Decomposition Of Austenite Research Articles

Investigation into the Influence of the Structure Dispersion and Homogeneity on the Properties of Powder Metastable Austenitic Carbide Steels and Diamond Tools

Diffusion and homogenization in “iron (5 μm–nickel (5 μm or 50 nm)” powder systems of various degrees of dispersion during sintering (900 and 1000°C), as well as spark plasma sintering, are investigated using the Matano–Boltzmann method. Calculated diffusivities in pairs of micron powders sintering without applying pressure (900°C, 6 h) and by the spark plasma method (900°C, 5 min) in these systems are equal to 7 × 10–10 cm2/s. It is shown that the use of nanodispersed nickel powder in diffusion pairs based on finely dispersed iron powder promotes a twofold increase in diffusivity at 900°C in contrast to the pair with the microdispersed nickel powder. Constants in the Ivensen sintering kinetics equation are calculated for the “iron–nickel” powder systems, by which the factors activating sintering of these systems are established. The dependences of the structural phase composition and physicomechanical properties of carbide steels of the Fe(base)–14 wt % Ni–8 wt % TiC system on the sintering temperature in range t = 900–1200°C and structure dispersity and homogeneity are determined. The dependences of the grain size, porosity, hardness, microhardness, fracture toughness, and bending ultimate strength on the sintering temperature are shown. Dependences of tribotechnical properties on the degree of homogeneity of the solid solution and volume of the phase transformation of metastable austenite into deformation martensite during abrasive friction turn out similar for carbide steels and diamond tools based on carbide steel. Optimal values of the variation coefficient of the nickel concentration in austenite and carbide steels of the same chemical composition but with different degrees of dispersity, which provide the maximal volume of the austenite decomposition and high values of the diamond-tool grinding coefficient, turn out equal to 5 in both systems, but the sintering parameters are different. It is shown that the physicomechanical properties of the studied systems depend on the structure porosity and dispersity, while tribotechnical properties depend on the structural homogeneity of steels.

Optimization of CCT Equations Using Calculated Grain Boundary Soluble Compositions for the Simulation of Austenite Decomposition of Steels

New CCT equations have been developed and optimized to simulate the start temperatures of the austenite decomposition process in low-alloyed steels using experimental CCT data published in the literature. Exceptionally, this optimization does not apply the nominal compositions of the steels, but the corresponding soluble compositions of the grain boundaries calculated using IDS software, depending on the reported austenitization treatments of the steels. These compositions, rather than the nominal ones, are expected to control the start of the austenite decomposition, which usually initiates at the grain boundaries. The new optimization treatment takes into account the solute microsegregation and the possible precipitate formation. Using IDS software, the new equations were validated with new experimental CCT data. Agreement was good not only for the austenite decomposition start temperatures, but also for the final phase fractions, indicating fairly reasonable predictions of phase transformation kinetics by the IDS. In addition, IDS simulations were compared with the experimental CCT data of five high-carbon steels, applying both the new equations based on grain boundary soluble compositions as well as the equations based on the nominal compositions. With the same experimental CCT data used in optimization, better agreement was obtained with the new equations, indicating the importance of determining the soluble compositions at the grain boundaries where the austenite decomposition process is likely to begin.

Ferritic Decomposition in Fe–36Ni–3Cr Invar Alloy

Nickel redistribution and austenite decomposition into ferrite with the formation of up to 15% α‑ferrite crystals 10–100 nm in size in a fairly stable Fe–36Ni–3Cr invar alloy were found. The austenite decomposition during heating in a 250–400°C temperature range is intensified by preliminary strong plastic deformation by rolling or shearing under high pressure.

Influence of Vanadium Microalloying on Isothermal Transformation Behavior of Eutectoid Steel

Abstract—The influence of vanadium as a microalloying element on the austenite decomposition behaviors and microstructure transformation of eutectoid steel were studied during isothermal cooling. Vanadium dissolved in γ matrix exhibited a negative effect on austenite decomposition to pearlite microstructure and led to a decrease in the lamellar spacing. Moreover, the results indicated that the dissolved vanadium segregated toward austenite grain boundaries and suppressed the formation of grain boundary ferrite. However, the precipitation of vanadium during the isothermal process led to a decrease in vanadium dissolved in matrix that indirectly promoted transformation to pearlite. The precipitates could act as nucleus for pearlite formation, which also accelerated the transformation. The lamellar spacing of pearlite increased as a result of carbon consumption of precipitates. The addition of nitrogen remarkably accelerated the transformation, because the chemical driving force for VN formation was significantly larger.

Observation of Phase Transformations in High-temperature Chamber under Microscope Objective Lens

Today, extensive information on a particular material can be obtained from its chemical composition using vari-ous computer programs. Such information includes phase transformation temperatures, such as Ac1 and Ac3 or the Ms and Mf, as well as the entire austenite decomposition sequence, as shown in TTT and CCT diagrams. When steel is heated, austenite grains begin to form, grow and eventually coarsen. On cooling, these grains de-compose into pearlite, ferrite, cementite and hardening phases. Transformations of this kind are well understood nowadays, namely the ways the individual phases form and the rates and temperatures involved. Yet, in-situ visual recordings of such transformations are relatively rare. Using the Linkam TS1400XY heating stage, which is capable to heat samples up to 1400°C and is integrated in an optical microscope featuring objective lenses with modified working distances, these phase transformations can be observed directly. This paper explores the po-tential of the heating stage for studying various materials and their specific heat treatments. It also discusses the method of sample preparation for use in the heating stage placed in an optical microscope.

Prediction of Dilution and Its Impact on the Metallurgical and Mechanical Behavior of a Multipass Steel Weldment

Abstract A three-pass groove weld made by gas-tungsten arc welding in a 20-mm thick SA508 steel plate is modeled to predict the thermal, metallurgical, and mechanical behavior. The dilution for each pass is estimated as the proportion of base material in the weld metal, based on the predicted cross-sectional areas for the fusion zone (FZ) associated with each individual pass. The temperature predictions are consistent with the thermocouple measurement data and cross-weld macrographs. The predicted microstructures are qualitatively compared with the observed microstructures in cross-weld optical micrographs. The measured hardness is then used to quantitatively validate the predictions for postweld microconstituents (e.g., the ferrite, bainite, and martensite fractions), based on a hardness-microstructure correlation. The predicted residual stresses are compared with those measured by neutron diffraction. The results show that the dilution significantly affects the metallurgical and mechanical properties of weld metal (either as-deposited or reheated), and its consideration notably improves the predictions for microstructure and residual stress in the multipass steel weldment. Furthermore, the increase in dilution promotes the formation of martensite, which enhances the hardness, and leads to lower tensile stresses (or higher compressive stresses) in the weld metal. Such behavior arises due to the higher hardenability of the base material employed in this study, coupled with delayed austenite decomposition on cooling.

Effect of quenching temperature on the microstructure and mechanical properties of 30MnBNbV hot stamping steel

A low carbon steel microalloyed with Nb-V subjected to hot stamping process, then quenching to different temperatures (30 °C, 230 °C, 300 °C) followed by air cooling to room temperature was investigated. The method in this article was significantly simplified compared with the conventional quenching & tempering (QT) process. The results shown that the best comprehensive mechanical properties could be obtained at 230 °C. The yield strength (YS) and ultimate tensile strength (UTS) were 1128.8 MPa and 1768.8 MPa respectively, and the total elongation was 8.8% without much compromise. The excellent properties were attributed to a multi-morphology microstructure which consisted of large amount of martensite (block fresh martensite and lath tempered martensite) and trace carbon-enriched retained austenite. In addition, the recovery of block fresh martensite (FM) and decomposition of unstable austenite were discussed during air cooling to room temperature process.

Correlation of 12% Chromium Ferritic-Martensitic Steel Heat Resistance with Supercooled Austenite Stability Indices

Development of new heat-resistant ferritic-martensitic steels with 12% chromium designed for operation at 700–720 °С is a complex multifactorial task. The main characteristics of such steels that define the possibility of their application at elevated temperature are high values of strength within the temperature range of 20–720 °С, and high values of creep resistance at temperatures of 650 °C and above with a satisfactory ductility level (at least 6–12% at 20–720 °С). A significant amount of time is required in the case of large range of steels investigated to determine the abovementioned properties. An important task is finding readily and quickly obtained properties by which it is possible to estimate the level of steel heat resistance properties. Diagrams for isothermal decomposition of supercooled austenite are analyzed and equations are found in the present work for their connection with yield strength and creep strength of the materials studied.

Инновационная технология термической обработки боковой рамы тележки грузового вагона, обеспечивающая повышение ее усталостной прочности

Basic regulations of the thermal strengthening technology of a bogie side frame in a freight car made of mild steel based on the procedure developed for the parameter definition of thermal treatment on the results of the computer simulation of kinetics of structural and phase transformations in steel are shown. The results of the computer simulation of structure formation in 20 GL steel containing 0.42 and 0.53% of silicon at the realization of different cooling intensity are shown. The computer simulation is carried out on the basis of the developed procedure formed on the basis of the austenite decomposition diagram taking into account different cooling rates. The experimental assessment of a bogie side frame state after realization of the technology developed is carried out.

FEATURES OF FORMATION OF THE STRUCTURE AND PROPERTIES OF STEEL 09Cr3NIMo3VNBr DURING THERMOCHEMICAL TREATMENT

Purpose. The article is aimed to investigate the structural state formation of the new 09Cr3NiMo3VNbr steel after cementation and nitration with subsequent heat treatment. To determine the rational mode of thermochemical treatment, which will ensure the creation of a uniform diffusion layer and high mechanical and operational characteristics. Methodology. Thermochemical treatment was carried out on primary and secondary hardening in four versions using the SIEMENS and ACEA furnaces. Nitration was carried out in two stages. To identify the microstructure of pilot production steel the nitric acid etching in a 4% solution of nitrous acid was carried out, the metallographic studies were performed on the cross section of the samples on Axio Observed.D1m microscope. To determine the effective depth of the diffusion layers after cementation and nitration, microhardness was measured on LECO LM248AT instrument, and hot hardness measurements were performed on Wilson Rockwell Hardness Tester instrument. Findings. To obtain a homogeneous structural state of steel 09Cr3NiMo3VNbr and maximize the results of mechanical and performance characteristics, thermochemical treatment was performed according to the following mode: cementation at 930°C with a carbon potential of 0.22, 13 hours, annealing 650°C, 5-7 hours, hardening from 1 030°С with cooling in oil, three-time tempering 530°С, cold treatment – 70°С with a holding time of 1 h. 30 min. and subsequent tempering at 510°C, 1 hour. After cementation, hardening was carried out in the temperature range of 1000–1030°C to prevent oversaturation of the working diffusion layer and formation of a carbide mesh. Subsequent three-time tempering at 530°С and cold treatment of – 70°С for 1 h. 30 min. contribute to the decomposition of residual austenite and the formation of fine-grained tempering martensite. Originality. We developed the optimal mode of thermochemical treatment, which will ensure maximum mechanical and operational properties of steel 09Cr3NiMo3VNbr due to a predetermined ratio of the structural components of the cemented layer. Practical value. The tested mode of thermochemical treatment of multicomponent alloyed 09Cr3NiMo3VNbr steel, produced by Motor Sich JSC and Ivchenko–Progress, which was developed for the gears of aviation gearboxes of gas turbine engines of the new generation, significantly increases heat and power load.

Revealing the Conditions of Bainitic Transformation in Quenching and Partitioning Steels

Isothermal decomposition of austenite during the partitioning process of quenching and partitioning (Q&P) steels is normally ignored but may play an important role in determining the condition of retained austenite in the final microstructure and hence the mechanical properties of the steel. To provide a theoretical basis and practical guideline for controlling the isothermal bainitic transformation and optimizing the competition with other transformation mechanisms, the isothermal bainitic transformation was systematically investigated and the associated time-temperature-transformation diagrams were constructed for 0.2C-1.60/2.82/3.92Mn-1.6Si steels during one-step and two-step quenching and partitioning processes. The influences of quenching temperatures, partitioning temperatures, and Mn concentrations on the bainitic transformation window were quantitatively analyzed by dilatometry and electron microscopy. The similarities and differences between one-step and two-step quenching and partitioning processes were discussed. The results reveal the conditions of isothermal bainitic transformation when subjected to different Q&P heat treatment scenarios and justify the relative importance of these conditions in determining the final microstructure with respect to the industrial practices of hot rolled and continuous annealing processes. The bainitic transformation process window obtained by this work could provide direct guidelines in optimizing the compositions and heat treatment parameters of Q&P steels.

Heat treatment behavior of alternator shafts steels

The present paper analyzes properties such as the hardness, the tensile and toughness behaviour and the microstructure for multiple bars of C40 and C45 steel used for shafts in alternators. The materials have been manufactured by rolling processes, followed by normalization treatments or forging processes followed by normalization treatments. Heat treatments are performed on steels to produce different microstructural phases that affect the mechanical properties. Microstructural changes during austenite decomposition depend on the transformation temperature region and cooling rate. Over the years, many experimental studies have been performed to reveal the microstructural changes during isothermal holding or continuous cooling of austenite for various kinds of steels. The isothermal transformation (IT) diagram, which is also called the time-temperature-transformation (TTT) diagram, and the continuous cooling transformation (CCT) diagram have been developed to graphically characterize the decomposition transformation as a function of time and temperature. However, when these diagrams are based on experimental measurements, several limitations are present. Each diagram is limited because the temperatures at which specific transformations occur can vary due to several factors, such as: the chemical composition of steel, the solid solution condition in the austenite prior to cooling and transformation, the presence of precipitates in austenite, the prior austenite grain size, and the applied stresses during the transformation.

Microstructure Evolution in Belt-Casted Strip of Grain Oriented Electrical Steel

This paper deals with the formation and decomposition of Widmanstätten austenite during solidification of the thin belt-casted strip made of a grain oriented electrical steel (GOES). Solidification of liquid steel starts with the formation of d-ferrite. Cooling in the delta + gama phase field results in the formation of a small fraction of Widmanstätten austenite by displacive transformation accompanied by carbon partition. Widmanstätten austenite laths have an orientation relationship with the ferrite grain into which they grow. Furthermore, they form a flat low energy interface along the ferrite grain boundary. In order to minimize the interfacial energy, ferrite grain boundaries in the vicinity of flat austenite/ferrite interface facets are forced to migrate which results in straightening of these grain boundaries. If parallel Widmanstätten austenite laths form in two adjacent ferrite grains, zig–zag ferrite grain boundaries arise. Precipitation of sulphides along ferrite/austenite interfaces make it possible to study the early stages of austenite decomposition under the delta + gama phase field. It starts with the formation of epitaxial ferrite accompanied by further partitioning of carbon into remaining austenite. The growth of epitaxial ferrite into the flat ferrite/austenite interface facets along ferrite grain boundaries results in a wavy shape of these ferrite grain boundaries. Finally austenite transforms either to pearlite or to plate martensite.

Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Martensite Embrittlement Regime

While tempering reactions under furnace conditions have been extensively studied, short-time tempering at higher temperatures, as might apply to induction tempering, has not been thoroughly explored. The mechanical behavior and phase development of short-time (1, 10, and 100 seconds) and conventionally (3600 seconds) tempered 4340 steel are compared at an equivalent degree of tempering defined by the Hollomon–Jaffe tempering parameter. The tempering parameter accurately predicts hardness values across short-time and conventional tempering conditions, but is less able to describe phase evolution associated with short-time tempering. Room temperature Charpy impact toughness and ductile-to-brittle transition temperature systematically improve with shorter tempering times at an equivalent tempering parameter. In particular, rapid tempering significantly increases toughness within the tempered martensite embrittlement regime. Relative to conventional tempering, shorter tempering times exhibit higher retained austenite content for a given tempering parameter, although no systematic or significant difference in cementite or transition carbide content is observed between time conditions. The retained austenite decomposition behavior indicates that the relationship between the classical tempering stages is altered at short tempering times, where increased overlap and/or “re-ordering” of stage II and III tempering are observed.

In Situ Quantitative Assessment of the Role of Silicon During the Quenching and Partitioning of a 0.2C Steel

Silicon is an essential alloying element added in quenching and partitioning (Q&P) steels to delay and/or suppress carbide precipitation. However, there is a strong industrial interest to reduce the silicon content as it has detrimental effects on the metallurgical route. This work investigates by means of in situ high-energy XRD (HEXRD) the effect of silicon on the microstructural evolution during quenching and partitioning of a commercial 0.2C-2.3Mn grade. The results of this study highlight the role of the bainite transformation during the reheating and partitioning steps for effective austenite retention. Silicon influences the kinetics of austenite decomposition into bainite and finally promotes the stabilization of austenite. This is explained by the ability of silicon to suppress carbide precipitation (i) at the interface between bainite and austenite and (ii) in the martensite matrix. Carbide precipitation at the bainite/austenite interface decreases the amount of carbon that diffuses from bainite to austenite, subsequently accelerating the bainite transformation kinetics and preventing austenite stabilization. Carbide precipitation in martensite reduces the amount of carbon available for partitioning in austenite, further preventing its stabilization. Additions of elements such as Cr or Mo could be therefore considered in order to reduce the austenite decomposition in low-silicon steel grades.

Tempering Reactions and Elemental Redistribution During Tempering of Martensitic Stainless Steels

Tempering reactions in the martensite phase of Fe-13Cr-0.47C (mass pct) stainless steel and its Si- and Mn-added modifications were studied by correlative dilatometry and magnetic measurements. Tempering for 5 minutes was performed at sequentially higher temperatures up to 923 K (650 °C). Classical tempering reactions including the segregation of C atoms at defects, precipitation of M3C and Cr-rich carbides, and austenite decomposition were clearly identified. The formation of M3C carbides was partially and entirely suppressed by Mn and Si additions, respectively. Compared to low-alloy steels, the decomposition of retained austenite in stainless steels was delayed to temperatures above 823 K (550 °C). The latter occurred concurrently with the formation of Cr-rich carbides in the martensite. In addition, non-classical tempering reactions such as the partial dissolution of C clusters at temperatures above 573 K (300 °C) and the short-range diffusion of substitutional elements including Cr and Mn to C clusters and M3C carbides in the temperature range of 673 K to 823 K (400 °C to 550 °C) were identified based on the associated increase in the magnetization.

Austenite decomposition and carbon partitioning during quenching and partitioning heat treatments studied via in-situ X-ray diffraction

High strength combined with excellent ductility can be achieved by quenching and partitioning (Q&P) microstructures containing martensite and a considerable amount of retained austenite. Since the mechanical properties are inherited from the microstructure, a thorough understanding of this relationship is indispensable. In the present work, in-situ synchrotron X-ray diffraction was used to investigate the transformation kinetics during Q&P processing. The effect of different heat treatment conditions on the microstructural evolution was examined and correlated to the mechanical properties obtained by tensile testing. The results showed that austenite decomposition occurred for all Q&P cycles, especially at the beginning of partitioning. The extent of this decomposition was affected by a change of the quenching temperature, while the partitioning temperature showed no significant influence. Regardless of the heat treatment parameters, carbon partitioning was clearly visible during the 2-step cycles, which led to enhanced work hardening with increasing strain. In contrast, this was not observed in the case of 1-step processing due to negligible carbon diffusion, and thus insufficient chemical stabilization of the austenite.

The Quality of Fe14Cr ODS Powder Alloys During Milling and Upon Heating and Its Impact on the Mechanical Properties of Consolidated Steels

Oxide dispersion-strengthened ferritic steels (ODSFSs) are promising structural materials for applications in fusion and fission power reactors, but further improvement in their (high-temperature) mechanical properties and ferrite phase stability is required. This work demonstrates that an approach to produce Fe14Cr ODSFSs with a stable ferrite phase and improved strength could involve grain size strengthening by long-term milling with a tiny amount of nitrogen. Fe-14Cr-3W-0.4Ti-0.25Y2O3 powders were ball-milled up to 170 hours under an argon atmosphere. In addition to X-ray diffraction, the change in product quality during milling and upon heating was thoroughly investigated by more sensitive magnetic and thermal analysis by measuring the saturation magnetization σs, coercivity Hc, Curie temperature Tc, and temperature of ferrite-austenite (α → γ) transition Tα→γ. A pronounced modification of magnetic and microstructure parameters was observed when milling over 70 hours and upon heating above 800 °C and was found to be generated by long-term milling with a tiny amount of nitrogen. Upon heating, the nitrogen, incorporated during milling, developed a α → γ transition region, with the decomposition of nitrides precipitated at the earlier stage of heating followed by austenite decomposition, nitrogen degassing, and microstructure refinement to a grain size of a few tenths of a nm (e.g., 28 nm by heating at 910 °C of 100-hour milled powder). The resulting ferrite phase with refined grains is highly stable to (further) heating. The powders milled for 70 and 100 hours containing 0.175 and 0.500 wt pct nitrogen, respectively, were consolidated at 1100 °C with subsequent annealing at 1050 °C and subjected to Vickers hardness and 3-point bending tests. The steel produced from the powder milled for 70 hours shows lower hardness, higher density (close to the theoretical value of 7.8 g/cm3), and fracture strain. The ductility of this ODS alloy (0.075 fracture strain) is comparable with Eurofer97 (0.075 fracture strain), whereas its strength (2070 MPa ultimate stress) is significantly higher than that of Eurofer97 (1222 MPa ultimate stress). This improvement was attributed to grain size strengthening—the refined grains (promoted by milling with nitrogen) could be effectively pinned by Y-Ti-O dispersoids.

The Effect of Rapid Heating and Fast Cooling on the Transformation Behavior and Mechanical Properties of an Advanced High Strength Steel (AHSS)

The major goal of this work was to study the effect of rapid heating and fast cooling on the transformation behavior of 22MnB5 steel. The effect of the initial microstructure (ferrite + pearlite or fully spheroidized) on the transformation behavior of austenite (during intercritical and supercritical annealing) in terms of heating rates (2.5, 30 & 200 °C/s) and fast cooling, i.e., 300 °C/s rate, were studied. As expected, the kinetics of austenite nucleation and growth were strongly related to the heating rates. Similarly, the carbon content of the austenite was higher at lower intercritical annealing temperatures, particularly when slower heating rates were used. The supercritical temperatures used in this study were similar to those used during commercial hot stamping operations, i.e., 845 and 895 °C, respectively, followed by a fast cooling rate. The prior austenite grain size (PAGS) was not strongly influenced by the nature of the initial microstructure, heating rate, reheating temperatures (845 or 895 °C), at 30 s holding time. The decomposition of austenite using fast cooling rates was examined. The results showed that 100% martensite was not obtained. The observed low temperature transformation products consisted of mixtures of martensite-bainite plus undissolved Fe3C carbides and small amounts of martensite-austenite (M-A). At higher supercritical temperatures, i.e., 1000 °C and 1050 °C, the final microstructure showed an increase in the volume fraction of martensite and a decrease in the volume fraction of bainite. The Fe3C and the M-A microconstituent were not observed. The best combination of tensile properties was obtained on samples reheated in the lower temperature range (845 to 895 °C). Interestingly, when the samples where reheated at the higher temperature range (1000 to 1050 °C) and fast cooled, the results of the mechanical properties did not exhibit significantly higher strength levels independent of heating rate or initial microstructural condition. This can be attributed to the change in the microstructural balance %martensite+%bainite as the reheating temperature increases. The results of this study are presented and discussed.

Advances in Modeling of Steel Solidification with IDS

IDS (Inter-Dendritic Solidification) is a thermodynamic-kinetic software package that simulates phase changes, compound formation/dissolution, and solute distribution during solidification of steels as well as during their cooling/heating process after solidification. The software package also simulates solid-state phase transformations related to the austenite decomposition process at temperatures below 900/600 °C, and calculates thermophysical material properties from the liquid state down to room temperature. These data are needed in other models, such as heat transfer and thermal stress models, whose reliability heavily depends on the input data. The software package also features a database for thermodynamic, kinetic and microstructure data, as well as for several material properties. Owing to the short calculation times, the IDS tool is suitable for online applications. This paper presents IDS and its modules with the latest developments and validations, along with examples of modeling results.