Bainite Packet Research Articles

Design of Quenching and Tempering Process and Elucidation of the Relationship between Microstructure and Properties of 20Mn2SiNiMo Bainitic Wheel Steel

The rapid development of train transportation toward higher speed and heavier load requires superior wheel materials with high strength and toughness. Herein, a 20Mn2SiNiMo bainitic wheel steel is developed and bainitic wheels are produced. The quenching and tempering process of bainitic wheels is studied considering the bainitic ferrite content and stability of retained austenite (RA). The quenching process includes water spraying and air cooling. Proeutectoid ferrite can be suppressed during the water‐spraying process. During air cooling, the bainitic ferrite transformation is promoted and the stability of RA is improved with the extension of cooling time. Corresponding microstructure of as‐quenched steel at wheel rim is bainite with different morphologies, martensite, and RA. The evolution of RA at lower and higher tempering temperatures is dominated by carbon enrichment and carbon consumption, respectively. Yield strength of bainitic wheel steel mainly comes from body‐centered cubic phase (containing martensite and bainitic ferrite). The increase of dislocation density and the decrease of effective grain size are beneficial for yield strength improvement. Bainitic packets with various direction, deformation‐induced martensite transformation of RA, and plastic deformation all play an important role in enhancing the toughness of bainitic wheel steel.

Correlation between microstructure and fracture mechanism of 960 MPa Grade ultra-high toughness steel

This study investigated the correlation between microstructure and fracture mechanisms in a 960 MPa grade ultra-high toughness high strength low alloy steel produced through controlled rolling and off-line heat treatment. The steel features a low-carbon composition (≤0.1 wt%) and a fine-grained(≤10 μm) bainitic microstructure. Through microstructural characterization and fractographic analysis, extensive quantitative analysis of grain, packet, dimple, and cleavage facet sizes was conducted. Fracture surfaces were examined to identify crack initiation sites. The findings indicate that grain refinement significantly enhances toughness. In fine-grained conditions, bainitic packets that approximate the size of the grains modulate crack propagation pathways. The near-oriented bainite across adjacent grains forms structures comparable to abnormally large grains, crucial for crack nucleation. Moreover, martensite/austenite (M/A) islands, while crucial to the initiation of cracks, require the synergistic interaction with grain boundaries and inclusions to significantly influence fracture initiation. This investigation elucidates the complex interplay between microstructural refinement and fracture mechanisms.

Effect of Wire Preheat and Feed Rate in X80 Steel Laser Root Welds: Part 1 — Microstructure

Laser welding with cold versus hot wire feed was employed as a root pass to weld X80 pipeline steel. The influences of wire feed rate and preheat on the fusion zone microstructure were investigated. Increasing the wire feed rate helped generate acicular ferrite in the weld metal, and preheating the wire further suppressed the formation of bainite. The acicular ferrite in the upper region of the fusion zone was finer than that in the lower region, which was due to an increase in nucleation sites available. Five fill and cap passes were applied by gas metal arc welding to fill the remaining top part of the groove. Compared to arc welding with a higher heat input, laser welding led to finer prior austenite grains and smaller bainite packet size in the coarse-grained heat-affected zone and limited the formation of martensite-austenite constituents in the intercritically reheated coarse-grained heat-affected zone.

Microstructure characterization and mechanical properties of 2.25Cr-1Mo steel pipes using pulsed current GTAW

The 2.25Cr-1Mo pipe was designed to withstand high pressures and temperatures in a variety of industries, including steam boiler pipes,offshore oil drilling, power generation, and petrochemicals. 2.25Cr-1Mo is a good choice for fusion welding and forming. Pulsed current GTAW providesthe advantage of joining thin sections with low heat input, leading to less deformation and warpage than TIG welding. It also allows for better weld pool management, enhanced weld penetration, and improved weld quality with less distortion and warpage. It also allows for better weld pool control, enhanced weld penetration, and improved weld quality. The goal of this research is to use a design of experiments technique to optimize process parameters and describe the microstructure and mechanical properties of 2.25Cr-1Mo pipes welded with pulsed current GTAW (DOE). The Taguchi method is used to choose the L9 (34) orthogonal array (OA) for four factors (pulse current (PC), background current (BC), pulse frequency (PF), and pulse on time (PO)) at three different levels on 2.25Cr-1Mo steel pipe joints. The pulsed current GTAW method is used to obtain the optimal combination for increased weld strength. To find the best process parameters for 2.25Cr-1Mo steel pipes, researchers used an analysis of variance (ANOVA). 220 A Pulse current, 120 A background current, 8 Hz pulse frequency, and 60 %pulse on are shown to be the best parameters for 2.25Cr-1Mo steel. Microhardness, Tensile, and Impact tests are done and compared to the parent material's properties. Some bainitic packets at the weld zone have finer lath structures, whereas others have coarser lath structures. The fracture morphology shows tear ridge, cleavage facets, and ductile dimples where failure occurs at the weld zone and it indicates ductility failure.

Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.

Microstructure, crystallography, and toughness in nugget zone of friction stir welded high-strength pipeline steel

Herein, we address the scientific gap in the relationship among the microstructure, crystallography, and toughness in the nugget zone (NZ) of friction stir welded X80 pipeline steel at various heat input parameters of 300, 400, and 600 rpm. The decreased heat input results in refined prior austenite accompanied with an increase in the Z-parameter. Lath bainite (LB) and granular bainite (GB) appear at a high heat input and a bulk of GB appears at a medium heat input, whereas at a low heat input, a complex phase of LB, GB, and ferrite is obtained. The highest ratio of the bainite packet boundaries was identified at a low heat input, which is attributed to the existence of three Bain groups. The strongest shear texture was detected in the NZ at a medium heat input. An excellent toughness of up to 92.5% of that of the base metal was achieved at a low heat input owing to the highest ratio of packet boundaries, finest martensite-austenite constituents, and soft second phase of ferrite.

Elongation improvement in nano bainite steel obtained from plastically deformed primary austenite

ABSTRACT Ausforming is being widely used to overcome the kinetics barrier in nanostructured bainitic steels and to enhance its practical industrial applications. It would also affect the microstructural characteristics and consequently the resultant mechanical performance. This article aims to investigate the tensile behaviour of nano bainite obtained from plastically deformed primary austenite at three different heat treatment stages. 10% of ausforming has been implemented and nanostructured bainite obtained after austempering at 300°C for 2, 4 and 6 h in salt bath furnaces. Results indicated that ausforming could successfully increase the volume fraction of bainite within the microstructure and refine the bainite packets sizes. Moreover, it has been demonstrated that higher mechanical stability of retained austenite and therefore more effective TRIP effect could be attained during the tensile test which consequently resulted in elongation improvement without deteriorating the strength properties. This claimed to be beneficial for materials performance during practical applications.

Microstructure and mechanical properties of hot-rolled low-carbon steel containing Ti–Ca oxide particles: a comparison between base metal and HAZ

The effect of inclusion-induced nucleation on hot-rolled steel base metal was evaluated in comparison with welding heat-affected zone (HAZ). Microstructure and mechanical properties of hot-rolled low-carbon steel containing Ti–Ca oxide particles were studied. The results showed that inclusions in Ti–Ca deoxidized steel distributed dispersely and were effective for intragranular acicular ferrite nucleation. Under hot rolling and controlled cooling conditions, microstructure in steel base metal was significantly refined and mainly consisted of acicular ferrite and intragranular bainite, which exhibited higher strength and excellent toughness. The microstructural evolution behavior followed the process that acicular ferrite plates divided the austenite grain, intragranular bainite packets formed between interlocking acicular ferrite plates, and the remaining austenite decomposed into fine polygonal ferrite grains. The resultant complex microstructure improved the impact toughness significantly. By comparison, in HAZ microstructure, laminar grain boundary ferrite having similar crystallography orientation showed adverse effect on toughness.

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H2SO4 + 10 mg L−1 As2O3, with an intensity current of 20 mA cm−2. Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values.

Effect of welding heat input on microstructure and impact toughness in CGHAZ of X100Q steel

The coarse-grained heat-affected zones (CGHAZs) of X100Q steel were reproduced via simulating their welding thermal cycles with the varying heat input (Ej) from 10 to 55 kJ/cm in Gleeble3500 system. The microstructures were characterized, and the impact toughness was estimated from each simulated sample. The results indicate that the microstructure in each simulated CGHAZ was primarily constituted of lath-like bainite. With the decreased heat input and accordingly the lowered Ar3 (the onset temperature for this transition), the prior austenite grain and the bainitic packet/block/lath substructure were refined remarkably, and the impact toughness was enhanced due to the microstructure refinement. The bainitic packet was the microstructural unit most effectively controlling the impact properties in CGHAZ of X100Q steel, due to their close correlation with the 50% fracture appearance transition temperatures, their size equivalent to the cleavage facet and their boundaries impeding the crack propagation.

Influence of the decomposition behavior of retained austenite during tempering on the mechanical properties of 2.25Cr-1Mo-0.25 V steel

The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 °C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180° to 650°C before the standard tempering at 700 °C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 °C tempering. The experimental results show that 455 °C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 °C tempering. Microstructural observations reveal that during the 455 °C pre-tempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 °C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.

Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels

We investigated the effects of C concentration variation from 0.028 to 0.058 wt pct on microstructure of the coarse grained heat-affected zone (CGHAZ) of low heat input girth welded Ti-Nb microalloyed steels by using electron microscope and atom probe tomography. It is found that the CGHAZ microstructure exhibits a systematic response to C variation. Increased C raises the temperature for precipitation of NbC. This leads to coarser (Ti, Nb)N-Nb(C, N) but finer delayed strain-induced NbC in the high-C steel than in the low-C steel. Fine strain-induced NbC are ineffective in preventing austenite grain coarsening in CGHAZ due to their fast dissolution upon heating. For a given inter-particle spacing originally determined by (Ti, Nb)N particles, increased epitaxial growth of Nb(C, N) on pre-existing (Ti, Nb)N in the high-C steel results in a smaller austenite grain size of 34 µm in the CGHAZ of the high-C steel than that of 52 µm in the low-C steel. Increased C promotes a microstructure consisting of bainitic lath structure with C Cottrell atmospheres at dislocation debris and martensitic layers of 30 to 100 nm in thickness at inter-lath boundaries in the CGHAZ. Increased C promotes configuration of crystallographic variants belonging to different Bain groups in the neighbors, preferentially twin-related variant pairs within a bainite packet.

Characterization of lower bainite formed below M S *

Abstract Lower bainite can indeed grow below martensite start temperature (M S ) in a commercial alloy. Ultrahigh-strength low-alloyed 42CrMo4 steels are austempered below M S . The preliminary martensite and lower bainite packets formed during austempering partition the prior austenite grains effectively. The obtained ultrafine-grained nanostructured microstructure enhances the mechanical properties of the alloys significantly. Morphology and overall microstructure are examined by reflection optical microscope and scanning electron microscopy. Substructures of lower bainite and virgin/tempered martensite are revealed by transmission electron microscopy. With increasing duration of austempering, a clear transition from cleavage to ductile rupture was observed by fractography. Effects of grain partitioning and the risk of embrittlement have been discussed concretely. The better understanding of the microstructure austempered below M S will feedback to the materials development activities both in conventional and new materials areas.

Effective microstructure unit in control of impact toughness in CGHAZ for high strength bridge steel

The high strength bridge steel was processed with the simulated coarse grain heat affected zone (CGHAZ) thermal cycle with heat input varying from 30 to 60 kJ/cm, the microstructures were investigated by means of optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD) and transmission electron microscope (TEM), and the impact properties were evaluated from the welding thermal cycle treated samples. The results indicate that the microstructure is primarily composed of lath bainite. With decreasing heat input, both bainite packet and block are significantly refined, and the toughness has an increasing tendency due to the grain refinement. The fracture surfaces all present cleavage or fracture for the samples with different heat inputs. Moreover, the average cleavage facet size for the CGHAZ is nearly equal to the average bainite packet size and the bainitic packet boundary can strongly impede the crack propagation, indicating that the bainitic packet is the most effective unit in control of impact toughness in the simulated CGHAZ of high strength bridge steel.

Effect of Austenitising Temperature on Mechanical Properties of Nanostructured Bainitic Steel.

Nanostructured bainite was obtained in high-carbon Si-Al-rich steel by low-temperature (220–260 °C) isothermal transformation after austenitisation at different temperatures (900 °C, 1000 °C, and 1150 °C). Improved strength-ductility-toughness balance was achieved in the nanostructured bainitic steel austenitised at low temperatures (900 °C and 1000 °C). Increasing the austenitising temperature not only coarsened prior austenite grains and bainite packets, but also increased the size and fraction of blocky retained austenite. High austenitising temperature (1150 °C) remarkably decreased ductility and impact toughness, but had a small effect on strength and hardness.

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

ABSTRACTIn this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.

Investigation of the Origin of Coarse-Grained Bainite in X70 Pipeline Steels by EBSD Technique

Structural inhomogeneities that occurred in pipeline steels during controlled rolling and were identified as elongated regions of coarse-grained bainite were found to promote a tendency for brittle fracture of these steels. In particular, it was shown by tint etching, with subsequent analysis in polarized light with the light-optical microscope, that their volume fraction exhibited a perfect correspondence with the failure behavior. The aim of the present investigation was to comprehensively characterize the microstructure and the local texture of these regions using the EBSD technique and thereby clarify their origin. It was shown that, whereas the local texture of fine-grained regions corresponded well to the macroscopic texture of the steel band, the local texture of the structural inhomogeneities corresponds to certain minor components of the macroscopic texture. The grains of the parent austenite, from which these inhomogeneities have been produced, had orientations concentrated close to the brass component of austenite rolling texture. Hot deformation of the austenite leads to a strong variant selection (predominant realization of certain lattice orientations among plausible orientations of α-phase) during phase transformation of these grains, that result in a significant increase of the bainite packet size. Formation of bainite in the upper temperature range of bainite transformation, in its turn, leads to such a variant pairing that low-angle boundaries are formed predominantly both inside and between the packets. This further enlarges the effective grain size and correspondingly enhances the deteriorative effect of the structural inhomogeneity.

NIOBIUM EFFECT ON BASE METAL AND HEAT AFFECTED ZONE MICROSTRUCTURE OF GIRTH WELDED JOINTS

The development of steels for line pipes during the last decades has been driven by the need to obtain improved combinations of high strength, toughness, weldability on industrial scale at affordable prices. The effect of niobium content on the heat affected zone (HAZ) microstructure is reported in this paper. Niobium, for its specific thermodynamic and kinetic attitude to form carbide and nitride precipitates, played a key role in the development of modern HSLA steels Results show that niobium addition is able to refine both the bainitic packet and cells size in the heat affected zone during welding. This implies that niobium addition leads to an improvement of both toughness and hardness of welded joints manufactured by Nb micro-alloyed steels.

New bainite kinetics of high strength low alloy steel in fast cooling process

Based on Kolmgorov-Johnson-Mehl-Avrami analysis, a new bainite kinetics of high strength low alloy steel in fast cooling process was developed by utilizing different experimental methods. Upper bainite transformation morphological evolutions at a cooling rate of 8. 3 K/s were directly observed by laser scanning confocal microscopy. This qualitative analysis suggests that bainite packet is more suitable to give a one-dimensional growth model if it is considered as a transformation unit. The nucleation rate of bainite packets in fast cooling process is assumed to give an a priori item. One-dimensional growth model with constant growth rate which is assumed as a function of cooling rate is adopted as well. Thus, the developed new bainite kinetics is simple in expression and contains an adjustable parameter and an empirical parameter. Experimental results show upper bainite and lower bainite transformations in fast cooling processes. Their referential phase volume fractions are calculated by the expanded lever rule on the first derivative dilatometer curves. For the similar transformation mechanisms, upper bainite and lower bainite are considered to give the same kinetics. With considering the Nakamura's equation, the bainite kinetics is fitted with experimental data. Results show that bainite volume fractions and bainite transformation rates can be expressed precisely by the newly developed bainite kinetics.

On the crystallographic characteristics of nanobainitic steel

This study aims to elucidate the crystallographic characteristics of bainite transformed in a temperature range of 200–350 °C, where a nanobainitic structure is formed. The microstructure, consisting of bainitic ferrite laths and retained austenite, became significantly refined and its crystallographic arrangement changed with a decrease in the phase transformation temperature. At 200–250 °C, the bainite packets mostly consisted of one or more blocks (i.e. bainitic ferrite laths and retained austenite lamellae) with different orientations, having a common habit plane. Some of the bainitic laths formed in this temperature range were composed of small segments with similar orientations, while others displayed a ragged morphology with small protrusions, suggesting face-to-face and face-to-edge sympathetic nucleation, respectively. At 300–350 °C, the latter nucleation mechanism appeared to be dominant, as bainite packets mostly consisted of two sets of bainitic ferrite laths with similar orientations and inclined to each other (i.e. having different habit planes). In general, all rational orientation relationships (ORs) ranging from Kurdjumov-Sachs (K-S) through to Nishiyama-Wassermann (N-W) were observed within the transformation temperature range. The N-W OR was dominant at 350 °C and progressively changed towards the K-S OR, which was prevalent at 200 °C. The five-parameter crystallographic approach was used to statistically measure the habit plane distributions for both bainitic ferrite and retained austenite, which were generally found to be irrational and exhibited a significant anisotropy. The bainitic ferrite interface plane distribution displayed a wide peak spreading from (101) to (535). The retained austenite revealed a maximum at the (111) orientation, extending towards the (554).