Formation Of Austenite Research Articles

Formation of manganese-enriched austenite at abnormally low temperatures for diffusion type processes at “cascade radiation shaking” of Fe–6.35 at.% Mn alloy with accelerated Ar+ (E = 15 keV) ions

Using the methods of Mössbauer spectroscopy and transmission electron microscopy, it was found that short-term (for 4 s) irradiation of cold-rolled foils of the Fe–6.35 at.% Mn composition with a thickness of 25 μm with Ar+ ions (E = 15 keV) with heating to 300–450 °C causes an α → γ phase transformation with the formation of the repeatedly enriched with manganese austenite (from 23.8 to 38.0 at.%). The estimates made do not allow us to explain these processes by the phenomena of thermal and radiation-enhanced diffusion. As an explanation, the role of nanoscale dynamic effects associated with shaking the medium by powerful post-cascade solitary waves is considered. Such waves, becoming undamped in metastable media and playing, in a certain sense, the role of temperature, cause a short-term giant increase in the mobility of atoms and the transition of these media to a state with minimum free energy. The concentrations of Mn c1 and c2 in the α-and γ-phases, calculated on the basis of decoding the Mössbauer spectra for irradiation temperatures 311, 378 and 449 °C, correspond quite well to the results of extrapolation of the lines of the equilibrium phase diagram to the low temperature region.

Cu-assisted austenite reversion and enhanced TRIP effect in maraging stainless steels

Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength, ductility, and toughness in high-strength steels. In this work, the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography, transmission electron microscopy, and mechanical tests. Our results indicate that Cu accelerates the austenite reversion kinetics in two manners: first, Cu, as an austenite stabilizer, increases the equilibrium austenite fraction and hence enhances the chemical driving force for the austenite formation, and second, Cu-rich nanoprecipitates promote the austenite reversion by serving as heterogeneous nucleation sites and providing Ni-enriched chemical conditions through interfacial segregation. In addition, the Cu precipitation hardening compensates the strength drop induced by the formation of soft reverted austenite. During tensile deformation, the metastable reverted austenite transforms to martensite, which substantially improves the ductility and toughness through a transformation-induced plasticity (TRIP) effect. The Cu-added maraging stainless steel exhibits a superior combination of a yield strength of ∼1.3 GPa, an elongation of ∼15%, and an impact toughness of ∼58 J.

Application of a Non-Isothermal Numerical-Analytical Model to Determine the Kinetics of Austenite Formation in a Silicon Alloyed Steel.

A non-isothermal transformation model was proposed to determine the austenite formation kinetics in a steel alloyed with 2.6% wt. Si by dilatometric analysis, considering that the nucleation mechanism does not change with the heating rate. From the dilatometric analysis, it was observed that the austenite formation occurs in two stages; critical temperatures, degree and austenite formation rate were determined. The activation energies associated with each of the stages were obtained employing the Kissinger method (226.67 and 198.37 kJ·mol−1 for the first and second stage) which was used in concert with the austenite formation rate in the non-isothermal model as a first approximation, with acceptable results in the second stage, but not in the first due to the activation energies magnitude. Then, the activation energies were adjusted by minimizing the minimal squares error between estimated and experimental austenite formation degree, obtaining values of 158.50 kJ·mol−1 for the first and 165.50 kJ·mol−1 for the second stage. These values are consistent with those reported for the diffusion of carbon in austenite-FCC in silicon steels. With these activation energies it was possible to predict the austenite formation degree with a better level of convergence when implementing the non-isothermal model.

Interaction Between Ferrite Recrystallization and Austenite Formation in Dual-Phase Steel Manufacture

In this publication, the effect of heating rate on microstructural evolution of manganese segregated cold reduced dual phase steels is systematically studied for different inter-critical temperatures and holding times. At slow heating rate, completion of ferrite recrystallization before austenite formation led to the preferential formation of austenite on the ferrite grain boundaries leading to a necklace austenite (now martensite) morphology. The slower austenite formation kinetics has been attributed to longer diffusion paths dictated by larger ferrite grain sizes. In medium heating rate condition, microstructure before austenite formation had partially recrystallized ferrite and partially spheroidized cementite. Rapid austenite growth occurred along the rolling direction in carbon-rich cementite regions and dislocation-rich recovered ferrite regions. The presence of partially recrystallized ferrite grains restricted the austenite growth in the normal direction and therefore enabled the formation of thin martensite bands parallel to the rolling direction. At fast heating rate, the microstructure before austenite formation predominately contained un-recrystallized ferrite and un-spheroidized cementite and therefore enabled faster austenite formation kinetics. Thicker martensite bands are formed at fast heating rates due to the absence of recrystallized grains, thereby, enabling the growth of austenite in all directions with a higher preference to the rolling direction.

Formation of austenite in ferrite-beinite, beinite-martensite and martensitic shipbuilding steels and its influence on the transformed structure

This article investigates the kinetics of austenite grain growth during heating and the features of phase transformations depending on the austenite grain size in ferrite-bainitic, bainite-martensitic and martensitic shipbuilding steels. The kinetics of dynamic and static recrystallization is studied depending on the holding time at a given temperature. The studies carried out made it possible to determine the effect of the austenite grain size in shipbuilding steels of manganese, manganese-nickel and nickel alloying composition on the transformed structure.

Design of tough, ductile direct quenched and partitioned advanced high-strength steel with tailored silicon content

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon contents (1.5, 0.75 and 0.25 wt.%) and Q&P processing parameters on microstructural development including carbide formation and retained austenite stabilization. Optimized Q&P parameters were selected to design a DQ&P processing route for laboratory hot-rolling trials based on the analyses of physical simulation data. The overall aim of the study was to produce ultrahigh-strength structural steels with yield strength ≥1100 MPa combined with high uniform and total elongation and impact toughness, achieved through designing a low-temperature quenching and partitioning route with effective carbon partitioning. The DQ&P steels gained an excellent combination of mechanical properties comprising of high yield strength of ∼1000–1200 MPa and tensile strength ∼2100–2300 MPa, good elongation (∼11–13%), and moderate impact toughness transition temperature T28J ∼ (−5 to +12 °C). Straining of austenite prior to DQ&P led to an extensive refinement of the final martensitic-austenitic nanostructure. Formation of nanoscale lath-martensite and fine film-like retained austenite structures enabled the observed improvement of mechanical properties. Besides the formation of nano-twinned martensite, inter-lath austenite and transitional carbides were comprehensively characterized. No adverse effect of prolonged partitioning during slow cooling, simulating coiling in actual industrial practice, has been noticed suggesting new possibilities for developing tough, ductile structural steels both for strip/plate products.

Evolution of microstructure and mechanical properties in 2205 duplex stainless steels during additive manufacturing and heat treatment

Metal additive manufacturing (AM) offers exceptional design freedom, but its high thermal gradients often generate non-equilibrium microstructures with chemical and interfacial instabilities. Steels that solidify as δ-ferrite often experience a further solid-state phase transformation to austenite during AM. The detailed nature of this phase transformation during AM is yet to be fully understood. Duplex stainless steel, which is known for its unique combination of high corrosion resistance and mechanical properties, is a suitable alloy to further study this phase transformation.The current study aims to gain novel insights into solid-state phase transformations and mechanical properties of duplex stainless steels during laser powder-bed fusion (LPBF). As-printed microstructures exhibit significant deviations when compared to conventionally manufactured counterparts in terms of phase balance and morphology, elemental partitioning, and interface character distribution. During LPBF, only a small fraction of austenite forms, mostly at the ferrite-ferrite grain boundaries, via a phase transformation accompanied by diffusion of interstitials. Austenite/ferrite boundaries are shown to terminate on {100}F//{111}A planes. This is due to the character of parent ferrite-ferrite boundaries which is dictated by the sharp <100> texture and geometry of austenite grains induced by directional solidification and epitaxial growth of ferrite. Benchmarking mechanical properties against a wrought counterpart demonstrates that AM offers high strength but relatively low ductility and impact toughness. A short heat treatment reverts the microstructure back to its equilibrium state resulting in balanced tensile and toughness properties, comparable to or even better than those of wrought counterparts.

Thermo mechanical and microstructural constituents of dissimilar S700MC-S960QC high-strength steel welded joints using overmatched filler wire

In this study, a mechanical and microstructural constituent of welded joints of dissimilar high-strength and ultra-high-strength steels (S700MC/S960QC) using overmatched filler wire was evaluated. Three different heat inputs (18 kJ/cm, 8 kJ/cm, and 10 kJ/cm) and overmatched filler wire were applied using the GMAW process. Micro-hardness measurement was conducted using Vickers hardness test, tensile test, and microstructural constituents by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed in this analysis. A dissimilar welded sample with a cooling time of 51s (18 kJ/cm) showed a substantial hardness reduction in the fine grain heat-affected zone (HAZ) of the S700MC steel side, which was between 220-240 HV5. The cause could be the increase of heat input but also the use of overmatched filler wire, which can be the result of an increase of ferrite to austenite formation in the HAZ. SEM/EDS results confirmed the increase of carbide clusters, and tempered Martensite in the CGHAZ of the S960QC side, and ferrite-bainite on the S700MC side. An increase in the presence of high carbide content and the formation of Ni, Mo, and Mn were observed on the S960QC side. The increase of carbide formation in the CGHAZ of both sides reduced the hardness and strength of the welded joints. The tensile test confirms the softening observed in the FGHAZ of the S700MC side, which caused the joint fracture on this side during the test. Â

Effect of Cold-rolling and Heating Rate on Austenite Formation in a Low–Carbon Steel

Effect of Cold-rolling and Heating Rate on Austenite Formation in a Low–Carbon Steel

Effect of Mn/N ratio on microstructure and mechanical behavior of simulated welding heat affected zone in 22% Cr lean duplex stainless steel

The effect of Mn/N ratio on microstructural evolution, mechanical and fracture properties in welding heat affected zone (HAZ) of 22% Cr lean duplex stainless steel (DSS) were investigated. At comparative low heat inputs for low nickel tested DSS, the reformed austenite evolution of HAZ was mainly controlled by N content, and increasing Mn/N ratio promoted the growth of widmanstatten austenite (WA) and inhibited Cr2N precipitates formation in HAZ, and then significantly reduced its strength and elongation due to weakened strengthening effect and the decrease of austenite fraction. The tensile and yield strength of HAZ for 17.80 Mn/N ratio DSS are higher than that of 2205 DSS, but the obstruction of dislocation movement resulted in a sharp decrease of plasticity at high heat input of 2.935KJmm-1. The impact fracture of HAZ changed from ductile to brittle mode with the increase of Mn/N ratio, and for 17.80 Mn/N ratio DSS with high nitrogen content, the widening gaps of hardness and elastic moduli between two-phases in HAZ increased crack tendency, and significantly reduced its toughness. More grain boundary austenite (GBA) and WA formation for 51.14 high Mn/N ratio DSS segmented and refined the ferrite matrix in HAZ and improve impact toughness at higher heat input, while the coarsening of WA and (Cr, Mn) O oxides adversely reduced it to a lesser extent.

The influence of the plasma-nitriding temperature on the microstructure evolution and surface properties of additive-manufactured 18Ni300 maraging steel

Maraging steel grade18Ni300 produced by powder bed fusion (PBF) in its as built condition was plasma nitrided at three different temperatures. The aim of the work was to investigate the impact of the nitriding temperature on the microstructural changes as well as on the surface properties such as hardness, wear and corrosion resistance. The microstructural features in the bulk as well as in the nitride layer were investigated using electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-Ray diffraction (XRD) analysis. The bulk microstructure consists of martensite with a small amount of retained austenite, the amount of which increases with a higher nitriding temperature. The nitriding process also causes the formation of precipitates and can therefore also act as an aging treatment. A specific lamellar structure occurs on the surface during the nitriding process, which in the majority of cases consists of the Fe4N phase. The retained austenite also transforms during nitriding to the nitride phase Fe4N. It was found that nitriding at higher temperatures leads to the formation of cracks in the nitride layer. The crack formation is related to nano and micro segregation during the LPBF. These segregations lead to austenite formation, which also takes place along the grain boundaries and transforms during nitriding to Fe4N. Higher nitriding temperatures lead to a thicker nitride compound layer and to better wear resistance. The impact of the cracks on the static mechanical properties is negligible. However, the corrosion resistance is governed by the formation of cracks at higher nitriding temperatures.

Phase transition kinetics and microstructures of A508-3 steel under continuous cooling process

The phase transition kinetics of A508-3 steel during continuous cooling were studied by high temperature transition instrument which was based on thermodilatometry in this paper. The microstructures during continuous heating and cooling were also observed. The transition points were determined to be austenite formation temperatures, A c1 = 757 °C, A c3 = 823 °C, and martensitic transformation start and final temperatures, Ms = 382 °C, Mf = 222 °C at the cooling rate of 5 °C/s. The curves of Austenite transition fraction and time during continuous heating show a typical S shape. And the Austenite will show a maximum transition rate value at the middle temperature. The volume fraction of new phases translated from Austenite as functions of time and temperature during the cooling process can be obtained using the lever law. The pearlite transitions can occur in high temperature region at cooling rate smaller than 0.1 °C/s. And Bainite can be gotten at a lower temperature region. The S type curves can be used to describe the relationships between the transition volume fraction and time. Increasing the cooling rate enhances the volume fraction of Martensite at the cooling rate ≥5 °C/s. And only Martensite transition can be observed at the cooling rate > 10 °C/s. And Ms and Mf increase with increasing cooling rate. The Martensite transition kinetics equation with changing temperature were obtained. Finally, the continuous cooling transformation curves of A508-3 steel was constructed, which can be used to predict the microstructure and design the heat treatment technologies.

Simulation of nucleation and growth of austenite in duplex stainless steels – metallurgical study on weld metal of duplex stainless steel SUS329J1

The microstructure of duplex stainless steels is formed by dendrite growth in the primary ferrite-phase and by subsequent solid-state transformation to the secondary austenite-phase. In this study, formation of austenite by solid-state transformation in duplex stainless steels was evaluated. The nucleation and initial growth of austenite was simulated by the software TC-PRISMA. The formation temperatures of austenite in previous works were corresponding to the calculation obtained by adjusting the interfacial energy. Moreover, the microstructure formation process both in grain boundary and intragranular was simulated using a multi-phase field method (MICRESS software). The growth of austenite was assumed to be under paraequilibrium considering the diffusion control of nitrogen and its morphology was assumed to be an acicular growth. The microstructure morphologies were relatively similar to those obtained by the experiments. The simulation method in this study can analyse complex microstructural morphology as well as predict the composition distributions. This method is considered to be highly effective in designing duplex stainless steels and in evaluating the microstructure morphology in welding process.

Role of nitrogen shielding environment on microstructure and corrosion behavior of 2205 duplex stainless-steel A-GTA welds

The present study aims to understand the role of the shielding environment viz., argon (Ar) and nitrogen (N2) on microstructure, general and pitting corrosion behavior of activated gas tungsten arc (A-GTA) welds of 2205 duplex stainless steel. A-GTA welds made with activated flux SiO2 along with 100% Ar and 95% Ar + 5 % N2 are compared with conventional gas tungsten arc (C-GTA) welds made with 100% Ar. All the metallographic specimens of C-GTA and A-GTA welds are recorded to observe the microstructural changes using an optical microscope connected with image analyser. The basic GillAC electro-chemical system is used to perform potentio-dynamic polarization studies for general and pitting corrosion behavior in a 1 M NaCl solution. From the present study, it is established that activated-GTA welds with SiO2 flux resulted in reduced weld width and enhanced depth of penetrationachieved in a single pass compared to C-GTA welds. The grain morphology of the weld zone (WZ) is observed to be a mixture of austenite and delta ferrite in all the weldments. Secondary austenite is found in the interior of the ferrite whereas at the interfaces of ferrite and primary austenite, various forms of primary austenite are noticed in the weld zone of A-GTA welds. From the corrosion studies, it is found that A-GTA welds made with 95% Ar + 5 % N2 show higher general and pitting corrosion resistance compared to C-GTA and A-GTA welds made with 100% Ar.Improved corrosion behavior may be due to the favourable microstructure and presence of nitrogen as shielding environment, which favors the formation of austenite in the ferrite matrix.

Study of mechanism of austenite formation in surface during cold drawing of wire made of austenitic-martensitic steel VNS9-SH

The study of the microstructure, microhardness distribution and phase composition of 0,36 and 0,95 mm diameter wire made of austenitic-martensitic trip steel VNC9-Sh after cold drawing was carried out. According to the results of the research, a mechanism for the formation of austenite in the surface was proposed. The mechanism of formation of structural components was indirectly confirmed by the method of computer modeling of the cold drawing process with the calculation of the distribution of temperature fields.

Cryogenic Process Optimization for Medium Carbon Spring Steels

Spring steels are extensively used in load bearing elements of road and railway vehicles. The basic microstructure of these parts, which are subjected to high cycle fatigue, is tempered martensite. The improvement of the mechanical properties of these steels can only be achieved by modifying the material microstructure. In the microstructure of steel parts produced in different diameters, the formation of retained austenite and residual stresses is inevitable. For spring steels operating under heavy load, it is very important to remove the residual austenite form. Although the cryogenic treatment process is applied as a standard process for high alloyed and high carbon steels, it is not optimized for low alloy and medium carbon steels. In this study, different types of cryogenic treatment (Deep Cryogenic Treatment (DCT) / Shallow Cryogenic Treatment (SCT)) and tempering applications were carried out on spring steels with different alloying levels. With the outputs obtained from these experiments, optimization of cryogenic process parameters for medium carbon spring steels is aimed.

Effect of undercooled austenite ausforming on the role of the M–A constituents in the CGHAZ toughness of the HSLA steels with bainite structure

The brittle martensite-austenite (M-A) constituents in the coarse-grained heat-affected zone (CGHAZ) of the bainite steel are known to have negative effect on toughness. To modify the microstructure of the M–A constituents and then improve the CGHAZ toughness, this work applied ausforming to the bainite steel during its CGHAZ simulating process when the temperature of the thermal cycle decreased to 600 °C. Morphology of the M–A constituents in the CGHAZ with and without the ausforming was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructure of the M–A constituents was characterized by transmission electron microscopy (TEM), the toughness of the specimens was assessed through instrumented Charpy impact test, and the crack behavior beneath the fracture was revealed by SEM. Results show that the ausforming contributes to the austenite formation within the M–A constituents, which helps to improve the CGHAZ toughness from 10.50 J to 33.70 J. This work proposed a nontraditional method that enables the microstructure modification of the M–A constituents and the toughness improvement in the CGHAZ.

BEHAVIORS OF OXIDATION AND DECARBURIZATION ON SURFACE OF HIGH-CARBON STEEL WIRE RODS

The quality of high-carbon wire rods is subject to high requirements in terms of the size of the decarbonized layer, the amount and composition of the scale formed by the interaction of the oxidizing furnace atmosphere with the surface of steel. In this connection, studies of the kinetics of high-temperature oxidation of high-carbon steel are of great practical importance, both in the development of new and adjustment of existing modes of heat treatment of steel. The results of experimental studies of hightemperature oxidation and decarburization of the surface of high-carbon steel grade 80P on the synchronous thermal analysis device STA 449 F3 Jupiter presented. Temperature intervals of intensification of scale formation and decarburization of the surface in various media (weakly oxidizing and oxidizing) have been established. Phase transformations in high-carbon steel grade 80P have been studied by differential scanning calorimetry and the temperatures of the beginning and end of the formation of homogeneous austenite during heating have been established. It was noted that in the range of heating temperatures of steel 720‒950°C, along with phase transformations, the intensification of the processes of scale formation and depletion of the surface layers with carbon begins. The exponential dependence of steel loss on the heating temperature of the sample was obtained by thermogravimetric analysis. It was shown that the oxidation rate of the surface increases by 3 times with an increase in the temperature of the workpiece from 900 to 1000°C, and up to 1200°C ‒ by 8 times. It was noted that the temperature at which the oxidation rate of steel is minimal in the temperature range from 900 to 1000 °C is about 929 °C, in the range of 1100‒1250°C ‒ 1157°C. The obtained results can be used to select the main technological parameters of the heat treatment mode of wire rods made of 80P steel, which ensure the formation of a minimum amount of easily removed scale on the metal surface.

Modelling the formation of austenite in the intercritical interval in ductile iron

In this work, the formation of austenite in the intercritical interval under continuous heating and isothermal holding in ductile iron with various chemistries was investigated using high-resolution dilatometry and quantitative metallographic analysis. The study was conducted using fully ferritic and fully pearlitic matrices as initial microstructures. Subsequently, a mathematical model based on Avrami's equation, that describes the formation of austenite at the intercritical range, was proposed and adjusted to the experimental data. The results show that austenite formation at the intercritical range is faster and happens at lower temperatures when the initial microstructure is pearlitic. Additionally, the kinetic of austenite formation did not change by adding Cu and it is accelerated by adding Ni to the alloy. Finally, the Avrami's equation allowed to model the austenite formation under continuous heating followed by isothermal holding with a good adjustment to the experimental data, which contributes to the understanding of the kinetic of austenite formation in ductile iron at the intercritical range.

Effect of N on corrosion resistance of Fe-Cr-Ni-Mo-Mn alloy

In order to develop the Fe-Cr-Ni-Mo-Mn-N corrosion resistant alloys, analyze the influence of nitrogen on the corrosion resistance of Fe-Cr-Ni-Mo-Mn alloy, adjust the N content in the alloy system, and Flux cored wires with different N content were prepared. They were surfaced on low carbon steel by MIG welding. The phase composition, microstructure and corrosion resistance of the cladding metal were analyzed to study the effect of N content on the structure and performance of the surfacing metal. The results show that the addition of nitrogen does not change the matrix structure (which is Fe-Ni-Cr austenite), but with the increase of nitrogen content, the precipitation of nitrides (Cr2(C,N) and BN)is accompanied; when the amount of nitrogen added reaches 1.0%, the corrosion resistance of the surfacing metal is the best. At this time, the self-corrosion current density is at least 1.569 × 10–9 mA cm−2 and the self-corrosion potential is at most −0.318 V. The addition of nitrogen can promote the formation of austenite and nitride, inhibit the appearance of carbides, avoid the phenomenon of poor Cr, and improve the corrosion resistance of the material.