Bainite Formation Research Articles

Effects of prior austenite grain size and phase transformation temperature on bainitic ferrite formation in multi-constituent microstructures of a strong ultra-low-carbon steel

Bainitic ferrite is a key microstructural constituent in High-Strength Low-Alloy (HSLA) steels, influenced by the prior austenite grain size and heat treatment conditions. This study investigates the decomposition of coarse- and fine-grained austenite under rapid step quenching heat treatments around the martensite-start (MS) temperature in an ultra-low-carbon HSLA steel. An isothermal-to-nonisothermal transition occurred in the austenite phase transformation behavior after the grain size refinement from ~69 μm to ~10 μm resulting in the formation of a complex combination of various ferritic products consisting of quasi-polygonal ferrite, granular bainite, bainitic ferrite, and martensite in the microstructure of the below-MS-treated samples. Fine bainitic ferrite laths with an average thickness of ~150 nm formed, which were about 70% thinner than those of the above-MS-treated samples, contributing in part to an ultimate tensile strength of 791 MPa along with a uniform elongation of 6.4% for the below-MS-treated samples. The results can give insight into the methodologies for transferring the concept of nanoscale bainite to ultra-low-carbon steels.

Microstructure evolution and acicular ferrite nucleation in inclusion-engineered steel with modified MgO@C nanoparticle addition

To investigate the effect of surface-modified nanoparticles (NPs) on the inclusion refinement and microstructure evolution, deoxidized experiment ingots with different amounts of modified NPs were manufactured under different cooling conditions. Laser scanning confocal microscope (LSCM) was hereby used for in-situ observation of the phase transition and microstructural evolution during heat cycle process. The results revealed that the inclusion size was always smaller under water quenching than under air cooling, and the number of inclusions was greater under water quenching. After NP addition, the nucleant inclusions were identified as MgAl2O4 spinel and irregular TiN inclusion from SEM-EDS measurement and equilibrium calculations using Factsage thermodynamic software. The higher cooling rate under water quenching resulted in less polygonal ferrite decrease and the formation of bainite in the steel. The LSCM experiments showed that ferrite side plates (FSP) always formed on the boundary prior to the formation of acicular ferrite (AF) on the intragranular inclusions, and the start transformation temperatures of FSP and AF phases both lowered after NP addition. The higher cooling rate and NP addition contributed to AF formation and increased the degree of interlocking of the AF phase. Finally, the relationship between the characteristics of inclusions and the kinetics of AF was investigated.

Bainite precipitation in a Cu–Al–Mn-Gd shape memory alloy

In this study, the aging effects and bainite formation kinetics in the Cu71.2Al18.6Mn10.2 and Cu69.7Al18.9Mn10.3Gd1.10 shape memory alloys are evaluated using different experimental techniques. The results show that the Cu71.2Al18.6Mn10.2 and Cu69.7Al18.9Mn10.3Gd1.10 alloys exhibit aging hardening during bainite precipitation, reaching hardness values at about 370 HV. It is verified that the ferromagnetic β3 phase is produced during the bainite precipitation on the aging. The kinetic process is evaluated, and for the Cu69.7Al18.9Mn10.3Gd1.10 alloy, the activation energy value is ~42 kJ.mol−1 while the alloy without Gd presents an activation energy value at about 62 kJ mol−1. Gadolinium modifies the reaction mechanism analyzed due to their interaction with β1 and α phases, and the phase decomposition with Gd. In the studied alloys the bainite formation shows to be strongly related to the diffusive processes.

Microstructure of the Heat Treated Advanced Low Carbon Steel

The microstructure of the advanced low carbon steel with a superior hardenability was studied. The steel contained the following main alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The dilatometer investigation of the steel under consideration revealed the only phase transformation occurring during continuous cooling (0.1...30 °C/s), which started at the martensite start temperature Ms. It was shown that the isothermal treatment of the studied steel led to the bainite formation above and below Ms. The temperature of the bainite morphology shift was determined.

Microstructural Characterizations and Kinetics Model of Isothermal Bainite Formation with the Presence of Prior Martensite in High‐Carbon Nanostructured Bainitic Steel

Quenching and subsequent isothermal bainite transformation (QBT) and direct isothermal transformation (DIT) procedures are performed to comparatively evaluate the impact of the introduced prior martensite on bainite transformation behaviors and microstructural characterizations in a high‐carbon nanostructured bainitic steel. A thermodynamic model under a QBT process based on displacive theory is also developed to expound the bainite formation kinetics. The results indicate that the modified kinetics model, by considering austenite/martensite interface nucleation, austenite grain boundary nucleation, and autocatalytic nucleation, evolved here can be used to accurately describe the bainite formation behavior in the presence of prior martensite. A certain fraction of prior austenite/martensite interface can serve as the preferential nucleation site for the bainite transformation and assist in an acceleration of transformation rate. The bainite nucleation event initially takes place at prior martensite plate boundary in the QBT process instead of austenite grain boundary in the DIT process and subsequently continues through autocatalytic nucleation at austenite/bainite interface generated at either austenite/martensite interface or austenite/austenite interface. The introduced prior martensite slightly refines bainite sheaves and lessens the blocky retained austenite size, presenting a feasible approach to shorten the required time for producing a nanosized bainitic microstructure.

Effect of introduced vanadium carbide at the bay region on bainite transformation, microstructure and mechanical properties of high-carbon and high-silicon steel

Obtaining a fine bainitic ferrite for high carbon steel at low temperature requires a long period. Thus, creating strategies to accelerate bainite transformation is important. This investigation aims to explore the introduction of vanadium carbide at the bay region on bainitic transformation kinetics and mechanical properties by comparing isothermal bainitic transformation treatment directly and in combination with bay treatment prior to bainite transformation. Results show that introduced fine VC precipitation during bay treatment notably accelerates the initial bainite transformation with the incubation period being reduced by 91%. Decrease in activation energy for bainite nucleation at vanadium carbide/austenite interfaces and generation of preferred nucleation sites lead to a favorable acceleration of bainitic formation kinetics. However, indirect refinement of grain size for intragranular bainitic ferrite nucleation and consumption of available vanadium carbide/austenite interfaces result in overall rate of the subsequent bainite transformation. For the sample with prior bay treatment, the lath directions of bainitic ferrite are more complex and the size of block RA is smaller than that of the conventional sample. Moreover, the introduction of vanadium carbide at the bay region can simultaneously enhance the tensile strength and impact toughness of the steel.

Microstructural changes by controlling austenitizing and tempering conditions on the J-R fracture resistance of SA508 Gr. 1A low alloy steels

The microstructure, tensile properties, and J-R fracture resistance of SA508 Gr.1A low alloy steels were evaluated for different heat-treatment conditions, and the relationship between them was analyzed to investigate how the resulting microstructures affected ductile crack propagation behavior. Heat-treatment was carried out by varying the austenitizing temperature and tempering time, which changed the sizes of grains and precipitates. The SA508 Gr.1A microstructure was determined to be composed of ferrite and tempered bainite. The fraction of tempered bainite and grain size increased as austenitizing temperature increased. The overall microstructure was not changed by varying tempering time, but precipitate coarseness increased as tempering increased. When the austenitizing temperature was increased, yield strength increased slightly due to the formation of bainite. Even though its grain size increased, J-R fracture resistance decreased. When tempering time was increased, strength decreased, and J-R fracture resistance increased despite the formation of coarse carbides. The grain boundaries act as obstacles to crack propagation, and therefore, the number of grains that form in the plastic zone at the crack tip is closely related to the crack propagation resistance. The J-R fracture resistance increases as the number of effective grains in the plastic zone increases, because the effective grain boundaries act as strain energy absorption sites against plastic deformation. They exhibited a linear relationship regardless of the formation of coarse carbides.

Revealing the Dynamic Transformation of Austenite to Bainite during Uniaxial Warm Compression through In-Situ Synchrotron X-ray Diffraction

In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing.

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s−1) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

Effect of cooling rate on mechanical properties of SA508 Gr.1A steels for main steam line piping in nuclear power plants

To improve mechanical properties of SA508 Gr1.A steels for application of leak-before-break (LBB) design in main steam line piping in nuclear power plants, the effect of cooling rate on microstructure formation, tensile properties, Charpy impact properties, and J-R fracture toughness of SA508 Gr.1A steels was analyzed. As cooling rate increased, coarse ferrite/pearlite microstructure changed to fine bainite and martensite, and coarse grain boundary carbides disappeared and fine carbides precipitated inside grains. When the cooling rate is faster, both strength and elongation at 286 °C improved owing to the formation of fine low-temperature-transformation phases. The increase in cooling rate also have positive effect to the ductile-brittle transition temperature (DBTT) and J-R fracture toughness at 286 °C. From the fractograph analysis of J-R tested specimens, it is revealed that large voids formed at the coarse grain boundary carbides and crack propagated along the ferrite/pearlite interface. The increase in the cooling rate suppressed the formation of coarse carbides and ferrite/pearlite and promoted the formation of bainite and martensite having fine effective grain size, which improved the strength and toughness of the SA508 Gr.1A steels.

Effect of quenching media on microstructural evolution, mechanical and wear properties of AISI4135 steel

The aim of this work is to study the effect of various quenching media on the microstructural evolution and properties enhancement of AISI 4135 alloy steel. The formation of dual microstructures and their effect on mechanical and wear properties are investigated in this work. An attempt is made to correlate the microstructure-properties relation based on the quenching method used. Steel is heated above the austenitic temperatures (A3) and subsequently cooled down through various quenching media to obtain the variations in microstructures and related properties. The heat treated samples were investigated for microstructural evolution through optical microscopy and scanning electron microscope; mechanical characterization through microhardness study, tensile testing and impact testing; and wear characterization through pin on disc tribometer. It was observed that increased cooling rates increased the volume fraction of martensitic structures whereas oil quenching enhanced fine pearlite and bainite formation. The air cooling led to the formation of fine pearlite along with ferrite structures. Strength, hardness and wear resistance is favored with martensite formation but toughness decreases.

The Use of Acoustic Emission and Neural Network in the Study of Phase Transformation below MS.

Acoustic emission and dilatometry were applied to investigate the characteristics of phase transformations in bearing steel 100CrMnSi6-4 during austempering below the martensite start temperature (MS 175 °C) at 150 °C. The aim of this study is to characterize the product of transformation occurring below the MS temperature using various research methods. Analysis of the dilatometric curves shows that, after the formation of athermal martensite below the MS temperature, the austenite continues to undergo isothermal transformation, indicating the formation of bainite. Additionally, tests were carried out with the use of acoustic emission during isothermal hardening of the adopted steel. The obtained acoustic emission signals were analyzed using an artificial neural network. The results, in the form of a graph of the frequency of acoustic emission (AE) event occurrence as a function of time, make it possible to infer about the bainite isothermal transformation. The results of this research may be used in the future to design optimal heat treatment methods and, consequently, may enable desired microstructure shaping.

Influence of Chromium and Niobium on the Press-Hardening Process of Multiphase Low-Alloy TRIP Steels

Press-hardening is an intensively developing forming technology which is mainly used for the production of car body parts. Because it is a hot forming technology, small forming forces can be utilized and, due to the lower spring-back effect, more accurate products are achieved. In car bodies, materials with high energy absorption and a sufficient hardening coefficient are mainly used in impacted parts. One of these materials is TRIP multiphase steels with different chemical composition. In these steels, it is possible to achieve an ultimate strength up to 1000 MPa with the ductility of 20-30%. In order to achieve the desired properties, it is necessary to select a suitable heat treatment that allows to achieve a multiphase structure. Phase transformations and mechanical properties are influenced by the use of suitable alloying elements. Three low-alloy, multiphase TRIP steels with different chemical compositions with a carbon content of 0.2% were chosen for the experimental program. The first steel was alloyed only with manganese and silicon, in the second niobium was added, and in the third the influence of chromium on increase of hardenability and strength was investigated. Press-hardening was performed in a heated forming tool. To describe the effect of the cooling rate, the forming was carried out in a tool at room temperature and after preheating to 425°C. The influence of holding time in the tool at 425°C to support the formation of bainite and retained austenite stabilization was also investigated. Mixed ferritic-bainitic-martensitic structures with some retained austenite content were obtained.

Effect of low-temperature isothermal holding on microstructure and mechanical properties of hot rolled high carbon Nb microalloyed steel plate

Two high carbon steels, one plain carbon (HC0) and the other, Nb microalloyed (HC1) are selected in the present research. Both experimental steels were initially hot rolled after complete homogenisation and then air cooled to room temperature. These hot rolled plates were then subjected to low temperature isothermal holding followed by air-cooling to room temperature. Isothermal holding above the martensite start temperature (Ms) results in the formation of bainite with some amount of high carbon martensite whereas, isothermal holding below Ms leads to the formation of predominantly martensite. The increase in isothermal holding temperature from 200°C to 300°C results in lowering the hardness, estimated yield strength and ultimate tensile strength values marginally for HC0 whereas, it decreases the hardness, estimated yield strength and ultimate tensile strength values significantly for HC1 steel which may be attributed to the expected formation of some amount of coarse niobium carbide (NbC).

Formation of Nodular Bainite in a Fe-9.10ni-0.06c (Wt. %) Alloy: A New Microstructure for Cryogenic Steels

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500 °C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships between ferrite and cementite were observed. Also, the microhardness of nodular bainite in the sample transformed at 500 °C (214 ± 4 VHN) was higher than that in the sample transformed at 550 °C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.

The formation of a mixed martensitic/bainitic microstructure and the retainment of austenite in a medium-carbon steel during ultra-fast heating

The current work focuses on the microstructure evolution of a medium-carbon steel during an ultra-fast heating cycle. Electron microscopy methods such as Scanning Electron Microscopy, Electron BackScatter Diffraction and Transmission Electron Microscopy are utilized for the characterization of microstructural constituents of the studied steel samples. The applied heating rate during this treatment was 100 °C/s and the heating temperature was 930 °C, slightly above the Ac3 temperature, and subsequent quenching in water. It is found that the microstructure consists of martensite (60 %), lower bainite (37 %) and retained austenite (3%). The formation of bainite during this treatment is attributed to the heterogeneity of the chemical composition in the microstructure due to the lack of time for carbon to diffuse from cementite in long distances. Regions that are located away from the former pearlitic colonies are not enriched in carbon and manganese along the austenitization process and, according to the Controlled Cooling Transformation diagram, will form bainite upon cooling. The retainment of austenite in films is observed between the bainitic plates and on the martensitic lath boundaries, where carbon and manganese are segregated. Such microstructures are utilized by the current 3rd Generation Advanced High Strength Steels for the optimization of the mechanical properties.

Study of phase transformations in complex phase steel using a mesoscale cellular automaton model Part I: Modeling Fundamentals

A two-dimensional mesoscale model based on the concept of hybrid cellular automata is developed to study phase transformations in a complex phase steel during continuous cooling. The model is capable of simulating microstructure evolution with carbon diffusion in the volume and along grain boundaries, γ/α interfaces migration into austenite, as well as formation of bainite and martensite islands during intensive cooling in lower temperatures. In contrast to the classic statistical approaches which are based on the assumption of modeling one point in the material with homogeneous microstructure, the proposed phase transformations’ model in the mesoscale accounts for material heterogeneity. The simulation results in the form of a digital material representation with microstructures and maps showing the carbon concentration field as well as microhardness distribution are presented. One of the main advantages of the model is that has only seven adjustment coefficients that are used in the fitting process.

Statistical Modeling for Prediction of CCT Diagrams of Steels Involving Interaction of Alloying Elements

The interaction of different alloying elements has a significant impact on the mechanical and microstructural properties of steel products due to the thermodynamic and kinetic effect. This article presents a statistically developed and validated model for austenite decomposition during cooling, based on a set of experimental continuous cooling transformation diagrams available in literature. In the model, two-way interactions of the alloying elements are included as add-on terms, and the procedure for the analysis ensures there is no overfitting. The model can be used to predict phase transformation temperatures and critical cooling rates for the formation of polygonal ferrite, bainite or martensite for the production of steel.

Study of the kinetics of austenite grain growth by dynamic Ti-rich and Nb-rich carbonitride dissolution in HSLA steel: In-situ observation and modeling

The effect of carbonitrides on the growth of austenite grains in high-strength low-alloy (HSLA) steel was investigated through microstructure analysis and high-resolution dilatometry. The austenite grain growth at various temperatures was measured in situ using high temperature laser scanning confocal microscopy. Two kinds of precipitates with different morphologies containing large amounts of Ti and Nb were observed in the austenitized samples at different temperatures. The smaller Nb-rich spherical carbonitrides dissolved at the higher temperature, while the lager Ti-rich rectangular carbonitrides have a higher thermal stability and can pin at the austenite grain boundary. This study reveals that the dissolution of Nb in austenite causes the rapid growth of austenite grains and slows down the ferrite phase transition, which is beneficial to the formation of bainite. A kinetic model of austenite growth considering the dynamic dissolution of different types of carbonitrides is proposed. The results of the model show the same overall trend as the experimental results, and the individual differences can be explained. The proposed model can be used to predict the austenite grain size during the austenitizing process of HSLA steel and can reflect the austenite grain change throughout the process more continuously.

Effect of Annealing Process on Microstructure, Texture, and Mechanical Properties of a Fe-Si-Cr-Mo-C Deep Drawing Dual-Phase Steel

Dual phase steel generally has poor deep drawing property with a low r value less than 1.0, making it difficult to be used for deep drawing automotive parts. In order to improve the mechanical properties of the steel through heat treatment, effect of heat treatments with different conditions on a Fe-Si-Cr-Mo-C deep drawing dual-phase steel was investigated with the aim of identifying effective heat treatment parameters for effective modification towards optimal properties. Relevant thermal dilation and heat treatment experiments were performed. Corresponding characters were investigated. The results show that island martensite can be obtained at low cooling rate. With the increase of cooling rate, the formation of pearlite and bainite is favored. During annealing at low temperatures, recrystallization of the steel is incomplete with the presence of the shear bands. With the increase of annealing temperature, the recrystallization process is gradually complete, and the number of high angle grain boundaries increases significantly. The ratio of gamma orientation components to alpha orientation components decreases first and then increases with the increase of annealing temperature. The strain hardening exponent and r value show an upward trend with respect to annealing temperature, and the r value is as high as 1.15.