Showing papers on "Bainite published in 2021"

Microstructure characterization and its relationship with impact toughness of C–Mn and high strength low alloy steel weld metals – a review

Abstract: Prediction of impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the innumerous parameters involved. The common practice that associates this property with the microstructure of the last bead of multipass weldments has been proven to be unsatisfactory, because the amount of acicular ferrite, the most desirable constituent, may not always be the main contributor to toughness. Parameters such as the recrystallized fraction, the presence of micro-phases and inclusions may also have a relevant role. Thus, to consider the influence of all these parameters, the method proposed by the International Institute of Welding (IIW) is not sufficiently comprehensive and so complementary techniques are necessary. This situation is more relevant for high strength steel weld metals, where very refined microstructures may not be adequately resolved, resulting in misidentification of the microstructure. The use of scanning electron microscopy as an auxiliary technique to optical microscopy has been successfully used for many decades to study C–Mn and low alloy weld metals, mainly when refined microstructures are to be assessed. Recently, in addition to the previously mentioned techniques, Electron Back Scattering Diffraction (EBSD) has also been used to enable a more effective analytical procedure. This technique, which provides valuable information about grain boundaries, is advantageous for refined microstructures to confirm constituents such as acicular ferrite, bainite, and martensite. This work proposes a contribution to the microstructural characterization of C–Mn and high strength steel weld metals based on the analysis carried out by optical microscopy, scanning electron microscopy and EBSD techniques, involving the influence of recrystallization in multipass welds, microstructural constituents, microphases, and inclusions. The microstructure/toughness relationship analysis of some experimental results obtained over the last decades for weld metals with ultimate tensile strength varying from 400 to 1000 MPa was done using the methodology proposed in this work to check its effectiveness and to explain the impact toughness behavior.

Role of bainitic microstructures with M-A constituent on the toughness of an HSLA steel for seismic resistant structural applications

Abstract: Fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of fusion welded HSLA steel is severely reduced at low temperatures with the combination of deleterious microstructures such as bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and the martensite-austenite (M-A) constituents. Toughness is one of the basic requirements for seismic resistant steels. The three heat input conditions (different cooling rate) were thermo-mechanically simulated, and the combined effect of M-A constituents with bainitic microstructures on toughness was examined. As the heat input changes from low to high, a combination of acicular ferrite (AF) and BF microstructures changes to a combination of PF, GB and M-A constituent. The former microstructure combination provides better toughness (ductile), whereas the latter makes the samples more brittle. The coarse BF, GB, and PF microstructures with low angle grain boundaries (LAGBs) make them brittle by easy crack propagation. Moreover, harder microstructures such as M-A can potentially act as cleavage initiation sites. However, a high frequency of high angle grain boundaries (HAGBs) in the AF microstructure can suppress the crack propagation, which turns the fracture to completely ductile. Also, it is observed that the M-A constituent in CGHAZ was abundant in martensite rather than the austenite.

Additive manufacturing of 24CrNiMo low alloy steel by selective laser melting: Influence of volumetric energy density on densification, microstructure and hardness

Abstract: 24CrNiMo low alloy steels were fabricated by selective laser melting (SLM), aiming at investigating the effect of volumetric energy density (η) on the densification behavior and microstructural evolution, and then establishing a rational densification-microstructure-microhardness relationship of as-built samples. The results showed that the densification level of the as-built samples was significantly improved with increasing the η, due to the inhibition of balling effects and internal pores. As-built samples mainly exhibited α-Fe (M) and Fe3C, and the diffraction angles of the main phase α-Fe (M) peaks slightly shifted to the left in comparison with their standard locations due to the lattice distortion. The microstructure of the as-built samples mainly composed of high-containing martensite (M) and low-containing lower bainite (LB), resulting from the rapid solidification and the heat accumulation. As the η increased, the size and number of LB gradually increased owing to the relatively decrease of cooling rate. EBSD maps depicted that the grain size of the as-built samples increased with the increase of η. The microhardness of the as-built samples exhibited an upward trend with the η increasing, implying that compared to the grain size, the internal defects played a dominant role in determining the hardness. The solid solution strengthening, dislocation strengthening and precipitate strengthening resulting from M/LB duplex microstructure resulted in the achieved high hardness, under the premise of ensuring relative density.

Role of inclusion and microstructure on corrosion initiation and propagation of weathering steels in marine environment

Abstract: The present research examined the effects of inclusions and microstructure on the initial marine corrosion and evolution of corrosion products in weathering steels by using first-principle modeling and various highly-sensitive analytical techniques including in-situ scanning vibrating electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and electrochemical workstation. The results demonstrated that CaS in the (Al, Mg)Ox-CaS inclusion formed in both Q500qE and Q370qE steels preferentially dissolved and triggered the initial corrosion. The acidic environment created between the inclusions and the iron matrix further promoted the dissolution of the inclusions. Moreover, due to the discrepancy in corrosion tendency, galvanic couples generated between the bainite ferrite (BF) phase and martensite/residual austenite (M/A) island in the Q500qE steel as well as the ferrite phase and pearlite phase in the Q370qE steel, accelerating the initial corrosion. In addition, pearlite facilitates a faster spread rate of local corrosion compared to bainite. Furthermore, with prolonged exposure, Q500qE steel exhibited more uniform and dense structure of the corrosion products layer, demonstrating a higher corrosion resistance than Q370qE steel. Finally, the mechanistic model was established to illustrate the influence of inclusions and microstructure on corrosion initiation and propagation of weathering steels.

Plastic Behavior of Ferrite–Pearlite, Ferrite–Bainite and Ferrite–Martensite Steels: Experiments and Micromechanical Modelling

Abstract: In this work, low carbon low alloy steel specimens were subjected to suitable heat treatment schedules to develop ferrite–pearlite (FP), ferrite–bainite (FB) and ferrite–martensite (FM) microstructures with nearly equal volume fraction of hard second phase or phase mixture. The role of pearlite, bainite and martensite on mechanical properties and flow behaviour were investigated through experiments and finite element simulations considering representative volume elements (RVE) based on real microstructures. For micromechanical simulation, dislocation based model was implemented to formulate the flow behaviour of individual phases. The optimum RVE size was identified for accurate estimation of stress–strain characteristics of all three duplex microstructures. Both experimental and simulation results established that FM structure exhibited superior strength and FP structure demonstrated better elongation while FB structure yielded moderate strength and ductility. The von Mises stress and plastic strain distribution of the individual phase was predicted at different stages of deformation and subsequent statistical analyses indicated that hard phases experienced maximum stress whereas, maximum straining occurred in soft ferrite phase for all three structures. Micromechanical simulation further revealed that strain accumulation occurred at the F–P and F–B interfaces while the same was observed within the martensite particles apart from the F–M interfaces for FM. These observations were further substantiated through the identification of void and crack initiation sites via subsurface examinations of failed tensile specimens.

Influence of the starting microstructure of an advanced high strength steel on the characteristics of Zn-Assisted liquid metal embrittlement

Abstract: High temperature tension tests were performed to investigate the influence of variations in the starting microstructure on zinc (Zn)-assisted liquid metal embrittlement (LME) susceptibility of an advanced high strength C–Mn–Si steel. Using a single alloy, the starting microstructure was modified by heat-treatment prior to zinc electroplating. The hot tension test data revealed comparable Zn-LME susceptibility for different martensite and bainitic-ferrite based microstructures generated through full austenitization, such as as-quenched martensitic, quenched and partitioned (Q&P), and transformation-induced-plasticity (TRIP)-assisted bainitic ferrite (TBF). LME cracks propagated through these microstructures via intergranular fracture along prior austenite grain boundaries, with no apparent involvement of microstructural components like retained austenite or carbide-free bainite in the LME cracking behavior. In contrast, a dual phase (DP) microstructure variant of the same steel, generated through intercritical annealing (partial austenitization), exhibited suppressed LME, revealing that the starting microstructure can influence Zn-LME susceptibility. The lower LME susceptibility of the DP steel compared to the martensitic, Q&P, or TBF steels is explained by the presence of ultrafine ferrite grains and discrete martensite islands, and a smaller area fraction of prior austenite grain boundaries in the DP microstructure.

Characteristics of bainitic transformation and its effects on the mechanical properties in quenching and partitioning steels

Abstract: Steel treated by quenching and partitioning exhibits an excellent combination of strength and ductility by taking advantage of the transformation-induced plasticity effect of retained austenite. Therefore, the precise tailoring of retained austenite is crucial for quenching and partitioning steels. However, austenite decomposition including bainitic transformation during partitioning is usually neglected, and consequently, its effects on the mechanical properties are not clear. Here, we elucidate the significant role of bainitic transformation during the partitioning process in the microstructure and mechanical properties of 0.2C–2.82Mn-1.58Si steel subjected to one-step quenching and partitioning process and two-step quenching and partitioning process. The results show that the occurrence of bainitic transformation during the QP as a result, the amount of retained austenite is increased, and more blocky austenite is retained due to the optimization of carbon redistribution during bainitic transformation. Moreover, an increase in the bainite volume fraction and the various morphologies of retained austenite lead to excellent work hardening performance, thereby significantly improving the elongation.

Improvement of mechanical properties for low carbon ultra-high strength steel strengthened by Cu-rich multistructured precipitation via modification to bainite

Abstract: This work focused on ultra-high strength steel strengthened by Cu-rich multistructured precipitates and toughened by bainite. A Cr–Mo bearing bainitic steel was designed and contrasted with a ferritic steel to examine the effects of Cr and Mo on microstructural transformation and precipitation evolution as well as the corresponding mechanical properties. By adding Cr and Mo elements, bainitic steel was obtained by an air cooling process after hot rolling, thus omitting controlled cooling and off-line quenching. Cr and Mo decreased the γ/α transformation temperature, such that the ferritic matrix was modified to fine bainite. Cu-rich multistructured particles and (Nb,Ti)C particles were found to be the two main precipitation phases in this bainitic steel. Cu-rich multistructured particles contained B2-ordered structure and transition-state structure between B2-ordered and 9R structure. Notably, this Cr–Mo bearing bainitic steel had better strength and toughness compared with ferritic steel. Its yield strength reached 1155 MPa, owing to precipitation and grain boundary strengthening, which were estimated to be 596 and 311 MPa, respectively. The impact absorbed energy of this steel at −40 °C was ~55.3 J and its fracture mode a brittle-ductile mixed mode, compared with the cleavage fracture mode of ferritic steel. The small grain size of this steel compensated for toughness deterioration caused by precipitation and contributed to high impact toughness.

Effects of different cooling rates on the microstructure, crystallographic features, and hydrogen induced cracking of API X80 pipeline steel

Abstract: Hydrogen-Induced Cracking (HIC) is a primary failure mechanism of pipeline-welded joints in the absence of external loading in the oil and gas exploration industries. Three different cooling rates after austenitization were used to simulate in the laboratory different regions of the heat-affected zone (HAZ) formed when welding an API X80 pipeline steel specially designed to enhance the HIC resistance. The samples were characterized with regard to microstructure and crystallography as well as HIC resistance. The HIC resistance test used NACE TM0284-2011 methodology. The microstructure and its homogeneity varied as a function of cooling rates. Samples containing inclusions and segregation zone from the segregation bands of specimens showed reduced HIC resistance, while specimens containing only acicular ferrite and granular bainite coupled with the absence of segregation zone showed significant improvement in HIC resistance. The best HIC resistance results came from samples presenting fine acicular ferrite consisting of fine interlocking plates, with divergent crystallographic orientations, preventing the formation of localized strain distribution inside the grain and at grain boundaries. It was also found that a large proportion of medium-angle boundaries prevent microcrack initiation and the transgranular mode of crack propagation.

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

Abstract: 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.

Effect of cooling rate on microstructure and mechanical properties of a low-carbon low-alloy steel

Abstract: The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

Abstract: High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

Tempering response and improved mechanical properties in secondary hardened steel by introducing an optimized austempering process

Abstract: The tempering response and mechanical properties of the austempered Cr4Mo4V steel with bainite/martensite multiphase structure were investigated During the tempering, the retained austenite (RA) decomposed almost entirely in the specimens without or with a short austempering, while massive blocky RA still remained in the specimens with a long austempering process (4 h) Meanwhile, the significant coalescence and coarsening of bainite occurred during the tempering of the specimens with longer austempering After the tempering, the diameter of packets in the specimens austempered for 1 h decreased from 195 to 106 μm, which could be attributed to the second division of austenite grain during austempering and the minimum degree of bainite coarsening during tempering Additionally, the phase transformation texture was weakened while the dislocation density was increased, as compared with the martensitic quenched specimens Due to the synergistic effect of the finer packet, higher dislocation density and weaker texture, the ultrahigh strength (2604 MPa) combined with an improved impact toughness (93 J) was achieved in the Cr4Mo4V steel austempered for 1 h without compromising hardness (630 HRC) However, with the increase of austempering time, the mechanical properties would deteriorate due to the coarsened packets and undecomposed blocky RA These results indicate that by introducing an optimized austempering process, the mechanical properties of Cr4Mo4V steel can be dramatically improved, while the content of bainite should be strictly regulated to prevent blocky RA and bainite coarsening

On Microstructure and Mechanical Properties of a Low-Carbon Low-Alloy Steel Block Fabricated by Wire Arc Additive Manufacturing

Abstract: In this study, wire arc additive manufacturing process is employed to fabricate a low-carbon low-alloy steel block, using an ER70S-6 solid wire. Three sets of samples with different orientations, including perpendicular (Vertical), parallel (Horizontal), and 45° (45-degree) relative to the deposition plane, were prepared in order to investigate the anisotropy in mechanical properties and microstructure of the fabricated part. Both Horizontal and 45-degree samples showed a uniform microstructure containing mostly ferritic grains with a small volume fraction of pearlite at their grain boundaries. Differently, a periodic microstructure was detected in the Vertical sample, consisting of a combination of acicular ferrite, bainite, and allotriomorphic ferrite formed in the interlayer regions in addition to polygonal ferrite within the melt pools’ center. Moreover, the uniaxial tensile and Charpy impact results exhibited isotropic tensile, yield, elongation, and impact properties for both Horizontal and 45-degree samples; however, the Vertical sample showed a lower mechanical performance. The improved mechanical properties of the Horizontal and 45-degree samples were correlated to their uniform ferritic microstructure.

Effect of laser welding heat input on fatigue crack growth and CTOD fracture toughness of HSLA steel joints

Abstract: High strength low alloy steels are employed in structural elements and, despite presenting good weldability, the welded joint is always a critical issue and its evaluation is fundamental in guaranteeing structural integrity. The mechanical properties of weld beads can be significantly different from the base material's properties due to metallurgical alterations caused by the welding process. One of the factors leading to significant impact is the heat input. This paper evaluates the hardness, fracture toughness and fatigue crack growth in weld beads obtained via a Laser process with two different heat inputs, which resulted in weld beads with distinct microstructures: one composed of ferrite presenting different morphologies and the other composed of martensite and bainite. The aims of this work are evaluating the effect of Laser welding heat input on microstructures in the weld beads, and the correlation of the microstructure with fatigue crack growth and crack-tip opening displacement fracture toughness. Fracture toughness presented itself to be more sensitive to the microstructural alterations caused by the heat input than hardness and fatigue crack growth. Weld beads showed higher resistance to fatigue crack growth when compared to the base metal, even though there were no significant differences between them.

Influence of Multiphase High Silicon Steel (Retained Austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) and Carbon Content of RA-Cγ on Rolling/Sliding Wear

Abstract: In this research work, heat-treatment processes have been utilized to obtain multiphase microstructure in the silicon rich steel samples, silicon in the steel helps in the development of multiphase microstructure and to keep away from carbide precipitation development through the austempering. The desired multiphase microstructure (Retained austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) consisting of continuous cooling (CC) for 0, 20 and 40 s respectively after austenization followed by austempering at (300, 350 and 400 °C) to form a high wear resistance multiphase steels with microstructure varies amount of F,B, P and RA during continuous cooling. Steels with varies retained austenite up to (5±1.1 to 18±1.9%) along with excellent specific wear rate (2.038 × 10−9-1.061 × 10−8 m3/N-m) were obtained. Further, the rolling/sliding wear rate has been obtained through the disc-on-disc experimental setup. The effect of continuous cooling on retained austenite, carbon content of retained austenite on specific rolling/sliding SWR (specific wear rate) has been studied; phase fraction (P, B and F) and the materials characterization with the help of XRD, AFM and FE-SEM.

Modeling of Phase Transformation Kinetics in Resistance Spot Welding and Investigation of Effect of Post Weld Heat Treatment on Weld Microstructure

Abstract: In recent years, the usage of dual phase (DP) steels, transformation induced plasticity (TRIP) steels and boron steels in some auto body parts has become a necessity because of their strength and lightweight. Resistance spot welding (RSW) being a process were steel is heated and cooled in a very short period of time, the resulting weld nugget is generally fully martensitic, especially in the case of DP, TRIP and boron steels but that also holds for plain carbon steels as AISI 1010 grade which is extensively used in auto body inner parts. Martensite in is turn must be avoided as much as possible when welding steel because it is the principal source of brittleness. Thus, this work aims in finding a mean to reduce martensite fraction and increase phase diversity in weld nugget. The prediction of phase transformation during RSW has been done. Simulations have been performed for 2 mm AISI 1010 sheets and results show that the application of post weld heat treatment leads to the reduction of martensite fraction, and formation of ferrite and bainite in the nugget. Welding experiments have been done in parallel and experimental weld nugget geometry is in good agreement with simulation results.

Effects of quenching temperature on bainite transformation, retained austenite and mechanical properties of hot-galvanized Q&P steel

Abstract: Over the last decade, demand has increased for developing the hot-galvanized quenching and partitioning (Q&P) steel to overcome the disadvantages associated with vehicle safety, fuel consumption and corrosion resistant. The present work aims to elucidate the effects of quenching temperature on phase transformation kinetics, microstructure evolutions and mechanical properties of a hot-galvanized Q&P steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) by combining modeling and experimental research. Using dilatometry, SEM, EBSD, EPMA, TEM, PED, XRD and Image-Pro Plus (IPP) software, we quantitatively investigated the microstructure evolution at different quenching temperatures. Results indicated that a larger fraction of primary martensite at lower quenching temperature could strongly promote subsequent bainite transformation kinetics, which is attributed to more martensite-austenite interfaces and defect density. By fitting the dilatometer curves and establishing the equation of transformation rate vs. quenching temperature, a Kolmogorov-Johnson-Mehl-Avram (KJMA) equation was established to describe insufficient bainite formation kinetics during high-temperature short-time overaging. Furthermore, a modified CCE model taking into account intercritical ferrite and short-time bainite transformation was proposed and the predicted RA fractions are more consistent with the experimental values. As the quenching temperature decreases, small-sized blocky RA along martensite boundaries and filmy RA between martensite laths increase, while coarse lath/blocky RA inside bainite structures or at bainite boundaries decreases gradually. In addition, the YS decreases from 763 MPa to 431 MPa with the increase of quenching temperature, while the UTS varies in a narrow range between 967 MPa and 1036 MPa. A stable TEL of 22.6–25.7% can be obtained at a wide range quenching temperature (150–275 °C), which is attributed to the joint effects of TRIP effect and multiphase structure. This research would be of guiding significance for the industrial practice.

Stability of Retained Austenite in Carbide Free Bainite during the Austempering Temperature and its Influence on Sliding Wear of High Silicon Steel

Abstract: The present work has been carried out to study the different heat-treatment processes to obtain carbide-free bainite in the high silicon spring steel. Silicon content helps to develop carbide-free bainite (compositions of steel is 0.551% C, 1.756% Si, 0.825% Mn, and 0.13% Cr) and also, to avoid carbide precipitation formation during austempering. The desired microstructure of treated samples has been achieved through heat treatment process at different austempering temperatures such as 300 °C, 350 °C, and 400 °C at holding time of 10, 20 and 30 min. Further, the wear rate of the base and treated samples has been analysed through the pin-on-disc testing machine. The retained austenite helps to resist the crack initiation and propagation through transformation during deformation, and it also improves wear resistant and hardness of the treated sample. The effect of fraction of retained austenite and its carbon content on the specific wear rate has been systematically studied. The phase fraction and retained austenite stability have been critically analysed with the help of X-ray diffraction, atomic force microscopy and scanning electron microscopy.