Austenite Size Research Articles

The evolution and toughening mechanism of austenite in high Co–Ni ultrahigh strength steel

The austenite, as a “soft” phase, has a significant impact on the mechanical properties in ultrahigh strength steel. The morphology, size, and distribution of austenite play a critical role in toughness of steel. This study concentrates on morphology, size, and distribution of austenite subjected to solid solution, cryogenic, and aging treatments and explores its evolution characterized by transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction and x-ray diffraction. The toughening mechanism of austenite with different characterization was investigated through charpy impact testing and fracture morphology analysis. The results indicate that filmy and blocky austenite existed at the lath boundary of martensite. Blocky austenite and some filmy austenite were eliminated after cryogenic treatment, while approximately less than 15 nm filmy reverted austenite was formed after aging treatment. Filmy reverted austenite at the boundary of martensite laths can enhance toughness. Retained austenite without cryogenic treatment can induce the formation of larger blocky austenite, which decreased toughness.

Realizing the strength-ductility balance of a warm-rolled 10 Mn steel via preparing dual nano-sized precipitates

A novel strategy involving warm rolling and tempering-annealing was introduced to optimize the strength-ductility balance of Fe-10.2Mn-2.2Al-0.41C-0.6 V (wt. %) steel. The V- and κ-carbide particles produced during the tempering are critical for tailoring the austenite characteristics during the subsequent annealing. Austenite that nucleates on Mn-rich κ-carbides inherit their high Mn content and fine grain size. Quantitative calculations suggest that the pinning effect exerted by the V-carbide particles strongly resists grain boundary motion and recrystallization grain growth, further refining the final microstructure. Moreover, the dual nano-sized particles induce abnormal variations in the grain size and fraction of austenite, influencing the deformation mechanisms. In the absence of deformation twins, the fresh martensite develops into “separating walls” to refine the microstructure and further enhances the work hardening in addition to the complex dislocation mobile. Finally, the tempered-annealed sample demonstrated a simultaneous increase in the yield and tensile strengths from 1166 to 1452 MPa and 1507–1727 MPa, but a slight decrease in the total elongation from 18.6% to 16.8% compared to annealed sample, achieving a good strength-ductility balance.

Strengthening mechanism of friction-stir welded Fe-17Mn alloy

In present study, the strengthening mechanism of friction-stir welded Fe-17Mn binary alloy was quantitatively investigated by means of the electron microscopic observation in stir zone center and the dislocation-based constitutive equation. As the rotation speed increased from 440 to 760 rpm, both fraction of thermal ε martensite and prior γ austenite size were elevated due to the severely frictional heat, whereas the dislocations were annihilated. Though different microstructural feature of welds, the stir zone center had similar hardness and yield strength regardless of rotation speed. It was because of notable increase in ε martensite hardening as well as marginal decreases in hardening in grain boundary and dislocation with increasing rotation speed.

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

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

Ti-Mo microalloyed medium Mn steels: Precipitation and strengthening mechanism

Medium-Mn steels possess an exceptional strength-ductility balance but normally relatively low yield strength, which hinders their practical applications especially in ultra-high strength structural components. In this study, the simultaneous addition of Ti and Mo was proposed to improve the yield strength of medium-Mn steels, together with the systematic experimental and theoretical investigations on the precipitation behavior of (Ti, Mo)C carbides and its effects on the microstructure-properties relationship. The addition of Ti and Mo increases yield strength by 150∼350 MPa and ultimate tensile strength by 55∼320 MPa without a significant loss of ductility. Based on the intercritically annealed microstructure close to equilibrium as well as the nucleation and growth kinetic theory, the Ti–Mo addition can reduce the fraction of austenite, but increase the mechanical stability of austenite by refining the mean grain size of austenite. Mo was found to increase the precipitate density of (TixMo1-x)C by lowering the interfacial energy between the nano-precipitates and the matrix, and reduce the mean size of precipitates. A maximum increment in yield strength (∼350 MPa) was achieved at 610 °C, which was mainly attributed to the combined effects of grain refinement and precipitation hardening due to the addition of Ti and Mo.

Effects of Ultrafine and Homogenization Process on Microstructure and Properties of H13 Die Steel

Abstract The mixed grain structure was found in H13 die steel due to improper production process parameters, and mixed grain structure has adverse effects on the performance of H13 die steel. The appearance of mixed grain structure is analysed. The heredity of mixed grain structure in H13 die steel was eliminated by using ultrafine and homogenization processes. The characteristics of the microstructure of H13 die steel were investigated, and mechanical properties were tested. The results indicated that the mixed grain structure transformed into fine equiaxed grains after the ultrafine and homogenization process and the average grain size of austenite is about 20.7 μm. After quenching and tempering treatment, the hardness of the sample reached 47.1 HRC, and the impact absorption energy was 10.8 J.

Study on the relationship between the microstructure characteristics, tensile properties and impact toughness of carbide-free bainitic steel

To understand the relationship between microstructure characteristics, tensile properties, and impact toughness of bainite structure, microstructure characterization, μ-DIC and mechanical tests were carried out on bainitic steel subjected to pre-quenching treatment, followed by austenitizing at 800 °C or 880 °C and then by austempering. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite. Compared with 880 °C-austenitized specimen, the final microstructure in the 800 °C-austenitized specimen is significantly refined, and the amount of large martensite/austenite (M/A) islands is also decreased. The finer microstructure of the 800 °C-austenitized specimen results in a relatively uniform strain distribution during the tensile process, delaying the occurrence of tensile necking. In contrast, the strain of the 880 °C-austenitized specimen tends to concentrate at the interface between the bainite region and M/A islands, leading to premature fracture and decreased plasticity. In addition, the smaller retained austenite size in the 800 °C-austenitized specimen enhances its stability, leading to persistent work hardening behavior and improved plasticity. Furthermore, the decrease in the amount of M/A islands in the 800 °C-austenitized specimen reduces the initiation of cracks, and finer packets and blocks increase the proportion of high angle grain boundaries, enhancing the resistance to crack propagation. Therefore, the total absorbed energy and toughness of the 800 °C-austenitized specimen are about twice higher than those of the 880 °C-austenitized specimen.

Effect of Nb content variations on the microstructure and mechanical properties of NiCrMo welded joints

The effects of varying the Nb content (1.04 wt%-1.55 wt%) in NiCrMo weld metals on the microstructure, tensile strength and cryogenic impact toughness were investigated. The microstructure of the three different weld metals was observed to be composed of austenite matrix and precipitates by characterization techniques such as SEM, EDS, EBSD, and TEM. The precipitates within the grains were NbC and Ni3Nb phases, and the precipitates on the grain boundaries were NbC phases. With the increase of Nb content, the average grain size of austenite decreased from 205.3 μm to 161.4 μm, and the area fraction of precipitates increased from 1.41% to 3.97%. The cryogenic impact value of the welded joints decreased from 97 J to 76.3 J, while the tensile strength increased from 682.5 MPa to 710 MPa, indicating that the increase in the area fraction of the Nb-rich phase had a deleterious effect on the cryogenic impact toughness and was conducive to the elevation of the tensile strength. When the Nb content in the weld metal was 1.25 wt%, the comprehensive mechanical properties of NiCrMo welded joints were the best, with a tensile strength of 695.5 MPa and a cryogenic impact value of 86.7 J.

Effect of Different Quenching Methods on the Microstructure and Mechanical Properties of 30MnB5NbTi

The effects of different quenching methods on the microstructure and mechanical properties of 30MnB5NbTi hot stamping steel are investigated, and the quenched microstructure is characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The mechanical properties are evaluated by uniaxial tensile test. The results reveal that, in comparison to die quenching, oil quenching, or air cooling, water‐quenched samples (tempered after water quenching, die quenching and oil quenching) exhibit the highest ultimate tensile strength (UTS) and yield strength (YS), reaching 2052 and 1422 MPa, respectively. Various quenching methods enable multi‐level strength control for the same steel grade, achieving a strength control range of ≈1000 MPa. The microstructure of samples quenched by water, die, and oil is fully martensite, and martensite exhibited block and lath morphology. With the increase of cooling rate, martensite laths become finer and the prior austenite size decreased. The microstructure of air‐cooled samples is martensite, ferrite, and retained austenite. The strengthening mechanisms of different quenched samples are calculated. The results show that dislocation strengthening and precipitation strengthening are the two dominant strengthening mechanisms in 30MnB5NbTi hot stamping steel. Therefore, the properties of hot stamping steel can be improved by changing the cooling rate by designing physical dies.

Effect of intercritical annealing temperature on microstructure and mechanical properties of V-Ti-Mo microalloyed medium Mn steel

The effect of intercritical annealing temperature on the microstructure and mechanical properties of V-Ti-Mo microalloyed medium Mn steel were investigated using a scanning electron microscope, electron backscatter diffraction, X-ray diffraction and transmission electron microscope. The results showed that the microstructure of the experimental steel in the hot rolled state and after annealing at different temperatures consisted of martensite and reverted austenite. As the annealing temperature increased, the content of reverted austenite increased linearly and reached a maximum value of 29.08 % after annealing at 650 °C. It was difficult to occur austenitic reverted transformation at an annealing temperature of 580 °C. In addition, block-like reverted austenite appeared after annealing at 600 °C, and the austenite size had the largest after annealing at 650 °C. With the increase of annealing temperature, the mass fraction of M3C particles decreased gradually, while that of MC particles increased progressively. Specifically, the mass fraction of 1–5 nm sized MC particles increased first and then decreased, indicating the precipitation strengthening effect was enhanced initially and then weakened. As the annealing temperature rose, the yield strength and tensile strength of the steels decreased gradually, whereas the elongation and impact energy gradually increased. The steel annealed at 650 °C exhibited optimal comprehensive mechanical properties, with a strength-ductility product reaching 26.8 GPa· %.

The Influence of the Second Phase on the Microstructure Evolution of the Welding Heat-Affected Zone of Q690 Steel with High Heat Input.

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150-300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite.

Effect of bainitic and retained austenite's length scale on the aqueous corrosion behaviour of nanostructured bainite in 3.5 wt% NaCl

Microstructural refinement offers immense advantages in terms of enhancement of strength, hardness and wear resistance in carbide-free bainitic steels. Lath thickness in bainite can be changed while maintaining the similar volume fractions of bainitic ferrite (BF) and retained austenite (RA) by modifying the grain size of austenite by changing the austenitizing temperature. However, the effect of this microstructural refinement on the corrosion response is difficult to predict and has not been studied. Hence, the current study aims to study the corrosion response of carbide-free bainitic steels formed by austenitization at 900°C (NSB_900) and 1000°C (NSB_1000) respectively and austempered at 250°C. Finer BF lath thickness and RA was obtained in NSB_1000 while the volume fractions of both BF and RA were the same in both the blocks of steels. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) technique have been used to evaluate the corrosion behaviour of the two steel blocks. Lower corrosion current density was found for NSB_1000 which had a finer microstructure although corrosion potential was similar for both the specimens due to similar bainitic volume fraction. The side-section SEM of NSB_1000 showed a thinner corroded layer and the top surface showed a better connected and adhered corrosion film as compared to NSB_900 which showed the corroded layer as disconnected patches leading to a significant part of the sample still being exposed to subsequent corrosion. This manifested in NSB_1000 showing higher polarisation resistance, solution resistance, charge transfer resistance and higher tendency towards an ideal capacitive behaviour.

Microstructural evolution and mechanical properties of 60Si2CrVNb spring steel under quenching-tempering heat treatment process

This study investigated the evolution of microstructure and mechanical properties of 60Si2CrVNb spring steel under different quenching and tempering processes. The morphology of martensite, precipitate phases, and carbides, as well as the analysis of mechanical properties, were examined. The results revealed that the highest ultimate tensile strength (2260 MPa) and yield strength (2091 MPa) of 60Si2CrVNb spring steel were achieved through austenitization at 900 °C and subsequent tempering at 350 °C, which is related to the maximum dislocation strengthening effect. The optimal combination of mechanical properties for 60Si2CrVNb spring steel was obtained by quenching at 900 °C for 30 min and tempering at 400 °C for 90 min. Under these conditions, the steel exhibited an ultimate tensile strength of 2084 MPa, yield strength of 1846 MPa, elongation of 10.8%, and area reduction of 41.5%. Increasing the quenching temperature resulted in the gradual dissolution of undissolved carbides, an increase in the average grain size of austenite, coarsening of martensite, and an initial increase and subsequent decrease in tensile strength and plasticity. Furthermore, the fracture type transformed from ductile fracture to mixed fracture of ductile and brittle. On the other hand, the increasing tempering temperature led to a decrease in dislocation density and gradual decomposition of martensite lath boundaries. Additionally, the type of carbides changed from small-sized M2C and MC carbides to large-sized M3C, M7C3, and M23C6 carbides. As a result, the tensile strength of the steel decreased significantly, while its plasticity steadily increased. Moreover, the fracture behavior transformed from brittle fracture to ductile fracture.

Corrosion and abrasion behavior of high-temperature carburized 20MnCr5 gear steel with Nb and B microalloying

This paper investigates the effect of Nb and B microalloying on the wear and corrosion resistance of 20MnCr5 steel after high-temperature carburization at 950 °C. The abrasion and corrosion performance of the steel was compared using electrochemical experiments and dry sliding wear tests. The results show that Nb microalloying reduces the size of residual austenite, thereby improving the wear resistance of 20MnCr5 steel. Furthermore, the combination of Nb and B microalloying reduces the amount of residual austenite, leading to further improvements in wear resistance. Both microalloying methods reduce the wear scar depth and debris accumulation. The corrosion potentials of 20MnCr5, 20MnCr5Nb, and 20MnCr5NbB are 0.645 V, 0.627 V, and 0.675 V, respectively. The addition of Nb to 20MnCr5 steel results in higher corrosion resistance after carburization due to the refinement of needle-like martensite. However, 20MnCr5NbB steel exhibits low corrosion resistance due to the coarsening of needle-like martensite caused by B microalloying. In summary, this study provides new insights into the effects of microalloying elements on the properties of steel. The findings contribute to a deeper understanding of how to design and manufacture gears and other components with superior wear and corrosion resistance.

Modeling Segregation of Fe–C Alloy in Solidification by Phase-Field Method Coupled with Thermodynamics

The primary carbide in high carbon chromium bearing steels, which arises from solute segregation during non-equilibrium solidification, is one of the key factors affecting the mechanical properties and performance of the related components. In this work, the effects of carbide forming element diffusion, primary austenite grain size, and the cooling rate on solute segregation and carbide precipitation during the solidification of an Fe–C binary alloy were studied by the phase-field method coupled with a thermodynamic database. It was clarified that increasing the ratio of solute diffusivity in solid and liquid, refining the grain size of primary austenite to lower than a critical value, and increasing the cooling rate can reduce the solute segregation and precipitation of primary carbide at late solidification. Two characteristic parameters were introduced to quantitatively evaluate the solute segregation during solidification including the phase fraction threshold of primary austenite when the solute concentration in liquid reaches the eutectic composition, and the maximum segregation ratio. Both parameters can be well-correlated to the ratio of solute diffusivity in solid and liquid, the grain size of primary austenite, and the cooling rate, which provides potential ways to control the solute segregation and precipitation of primary carbide in bearing steels.

Failure analysis of ball screw in the progressive induction hardening and finishing machining

Some failure modes, such as wear, quenching and finishing cracks, are easy to produce on the surface of the ball screw during the progressive induction hardening, and can shorten the fatigue life of ball screw. The hardened layer, microstructure, tensile strength and cracks of a 55CrMo steel ball screw after induction hardening are tested or observed by experiment. Based on the results of experiment and numerical simulation, the effect of heating temperature on the austenitization, microstructure, tensile strength, grain size of austenite and stress state of the ball screw are analyzed and discussed. In the induction hardening process, high heating temperature can result in some problems such as coarse grains and melting of the eutectic Fe-FeS-MnS produced on the grain boundary, which will impact the quenching quality and service performance of ball screw. Before the phase transformation of austenite transformed to martensite, the channel top of ball screw is in the state of three-dimensional tensile stress, which leads to brittle fracture during the cooling process. In the cutting process, the mechanical and thermal stress successively generate the tensile stress in the inner and outer layers of the ball screw channel, resulting in the internal cracks formed during quenching gradually extending to the surface. The essential cause of cracks in the finishing of the ball screw is the cracks formed during the induction hardening.

Achieving high strength and ductility in Fe–Mn–Al–C austenitic steel via vanadium microalloying and aging

An investigation was performed to evaluate the effect of vanadium (V) addition on the microstructure and the tensile behavior of Fe–27Mn–8Al-1.0C austenite lightweight steel after annealing and subsequently aging. The grain size of austenite in V-added steels was effectively refined and many nano-sized VC particles were observed in the austenite grains, compared to V-free steels, under annealed conditions. Subsequent aging promotes the precipitation of κ-carbide from the austenite matrix, and the dual-nanoprecipitation consisting of nano-sized VC and κ-carbide particles further improves the strength of the steel. Because of this, the current steels have good ductility (up to 41% total elongation) even when aged and high yield and ultimate tensile strengths of up to 1009 MPa and 1243 MPa, respectively. By examining the dislocation structure of the specimens during deformation, it was also possible to determine how precipitates affected the rate of strain hardening. As a result, the strain hardening rate of the V-added steel after aging is effectively improved at the initial deformation by the precipitation of VC particles, which compensates for the decrease in strain hardening rate due to shearable κ-carbide. This study provides a strategy to obtain an excellent combination of strength and ductility in lightweight steel by V microalloying and simple thermomechanical treatment.

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.

Microstructure Evolution and Mechanical Properties of Ferrite-Austenite Duplex Fe-Mn-Al-(Cu)-C Steel under Different Annealing Temperatures.

The effect of Cu addition and the intercritical annealing (IA) temperature on the microstructural evolution and mechanical properties of Fe-0.4C-7Mn-4Al (wt%) was investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and nanoindentation tests. The results showed that the volume fraction and the average grain size of austenite, and the fraction of high angle grain boundaries, increased with IA temperature increase in the range of 650 °C to 710 °C. The addition of Cu facilitates the formation of Cu-rich nanoparticles, raises the volume fraction of austenite, and delays the recrystallization of austenite. As IA temperature increased, the yield strength (YS), ultimate tensile strength (UTS), and Lüders bands strain (LBS) decreased in both experimental steels. The Cu addition not only increases the YS of medium Mn steel but also benefits the decrease of LBS. The best comprehensive mechanical properties were obtained at the IA temperature of 690 °C in the studied steel, with Cu addition. According to nanoindentation experiments, the Cu addition raises the hardness of ferrite and austenite from 4.7 GPa to 6.3 GPa and 7.4 GPa to 8.5 GPa, respectively, contributing to the increase of YS of medium-Mn steel.

The coupling machine learning for microstructural evolution and rolling force during hot strip rolling of steels

Microstructural evolution during hot rolling, which is complex and unperceptive but has direct effects on rolling forces, determines the final microstructure and mechanical properties of steel products. In this paper, a comprehensive set of machine learning (ML) models was developed through rolling forces to reveal the evolutions of recrystallization and grain size of austenite, in which the grain size was estimated by considering the grain shape effect after each rolling pass. By using the backpropagation neural network (BPNN) and genetic algorithm (GA) combined with industrial data, the quantitative connections of parameters in the models for static recrystallization (SRX), meta-dynamic recrystallization (MDRX), and mean flow stress (MFS) were established with steel chemical compositions and deformation conditions. Through the machine learning (ML) approach, softening kinetics and rolling forces can be more accurately predicted than traditional models. Also, the recrystallization behavior and evolution of austenite grain size during hot rolling were analyzed by using the proposed ML models, which are consistent with experimental results.