Bainitic Transformation Temperature Research Articles

The tale of upper and lower bainite: A computational analysis of concurrent C-diffusion and precipitation

Carbon partitioning in bainite is a central mechanism controlling this transformation. Depending on whether C atoms form precipitates in the ferrite platelets or remain dissolved in a solid solution, the resulting microstructure is denoted as either lower or upper bainite. In the present analysis, a simulation study is conducted with long-range diffusion of C and simultaneous precipitation of carbides. The computational analysis suggests that carbides always form within the ferritic subunits before carbon redistribution into the surrounding austenite commences, independent of temperature. Since these carbides can rapidly dissolve again at higher bainite transformation temperatures, they are commonly not experimentally observed in upper bainite. At lower temperatures, carbide re-dissolution takes significantly more time, and carbides are, therefore, commonly identified, with the corresponding microstructure denoted as lower-bainite. Since carbides shall always form in the ferrite platelets, a classification of upper and lower bainite based on the existence of carbides should be reconsidered.

Ultrastrong and ductile steel welds achieved by fine interlocking microstructures with film-like retained austenite

The degradation of mechanical properties caused by grain coarsening or the formation of brittle phases during welding reduces the longevity of products. Here, we report advances in the weld quality of ultra-high strength steels by utilizing Nb and Cr instead of Ni. Sole addition of Cr, as an alternative to Ni, has limitations in developing fine weld microstructure, while it is revealed that the coupling effects of Nb and Cr additions make a finer interlocking weld microstructures with a higher fraction of retained austenite due to the decrease in austenite to acicular ferrite and bainite transformation temperature and carbon activity. As a result, an alloying design with Nb and Cr creates ultrastrong and ductile steel welds with enhanced tensile properties, impact toughness, and fatigue strength, at 45% lower material costs and lower environmental impact by removing Ni.

Microstructure and mechanical properties of 0.2C-3.9Al-1.12Mn-0.3Mo δ-TRIP steel as a function of isothermal bainitic transformation temperature

The present investigation employed a two-stage heat treatment on a δ-transformation-induced plasticity (TRIP) steel comprising 0.2C–4Al–1.2Mn–0.3Mo (wt%). The mechanical and microstructural characteristics that result from varied isothermal bainitic transformation (IBT) temperatures following inter-critical annealing at 820℃ for 10 min are thoroughly analyzed. The microstructure of the steels consisted of δ-ferrite, α-ferrite, bainitic ferrite, retained austenite (RA), and martensite, resulting in an optimum combination of the ultimate tensile strength (UTS) and total elongation (TE). The results of the investigation showed that IBT temperature had an effect on the stability of RA and the martensitic transition. Due to the increased mechanical stability of RA, the study revealed that the TRIP phenomenon was more prominent at lower IBT temperatures. Both tensile and yield strengths, as well as elongation, decreased as a consequence of the increase in IBT temperature. Maximum values of UTS, TE, and the product of these two properties (PSE) are attained (860 MPa, 41%, and 35260 MPa∙%, respectively) under optimal processing conditions (at 350℃ IBT temperature in 10 min).

Enhanced stretch flangeability and crack propagation behavior of an 1100 MPa grade TRIP-aided bainitic ferrite steel

In order to face the challenges of lightweight, safety, and emission reduction in the automotive industry, transformation-induced plasticity aided bainitic ferrite (TBF) steel was fabricated by controlling the size and shape of bainitic ferrite and compared with quenching and partitioning (Q&P) steels. The effects of bainite transformation on the mechanical properties, microvoid formation, and crack propagation were analyzed by dilatometry measurement and microstructure characterization. At lower isothermal bainite transformation (IBT) temperature, the lathy bainitic matrix can prevent crack formation and limit the crack propagation inside its laths, in favor of improving the strength and stretch flangeability simultaneously. In addition, granular bainite, coarse martensite, and austenite (M-A) islands would increase the risk of void formation at higher IBT temperatures. These voids provided the initial sites for crack formation, which was also responsible for the reduction of the hole expansion ratio (HER). In conclusion, the outstanding properties were achieved in the T-375 samples with the HER of 33.5%, the total elongation of 16.9%, the tensile strength of 1191 MPa, and the yield strength of 934 MPa.

Structure-property relationship in vanadium micro-alloyed TRIP steel subjected to the isothermal bainite transformation process

Structure-property relationship in vanadium micro-alloyed TRIP steel is investigated focusing on austenite transformation, precipitation behavior and mechanical performance. Microstructural evolution of the steel during processing was characterized mainly using EBSD, TEM and APT techniques; and its influence on mechanical properties was discussed. The TRIP steel exhibits complex microstructure including intercritical ferrite (IF), bainitic ferrite (BF), retained austenite (RA) and martensite. After identical intercritical annealing (IA), IF fraction keeps constant, BF and RA fractions increase while martensite fraction decreases with decreasing isothermal bainite transformation (IBT) temperature from 470 to 380 °C. The initially precipitated vanadium carbides (VC) in the hot-rolled specimen undergo coarsening during IA process, accompanied with some newly precipitated VC in either ferrite or in austenite, confirmed by the co-existence of small-sized and coarsened VC after IA process, as well as occurrence of K–S orientation relationship between VC and the α-Fe matrix. Ultra-fine average grain size of less than 1 μm in the TRIP heat-treated steel is achieved by the refining effect of these nano-scale VC precipitates, which can inhibit phase boundary (PB) migration during IA process and promote bainite transformation during subsequent IBT process. The higher volume fraction of RA accompanied with higher carbon enrichment intensifies sustainable strain hardening and finally increased ductility of the specimens treated under lower IBT temperature. In the IBT-380 specimen with the highest RA and BF fractions, the occurrence of micro-yielding phenomenon is a result of the Cottrell atmosphere in soft domain BF and stress relaxation due to RA to martensite transformation, during which the soft domain IF/RA/BF and hard domain martensite yields step by step. An excellent mechanical performance is thus achieved in the IBT-380 specimen, meeting the requirements for the 3rd generation advanced auto-steel.

Controlling the Thermal Stability of a Bainitic Structure by Alloy Design and Isothermal Heat Treatment.

The aim of this work was to develop a novel bainitic steel that will be specifically dedicated to achieving a high degree of refinement (nano- or submicron scale) along with increased thermal stability of the structure at elevated temperatures. The material was characterized by improved in-use properties, expressed as the thermal stability of the structure, compared to nanocrystalline bainitic steels with a limited fraction of carbide precipitations. Assumed criteria for the expected low martensite start temperature, bainitic hardenability level, and thermal stability are specified. The steel design process and complete characteristics of the novel steel including continuous cooling transformation and time-temperature-transformation diagrams based on dilatometry are presented. Moreover, the influence of bainite transformation temperature on the degree of structure refinement and dimensions of austenite blocks was also determined. It was assessed whether, in medium-carbon steels, it is possible to achieve a nanoscale bainitic structure. Finally, the effectiveness of the applied strategy for enhancing thermal stability at elevated temperatures was analyzed.

Investigate on ultrahigh-strength TRIP-aided autosteel plate under the framework of ICME

A novel low-carbon (C) low-alloy transformation-induced plasticity (TRIP) steel with a nominal composition of Fe-0.3C-1.9Mn-1.0Si-0.7Al wt.% was successfully designed by combining high-throughput thermodynamic and kinetic calculations and feature engineering based on the Integrated Computing Materials Engineering (ICME) framework. The designed TRIP steel was subjected to a series of two-stage heat treatments according to the target intercritical annealing temperature determined by estimation in an attempt to optimize phase constituents to obtain an ideal strength-plasticity balance. After annealing at 780 °C, the ultimate tensile strength (UTS), total elongation (TEL) and the product of strength and elongation (PSE) of the experimental steel can reach ∼980 MPa, ∼30% and ∼30 GPa•% respectively, because of the appropriate phase fraction and size of retained austenite (RA). Annealing at 820 °C, when the isothermal bainitic transformation (IBT) temperature is low, the short distance diffusion of C made it enriched in a small region, the stability of different RA grains varied greatly, which was not conducive to ensure stable and continuous TRIP effect. Moreover, by evaluation of the effect multiphase on yield strength (YS) and UTS, the presence of more bainite led to samples having higher tensile strength but deteriorated their plasticity.

Strengthening behaviour and failure analysis of hot-rolled Nb+V microalloyed steel processed at various coiling temperatures

The present work investigated the role of thermomechanical processing and coiling temperature on various strengthening mechanisms in a low carbon Nb + V microalloyed steel and its mechanical properties. The sample was subjected to 50 % thickness reduction by hot-rolling at 1100 °C and then air cooling directly to a coiling temperature maintained in the range of 650–500 °C. At the coiling temperature, an isothermal holding period was maintained for 1 h followed by water quenching to room temperature to preserve the sample microstructure. The microstructural characterization reveals a duplex/triplex phase mixture depending on the coiling temperature affecting its mechanical properties. For the coiling temperature range of 650–550 °C, the pro-eutectoid ferrite + pearlite phase mixture is evolved resulting an adequate ductility of ∼28 % with a tensile strength of ∼650 MPa. The strength-ductility combination was not altered much at the coiling temperature of 500 °C (i.e., near the bainitic transformation temperature domain), though a trace quantity (∼5 %) of Widmanstatten ferrite (WF) is nucleated along with the pro-eutectoid ferrite + pearlite microstructure. The isothermal holding further led to the nucleation and growth of niobium and vanadium-rich fine carbides with fcc crystal structure, which are mostly incoherent with the ferrite matrix phase. Though these precipitates were nucleated within the matrix grains due to induction of strain, the volume fraction was too low (∼1 %) to offer a substantial amount of precipitation strengthening (26–35 MPa) as compared to the grain refinement strengthening (126–156 MPa) and dislocation strengthening (115–150 MPa). The SEM analysis indicated that the segregated Mn and S atoms formed MnS inclusion at the phase interface of ferrite and pearlite, which operates as potential nucleation sites for crack initiation during tensile deformation.

Nanomechanics of Retained Austenite in Medium-Carbon Low-Temperature Bainitic Steel: A Critical Analysis of a One-Step versus a Two-Step Treatment.

A two-step bainitic treatment with a final isothermal temperature below MS was adopted to obtain bainitic steel with abundant retained austenite (RA). Nanoindentation testing was used to investigate the stability of RA in bainite steel and clarify the effect of RA on the deformation of medium-carbon steel. The results showed that, in contrast to the traditional one-step approach, a greater amount of nanoscale RA film was obtained using the two-step treatment. This was due to a lower final bainitic transformation temperature, which induced a higher carbon concentration in the untransformed austenite in the stasis stage; this resulted in untransformed austenite with a higher carbon content existing as RA rather than forming martensite in the subsequent cooling process. In addition, it was determined that the increased stability of RA during the two-step transformation delayed the pop-in point.

Influence of Austenitisation Temperature and Time on Martensitic and Isothermal Bainite Phase Transformation of Spring Steel

The influence of austenitisation temperature and time on the martensitic and isothermal bainite transformation of 51CrV4 spring steel was analysed. Based on the analysis of dilatometric curves, the martensite start temperatures (MS) were determined at different austenitisation temperatures (800–960 °C) and austenitisation times (5–30 min). At a temperature of 800 °C, a partial austenitic transformation occurred, and undissolved chromium carbides were present in the matrix. At higher temperatures, the austenitic transformation was complete, and the temperature MS increased with the austenitisation temperature. The temperature of the isothermal phase transformation has a stronger effect on the bainite transformation, which has different effects on the stability of the austenite and the diffusion processes. The microstructure of isothermal bainite transformation samples at 330, 430 and 520 °C was characterised by optical microscopy and dilatometric curves. Lower bainite was formed at a bainitic transformation temperature of 330 °C, and a combination of upper and lower bainite was characterised at a transformation temperature of 430 °C. In the samples transformed at 520 °C, a smaller proportion of lower bainite formed in addition to the upper bainite and martensite. Some allotriomorphic ferrite formed along the boundaries of the austenitic grains.

Effects of Mo and Nb on Microstructure and Charpy Impact Properties of CGHAZ in HSLA Steels

In this study, the effects of Mo and Nb on the microstructure and Charpy impact properties of coarse-grain heat affected zone (CGHAZ) in high-strength low-alloy (HSLA) steels were investigated. The high-Mo steel had a lower volume fraction of acicular ferrite (AF) and higher volume fraction of granular bainite (GB) than those of the low Mo steel. The high-Mo CGHAZ specimen had a lower volume fraction of AF and higher volume fraction of GB compared to the low-Mo CGHAZ specimen. This is because, an increase in Mo lowers the bainite transformation temperature, induces the formation of GB and bainitic ferrite (BF), and reduces the diffusion velocity of carbon, thereby inhibiting the formation of ferrite. The Nb-added steel had a volume fraction similar to that of AF but smaller packet sizes of GB compared to the non-Nb steel. Compared to the non-Nb CGHAZ specimens, the Nb-added CGHAZ specimen had a lower volume fraction of AF, a higher volume fraction of GB, smaller packet sizes of GB and BF, and a smaller prior austenite grain size. This is because, Nb forms Nb(C,N) to inhibit the grain growth, thus inhibiting the formation of GB and BF. The Nb-added low-Mo CGHAZ specimen with the highest AF fraction and smallest average grain size had the highest Charpy-absorbed energy. However, articles in the Journal of Welding and Joining reported that the other CGHAZ specimens with high volume fractions of GB and BF with coarse packets showed low Charpy-absorbed energy.

Morphology and crystallography of microstructures in Mg-deoxidized offshore engineering steels after simulated welding thermal cycles

ABSTRACT The morphology and crystallography of microstructures in heat-affected zone of offshore engineering steels with 0.0002 and 0.0042 wt% Mg were studied. With increasing Mg content from 0.0002 to 0.0042 wt%, the prior austenite grain (PAG) size increases from 92 to 163 μm, which decreases the PAG boundary density and increases the bainite transformation temperature. So the bainite nucleation sites and driving force reduce, affecting bainites with morphology and crystallography. For morphology, the sympathetic nucleation reduces and widening growth rate of bainitic laths increases, coarsening microstructures. For crystallography, the variant selection is intensified, coarsening the unevenly distributed Bain zones with specific variants and reducing the high-angle grain boundary (HAGB) density from 1.4 to 0.6 μm–1. The coarsened microstructure with lower crack resistance increases the ductile-brittle transition temperature, contributing to the ductile fracture transforming to cleavage fracture and the HAZ toughness at –40 °C decreasing from 201 to 58 J.

The Impact of Retained Austenite on the Mechanical Properties of Bainitic and Dual Phase Steels

This paper presents the microstructural changes and mechanical properties of carbide-free bainitic steel subjected to various heat treatment processes and compares these results with similarly treated ferritic–pearlitic steel. A key feature of the investigated steel, which is common among others described in the literature, is that the Si content in the developed steel was >1 wt.% to avoid carbide precipitation in the retained austenite during the bainitic transformation. The phase identification before and after various heat treatment conditions was carried out based on microstructural observations and x-ray diffraction. Hardness measurements and tensile tests were conducted to determine the mechanical properties of the investigated materials. In addition, following the tensile tests, the fracture surfaces of both types of steels were analyzed. Changing the bainitic transformation temperature generated distinct volume fractions of retained austenite and different values of mechanical strength properties. The mechanical properties of the examined steels were strongly influenced by the volume fractions and morphological features of the microstructural constituents. It is worth noting that the bainitic steel was characterized by a high ultimate tensile strength (1250 MPa) combined with a total elongation of 18% after austenitizing and continuous cooling. The chemical composition of the bainitic steel was designed to obtain the optimal microstructure and mechanical properties after hot deformation followed by natural cooling in still air. Extensive tests using isothermal transformation to bainite were conducted to understand the relationships between transformation temperature and the resulting microstructures, mechanical properties, and fracture characteristics. The isothermal transformation tests indicated that the optimal relationship between the sample strength and total elongation was obtained after bainitic treatment at 400 °C. However, it should be noted that the mechanical properties and total elongation of the bainitic steel after continuous cooling differed little from the condition after isothermal transformation at 400 °C.

Relationship between the Microstructure and Impact Toughness of the Coarse‐Grained Heat‐Affected Zone for Offshore Engineering Steels with Different Mg Contents

The relationship between the microstructure and impact toughness of coarse‐grained heat affected zone for offshore engineering steels with 0.0002 and 0.0042 wt% Mg, termed as 2 and 42Mg, respectively, is clarified. With increasing the Mg content from 0.0002 to 0.0042 wt%, the average impact energy at −40 °C decreases from 201 to 59 J. The prior austenite grain size (PAGS) increases from ≈92 to ≈163 μm, the average thickness of lath bainites increases from ≈234 to ≈361 nm. The addition of Mg makes the particle increase in number and refine in size, but the amount of the effective pinning particles at the temperature close to 1350 °C in 42Mg is smaller than that in 2Mg, which leads to the larger austenite grain growth rate and larger PAGS in 42Mg. The larger PAGS with the higher bainite transformation temperature and smaller PAG boundary density in 42Mg makes the bainite coarser. So, in 2Mg, the effective bainitic grain size with misorientation above 45° is smaller and the density of high‐angle grain boundaries (HAGBs) is larger, resulting in its higher toughness. Besides, the coalesced bainite containing cementite colonies in 42Mg makes the bimodal bainitic lath thicknesses and heterogeneous microhardness distribution, reducing the toughness.

Effect of austempering time on microstructure and properties of a low-carbon bainite steel

The effect of austempering time after the bainitic transformation on the microstructure and property in a low-carbon bainite steel was investigated by metallography and dilatometry. The results showed that by prolonging the austempering time after the bainite transformation, the amount of large-size martensite/austenite islands decreased, but no significant change of the amount and morphology of bainite were observed. In addition, more austenite with a high carbon content was retained by prolonging the holding time at the bainite transformation temperature. Moreover, with a longer holding time, the elongation was improved at the expense of a small decrease in tensile strength. Finally, the Avrami equation BRF = 1− exp(−0.0499 × t0.7616) for bainite reaction at 350°C was obtained for the tested steel. The work provided a reference for tailoring the properties of low-carbon steels.

Regulating tensile properties through bainitic transformation temperature in a hot-rolled δ-TRIP steel

This study aims to clarify effects of isothermal bainitic transformation (IBT) temperature on microstructure and mechanical properties of a high Al–low Si δ-TRIP steel. For this purpose, the alloy was austenitised at the intercritical annealing temperature of 820°C for 600 s and then was isothermally cooled down into a salt bath and held at the temperatures between 350 and 450°C for 360 s. X-ray diffraction indicates that by increasing the IBT temperature, the volume fraction of retained austenite increases. In addition, it was found that the optimum combination of strength and ductility, i.e. tensile strength of 1046 MPa and total elongation of 54%, was achieved in the sample which was isothermally held at 450°C for 360 s.

Influence of chemical composition and parameters of heat treatment on the mechanical properties and microstructure of TRIP steels

Good mechanical properties of steels, in which appropriate heat treatment can produce mixed hardening structures, make them candidates for a broad range of applications, namely in the automotive industry. TRIP steels (in which transformation-induced plasticity operates) with a carbon content of approximately 0.2 % are one of such steels. Heat treatment of these steels comprises two stages. The first involve heating to the intercritical region between the A1 and A3. It is followed by cooling to a bainitic transformation temperature and holding. The resulting mixed microstructure consist of ferrite, bainite and retained austenite. Thanks to the presence of ferrite and retained austenite, the ultimate strength and elongation can reach 1500 MPa and 25-40 %, respectively. The experiments presented in this paper were performed on two steels which chemistries were specially adjusted to support formation of TRIP microstructure. The main difference between them was the level of chromium. Intercritical annealing was carried out on both steels. Aspects of interest included mainly the effect of the cooling rate above the bainitic transformation temperature and the holding time on mechanical properties and final microstructure. The heat treatment led to microstructures ferrite, bainite and retained austenite. The strength was under 1100 MPa and elongation reached 28 %.

Resistance Increase to Hydrogen-Induced Sheet Cracking for Gas and Oil Pipes Based on the Structure Formation Control in the Central Segregation Zone

In sheets of low-alloy pipe steels with high purity by harmful impurities and non-metallic inclusions, the nucleation and propagation sites of hydrogen-induced cracking (HIC) are extended segregation bands of increased hardness in the axial zone, with sections consisting of high-carbon structures. The use of intensive cooling after controlled rolling from the austenitic region to the bainitic transformation temperatures and a decrease in the concentration of liquor carbon and manganese in steel helps to increase the sheet resistance to HIC due to reducing central segregation heterogeneity.

Investigation of microstructural evolution and bainite transformation kinetics of multi-phase steel

In this study, multi-phase steel with a mixture of ferrite, bainite, retained austenite and martensite is obtained after specific heat treatments, which include intercritical annealing, isothermal holding and final quenching. The microstructural constituents and individual phase fractions are precisely quantified based on multi-scale characterizations. The focus is on bainite transformation kinetics and its impact on the microstructural evolution, which is systematically analyzed according to the assumption of displacive modeling theory. Bainite transformation begins with austenite grain-boundary nucleation and continues through autocatalytic nucleation based on nucleation kinetics of bainitic sub-units. A small discrepancy between the modeling and experimental results is indicative of carbon partitioning into austenite, which reduces the driving force and/or the rate parameter κ for bainite transformation. In addition, the continuous carbon enrichment in austenite decelerates the bainite transformation and prolongs the duration time. Moreover, the maximum carbon concentration in austenite is limited by the T0 line. Austenite with a lower carbon content but higher volume fraction will not transform to bainite at higher isothermal bainite transformation temperatures, and such mechanically unstable austenite is more likely to transform to martensite during final quenching. As a result, with increasing isothermal holding temperature, the bainite and retained austenite fractions decrease while the martensite fraction increases in the sepcimens after final quenching.

The significance of variant pairing in governing toughness of coarse-grained heat affected zone (CGHAZ) in Nb-bearing high strength structural steels

We have addressed here a scientific gap of elucidating the significance of Nb on impact toughness of coarse-grained heat affected zone (CGHAZ) in a structural steel, from the perspective of crystallographic structure, particularly, digitization and visualization of variant pairing. At Nb content of 0.03 wt%, there was a dominance of V1/V8 pairing such that the density of low angle grain boundaries (LAGBs) was high. However, with the increase of Nb content to 0.06 wt%, the bainite transformation temperature was decreased in conjunction with the significant decrease in V1/V8 pair and increase in V1/V2 pair. This transition in variant pair from V1/V8 to V1/V2 with increase in Nb content was responsible for superior toughness in CGHAZ together with high density of high angle grain boundaries (HAGBs).