Bainitic Plates Research Articles

Effect of two-step ausforming on bainite transformation and retained austenite in a medium-carbon bainitic steel

It is commonly accepted that single ausforming with small strain at low temperature promotes isothermal bainite transformation and leads to more retained austenite. In the present study, the effects of two-step ausforming on the bainite transformation and retained austenite in a Fe-C-Mn-Si medium-carbon bainitic steel were investigated by thermo-mechanical simulator, x-ray diffraction, scanning electron microscope and transmission electron microscope. The results show that the two-step ausforming cannot further promote the bainite transformation compared with single ausforming. However, the two-step ausforming enhances the mechanical stability of residual austenite due to more dislocations induced by the second deformation and therefore leads to more retained austenite. In addition, when the total strain is constant, the fraction of retained austenite increases with the increasing second strain. Moreover, the thickness of bainite plates treated with two-step ausforming is larger compared with single-step ausforming when the total strain is the same.

Diffusion-Controlled Lengthening Rates of Bainitic Ferrite a Part of the Steel Genome

As a step in the further development of models and databases to support design of new steels, i.e., the “steel genome”, the growth of bainitic ferrite plates is accounted for by a thermodynamic and kinetic approach. The thermodynamic aspects are represented by CALPHAD databases and a Gibbs energy barrier for growth Bm. Experimental information on ferrite-plate growth rates for a number of Fe-C alloys, some of high-purity, are analyzed in terms of a modified Zener–Hillert model and the barrier as well as some kinetic parameters are evaluated. It is found that the barrier varies in a smooth way with carbon content and lengthening rate. In order to improve the agreement with the experimental information it was necessary to adjust the diffusion coefficient of carbon in austenite at low temperatures. It is concluded that the representation of the experimental data is satisfactory.

Visualizing the Fine Structure of Surface Bainite Layers on the Basis of EBSD Data

Electron backscatter diffraction (EBSD) is widely used to investigate the mutual crystallographic orientation in the structural components of steel, including bainite. Nevertheless, the fine structure of bainite (subplates and plates, packet components) is not investigated by that means. A method is proposed for visualization of the surface structure of bainitic structural steel by means of the Euler angles determined by EBSD. For the example of 25G2S2N2MA steel, a three-dimensional map of the bainite substructure is obtained. This result agrees with data obtained by scanning-probe microscopy.

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.

A Study of the Microstructure of Bainite in Steel 25G2S2N2MA by the Method of Atomic Force Microscopy

The methods of optical, electron, and atomic force microscopy are used to study the morphology of bainite formed in high-strength structural steel 25G2S2N2MA (HY-TUF) under isothermal quenching. It is shown that the temperature of the transformation affects the substructure of the bainite, which is represented by ordered layers of plates with a width obeying a lognormal distribution law. It is shown that the bainite plates consist of nanosize subplates, the size of which is determined by the temperature of the bainitic transformation.

An Artificial Neural Network Model to Predict the Bainite Plate Thickness of Nanostructured Bainitic Steels Using an Efficient Network-Learning Algorithm

Nanostructured bainitic steel has an extraordinary ultrahigh strength of about 2.0 GPa along with good toughness of 30 MPa m1/2. The finer thickness of plate 20-80 nm is largely responsible for achieving such a large hardness of 690-720 HV. In this work, a multilayer perceptron-based artificial neural network (ANN) model has been developed to predict the thickness of bainite plate pertaining to nanostructured bainitic steels. The inputs of the ANN model are, namely, Gibbs free energy for bainitic transformation, austenite strength, transformation temperature for bainite and carbon concentration in the steel. The model prediction revealed that the bainite plate thickness critically depends on austenite strength and Gibbs free energy. From the neural prediction, it is concluded that formation of nanostructured bainitic steel is feasible only if sufficient austenite strength (above 165 MPa) has been achieved. This can be accomplished with a minimum carbon content of 0.5 wt.% in steel and transformation temperature below 300 °C. Higher driving force (Gibbs free energy) below − 1800 J/mol is another prerequisite condition of formation of nanostructured bainite steel. The network-learning architecture has been optimized using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm to minimize the network training error within eight training cycles. The algorithm facilitates a faster convergence of network training and testing errors.

Microstructure and mechanical properties of nanobainitic steel subjected to multiple isothermal heat treatments

Nanostructured bainite in 62MnSiCr steel was prepared by two-stage transformation process at different temperatures for less than 2 h. Microstructures, phase distribution and mechanical properties of the obtained steel were investigated. The results showed that the thickness of bainite plate and the amount of retained austenite decreased obviously after the two-stage transformation, while the carbon concentration in the retained austenite showed a small change. With increase in the second holding temperature within the bainite transformation range, all of them increased slightly. The additional formation of bainite at the second transformation stage is beneficial to refining the austenite and further enriching it with carbon, resulting in the enhancement of the mechanical stability. Bainite transformed in two-stage process showed a better comprehensive performance. Absorbed impact energy of 88 J and an ultimate tensile strength of 1818 MPa have been achieved by isothermal heat treatment at 300 °C followed by 260 °C. Meanwhile, there was a slight change in mechanical properties when the second transformation temperature varied from 260 to 220 °C.

Cyclic Properties of Superelasticity in Cu–Al–Mn Single-Crystalline Sheets with Bainite Precipitates

Microstructure, Vickers hardness and cyclic superelastic properties at room temperature were examined for Cu-Al-Mn single-crystalline sheets aged at 200 °C for various time periods. The bainite plates started to appear from about 9 ks and the density drastically increased from 18 to 30 ks and the Vickers hardness was strongly affected by the density of the bainite plates. In the sample with a little amount of bainite plates, the superelastic properties were basically similar to those in the bainite-free sample. In the samples with a high density of bainite, however, the transformation critical and hysteresis stresses and the dissipated energy were high and large for the initial stage of cycles, respectively. They drastically and monotonically decreased with increasing cycle number and the residual strain increased from the initial stage, unlike those in the bainite-free sample.

An In-Situ LSCM Study on Bainite Formation in a Fe-0.2C-1.5Mn-2.0Cr Alloy

Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol−1 is considered.

Comparative Study of the Uniaxial Cyclic Behaviour of Carbide-Bearing and Carbide-Free Bainitic Steels

Bainitic steels play an important role in the modern automotive and rail industries because of their balanced properties. Understanding the relationship between the bainitic microstructure features and the fatigue performance is a fundamental ingredient in developing safer and durable products. However, so far this relationship is not sufficiently clear. Therefore, there is the need to strengthen the knowledge within this field. The present paper aims at comparing the uniaxial cyclic behaviour of carbide-bearing and carbide-free bainitic steels. To meet this goal, fully-reversed strain-controlled tests at various strain amplitudes were performed. After the final failure, fracture surfaces were observed by transmission electron microscopy to relate the bainitic morphology to the fatigue performance. The main findings of this work show that the carbide-free lower bainite has superior fatigue performance compared to the carbide-bearing lower bainite. This is explained by the presence of stable carbides and thick bainite ferrite plates.

Effects of lanthanum on hot deformation behaviour of Mn-Cr-Mo bainitic rail steel

Effects of lanthanum (La) as micro-alloying element on the hot deformation behaviour of the high strength Mn-Cr-Mo bainitic rail steel were investigated under a range of deformation conditions. The results indicate that La increases the flow stress by 10–30 MPa through strengthening nanoscale strain induced precipitation (SIP) θ-(Fe,La)3C during hot deformation. The hot deformation activation energy increases by 10–40 kJ/mol due to the “Zener effect” of SIP and the dynamic recrystallization (DRX) is retarded due to the competitive behaviour between SIP and DRX. Bainite plates in the DRX domain can be refined by adding La, resulting in the improvement of hot workability. The DRX domain with peak power dissipation efficiency of 52% is determined to be the optimal processing region for Mn-Cr-Mo-La bainitic rail steel.

Effects of microstructures on the fatigue crack growth behavior of laser additive manufactured ultrahigh-strength AerMet100 steel

In order to evaluate the effects of microstructure characteristics on fatigue crack growth (FCG) resistance of laser additive manufactured (LAM) AerMet100 steel, microstructures and FCG behaviors (in Paris region) of as-deposited specimen and three types of tempered martensite specimens were examined. Results indicate as-deposited specimens of LAM AerMet100 steel have apparent texture characteristics of epitaxy unidirectional growth prior-austenite columnar grains and grain-interior inter-dendritic blocky retained austenite with [001] crystallographic orientation. And poor boundary cracking resistance of these texture characteristics along deposition direction mainly contributes to the FCG rate anisotropy of as-deposited specimens. After post-LAM heat treatments, the FCG resistance of all heat-treated specimens apparently improves with the fracture mode of transgranular cracking. With the increase of yield strength, the value of Paris coefficient C of the steel increases, but the value of Paris exponent m decreases. Compared to the poor dislocation slip resistance of bainite plates in as-deposited specimens, the improved dislocation slip resistance of martensite plates is mainly related to the strong dislocation pinning effect of fine dispersive rod-like coherent M2C carbides, resulting in the stronger FCG resistance of the heat-treated specimens. In the Paris region of low ΔK (< ~ 20 MPa m1/2), fatigue cracks mainly propagate along the bainite (or martensite) plate interfaces, and the FCG rate of the steel can be effectively decreased by containing higher contents of thick film-like retained/reverted austenite; with the increase of ΔK, besides propagating along the soft inter-plate film-like austenite, fatigue cracks can also directly pass through the harder bainite (or martensite) plates with the striations and secondary cracks observed on fracture surfaces; in the Paris region of high ΔK (> ~ 70 MPa m1/2), higher contents of retained/reverted austenite inversely accelerate the FCG rate of heat-treated LAM AerMet100 steel. In contrast, grain refinement has the little influence on the FCG rate (in most of Paris region) of the heat-treated specimens.

In situ synchrotron X-ray study of bainite transformation kinetics in a low-carbon Si-containing steel

ABSTRACTThe bainite transformation in a low-carbon Si-containing steel has been studied in situ by synchrotron X-rays. While the austenite is homogeneous prior to transformation, the carbon distribution becomes nonuniform as bainite plates form. This is because of the different degrees of physical isolation of films and blocks of residual austenite. The method for converting dilatational strain into bainite volume fraction, using lattice strain as a reference, during isothermal transformation was found to overestimate it. The bainitic and martensitic ferrite did not exhibit a tetragonal unit cell due to the low-carbon content of the steel and the high transformation temperature.

Redistribution of carbon in the deformation of steel with bainite and martensite structures

Recent years have been marked by considerable increase in the use of high-strength steel—especially martensitic and bainitic steel—for the manufacture of key industrial components and structures. The high strength depends on strain hardening of the steel. It is important to understand the strain hardening of steel of different structural classes with active plastic deformation, in order to ensure specified structural and phase states and mechanical properties. In the present work, by transmission electron-diffraction microscopy, the evolution of the structure, the phase composition, and the state of the defect substructure is compared for steel samples with martensite and bainite structure in active plastic deformation to failure. After austenitization at 950°C (1.5 h) and subsequent quenching in oil (for 38KhN3MFA steel) and cooling in air (for 30Kh2N2MFA steel), multiphase structure (α phase, γ phase, and cementite) based on packet martensite (38KhN3MFA steel) and lower bainite (30Kh2N2MFA steel) is formed. Quantitative results for the structural parameters of steel in plastic deformation permit analysis of the distribution of the carbon atoms in the structure of the deformed steel. The points of localization of carbon atoms in the martensite (quenched 38KhN3MFA steel) and bainite (air-cooled 30Kh2N2MFA steel) are identified. Deformation of the steel is found to be accompanied by the destruction of cementite particles. For quenched martensitic steel, the total quantity of carbon atoms in the solid solution based on α and γ iron is reduced, while their content at structural defects is increased. The redistribution of carbon atoms in the bainitic steel with increase in the strain involves the increase in the quantity of carbon atoms in the α iron, defects in the crystalline structure, and cementite at the intraphase boundaries; and the decrease in the content of carbon atoms in the cementite particles within the bainite plates and the γ iron.

Morphology and Crystallography of Ausferrite in Austempered Ductile Iron

The microstructure of austempered ductile iron was investigated by electron backscatter diffraction technique. The results show that the orientation relationship between acicular bainitic ferrite and austenite is Greninger–Troiano relationship. A single austenite grain is divided into four packets and each packet contains six variants that share a {011}α (i.e., {111}γ) plane. When two γ grains are twinned, the twins share a {111}γ plane and have seven packets. The adjacent acicular bainitic ferrite plates (or laths) sharing a 〈 001 〉 γ axis have small misorientation of about 5.7°. The adjacent acicular bainitic ferrite plates (or laths) not sharing a 〈 001 〉 γ axis have two high misorientation angles of ~54.3° and ~60.0°. Further, the low angle boundary to high angle boundary ratio is far less than the ratio of the variant pairs with small misorientation to the ones with large misorientation. This work is available for structures obtained as a consequence of the heat treatment of austempering.

Effect of Pre-Existing Martensite on Bainitic Transformation in Low-Temperature Bainite Steel

The effect of different volume fractions of pre-existing martensite on the low-temperature bainitic transformation and microstructures was quantitatively analyzed by dilatometer, optical microscope and scanning electron microscope. The results showed that pre-existing martensitic transformation accelerated the subsequent low-temperature bainitic transformation, and the incubation period and completion time of bainitic reaction were significantly shortened. This phenomenon was attributed to the increasing nucleation sites caused by the introduced dislocations in austenite due to the formation of pre-existing martensite. However, it was noteworthy that, because of the increased bainitic plates adjacent to the pre-existing martensitic plates, the probability of the impingement of bainitic plates during growth was increased, which resulted in a decrease in the maximum attainable volume fraction of bainite.

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

Concurrent enhancement of ductility and toughness in an ultrahigh strength lean alloy steel treated by bainite-based quenching-partitioning-tempering process

A low-carbon Mn-Si-Cr-Mo alloyed steel was treated by two different heat treatment routes, namely bainite-based quenching plus tempering (BQ-T) and bainite-based quenching- partitioning- tempering (BQ-P-T). The strength, ductility and toughness were enhanced concurrently after BQ-P-T treatment (i.e., ultimate tensile strength: 1416MPa, the PSE: ~ 25.5GPa%, the CVN impact energy at 20°C and − 40°C: ~ 95Jcm−2 and ~ 45Jcm−2, respectively). These enhanced mechanical properties were attributed to the refined ductile bainite/martensite and the filmy retained austenite multiphase microstructure. The microstructural characterization were carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, transmission electron microscopy and dilatometry. The carbon partitioning between bainite/martensite and austenite can not only stabilize the austenite films but also hinder the coalescence of the bainitic plates and promote the formation of ultrafine bainitic plates. Besides, both the carbon partitioning and the enhanced tempering of martensite/bainite in the BQ-P-T condition contribute to generating the ductile bainite/martensite. Finally, the microstructural factors in controlling the strength, ductility as well as impact toughness were investigated through the analyses of the fracture surface morphology and the retained austenite evolution beneath the fracture surface.

Hot-rolled and continuously cooled bainitic steel with good strength–elongation combination

The current work demonstrates the microstructural evolution and mechanical property evaluation of a newly designed steel composition after hot rolling in laboratory-scale rolling mill, followed by continuous cooling. The steel thus developed has typically about 80% carbide-free bainite; about 20% retained austenite and can deliver ∼1400 MPa ultimate tensile strength along with more than 20% total elongation. The presence of ultra-fine bainite plates (∼100–130 nm thick) with high dislocation density was thought to be responsible for ultra-high strength. Excellent ductility at such strength level could be due to the presence of sufficient amount of retained austenite (∼20%) thermally stable at room temperature but starts transforming to martensite during deformation exhibiting transformation-induced plasticity effect.

Defeat mechanisms provided by slotted add-on bainitic plates against small-calibre 7.62 mm × 51 AP projectiles

Thin steel plates with an array of holes, i.e. perforated plates, are used as a passive add-on improving protective properties of armours against small-calibre projectiles. A number of holes in such plates increases the probability of asymmetrical contact between the plate and the projectile, due to which small-calibre projectiles may be destabilized or fragmented before they reach the main-armour. The aim of the study is to analyse the defeat mechanisms provided by 4-mm-thick slotted super-bainitic plates (Pavise™ SBS 600P) against hard-core 7.62mm P80 0.30 AP×51 (0.308 Win) projectiles. To show the dependence between the hit-point and projectile failure, moments when projectiles hit the pre-armour plate were recorded by an ultra-high speed camera and their behaviour after the impact was captured by the flash X-ray radiography. The obtained results complemented by the Lagrangian FEM analysis confirm that slotted steel plates have high protection effectiveness against small-calibre projectiles.