Transformation Of Austenite Research Articles

Study on Austenite Transformation and Growth Evolution of HSLA Steel.

HSLA steel is widely used in various applications for its excellent mechanical properties. The evolution of austenite transformation and growth has been systematically studied in HSLA steel Q960 during the heating process. A thermal expansion instrument and optical microscope were adopted to analyze the kinetics of austenite transformation, which is a nonlinear continuous process and was accurately calculated by the lever rule based on the dilatation curve at the holding time within 10 min. The austenite growth behavior at temperatures above Ac3 was explored using TEM and DSC. The main precipitates in austenite were Nb-rich and Ti-rich (Nb, Ti)(C, N), and the particle size increased and amount decreased with the increase in the heating temperature, which resulted in the rapid growth of austenite. With the increase in holding temperature and time, the growth of austenite progressed through three stages, and a heat treatment diagram was established to describe this evolution.

Microstructural manifestation and kinetics of dynamic transformation in a W-modified 9%Cr steel

Dynamic transformation (DT) of austenite to ferrite is shown to occur in a W-containing 9%Cr steel during hot deformation across a wide range of strain rates. A new facet of DT, namely its dependency on process kinetics, is revealed by examining work-hardening rate and strain rate. The kinetics of DT is associated with competing processes of dislocation immobilisation and remobilisation, both of which have a complex dependence on strain rate. The microstructural manifestation of DT-ferrite is examined and contrasted with that of conventional ferrite on the basis on morphology. This contrast arises from different modes of transformation and resultant differences in crystallography. DT is placed in the practical context of thermo-mechanical processing by addressing issues of process control and stability.

In situ observation of solidification in peritectic steels and evaluation of the strains induced in the solidifying shell

In the continuous casting of peritectic steels, unevenness and cracks of the solidifying shell are typical casting defects. This study focuses on the ferrite–austenite transformation and the influence of the transformation on the solidifying shell deformation. In situ observations revealed that the austenite hardly nucleated and, consequently, the undercooled ferrite massively transformed to the austenite in the solid state. Simultaneously, the austenite rapidly solidified in the remaining liquid. The volume changes due to the ferrite–austenite transformation and the solidification shrinkage were measured using time-resolved tomography. On the basis of the observations and measurements, a model that includes the ferrite–austenite transformation and the austenite solidification was proposed to evaluate the volumetric strains in the solidifying shell. There was a sharp peak in the volumetric strains derived from the model at hypo-peritectic compositions. This work demonstrates the need to include the ferrite–austenite transformation in the solid state and the rapid austenite solidification for understanding the solidifying shell deformation in peritectic steels.

Microstructure and Wear Resistance in Medium and High C Steels Treated by Quenching and Partitioning

The quenching and partitioning (Q&P) steels have shown to be promising candidates to be applied in fields where wear resistance is required. In this study, a medium and a high C steel are heat treated by Q&P and the resulting microstructure, hardness, and wear resistance are characterized. The mechanical stability of the austenite phase under wear test conditions is investigated. It is found that the stability of austenite is very high in the high C steel and decreases in the medium C steel. Additionally, the hardness and wear behavior of the Q&P‐treated steels are compared with the results obtained for quenching and tempering (Q&T) treated samples, showing that, although the hardness of Q&P steels is quite lower, the obtained wear rates are similar. It means that in the studied Q&P steels, although the austenite transformation into martensite does not occur considerably, the presence of austenite might play a key role in the wear resistance.

In Situ Observations of the Microstructural Evolution during Heat Treatment of a PH 13‐8 Mo Maraging Steel

The standard heat treatment of PH 13‐8 Mo maraging steels consists of solution annealing and subsequent aging. Herein, it is investigated how an additional intercritical annealing step prior to aging affects the microstructure, and, consequently, the mechanical properties of a PH 13‐8 Mo maraging steel. In situ techniques by means of high‐temperature electron backscatter diffraction and high‐temperature X‐ray diffraction are applied to study the microstructural changes during intercritical annealing and subsequent aging. In addition, high‐resolution investigation methods, such as transmission electron microscopy and atom probe tomography supplemented by transmission Kikuchi diffraction, are used for an in‐depth characterization of the microstructure. The results reveal that a diffusion‐controlled martensite to austenite transformation accompanied by partitioning of the substitutional atoms Cr, Ni, and Mo takes place during intercritical annealing. As a result of partitioning during intercritical annealing, an inhomogeneous distribution of Ni remains in the microstructure after the martensitic transformation. Consequently, the formation of reverted austenite is facilitated during subsequent aging due to existing Ni‐enriched zones in martensite. Since the fracture toughness is significantly enhanced compared to the standard heat treatment, it is suggested that this improvement is related to the increased phase fraction of reverted austenite.

ON α- MARTENSITE NUCLEATION IN STEELS AND IRON ALLOYS

Purpose. To investigate the possibility of martensite nucleation on magnetic (concentration) inhomogeneities of chrome-nickel steels austenite.
 Research methods. Generalization of experimental and theoretical studies of the strong magnetic field effect on the martensitic transformation in steels and iron alloys, taking into account the magnetic state of austenite
 Results. The analysis of the magnetic state of austenite and the abnormally large strong magnetic field effect on the martensite point displacement in 40X2H20 steel indicates that the transformation of austenite into martensite in it is a “magnetic first order phase transition”.
 Scientific novelty. Some features of the martensitic transformation in steels and iron alloys are explained. It is assumed that martensite during the “magnetic γ→α- transition” originates on magnetic inhomogeneities with a disoriented magnetic structure of the spin glass type. Forced magnetostriction during paraprocess and spontaneous magnetostriction below the Curie point lead to the emergence of collinear ferromagnetism with a change in the coordination number from 12 to 8. High all-round pressure leads to the occurrence of collinear antiferromagnetism and the γ-phase stabilization. In paramagnetic austenite the nucleation of α-martensite occurs on dislocation nuclei, the sizes of which have the same order of magnitude (by volume) as the magnetic inhomogeneities in the Fe-Ni system.
 Practical value. The results obtained in the work expand the concept of martensitic transformation in steels and provide grounds for explaining some kinetic features in them.

Austenite reversion suppression with deep cryogenic treatment: A novel pathway towards 3rd generation advanced high-strength steels

This study investigates the impact of deep cryogenic treatment (DCT) on microstructure and properties of a martensitic stainless steel AISI 431. The reasonably simplistic microstructure tailoring with DCT delivers a novel pathway towards advanced 3rd generation high-strength steels (AHSS), whilst utilizing existing alloys and chemistry. The resulting microstructural changes deliver improved strength and hardness of the material, whilst still sustaining good elongation properties. DCT reduces the material's toughness but improves its deformation resistance and wear resistance. The study reveals that DCT modifies the microstructural evolution during tempering. During the conventional treatment, AISI 431 develops austenite reversion transformation (ART), which allows the high strength and elongation capabilities formed with transformation-induced plasticity (TRIP). In contrast, DCT progresses the microstructure even further by retransforming the reverted austenite formed through ART into tertiary α-martensite, ε-martensite and carbides that increase the strength and deformation behavior of AISI 431. The novel microstructural modification that we have entitled cryogenic austenite retransformation (CAR), is both experimentally and theoretically assessed. The underlying transformation mechanisms of CAR and individual stages of DCT were evaluated with ex-situ electron backscatter diffraction and transmission electron microscopy. The impact of DCT on the modified transformation capabilities of AISI 431 is discussed in relation to possible implementation in automotive industry as a universally applicable treatment procedure.

Strategic Optimization of Post Weld Heat Treatment for Dissimilar TIG Weldment of P22 and P91 Steels

The Gas Tungsten Arc Welding (GTAW) between P22 (2.25Cr-1Mo) and P91 (9Cr-1Mo) steels are quite common in heavy vehicle structures like; railway locomotives and war (or combat) vehicles. After welding, high hardness values of the Heat-Affected Zones (HAZs) of these dissimilar weld joints are possibly obtained. The uneven hardness in HAZ comes from the austenite transformation to Martensite, because of highly uncontrolled cooling rates. The considerable difference in hardness among various zones leads to prior cracking and is bound to failure during high-temperature operations. Therefore, there is a dire need to establish an appropriate Post Weld Heat Treatment (PWHT) to temper the dissimilar weld joints for required durability. The study investigates the microstructure and mechanical properties of various erupted zones of steel, before and after the application of PWHT. Before and after PWHT, weld zone and HAZ in P22 and P91 are mechanically tested and metallurgically examined. Further PWHT is tried to be optimised strategically and statistically, for the least variation in hardness among different zones. So that any failure due to thermal stresses or strains, during normal running conditions can be avoided.

Effectively improving the hardness-strength-toughness of carburized bearing steel via nanoprecipitates and fine grain structure

Cryogenic treatments are fast and efficient methods used to improve the mechanical properties of metal components by promoting retained austenite (RA) transformation. However, the relationship between the evolution of RA with the gradient changes in content and hardness-strength-toughness improvements during this period are unclear. The present study investigated the beneficial effects of a cryogenic treatment on the microstructural evolution and mechanical properties of a gradient-structured carburized M50NiL bearing steel. The results revealed that the RA average sizes, the densities of the high-angle grain boundaries (DHAGBs) and low-angle grain boundaries (DLAGBs), and the hardness of the carburized layer changed with increasing distance from the surface. Compared with the untreated samples, the cryogenically treated samples demonstrated a 30% greater effective hardened layer thickness as well as 15.6% and 22.9% higher strength and toughness, respectively. The synergistic enhancements of the hardness, strength, and toughness provided by the cryogenic treatment were attributed to the increased nanoprecipitate content and finer grain structure generated in situ during transformation and further decomposition of the RA, which resulted in reductions of up to 83.6% in the RA content. This work provided new ideas for innovative enhancements of gradient-structured materials with superior mechanical properties.

Microstructure and mechanical properties of medium manganese steel in intercritical annealing

A new type of cold-rolled medium manganese steel was investigated. The test steel was treated by Q&P process. With the increase in annealing temperature, the grain size gradually increases, the carbides dissolve progressively and the fine lath grains transform into massive or equiaxed grains. Both the volume fraction and the stability of the retained austenite are continuously decreasing, which together affect the change of mechanical properties. At the intercritical annealing temperature of 680°C, the specimens have higher retained austenite content and transformation rate. At the same time, due to the discontinuous TRIP effect caused by the retained austenite stability gradient inside the specimen, it exhibits excellent comprehensive mechanical properties, and the product of strength and elongation is 43.5GPa·%.

Effect of precipitation evolution of NiAl and Cu nanoparticles on strengthening mechanism of low carbon ultra-high strength seamless tube steel

We report an alloy design strategy, rolling and heat treatment processes for a new low-carbon ultra-high strength seamless tube steel. The evolution of multiphase microstructure and mechanical properties of experimental steel in the rolled, quenched, and quenched-tempered states has been investigated based on the reverse transformation of austenite correlated with the transformation of secondary martensite and co-precipitation behavior. Microstructure observation results show that the microstructure of rolled state is composed of 88.1 ± 0.1% lath bainite (LB) and 11.9 ± 0.1%% granular bainite (GB), the quenched state is composed of 82.1 ± 1.1%% lath martensite (LM) and 17.9 ± 1.1% GB, while the QT steel is composed of tempered martensite (TM) and GB, and the reversed austenite content increases from 0.66 ± 0.03%% at 500 °C to 5.39 ± 0.05%% at 650 °C, and the high reversed austenite content at 650 °C is the basis of secondary martensitic transformation. The co-precipitation sequence of nanoparticles observed by high-resolution transmission electron microscope (HRTEM) is “supersaturated solid solution → B2 NiAl + bcc Cu → B2 NiAl + bcc Cu + B2 core-9R shell → B2 NiAl + fcc Cu”. Aged at 550 °C for 1 h, the experimental steel obtained a maximum yield strength of 1483.5 ± 3.5 MPa, which was attributed to the combined effect of shear and Orowan strengthening mechanisms, and the maximum strengthening increment reached ∼750.5 MPa.

Modelling deformation-induced martensite transformation in high-carbon steels

The transformation behaviour of retained austenite in steels is known to differ according to chemical composition and other microstructural attributes. Earlier research indicated that austenite in high-carbon steels transforms into martensite only when the applied stress exceeds a critical value, contrary to low-carbon steels where transformation occurs in the early stages of deformation. Although transformation models have been proposed, most are optimised for low-carbon steels. Here, we propose physics-based models applied to high-carbon steels to overcome previous limitations. The models have fewer free parameters (4) compared to previous approaches (6), exhibiting improvements in the numerical and physical interpretation of the austenite transformation process. We envision the use of these models as tools for alloy design, also highlighting their scientific and technological value.

In Situ Observation of Solidification and Crystallization of Low-Alloy Steels: A Review

Crystallization during the solidification process of steels is of vital importance for controlling the quality of final products. This paper summarizes the in situ characterization research activities of crystallization behaviors of low-alloy steels during the solidification process. The results obtained using high-temperature confocal laser scanning microscope (HT-CLSM) are critically reviewed, and other relevant methodologies, i.e., either classical method using differential scanning calorimetry (DSC) or large-scale facility (LSF), are also briefly mentioned. The evolution of the crystallization front from a planar to a cellular and further to a dendritic one, and subsequential microstructure evolutions, i.e., delta-ferrite (δ) formation from the liquid, austenite (γ) transformation and decomposition, are mainly discussed. The current review aims to highlight the state-of-the-art research outputs obtained by the novel in situ characterization techniques, and the obtained knowledge aims to shed light on the further development of the quality low-alloy steel products by controlling the processing and structure correlation.

Modeling of welding residual stress in a dissimilar metal butt-welded joint between P92 ferritic steel and SUS304 austenitic stainless steel

In this study, a butt-welded joint between P92 ferritic steel and SUS304 austenitic stainless steel was fabricated with a nickel-based filler material. Based on Sysweld software, three-dimensional thermal-metallurgical-mechanical coupled finite element analyses were performed to investigate the hardness and welding residual stress in the welded joint. In the material behavior model, various factors influencing the formation and distribution of hardness and welding residual stress were carefully considered, including the austenitic transformation, martensitic transformation, and the tempering of martensite in P92 steel; the isotropic strain hardening, annealing effect, and melting effect of all materials; and the differential thermal expansion between dissimilar metals. The neutron diffraction method and hole-drilling strain gauge method were applied to measure the profiles of residual stress of the welded joint, respectively. Hardness mapping was performed on a weld cross-section specimen. The predicted results of welding residual stress and hardness are in good agreement with the measurements, respectively. The peak residual stresses in longitudinal and transverse directions are above 600 MPa in P92 steel. There are high tensile residual stress and significant cold work near the weld root in SUS304 stainless steel. The solid-state phase transformation induces compressive longitudinal residual stress in the heat-affected zone, high tensile transverse residual stress near the weld toe in P92 steel, and a significant stress gradient. The tempering effect can reduce the hardness and tensile residual stress in the heat-affected zone of P92 steel. The formation mechanism of welding-induced residual stress was illustrated based on the Satoh test. The effects of peak temperature, internal restraint, and differential thermal expansion are found to be decisive for the formation of residual stress. Large tensile residual stress can be generated with internal restraints, which promote the stress increase and lead to material yielding. A restraint factor was proposed for the quantitative assessment of stress evolution.

Comparison of the tribological behavior of quench-tempered ductile iron and austempered ductile iron with similar hardness

The tribological behavior of quench-tempered ductile iron (QTDI) and austempered ductile iron (ADI) with similar hardness was investigated. QTDI was composed of cementite particles, ferrite and spheroidal graphite, whilst ADI was consisted of acicular ferrite, retained austenite and spheroidal graphite. A higher yield strength was obtained for QTDI compared with ADI. Wear tests indicated that QTDI exhibited a lower friction coefficient than ADI. However, ADI had a better wear resistance than QTDI, which was because the transformation of residual austenite into martensite induced by friction stress resulted in the increase of surface hardness. Worn surface morphology indicated that the main wear mechanisms of QTDI was adhesive and abrasive wear. However, the surface fatigue wear was observed for ADI.

In Situ Observation of Retained Austenite Transformation in Low-Carbon Micro-Alloyed Q&P Steels

Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

Impact abrasive wear behaviour of the multiphase microstructure in a medium carbon quenched and partitioned bainitic steel

To enhance the impact abrasive wear resistance of steel for mining and metallurgy industries, a 0.3C steel with a multiphase microstructure of bainitic, martensitic and retained austenite was produced by a Q&P process involving bainitic transformation, and its wear performance under impact abrasive conditions was evaluated in contrast with fully martensitic and fully bainitic steels. The results revealed that the multiphase microstructure demonstrates outstanding combination of strength (1612 MPa), toughness (105 J), hardness (48.7 HRC) and elongation (19.2%), along with superior impact abrasive wear resistance. After 90 min of impact abrasive wear test, the multiphase microstructure shows lower wear loss than that for the fully martensitic and fully bainitic steels, which can be attributed to its high surface microhardness and increased toughness. The formation of nanograins and an increase in dislocation density as well as the retained austenite transformation leads to the higher surface microhardness. In addition, some larger craters can be observed on the wear surface of fully martensitic and fully bainitic steels, and the crack propagation depth is larger under the wear surface. In contrast, the wear surface of the multiphase microstructure presents some delamination damage, and the crack propagation depth is shallow because of the thicker deformed layer and presence of film-like austenite retained in the deformed microstructure. These results suggested that the multiphase microstructure can be used as a potential candidate structure for enhanced resistance to impact abrasive wear.

The effect of microstructural and texture evolutions during thermomechanical treatment on corrosion resistance of 310s austenitic stainless steel

In this study, the evolution of the microstructure and texture during thermomechanical treatment and its effect on corrosion properties of 310s austenitic stainless steel were investigated. This stainless steel was cryo-rolled at 50 and 90% thickness reductions, and then the 90% cryo-rolled sample was annealed at 750 °C for 5 and 30 min. SEM and optical microscope images were used to examine the microstructure of the samples. Fritoscopy test was also used to calculate the volume fraction of the martensite phase. Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and cyclic potentiodynamic polarization tests were performed in the 3.5 wt.% NaCl solution to investigate the corrosion behavior of the studied steel. The results showed that the cryo-rolling process caused the reduction of grain size, texture strengthening and transformation of austenite to strain-induced αʹ-martensite phase. Decreasing grain size and increasing texture components containing dense planes are beneficial factors and the formation of the αʹ-martensite phase is a harmful factor for corrosion resistance. It was observed that annealing at 750 °C for 30 min caused the grain growth and texture weakening, while a favorable condition is developed in the annealed sample for 5 min. After 90% cryo-rolling and subsequent annealing at 750 °C for 5 min, the corrosion resistance was significantly improved compared to the as-received sample and reached 37 kΩ.cm2. Formation of the sub-micron microstructure along with the high volume fraction of Brass and Goss texture components were the main reasons for improving corrosion resistance at 750 °C–5 min.

DIE STEEL 4Х4Н5М4Ф2 (WITHOUT FORGING TREATMENT) FOR HOT DEFORMATION OF ALUMINUM ALLOY

Purpose. Production of steel with controlled austenitic transformation during operation of the 4Х4Н5М4Ф2 grade (without forging treatment) using the technology of electroslag remelting and establishing optimal modes of heat treatment (annealing, quenching and tempering). Production of tools (matrices) from 4Х4Н5М4Ф2 steel (without forging treatment) for hot deformation of aluminum alloy at operating temperatures below the critical point A1.
 Research methods. Metallographic analysis of experimental die steels 4Х4Н5М4Ф2 (without forging treatment) and forged 4Х5МФ1С: study of metal structure; determination of specific resistance; determination of tensile strength; determination of hardness and impact strength.
 Results. The results of research on the optimization of heat treatment modes (quenching and tempering) of 4Х4Н5М4Ф2 steel (without forging treatment) are given. Adjusted chemical composition (4Х4Н5М4Ф2) and optimized mode of heat treatment of steel, which allowed to increase heat resistance up to a temperature of 650 °С. The tempering temperature of steel (475±25 °С) at which irreversible temper brittleness occurs is established. Experimental and industrial tests of steel 4Х4Н5М4Ф2 (without forging treatment) for hot deformation of AK7ч aluminum alloy are presented. It is shown that compared to forged steel H13 (an analogue of 4Х5МФ1С), which is used at one of the Chinese enterprises, the experimental steel has practically the same resistance .
 Scientific novelty. In the process of partial recrystallization (incomplete annealing at a temperature of 750±20 °C) of 4Х4Н5М4Ф2 steel (without forging treatment), a spheroidized carbide component is formed, which leads to an improvement in the mechanical processing of the workpiece for the manufacture of matrices.
 Practical value. The possibility of using steel with an adjustable austenitic transformation during operation of the 4Х4Н5М4Ф2 grade (without forging treatment) is shown for a wide range of operating temperatures of hot deformation: copper-nickel (at an operating temperature above the critical point A3) and aluminum alloy (at an operating temperature below the critical point A1) with increased service life compared to forged steel 4Х5МФ1С.

Microstructure evolution induced by solidification and ferrite–austenite massive-like transformation in Fe–C alloys

The coarsening of austenite grains produced through a massive-like transformation from the ferrite to the austenite phase during/after solidification in Fe–0.45mass%C steel was examined through time-resolved X-ray diffraction measurements using time-resolved computed tomography. The δ phase solidification resulted in a single δ grain in the observation region (0.6 mm in diameter, 0.1 mm in height). The massive-like transformation produced multiple γ grains, and coarsening immediately followed the transformation. The coarsening in the observation region was completed even within 200 s. A correlation of crystallographic orientation between the γ grains was found in the γ grain structure. The massive-like transformation frequently produced fcc (111) twinned grains, and the twinned austenite grains tended to remain during coarsening because the twin interface energy is lower than the grain boundary energy. The massive-like transformation following the solidification contributed to the texture formation in the γ grain structure.