Amount Of Bainite Research Articles

Cyclic re-austenitization of copper-bearing high-strength low-alloy steels fabricated by laser powder bed fusion

For the first time, cyclic re-austenitization is carried out for additively manufactured high-strength low-alloy (HSLA) steels in order to refine the microstructure by reducing the prior austenite grain (PAG) size. In this work, HSLA-100 steels processed using laser powder bed fusion (LPBF) technique are subjected to several cycles of re-austenitization using quenching dilatometry. Microstructure characterization for every cycle revealed the presence of bainite, martensite and martensite/austenite (M/A) islands. From the analysis of the dilatometry curves and extensive microstructure characterization, it was found that till the 2nd cycle of re-austenitization, both PAG size and martensite start (Ms) temperature reduces, while the amount of bainite transformed decreased and the retained austenite content increased. Concomitantly, the highest microhardness along with peak nanohardness of the constituent phases was achieved at the 2nd cycle. Conversely, from the 3rd cycle, the microhardness, as well as the nanohardness of the constituent phases, are found to decrease due to an increase in the PAG size. This behavior is in contrast to the general tendency where a saturation limit is reached after the peak refinement is achieved. It is found that retained austenite can act as a pinning particle to obstruct the PAG boundary movement and its fraction is found to decrease from the 3rd cycle. Hence, the increase in PAG size after the 3rd cycle can be attributed to the destabilization of effective pinning particles to hinder the PAG boundary movement during the re-austenitization.

UP-quenched SAE 4130 steel: Mechanical properties assessment and bainite formation

This work focuses on the effect of Up-Quenching experimentation on mechanical properties of SAE 4130 steel. In that sense, the effect of time and temperature of Up-quenching on hardness, yield strength (YS), ultimate tensile strength (UTS), elongation, and toughness of this automotive steel were investigated. Up-quenching of the austenitized specimens were performed in the temperatures range between 350 to 500 °C for the times period between 20 to 60 min. Mechanical properties were evaluated by means of tensile and hardness tests. In addition, Charpy impact tests were done to measure the fracture energy of the specimens. Microstructural analyses, including texture development and fracture surfaces analyses, were done using Scanning Electron Microscope (SEM) equipped with Electron backscattered diffraction (EBSD) and Transmission Electron Microscope (TEM). Results showed that UP-quenching has positive contribution on hardness, UTS, Elongation% (EL%) and toughness. The results of the present work are useful to control the mechanical properties of SAE 4130 steel via changing the heat treatment parameters. Electron microscopy results showed that the maximum amount of bainite was observed at 450 °C. And the optimum microstructure, that provides the best mechanical properties, is formed in the case of Up-quenching at 450 °C for 20 min {100} pole figures for the Up-quenching in 20 min were constructed and showed that at different Up-quenching temperatures, textured structures with the main components of S, Brass, Copper and Cube are attainable. Texture components were substantially weak in the cases of non-optimum temperatures of Up-quenching. Texture intensity was quite high for the Up-quenching case situation of 450 °C. As well, non-cube textures of S, Brass, and to some extent Copper, were developed for the Up-quenching condition of 450 °C, which clarified the acceptable combination of mechanical properties in this condition. Finally, fracture surfaces examination showed that increasing the temperature and time of Up-quenching leads to lower of porosity that is better distributed in the microstructure.

Effect of cooling temperature on the microstructure and corrosion behavior of X80 pipeline steel

Dual-phase accelerated cooling (DPAC) was applied to X80 pipeline steel to obtain its microstructure with different amounts of bainite and ferrite. The microstructure, hardness, and polarization behaviors of the steel, cooled to different temperatures, were investigated. Results showed that, with decreasing cooling temperature, the amount of polygon ferrite (PF) increased while that of acicular ferrite (AF) decreased. The amount of bainite correspondingly decreased, except when cooled to 760°C. Moreover, the grain size of ferrite increased. The corrosion behaviors of different phases were distinct. Martensite/austenite (M/A) islands presented at the grain boundary of the PF phase caused small pits. Numerous micro-corrosion cells were formed in the AF and bainite phases, where micropores were prone to form. X80 pipeline steel cooled to 700°C had the best corrosion resistance in the simulated seawater. The decreased amount of the PF phase reduced the area of cathode, resulting in slight corrosion. About 40vol% of the bainite phase provided strength while the PF phase provided adequate ductility to the X80 steel. It was concluded that the appropriate cooling temperature was 700°C for ideal corrosion resistance and mechanical properties.

Understanding the effect of prior bainite/martensite on the formation of carbide-free bainite

ABSTRACTThe isothermal bainite transformation in low-carbon low-alloy steel under direct and step-quenching conditions have been investigated in the present work. Varying quantities of prior bainite and prior martensite were formed separately by quenching above and below temperature to understand their effects on the extent and kinetics of subsequent bainite transformation in regime. Formation of prior bainite at higher temperature has been found to stabilise part of remaining austenite against subsequent bainite transformation at lower temperature showing thermal stabilisation of austenite. Hence, the total amount of bainite in two-step treatment is less than that obtained in single-step isothermal bainite transformation at the latter temperature, indicating the inapplicability of ‘additivity rule’ for continuously cooled bainite. A decrease in the bainitic ferrite plate volume due to higher strength of parent austenite and higher nucleation rate, together with limited number of bainite nucleation sites at the latter temperature are believed to be responsible for the thermal stabilisation of austenite. Prior martensite also stabilises part of remaining austenite against further bainite transformation through fragmentation of austenite grains and increased defect density in austenite. The kinetics of bainite transformation in each of the above-mentioned conditions have been analysed quantitatively using an established model and the results are correlated with the microstructures.

The effect of laser scanning speed on microstructural evolution during direct laser deposition 12CrNi2 alloy steel

This paper focused on the effect of different laser scanning speed (4 mm/s, 5 mm/s, 6 mm/s and 7 mm/s) on the microstructural evolution of direct laser deposition (DLD) 12CrNi2 alloy steel, and analyzed the relationship between microstructure and performance of DLD-processed samples. The results showed that the microstructure in the middle of as-deposited samples consisted of a large amount of bainite, a small amount of martensite (M) and ferrite (F). With the increase of laser scanning speed, the fraction of ferrite decreased from 55.6% to 14.7%, while that of martensite increased from nearly 0% to 4.9%. Besides, as increasing the laser scanning speed, granular bainite (GB) transformed into lath bainite (LB) due to the increase of cooling rate, and the fraction of LB reached the maximum of 29.9% when the scanning speed was 7 mm/s. In addition, the functions about the relationship between laser scanning speed and phase fractions were fitted in order to provide a theoretical basis for the design of DLD process parameters. EBSD maps of as-deposited samples exhibited anisotropy due to the complex heat flux direction during the multi-layer laser deposition process. With the increase of laser scanning speed, the grain size showed a downward trend from 5.89 µm2 to 3.44 µm2. The sample fabricated at 7 mm/s contained more LB and M, leading to the highest mean microhardness of 355 ± 6 HV0.2. The sample fabricated at 6 mm/s exhibited the best wear resistance due to its optimum combination of hardness and toughness. Because of a large amount of ferrite with optimal toughness, the sample fabricated at 4 mm/s had the best impact toughness of aku = 80 J/cm2.

The Effect of Stress on Bainite Transformation, Microstructure, and Properties of a Low‐Carbon Bainitic Steel

The effects of applying external stress on bainite transformation, microstructure, and mechanical properties are investigated by metallography, dilatometry, etc. Results show that compressive stress during bainite transformation accelerates the bainite transformation and increases the final bainite amount due to the additional mechanical driving force. The lath‐like bainite becomes shorter, and smaller blocky martensite/austenite islands form under stress during the bainite reaction. Different from the results with stress during bainite transformation, applying stress after bainite transformation during isothermal holding cannot increase the volume fraction of bainite, and no obvious change in bainite morphology is observed. In addition, the subsequent martensite transformation is promoted during the cooling process. Moreover, the less amount of retained austenite (RA) but more carbon content in RA is obtained with applying stress after the bainite reaction. Finally, the tensile strength increases, while the elongation decreases when the external stress is imposed after bainite transformation.

Effect of Mn content on microstructure and properties of wear-resistant bainitic steel

The microstructure, hardness and wear resistance of bainitic steel containing 1.0%–2.8% Mn were investigated by means of optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, x-ray diffractometer, hardness tester and M-200 wear tester. The results show that the as-cast structure is mainly composed of pearlite and a small amount of bainite when the Mn content is less than 1.6%. When the Mn content reaches 2.2%, the as-cast steel is mainly composed of acicular lower bainite and retained austenite. With the increase in Mn content, the amount of retained austenite decreases, the acicular lower bainite content increases, and the hardness tends to increase. After normalizing at 900 °C, the microstructure of the steel is mainly acicular lower bainite and film-like retained austenite. As the Mn content increases, the bainitic transformation curve moves to the right and down, the bainitic transformation start temperature (Bs) decreases, the bainitic laths are more refined, the retained austenite content is reduced, and the hardness is increased. The wear resistance of normalized bainitic steel increases obviously with the increase in Mn content.

Influence of carbon content on microstructure and mechanical properties of 1000 MPa deposited metal by gas metal arc welding

The effects of carbon content (0.078–0.100 wt.%) on the microstructure and properties of 1000 MPa grade deposited metal produced by gas metal arc welding have been investigated. Experimental results show that the microstructure of the deposited metal was mainly composed of martensite, bainite and retained austenite. With increasing carbon content, the proportion of martensite increased, and the amount of bainite was reduced. High carbon content is beneficial to strength, but harmful to impact toughness, and thus, carbon reductions lead to the increase in impact toughness. When the carbon content was 0.100 wt.%, the lowest Charpy absorbed energy of 47 J at − 40 °C for the deposited metal was achieved, the highest yield strength of 1038 MPa was attained, and the yield-to-tensile ratio was more than 0.88, while the highest Charpy absorbed energy of 55.7 J at − 40 °C and the lowest yield strength of 915 MPa were obtained when the deposited metal contains 0.078 wt.% C, and the yield-to-tensile ratio was less than 0.85. It is concluded that bainite fraction and fine effective grain size were the dominant factors to achieve good comprehensive mechanical properties (the required strength and an acceptable toughness) of deposited metals with various carbon contents.

Production of Heavy Plates of Pipe Steel with Increased Strength and Cold Resistance on the 5000 Rolling Mill at PJSC MMK

Abstract—On the basis of laboratory experiments on an ITs Termodeform-MGTU test system, the production of rolled plates with large thickness from pipe steel of improved strength (Rt 0.5 = 505–610 MPa, Rm = 570–680 MPa) and cold resistance (KV–48 ≥ 170 J, DWTT–55 ≥ 90%) is introduced in the 5000 rolling mill at PJSC MMK. To ensure high cold resistance, the formation of predominantly quasi-polygonal and polygonal (polyhedral) ferrite is optimal. It is found that, if the steel microstructure includes a considerable quantity of bainite, the performance of the steel in drop tests is impaired. By means of the Gleeble 3500 test system, the thermokinetic diagram of supercooled-austenite decomposition is plotted for the new low-carbon pipe steel, with up to 1% Ni.

Effects of Ni and Cr on Cryogenic Impact Toughness of Bainite/Martensite Multiphase Steels

In the present research, the effects of Nickel (Ni) and Chromium (Cr) on cryogenic impact toughness (CIT) of low-carbon bainite/martensite multiphase steels [processed by two different cooling processes: isothermal transformation process (ITP) and continuous cooling process (CCP)] were investigated. It was found that due to the formation of carbides during isothermal treatment, the addition of Ni and Cr yielded no significant improvements in CIT. However, during CCP treatment, the addition of Ni manifested a considerable enhancement in CIT, whereas the addition of both Ni and Cr caused a decrease in CIT. Further, after ITP treatment, the microstructure of all steels consisted of bainite and martenite, while Ni + Cr steel contained the largest amount of bainite. The microstructures of the CCP-treated steels mainly also consisted of bainite and martensite, but no retained austenite and carbides were observed, thus resulting in a superior CIT.

Critical ausforming temperature to promote isothermal bainitic transformation in prior-deformed austenite

ABSTRACTThe effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3 at 300°C presents accelerated kinetics of bainitic transformation. However, the amount of bainite in ausformed austenite then reduces with the increase in deformation temperature. A critical deformation temperature, determining whether the bainitic transformation can be promoted, was found in deformed austenite. In addition, the thickness of bainite plate in deformed austenite reduces with the decrease in ausforming temperature. The adjacent bainite ferrite plates grow up interactively, and the intersection angle is about 60–73°. A lower ausforming temperature contributes to a more serious cross-growth phenomenon of bainite plates.

Combined Effects of Deformation and Undercooling on Isothermal Bainitic Transformation in an Fe-C-Mn-Si Alloy

Both ausforming and transformation temperature affect the successive bainitic transformation and microstructure. The individual influence of each case is clear, whereas the combined effects are still unknown. Thermomechanical simulation and metallography were used to investigate the combined effects of ausforming and transformation temperature on bainitic transformation and microstructure. The kinetics of isothermal bainitic transformation in non-deformed and deformed materials was analyzed. A lower transformation temperature can lead to more bainite formation without deformation. However, ausforming with small strains can partially compensate for the decrease of bainite amount caused by the decreased undercooling. The larger the applied strain is, the smaller the difference between the final amounts of bainite with different undercooling. Ausforming at a relatively higher temperature is more favorable to the acceleration of subsequent isothermal bainitic transformation. The results in the present work provide reference for optimizing the fabrication technology of medium-carbon nanobainite steels.

The Effects of Hot Deformation of Austenite on the Bainite Transformation in a Fe-C-Mn-Si-Cr Steel

The effects of hot deformation conditions in the stable austenite state on the transformation kinetics and morphology of bainite were examined using dilatometry, electron back scatter diffraction (EBSD) and X-ray diffraction (XRD) measurements in a carbide-free bainitic steel with a composition of Fe-0.34C-2Mn-1.5Si-1Cr (in wt.%). Both uniaxial tensile and compression tests were applied to study the effect of the deformation mode. The temperature, strain and strain rate of deformation were varied in the ranges of 820-1000 °C, 0.1-0.6 and 0.001-0.1/s respectively. It has been revealed that hot tensile deformation retards the austenite transformation to lower bainite. The overall transformation kinetics slows down and the final attained amount of bainite decreases after completion of the isothermal transformation at 350 °C. However, hot compression deformation accelerates the bainite transformation, increasing both the bainite transformation rate and the final amount of bainite formed. The total amount of bainite increases with decreasing the strain rate irrespective of the mode of deformation. The effect of the deformation temperature and strain on the bainite transformation is in a complicated manner depending on the deformation mode.

Transformation Behavior and Properties of Carbide‐Free Bainite Steels with Different Si Contents

The bainite transformation behavior and properties of low carbon carbide‐free bainitic steels containing different silicon (Si) contents are investigated by two different types of heat treatment processes: isothermal transformation process (ITP) and continuous cooling process (CCP). The results indicate that for ITP and CCP, the transformation kinetics of bainite is retarded and the final bainite amount decreases with increasing Si content. However, both the strength and total elongation improve with the increase of Si content in the range of 1.0–2.0 wt%, resulting in an apparent increment in comprehensive property of bainitic steels due to the more film‐like RA and less carbides. It can be attributed to the increase of shear strength and stability of undercooled austenite and the formation of Cottrell atmosphere, as well as the solid solution strengthening of Si because of higher Si content. In addition, for the same samples, better mechanical properties can be achieved by a lower austempering temperature. Moreover, the increase of Si content resulted in an increase in the temperatures of Ac1 and Ac3.

Effect of Strain Rate on the Bainitic Transformation in Fe-C-Mn-Si Medium-Carbon Bainitic Steels

The effect of strain rate on the kinetics of the bainitic transformation in a Fe-C-Mn-Si medium-carbon bainitic steel was investigated by dilatometry, metallography, and X-ray diffraction. The results indicate that the bainitic transformation is affected by the strain rate. The bainitic transformation is significantly promoted by deformation at a low strain rate of 0.01 s−1, and the amount of bainite increases further when the strain rate increases to 0.1 s−1. However, as the strain rate increases further to 1.0 and 5.0 s−1, the extent to which deformation promotes the bainitic transformation decreases, and the amount of bainitic transformation is smaller than that at low strain rates. The amount of bainite tends to be constant with increasing strain rate from 1.0 to 5.0 s−1. In addition, the volume fraction of retained austenite shows the same trend as the amount of bainite, indicating that chemical stabilization plays a key role in determining the amount of retained austenite. Moreover, the strain rate has no significant effect on the bainite morphology.

EFFECTS OF Q&P PROCESS PARAMETERS ON PROPERTIES OF 42SiCr STEEL

<p class="AMSmaintext"><span lang="EN-GB">The requirement for high strength and good ductility poses problems in today’s advanced steels. This problem can be tackled by appropriate heat treatment which produces suitable microstructures. By this means, ultimate strengths of about 2000 MPa and elongations of more than 10% can be obtained. One of such advanced heat treatment techniques is the Q&P (Quenching and Partitioning) process. It produces a mixture of martensite and retained austenite, where the latter is an important agent in raising the ductility of steel. </span></p><p class="AMSmaintext"><span lang="EN-GB">In this experiment, a low-alloy steel with 0.41% carbon and manganese, silicon and chromium was used. An air furnace and a salt bath were employed for heat treatment and quenching, respectively. In order to obtain the best ultimate strength and elongation levels, partitioning temperatures of 250°C and 300°C were applied. Partitioning involves carbon diffusion from super-saturated martensite into retained austenite, and tempering of hardening microstructure. Effects of the quenching temperatures of 200°C and 150°C were studied as well. To map the impact of the Q&P process on mechanical properties, an additional schedule with conventional quenching and tempering was carried out. Upon optimization of the parameters, the process produced martensite with a small amount of bainite and retained austenite. The ultimate strength was between 1930 and 2080 MPa and the elongation levels were from 9 to 16%.</span></p><p class="AMSmaintext"><span lang="EN-GB"> </span></p>

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.

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s (process A) or rapidly cooled to 350°C and then reheated to 450°C (process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel (DP600) was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength (YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient (n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength (UTS) and elongation (A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties (YS = 362 MPa, UTS = 638 MPa, A80 = 24.3%, n = 0.17) was obtained via process A.

Effect of the Austempering Process on the Microstructure and Mechanical Properties of 27MnCrB5-2 Steel

AbstractThe effect of austempering parameters on the microstructure and mechanical properties of 27MnCrB5-2 steel has been investigated by means of: dilatometric, microstructural and fractographic analyses; tensile and Charpy V-notch (CVN) impact tests at room temperature and a low temperature.Microstructural analyses showed that upper bainite developed at a higher austempering temperature, while a mixed bainitic-martensitic microstructure formed at lower temperatures, with a different amount of bainite and martensite and a different size of bainite sheaf depending on the temperature. Tensile tests highlighted superior yield and tensile strengths (≈30%) for the mixed microstructure, with respect to both fully bainitic and Q&T microstructures, with only a low reduction in elongation to failure (≈10%). Impact tests confirmed that mixed microstructures have higher impact properties, at both room temperature and a low temperature.

Effects of Bainitic Phase on Mechanical Properties of Bainite−Aided Multiphase Steels

Advanced high strength steels play an important role in the automotive industry. However, they still show inferior formability such as low stretch−flangeability. Therefore, in this work, various heat treatment processes are performed for generating multiphase steels with enhanced mechanical and forming behavior. The bainitic phase with different amounts is included in dual phase (DP) steels by controlling oil−quenching temperatures. In other steels, varying bainitic transformation temperatures are applied. Microstructures and phase fractions of the investigated multiphase (MP) steels are characterized by optical and scanning electron microscope. Tensile test, hole expansion test, and micro−hardness measurement are carried out for the steel sheets. It is found that increased oil temperature leads to larger amount of bainite occurred in the DP steels. The bainite−aided DP steels exhibit lower strength, but considerably higher elongation, improved hole expansion ratio, and larger damage tolerance value. In case of MP steels with retained austenite, increasing the bainitic transformation temperature causes ferrite in the steels to be replaced by bainite and retained austenite with various amounts. It seems that the MP steels with higher bainitic phase fraction show deteriorated edge–flangeability and fracture resistance.