Isothermal Transformation Time Research Articles

Microstructural Design and Mechanical Enhancement of δ-TRIP Steel Through Mo Micro-alloying and Isothermal Bainitic Transformation

This research was done to investigate third-generation Advanced High-Strength Steel (AHSS) to improve mechanical properties and reduce production costs in the automotive industry. The potential of TRIP-assisted steel has been studied to address this issue, with a special focus on δ-TRIP steels. This research studied the δ-TRIP samples, which were micro-alloyed with 0.05 and 0.30 wt.% Mo, with isothermal bainitic transformation (IBT) time parameters. Microstructural analyses were conducted using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectrometry (EDX). The volume percentage of retained austenite (RA) phase, critical to achieving the requisite mechanical characteristics, was determined using X-ray diffraction (XRD). The analysis of the samples revealed a complex, multi-phased structure comprising δ-ferrite matrix, α-ferrite, bainite, RA, and martensite. The quantity and morphology of RA significantly influenced the resulting mechanical characteristics. The experimental results demonstrated yield stresses (YS) of 400–440 MPa, ultimate tensile strength (UTS) of 790–870 MPa, and total elongations to failure (TEL) of 24–34% in δ-TRIP steels containing 0.05 and 0.30 wt.% Mo alloy. The optimal combination of UTS and TEL (UTS × TEL) is achieved at an impressive 850 MPa × 32% (27200 MPa.%). This remarkable achievement was made possible by adding only 0.30 wt.% Mo and IBT, which were carried out at 450°C for 10 min. Focusing on the effects of Mo-alloy content and heat treatment duration on microstructure and mechanical characteristics, this study provided insights into enhancing the quality of δ-TRIP steel while decreasing manufacturing costs.

The effect of isothermal bainitic transformation time on austenite stability of TRIP-980 steel with high ductility

• The effect of IBT time on the microstructure and properties was studied. • The TRIP effect is jointly controlled by the content and stability of RA. • The elongation is 31.5%, while its strength meets the demand of TRIP-980 steel. Insufficient ductility is the main reason that limits the application of transformation-induced plasticity (TRIP) steel in the automotive industry. In this work, by designing a two-stage heat treatment method of intercritical annealing (IA) + isothermal bainitic transformation (IBT), the effect of IBT time on the microstructure and properties was studied. The results show that the volume fraction of retained austenite (RA) has almost no difference at different IBT times. When the IBT time is 4 min, the RA has the highest stability. Finally, the perfect combination of strength and ductility is achieved in this sample. While the strength meets the requirements of TRIP-980 steel, the total elongation reaches more than 30 %.

Nanobainite Generated in Low‐ and Medium‐Carbon Steels via an Economical Alloying Strategy

A low‐cost strategy for generating fast transforming nanobainitic (nB) steels in low‐ and medium‐carbon alloys is investigated. Accelerating the bainite transformation relies on adding 0.7–3 wt% Al. Alloys and heat treatments are designed via thermodynamic calculations and dilatometry. nB microstructures are generated via isothermal holding and continuous cooling. The microstructures generated are investigated via microscopy and mechanical characterization. Incubation periods less than 150 s as well as isothermal transformation times ranging from 2000 to 4000 s are recorded for all conditions. Increasing the Al content from 0.7 to 2.8 wt% lowers the incubation and transformation times from 150 to 15 s and 3000 to 2000 s, respectively, at a cost of a reduction in tensile strength and elongation % (EL%) from 1330 to 1270 MPa and from 13.5 to 7.7%, respectively. The introduction of δ‐ferrite to the microstructure of the high Al alloy increased EL% up to 16% and reduced the tensile strength to 1105 MPa. Continuous cooling at a rate of 0.03 K s−1 increased the tensile strength by 100 MPa at similar EL%. Lowering the cooling rate to 0.003 K s−1 yielded similar properties as isothermal treatment because most of the transformation is concluded near the starting temperature.

Effect of Isothermal Transformation Times below Ms and Tempering on Strength and Toughness of Low-Temperature Bainite in 0.53 C Bainitic Steel.

This study aims to investigate the microstructures, strength, and impact toughness of low-temperature bainite obtained by isothermal transformation at temperature below Ms (Martensite Starting temperature) for different times and tempering process in 0.53 C wt% bainitic steel. By using the optical microscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), electron back scatter diffraction (EBSD), and mechanical property test, it was found that the microstructures after heat treatment consist of small amounts of martensite, fine bainite, and film retained austenite. After tempered at 250 °C for 2 h, the volume fraction of retained austenite (10.9%) in the sample treated by isothermal transformation at 220 °C for three hours is almost the same as that of the sample without tempering. In addition, the retained austenite fraction decreases with the increase of holding times and is reduced to 6.8% after holding for 15 h. The ultimate tensile strength (1827 MPa), yield strength (1496 MPa), total elongations (16.1%), and impact toughness (up to 58 J/cm2) were obtained by isothermal transformation at 220 °C for three hours and tempered at 250 °C. Whereas, the impact toughness of sample without tempering is 28 J/cm2. After holding for 15 h, the impact toughness raises to 56 J/cm2, while the ductility and strength decreases. These results indicate that the tempering process is helpful to improve the impact toughness of low-temperature bainite.

Intercritical annealing pre-treatment used in a hot-dip galvanised TRIP steel

ABSTRACTAn intercritical annealing pre-treatment was added before the conventional two-step heat treatment process, and the effect of the isothermal bainitic transformation (IBT) time on the steel's microstructure and mechanical properties were investigated. The microstructure was investigated using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, while the mechanical properties were evaluated using tensile testing. The microstructure of the three-step hot-dip galvanised transformation induced plasticity (TRIP) steel consists of ferrite, bainite, retained austenite, and martensite. The mechanical properties of the steel after the three-step heat treatment process are excellent, with a tensile strength above 770 MPa and elongation above 29%. The effect of IBT time on the mechanical properties was insignificant because the intercritical annealing pre-treatment increases the bainitic transformation rate.

Effects of Undercooling and Transformation Time on Microstructure and Strength of Fe–C–Mn–Si Superbainitic Steel

A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation, microstructure, and strength of Fe–C–Mn–Si superbainitic steel. The results indicate that the strength of the samples decreases and the elongation increases with the isothermal transformation time, resulting in an increase in the product of the tensile strength and the total elongation. Therefore, the prolongation of the transformation time can improve the comprehensive property of the sample. In addition, the morphology of bainite changes from granular bainite to lath-like one with a decline in the isothermal transformation temperature, leading to an increase in strength. The elongation first increases and then decreases with a decrease in the isothermal transformation temperature. Finally, the sample shows the best comprehensive property when it is austempered at an intermediate temperature (350°C). The data presented here are instrumental in optimizing the technology of austempering treatment in industrial production.

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

Glasses of three alkyl phosphates show a range of kinetic stabilities when prepared by physical vapor deposition.

In situ AC nanocalorimetry was used to characterize vapor-deposited glasses of three phosphates with increasing lengths of alkyl side chains: trimethyl phosphate, triethyl phosphate, and tributyl phosphate. The as-deposited glasses were assessed in terms of their reversing heat capacity, onset temperature, and isothermal transformation time. Glasses with a range of kinetic stabilities were prepared, including kinetically stable glasses, as indicated by high onset temperatures and long transformation times. Trimethyl phosphate forms kinetically stable glasses, similar to many other organic molecules, while triethyl phosphate and tributyl phosphate do not. Triethyl phosphate and tributyl phosphate present the first examples of non-hydrogen bonding systems that are unable to form stable glasses via vapor deposition at 0.2 nm/s. Based on experiments utilizing different deposition rates, we conclude that triethyl phosphate and tributyl phosphate lack the surface mobility required for stable glass formation. This may be related to their high enthalpies of vaporization and the internal structure of the liquid state.

Method to Evaluate the Kinetics of Bainite Transformation in Low-Temperature Nanobainitic Steel Using Thermal Dilatation Curve Analysis

The aim of this work was to develop a method to evaluate the kinetics of bainite transformation by theoretical deduction and thermal dilatation curve analysis. A Gleeble-3500 thermomechanical simulator and dilatometer (DIL805A) were employed to study the isothermal transformation in deformed (360 ∘ C , 600 ∘ C , and 860 ∘ C ) and undeformed conditions. The thermal dilatation information during isothermal transformation was recorded, and the dilatation curves were well smoothed. By taking a derivative of the dilation curve with respect to the transformation time, the peak time of transformation rate (PTTR) was obtained, which can serve as the essence of isothermal transformation time. The relative change of length ( Δ L / L ) due to phase transformation was theoretically deduced, and the effect of temperature was taken into consideration. Combing experimental data, the volume fraction of bainite in isothermal transformation was calculated. Making a graph of volume fraction versus PTTR was a good method to evaluate the kinetics of bainitic transformation clearly and concisely which facilitated optimization of the preparation technique for low-temperature nanobainitic steel.

Limited surface mobility inhibits stable glass formation for 2-ethyl-1-hexanol.

Previous work has shown that vapor-deposition can prepare organic glasses with extremely high kinetic stabilities and other properties that would be expected from liquid-cooled glasses only after aging for thousands of years or more. However, recent reports have shown that some molecules form vapor-deposited glasses with only limited kinetic stability when prepared using conditions expected to yield a stable glass. In this work, we vapor deposit glasses of 2-ethyl-1-hexanol over a wide range of deposition rates and test several hypotheses for why this molecule does not form highly stable glasses under normal deposition conditions. The kinetic stability of 2-ethyl-1-hexanol glasses is found to be highly dependent on the deposition rate. For deposition at Tsubstrate = 0.90 Tg, the kinetic stability increases by 3 orders of magnitude (as measured by isothermal transformation times) when the deposition rate is decreased from 0.2 nm/s to 0.005 nm/s. We also find that, for the same preparation time, a vapor-deposited glass has much more kinetic stability than an aged liquid-cooled glass. Our results support the hypothesis that the formation of highly stable 2-ethyl-1-hexanol glasses is inhibited by limited surface mobility. We compare our deposition rate experiments to similar ones performed with ethylcyclohexane (which readily forms glasses of high kinetic stability); we estimate that the surface mobility of 2-ethyl-1-hexanol is more than 4 orders of magnitude less than that of ethylcyclohexane at 0.85 Tg.

Vapor-deposited alcohol glasses reveal a wide range of kinetic stability.

In situ AC nanocalorimetry was used to characterize vapor-deposited glasses of six mono- and di-alcohol molecules. Benzyl alcohol glasses with high kinetic stability and decreased heat capacity were prepared. When annealed above the glass transition temperature Tg, transformation of these glasses into the supercooled liquid took 103.4 times longer than the supercooled liquid relaxation time (τα). This kinetic stability is similar to other highly stable organic glasses prepared by vapor deposition and is the first clear demonstration of an alcohol forming a stable glass. Vapor deposited glasses of five other alcohols exhibited moderate or low kinetic stability with isothermal transformation times ranging from 100.7 to 102 τα. This wide range of kinetic stabilities is useful for investigating the factors that control stable glass formation. Using our current results and literature data, we compare the kinetic stability of vapor deposited glasses prepared from 14 molecules and find a correlation with the value of τα at 1.25 Tg. We also observe that some vapor-deposited glasses exhibit decreased heat capacity without increased kinetic stability.

Advanced Properties of Ausferritic Ductile Iron Obtained in As-Cast Conditions

In previous studies, the process parameters to obtain ausferritic ductile iron in as-cast conditions by means of engineered cooling were defined, that is, without an austempering heat treatment. This material was fundamentally characterized, and its mechanical properties were determined. It was demonstrated that obtaining fully ausferritic microstructures by means of engineered cooling was feasible and that the properties met the requirements of the conventionally produced austempered ductile iron. Additionally, an experimental model was developed to define the optimal processing parameters of castings presenting different thermal moduli, in terms of chemical composition, temperatures and time parameters. The aim of the present work is to go into detail about the physical properties of the ausferritic as-cast materials. The chemical composition of the samples was defined by means of the experimental model. The isothermal transformation temperature was changed from 300 to 400 °C, while the other process parameters (shakeout temperature and isothermal transformation time) remained constant. Due to the excellent strength/toughness ratio of these materials, they are prone to being used on different applications such as automotive suspension components, rail components in low temperature environments and pumps and engines exposed to corrosive marine conditions among others. With the aim of responding to this demand, an advanced characterization of the material’s low temperature, corrosion or dynamic properties was performed on this work. These results were compared to the conventionally heat-treated austempered ductile iron as well as other nodular iron ferritic–pearlitic grades found in the literature.

Heat treatment, microstructure and mechanical properties of a C–Mn–Al–P hot dip galvanizing TRIP steel

Heat treatments of a hot dip galvanizing TRIP (Transformation induced plasticity) steel with chemical composition 0.20C-1.50Mn-1.2Al-0.07P(mass%) were performed in a Gleeble 3500 laboratory equipment. The heat treatment process parameters were varied to investigate the effect of intercritical annealing temperature as well as isothermal bainitic transformation (IBT) temperature and time, on the microstructure and the mechanical properties. The microstructure was investigated using scanning electron microscopy, transmission electron microscopy and x-ray diffraction, while mechanical properties were evaluated by tensile testing. Furthermore, to generate a better understanding of the phase transformations during heat treatment, dilatometry trials were conducted. The desired microstructure containing ferrite, bainite, retained austenite and martensite was obtained after the heat treatments. It was further found that the IBT is critical in determining the mechanical properties of the steel, since it controls the fraction of bainite. With increasing bainite fraction, the fraction of retained austenite increases while the fraction of martensite decreases. The mechanical properties of the steel are excellent with a tensile strength above 780MPa (expect in one case) and elongation above 22%.

The Novel Technique of Grain Refinement in the Aluminum-Free Ultrahigh Carbon Steel

In order to achieve high strength and toughness of the aluminum-free ultrahigh carbon steel (UHCS), the effect of a novel three-parameter combination technique including heating temperature, cooling rate, and isothermal eutectoid transformation time on the phase composition, grain morphology, and mechanical properties is investigated by X-ray diffraction analysis, optical microscopy, and mechanical tests. The result shows that when the steel is cooled from 860 °C to the eutectoid transformation temperature with the rate of 1.0 °C s−1 and held for 1.5 h, the excellent mechanical properties are obtained due to the grain refinement of carbide and matrix.

Microstructure and mechanical properties of a mixed Si–Al TRIP-assisted steel subjected to continuous galvanizing heat treatments

Heat treatments with an isothermal bainitic transformation temperature of 465°C compatible with the continuous galvanizing process were performed on a 0.2C–1.5Mn–1.0Si–0.5Al (wt%) TRIP-assisted steel. The effect of intercritical annealing temperature and isothermal bainitic transformation (IBT) time at 465°C on mechanical properties and microstructure were determined. The best combination of strength and ductility was obtained for the lower intercritical annealing temperature of 750°C for an isothermal bainitic transformation time of 90s at 465°C. However, after intercritical annealing at a higher temperature of 797°C, optimum mechanical properties were obtained for a shorter isothermal bainitic transformation time of 60s at 465°C, likely due to the lower C content of the intercritical austenite bringing about faster transformation kinetics. It was concluded that varying the intercritical temperature and IBT time can play an important role in the development of a suitable process window to obtain the desired mechanical properties in low alloy TRIP-assisted steels given the constraints associated with the continuous galvanizing process.

EFFECT OF CARBON CONTENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-ROLLED C-Mn-Al-Si TRIP STEEL

The low-alloyed transformation induced plasticity(TRIP) steels demonstrate an improved combination of strength and ductility,and have became a promising candidate for the application of automotive bodies to reduce the weight without the loss of crash-worthiness.The typical microstructure of TRIP steels consists of the ferrite matrix and a dispersion of bainite.niartensite and the retained austenite.The existence of an amount of metastable retained austenite is responsible for the improved mechanical properties,resulted from the enhanced strain hardening capabilities of TRIP steels due to the strain-induced martensitic transformation during straining.The carbon content is considered as an important factor that influences the amount and stability of the retained austenite. In the present work,two cold-rolled C Mn Al-Si TRIP steels with different carbon contents, (0.1%and 0.2%,mass fraction) were fabricated by intercritical annealing and isothermal transformation. The microstructures and the mechanical behaviors of the used steels were investigated by OM,XRD and uniaxial tensile tests at room temperature.The results indicated that with the same isothermal transformation time at 400℃.the steels with high carbon content obtained lower fraction of bainite and larger fraction of martensite,and demonstrated higher strength and larger elongation than those of steels with low carbon content.The excellent ductility of steels with high carbon content was mainly attributed to its strong TRIP effect during deformation,resulted from the larger fraction of retained austenite as well as the higher carbon content of retained asutenite in the multiphase microstructure.The value of the product of tensile strength and total elongation,representing the combination of strength and ductility of steels,was increased linearly with the increase of the value of the product of volume fraction and carbon content of retained austenite,which could be used to characterize the TRIP effect.Variation of the formation rate of strain-induced martensite was similar to that of the incremental strain hardening exponent with strain during deformation,further proved the important role of TRIP effect in influencing the strain hardening capabilities of TRIP steels.

Development and experimental validation of a neural network model for prediction and analysis of the strength of bainitic steels

In this investigation, a neural network model is developed to predict the hardness of high carbon steels. The inputs to the neural network include the weight percentage of nine alloying elements and the heat treatment conditions such as austenitization temperature and isothermal transformation temperature and time. For the model development, 15 steels from the literature were used. The developed model was validated with respect to eight other steels from the literature that were not used for the model development. Additionally, the model was also employed to predict the hardness of five newly designed bainitic steels. Further, from the new experimental data, identifying the steel containing Co and Al as potentially viable for mass production, a computational analysis of this steel using the developed neural network model indicates the possibility of minimizing the processing costs by adjusting the alloying element content, Co in particular.

Austenite-Bainite Transformation Kinetic Model for the Powder-Metallurgical Steel Astaloy 85 Mo

A new approach for case hardening of powder metallurgical steels is surface densification prior to heat treatment, hence avoiding hardening to the core caused by open porosity. With regard to this process chain a porosity and carbon dependent model of the transformation kinetics is essential. In powder metallurgical materials the transformation behavior is mainly influenced by the chemical composition, homogeneity and porosity. Using a prealloyed powder, e.g. Astaloy 85 Mo, a homogeneous distribution of alloying elements after sintering can be assumed and the transformation behaviour is mainly determined by pores and the carbon profile caused by case hardening. The effect of carbon is widely known but up to now, only a few details about the effect of porosity on the transformation can be found in literature. It is reported that a decreasing relative density causes a reduction of incubation and overall isothermal transformation time. In the present study, the transformation kinetics of a powder metallurgical steel based on Astaloy 85 Mo were investigated for the carbon levels 0.5 and 0.8 wt% as well as the relative densities 6.8, 7.2 and 7.8 g/cm³. The investigations were carried out using a high-speed quenching dilatometer. The isothermal time temperature transformation diagrams for this powder-metallurgical alloy are presented and Avrami-type equations are fitted to the measured data. A good correlation can be found for the transformation model and the experimental results verifying the used modeling approach showing the potential to be applied within case hardening simulations.

The Effect of Isothermal Heat Treatment Time on the Microstructure and Properties of 2.11% Al Austempered Ductile Iron

In this article, the bainitic transformation during austempering was studied for a 2.11% Al containing ductile iron under different isothermal holding times. The austenitizing time and temperature were selected to be 60 min and 920 °C, respectively, referring to previous studies. The isothermal austempering heat treatments were performed at 350 °C for different durations. Microstructures have been examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Microstructural investigations revealed that austempering treatment at 350 °C for durations up to 100 min results in microstructures consisting of carbide-free bainitic ferrite with considerable amounts of retained austenite while the extension of isothermal transformation time leads to precipitation of carbides. Hardness measurements were also carried out the results of which were shown to be consistent with microstructural evolutions.

Determination of volume fraction of bainite in low carbon steels using artificial neural networks

Artificial neural networks have been used to estimate the volume fraction of bainite in low carbon steels containing various alloying elements. The network predicts the volume fraction for a given composition, isothermal transformation temperature and isothermal transformation time. Additionally, the maximum transformation temperature at which bainite formation takes place is also provided as an input to the neural network. The network was trained using the experimental data from three low carbon steels and it was found to perform quite well in predicting the volume fraction of bainite. The impact of the composition of alloying elements on the volume fraction of bainite was also studied and the results were in agreement with the known metallurgical theory.