the increasing of the percentage of cold working value i.e. 8.3%, 21.6%, 29.6% and 37.1%, respectively. The hardness of AISI 316L austenitic steel increased with the increasing of the cold working percentage. AISI 316L double phases which covers of austenite-martensite intermix phase structure with higher hardness mechanical properties has been successfully developed.

Influence of Deep Cryogenic Treatment on Microstructural Evolution and Transformation Kinetics Simulation by Finite Element Method of Low‐Carbon High‐Alloy Martensitic‐Bearing Steel

Nowadays, deep cryogenic treatment (DCT) is taken as a promising technique for improving the performance of steel. This article is focused on the influence of different heat treatment processes involving DCT on the microstructural evolution of a high Cr–Co–Mo high‐temperature‐bearing steel. Moreover, a multiphysical field coupling numerical model is built to investigate the martensitic transformation kinetics during DCT. The results show that DCT significantly improves the hardness and promotes the transformation of retained austenite to martensite at low temperatures. DCT not only refines the retained austenite as thin‐film morphology but also increases the precipitation of carbides and induces the carbides more homogenous distribution. This indicates that DCT promoted the segregation of carbon atoms and reduced the carbon content of martensite. The finite element model simulation shows that the retained austenite transformation is still incomplete, and the simulation results agree well with the experimental data by X‐Ray diffraction and transmission electron microscope analysis. This implies that the finite element simulation can better reflect and verify the experimental results.

Heterogeneous quenching and partitioning from manganese-partitioned pearlite: retained austenite modification and formability improvement

Increasing quenching temperature in conventional quenching and partitioning (Q&P) process usually increases the fraction of retained austenite (RA) being dominant by blocky morphology, which ensures a good global formability but deteriorates the local formability. Instead of homogeneous austenite in conventional Q&P process, we propose a Q&P process purposely based on Mn-heterogeneous austenite inherited from Mn-partitioned pearlite. Therein, Mn-depleted austenite areas originated from ferrite lamellae are readily transformed into lath martensite during quenching; meanwhile, Mn-enriched austenite areas originated from cementite lamellae are readily retained as film RA at room temperature. Consequently, this heterogeneous Q&P process unusually achieves a large RA fraction being dominant by film morphology. The large RA fraction ensures a large total elongation and the dominant film morphology ensures a large post-uniform elongation, indicating good global and local formability, respectively. Additionally, the microstructure is refined through the above divided transformation of heterogeneous austenite, predominantly contributing to the increased yield strength by 200 ∼ 300 MPa. Introducing heterogeneous austenite into Q&P process provides a feasible way to achieve a large RA fraction being dominant by film morphology, leading to the successful production of high-strength Q&P steels simultaneously with good global and local formability.