Bainite Formation Research Articles

Combined effect of Cu partitioning and nano-size precipitates on improving strength-ductility balance of Cu bearing Q&P steel

Cu was intentionally added to the traditional Q&P steel for further improving comprehensive mechanical properties through Cu partitioning, nano-size precipitates and microstructural refinement. Substantial enrichment of C, Mn, Cu and Ni atoms in newly formed austenite during intercritical annealing effectively inhibited the formation of bainite during the partitioning stage. The final microstructure of Cu bearing Q&P steel (Cu-QP steel) was significantly refined because of the refined prior austenite grains. Moreover, the modified constrained carbon equilibrium (CCE) model considering intercritical ferrite, Cu partitioning and the refinement effect of grain size was newly developed to better predict retained austenite (RA) fractions at different annealing temperatures in Cu-QP steel and Cu free Q&P steel (base steel). Both experiments and modeling results indicated that Cu-QP steel exhibited stronger austenite retention ability. The discrepancy of solid solubility between α and γ phases resulted in 14–18 nm Cu-rich precipitates randomly dispersed at the grain boundaries and in the interior of tempered martensite and intercritical ferrite after partitioning holding. The outstanding mechanical properties with ultra-high yield strength of 706–962 MPa, tensile strength of 1065–1139 MPa, and total elongation of 22–26% were obtained after the intercritical Q&P process. More volume fraction of tempered martensite and/or fresh martensite was formed in Cu-QP steel as a substitute for softer bainite, which remarkably improved the tensile strength. High density of nano-size Cu-rich precipitates and the refined microstructure enabled an appreciable increase of yield strength, while larger-fraction of ultrafine RA with higher stability contributed to better ductility. • Multi-element synergistic partitioning inhibited the formation of bainite. • The partitioning process facilitated carbon partitioning and Cu-rich precipitation. • The modified CCE model was newly developed to better predict RA fractions. • Multi-scale RA with various morphologies was beneficial to stronger TRIP effect.

Effect of prior martensite formation on the bainite transformation kinetics in high-strength 3% Mn multiphase steel

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10).

Comparison of Fracture Toughness in the Coarse-Grain Heat-Affected Zone of X80 Pipelines Girth-Welded under Conventional and Ultra-Low Heat Input.

The coarse-grain heat-affected zones (CGHAZs) of X80 girth-welded steel pipelines are prone to embrittlement, which has an extremely adverse effect on their structural integrity. In the present work, the fracture behavior of the CGHAZs of X80 girth welds under the conditions of conventional and ultra-low heat input was studied. The fracture toughness of CGHAZs was evaluated using the crack tip opening displacement (CTOD) test at -10 °C, and the fracture behavior mechanism of CGHAZs were clarified by analyzing microstructural characteristics at prefabricated fatigue cracks containing fracture cloud image, scanning electron microscopy (SEM), and electron back-scatter diffraction (EBSD) figures. The results illustrate that the average fracture toughness (CTOD) value of the ultra-low heat input CGHAZ is 0.6 mm, and the dispersion of CTOD values is small, while the CTOD value of conventional heat input is only 0.04 mm. The ultra-low heat input makes the high-temperature residence time of the coarse-grained region short, reduces the proportion of prior austenite grain boundaries, and inhibits the formation of strip-like bainite and island-like M-A components. The reduction of these deleterious ductile microstructures increases the plastic reserve and deformation capacity of the CGHAZ.

Influence of vacuum graded austempering treatment on microstructure and mechanical properties of M50 steel

By means of microstructure observation and mechanical property tests, the effects of vacuum graded austempering treatment on carbide precipitation, bainite transformation, hardness and impact toughness of M50 steel were studied. Results show that after the steel was vacuum grading treated at 560 °C for 10 min, 20 min, 30 min and 60 min and austempered at 200 °C for 2 h, the microstructure is mainly composed of martensite, acicular lower bainite, retained austenite, and carbides (C) in different shapes. With the extension of the grading time from 10 min to 60 min, the content of precipitated carbides after austempering treatment increases from 2.38% to 3.79%, that of the lower bainite increases from 13.14% to 16.03%, and the hardness decreases from 65.06 HRC to 64.26 HRC. After tempering treatment at 550 °C × 2 h for 3 times, the content of precipitated secondary carbides increases from 3.86% to 5.37%, and the hardness of the steel decreases compared to the corresponding austempered steel. Thereinto, the hardness of the tempered steel with the grading time of 20 min is the highest to be 62.92 HRC. After vacuum grading treatment at 560 °C × 10 min, austempering and tempering treatments, the impact toughness of the M50 steel is 16.63 J/cm2, which is enhanced by 50.6% compared to the steel only with vacuum quenching and tempering. This is related to the formation of bainite after graded austempering treatment and the uniform precipitation of fine carbides after tempering.

Post-Rolling Cooling Phase Transformation and Microstructure of High-Strength Anti-Seismic Rebars with Different Solute Nb and Austenite Microstructure

Three types of 20MnSiNb high-strength anti-seismic steels with different solute niobium and austenite microstructural characteristics were obtained through thermal simulation experiments which involved changing the reheating temperature and finish-rolling temperature; the dynamic CCT curve, microstructure transformation, precipitates and hardness of the steels were also studied. The results show that the ability of solute Nb to refine austenite grains is stronger than that of undissolved Nb. Lowering the reheating temperature and finish-rolling temperature can refine the prior austenite microstructure and keep the deformed austenite after hot rolling, respectively, and make the dynamic CCT curve move to the upper left, which promotes the formation of proeutectoid ferrite and pearlite, and inhibits the formation of bainite at low cooling rates. The increase in solute Nb and the deformed austenite grains effectively promote the precipitation of Nb-containing precipitates during the cooling process after rolling, and improve the precipitation strengthening effect of Nb. This research provides a theoretical basis for the production of Nb microalloyed high-strength anti-seismic rebars.

Effect of Mn on the chemical driving force and bainite transformation kinetics in medium-manganese alloys

This work presents insights into the manganese influence on the driving force and bainite transformation kinetics. Three different medium-Mn steels were subjected to theoretical calculations and dilatometric study in order to determine the Mn impact on bainite formation. The theoretical approach shows that the increase of manganese leads to a lower bainite fraction formed during the isothermal stage. This implicates the carbon enrichment of the austenite during thermal treatment. The less bainite is formed, the higher is the fraction of residual austenite which enrichment of carbon is globally low. Meanwhile, the manganese influences the incubation and transformation time. As the manganese content increases, the incubation period and formation time of bainite are longer because the chemical driving force essential to start and complete austenite into bainite transformation decreases. This was proved by theoretical calculations and dilatometric analysis, which show that even a small increase in manganese content leads to a longer time necessary to occur the bainitic transformation. For the steel containing 5% manganese, the driving force was too small that the transformation could occur even after 3 h. Additionally, the XRD analysis was conducted to determine the retained austenite fraction and its carbon enrichment. These results were compared with the theoretical values to determine the accuracy of the applied model.

Influence of chemical segregation on bainitic microstructures in a carburized bearing steel

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (zcase) and 0.16 wt% C (zcore). The microstructure of zcase is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For zcore, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of zcore are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.

Batch annealing enhances the yield strength of a high quenching temperature Q&P steel through accelerated bainitic transformation

Hot-dip galvanized quenching and partitioning (Q&P) steels exhibit excellent corrosion resistance and strength-ductility synergies. However, production using conventional continuous galvanizing lines (CGLs) is challenging because these lines cannot meet the required quenching and reheating capacity. The presence of ferrite and absence of sufficient primary martensite led to a substantial decrease in the yield strength of the material. To solve this problem, we propose a simple batch annealing (BA) treatment prior to continuous annealing and galvanization. The BA treatment resulted in spheroidization of cementite in the coiled samples and enrichment of Mn. During the intercritical annealing process, the higher Mn content in the cementite of the BA sample retards cementite dissolution. The retained cementite particles served as nucleation sites and significantly promoted bainite formation during quenching and partitioning. Consequently, the yield strength increased by approximately 90%, from 438 MPa to 821 MPa. The total elongation of the BA samples was not affected due to the simultaneous stabilization of the retained austenite. The effect of batch annealing time was also investigated, and the balance between Mn enrichment in cementite and the number density of spheroidized cementite before Q&P treatment was revealed.

Effect of Cu Content on Microstructure and Mechanical Properties for High-Strength Deposited Metals Strengthened by Nano-Precipitation

With the rapid development of low-carbon high strength steel, higher requirements are put forward for the matching welding consumables. The deposited metals with 0.62–2.32% Cu addition was prepared by tungsten inert gas welding via metal cored wire. The effect of Cu element on microstructure and mechanical properties of deposited metals were investigated. The multiphase microstructure of deposited metals consists of bainite, martensite, residual austenite, and martensite-austenite constituents. It is found that Cu decreases the start temperature of martensite (Ms) and enlarges the temperature range of bainite from 372 K to 416 K, improving the formation of bainite. With the increase of Cu content, the fraction of martensite decreases and the shape of M-A constituents changes from strip into granular. There are BCC and FCC Cu precipitates in deposited metals. The diameter of Cu precipitates is 14–28 nm, and the volume fraction of it increases with the increase of Cu content. Meanwhile, the deposited metals with 1.79% Cu can achieve a 10% enhancement in strength (yield strength, 873–961 MPa, ultimate tensile strength, 1173–1286 MPa) at little expense of impact toughness (64.56–56.39 J at −20 °C). Cu precipitation can effectively improve the strength of the deposited metals, but it degrades toughness because of lower crack initiation energy. The deposited metal with 1.79% Cu addition shows an excellent strength-toughness balance.

Effects of laser-arc hybrid welding on microstructure and mechanical properties of dissimilar steel joint

Laser-arc hybrid welding can improve production efficiency, and thus, it is being used in an increasing number of industries. Herein, the effects of laser-arc hybrid welding process parameters on the microstructure and mechanical properties of welded joint of low- and medium-carbon steels were investigated. Welding seam with best quality and no defects was obtained at welding current, arc voltage, laser power, and welding speed of 260 A, 24.8 V, 3300 W, and 0.8 m/min, respectively. Moreover, the enrichment of Mn in the weld zone promoted bainite formation. Rapid cooling led to the pseudo-eutectic phenomenon in the heat-affected zone (HAZ) on both sides of the weld; therefore, when an HAZ was closer to the weld on both sides, the pearlite content was higher. For low-carbon steel, the microstructure of the HAZ comprised ferrite, pearlite, and ferrite widmanstatten. For medium-carbon steel, the microstructure of the HAZ comprised a large amount of pearlite and a small amount of ferrite. In welded joint of both types of steels, differences in microstructure and internal stress altered the nanostructure morphology and distribution characteristics of cementite. The welded joint also had considerably better tensile strength due to a large number of dislocations and better grain refinement, which led to tensile fractures in the base metal zone of low-carbon steel. For a prefabricated weld notch, the tensile fracture mode was a ductile fracture with massive dimples.

Kinetic Model of Isothermal Bainitic Transformation of Low Carbon Steels under Ausforming Conditions

Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the kinetics of incomplete bainitic and martensitic phase transformations with and without austenite deformation. A 0.26C-1Si-1.5Mn-1Cr-1Ni-0.003B-0.03Ti steel and a 0.18C-1Si-2.5Mn-0.2Cr-0.2Ni-0.02B-0.03Ti steel were investigated with different levels of ausforming. The concept of ausforming is expected to accelerate the onset of the bainitic transformation and to enhance the thermodynamic stability of austenite by increased dislocation density. The phase transformation kinetics of both steels is quantitatively analyzed in the study by dilatometry and X-ray diffraction so that the carbon concentration in the retained austenite and bainitic ferrite, as well as their volume fractions, is determined. A critical comparison of the numerical and experimental data demonstrates that the isothermal kinetics of bainite formation and the variation of driving energy can be satisfactorily described by the developed model. This model captures the incompleteness of the bainite phase transformation and the carbon enrichment in the austenite well. A fitting parameter can be used to elucidate the initial energy barrier caused by the ausforming. An increase in austenite stability can be described by the nucleation reaction and the thermodynamic energies associated with the change of dislocation density. The proposed model provides an in-depth understanding of the effect of ausforming on the transformation kinetics under different low-carbon steels and is a potential tool for the future design of heat treatment processes and alloys.

Surface modification by in-situ grown TiC in the bainitic matrix using TIG arcing

In the current study, surface modification of the steel was carried out with the in-situ formation of TiC in the bainite matrix using tungsten inert gas (TIG) arcing. The actual arcing parameters were optimised after thermal simulation considering the final phase transformation. Various characterisation techniques revealed the formation of fine nano as well as micron-sized in-situ grown titanium carbide precipitate within the bainite matrix. The morphology and coherency of fine TiC were analysed by high-resolution transmission electron microscopy. Mechanical properties of the modified surface were significantly improved owing to the combined effect of bainite and TiC formation. The coherent precipitates indicate superior wear resistance with the absence of pull-out particle during sliding wear.

High-Cycle Fatigue Life and Strength Prediction for Medium-Carbon Bainitic Steels

High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based quenching plus partitioning (BQ&P), bainite austempering (BAT) and “disturbed bainite austempering, DBAT”. The interior inclusion-induced crack initiation (IICI) and noninclusion-induced crack initiation (NIICI) modes were found after fatigue failure. The fracture surface of IICI is characterized by a “fish-eye” surrounding a “fine granular area, FGA” in the vicinity of an inclusion. In contrast, a microfacet, instead of an inclusion, is found at the center of FGA for the NIICI fracture surface. The predications of fatigue strength and life were performed on the two crack initiation modes based on fracture surface analysis. The results showed that a majority of fatigue life is consumed within the FGA for both the IICI and NIICI failure modes. The fatigue strength of the NIICI-fatigued samples can be conveniently predicted via the two parameters of the hardness of the sample and the size of the microfacet.

The Influence Mechanism of HSLA Weld Metal with Different Composition on High‐Temperature Strength

In order to study the influence mechanism of HSLA weld metal with different composition on high‐temperature strength, four kinds of HSLA weld metal are designed by using low‐carbon Si–Mn and Si–Mn–Ni alloy system. Different samples are prepared by simple process of normalizing and high‐temperature tempering. The experimental results show that the weld metal is mainly composed of polygonal ferrite, quasipolygonal ferrite, pearlite, and granular bainite. Ni can change the microstructure shape of reheating zone and inhibit the formation of granular bainite. When the tempering temperature is lower than 580 °C, the strength increases. After tempering at 580 °C, the tensile strength and yield strength reach the peak value. When the tempering temperature exceeds 580 °C, the strength begins to decrease. The microhardness of the two alloy systems decreases gradually from columnar crystal zone to recrystallization zone. The impact absorption energy of the four samples is greater than 200 J at 600 °C. Under different temperature conditions, the content of Ni in all second‐phase particles is less than 10%, the content of Mn is lower, and the content of Si is higher.

Formation of lower bainite in a high carbon steel – an in-situ synchrotron XRD study

The microstructural evolution of and simultaneous dimensional changes in high-carbon SAE 52100 bearing steel were monitored continuously during austempering for 120 min at selected temperatures in the range of 210 °C-270 °C, and also during its subsequent tempering to 340 °C for an additional 120 min, via high-energy X-ray diffraction in real time and in-situ dilatometry. The austenite-to-bainitic ferrite transformation induces lattice defects and internal lattice stresses that increase with austempering time and at lower austempering temperatures. These changes are evidenced by the increase in the full-width half-maximum of the relevant reflections in X-ray diffraction. The lattice parameter of bainitic ferrite takes its highest value during the early stages of austempering, and then gradually decreases as the transformation progresses. This observation points to an initial state of carbon supersaturation in the ferritic lattice that is likely reducing due to carbon segregation close to dislocations, fine carbide precipitation within the bainitic ferrite, and carbon partitioning into the surrounding austenite. The carbon partitioning into austenite is evidenced in particular at the higher austempering temperatures of 240 °C and 270 °C, at which there is a noticeable increase in the lattice parameter of the remaining austenite at longer times. The dimensions of the bearing steel specimens are governed by the volume change due to the formation of bainitic ferrite during austempering and by the relaxation of its lattice distortion during tempering at 340 °C in the absence of further phase transformation.

Non-isothermal kinetics and the effects of alloying elements on bainite precipitation in the Cu74.5Al15.0Mn10.5 alloy

In as-quenched CuAlMn-based alloys, the bainitic precipitation can occur between 450 and 625 K on heating. This reaction can promote diffusion into the martensitic phase and can damage the shape memory effect. In this study, the effect of the fourth alloying element (Ag, Ga, and Gd) on bainitic precipitation kinetics in the Cu74.5Al15.0Mn10.5 alloy was evaluated using differential scanning calorimetry, optical microscopy, X-ray diffraction, and the non-isothermal isoconversional method. The study of the Gd addition to the Cu74.5Al15.0Mn10.5 alloy indicated that the Cu5Gd phase increased the number of nucleation sites, thereby reducing the average isoconversional activation energy for the bainitic reaction. Nonetheless, the Ag and Ga additions promoted a more effective change in the kinetic mechanism of the bainitic precipitation. The Ga addition reduced the isoconversional activation energy value and induced the bainitic phase formation at lower temperatures, but it also introduced parallel reactions that turned their lateral growing slower. On another hand, Ag elevated the initial isoconversional activation energy value and shifted up the bainitic precipitation starting, but the presence of Ag precipitates induced the bainite formation in a narrower temperature range. Therefore, the additions of Ag, Ga, and Gd to the Cu74.2Al15.0Mn10.5 alloy changed bainitic precipitation kinetics, which can be associated with modifications in the Cu diffusion energy and the number of nucleation points.

Influence of chemical composition and thermomechanical treatment of low-carbon steels on the microstructure and mechanical properties of their laser welded joints

The paper reports microstructures (revealed by transmission electron microscopy) in various zones of laser welds of the X70 and X80 low-carbon steels with different initial microstructures, as well as chemical and phase compositions. In the X70 steels with 0.13% C, the microstructure refinement has been achieved through helical rolling at temperatures of 920 °C and 850 °C (designated as X70-920 and X70-850, respectively). For all studied cases, both initial steel microstructures and phase compositions have determined the formation of different microstructures with various microhardness levels in the weld metal and heat-affected zones. For the X70-850 steel with a more dispersed and homogeneous microstructure (dF = 3.3 μm), a lower microhardness level of 340 HV has been observed in the weld metal, compared with the X70-920 one (dF = 5.5 μm, 370 HV). The reason has been the formation of both bainite and martensite laths in the X70-850 weld metal, while only lath and lamellar martensite has formed in the X70-920 one. For the X80 steel (0.55% C), lowering the carbon content and additional microalloying with chromium, molybdenum and nickel have enabled to decrease the microhardness level down to 295 HV in the weld metal due to the degenerate upper bainite formation and the carbon level reduction in martensite. The dispersed and homogeneous initial microstructures of the X70-850 and X80 steels has provided the formation of granular ferrite-bainite microstructures in the intercritical heat-affected zone. They have possessed a lower proportion of residual austenite regions and small sizes of twinned martensite areas. The welded X80 steel specimen has been characterized by higher ductile properties compared to both X70 ones.

Dilatometric study of the effect of soluble boron on the continuous and isothermal austenite decomposition in 0.15C–1.6Mn steel

Abstract The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac1 temperature during reheating. Isothermal transformations in the 450–660 °C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partitioning of Mn. The kinetics of bainite formation was not affected by the presence of soluble B. Upper bainite was found to be acicular ferrite in the CMnB steel as a result of the heterogeneous nucleation of ferrite on TiN precipitates.

Impact Toughness of Spring Steel after Bainite and Martensite Transformation

It has been reported that a multiphase microstructure with bainite, martensite, and retained austenite obtained by austempering, or quenching and tempering of spring steel containing Si, Mn, and Cr exhibits high strength and ductility. However, little research has been conducted on the bainite formation and impact fracture behavior of next-generation spring steel from the perspective of engineering and industrial applications. The microstructural transformation characteristics of bainite and martensite related to the heat treatment cycle on the maker side were quantitatively analyzed using electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) analyses. Moreover, the effects and mechanical properties of bainite and martensite formation in response to changes in lath length and width were studied and analyzed. That is, to obtain the mechanical properties of spring steel with the highest quality, tensile and impact specimens, whose microstructure and notch shape change according to the heat-treatment cycle, were prepared and studied.

The effects of die temperature and cooling condition on friction behavior of bare 22MnB5 boron steel in hot stamping

ABSTRACT Hot stamping is an effective method to improve both the elongation and plasticity of ultra-high strength steels, thus a higher specific strength could be achieved, making it possible for weight reduction in motor vehicles. However, due to the die temperature rise and the cooling liquid within hot stamping applications, the friction in its forming stage is always a non-isothermal mode. Therefore, a high-temperature sliding-on-sheet-strip (SOSS) tribo-tester to simulate sliding friction in hot stamping of 22MnB5 hot stamping steel sheet was designed and custom-made. Then, it was employed to exploit High-temperature friction behavior and mechanism of bare 22MnB5 hot-stamping boron steel sheet under different die temperature and extra cooling conditions. Results show that the friction coefficient obtained under the pure die cooling conditions is lower than that obtained under extra cooling conditions, which is attributed to the formation of thicker oxide layers on the surface of specimen during the cooling process. In addition, the friction coefficient is obviously decreased when the die temperature increased to 200 °C, which is ascribed to the co-action of the destruction of the oxide layers and the formation of bainite.