Showing papers on "Bainite published in 2020"

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

Abstract: This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.

Recent progress in third-generation low alloy steels developed under M 3 microstructure control

Abstract: During the past thirty years, two generations of low alloy steels (ferrite/pearlite followed by bainite/martensite) have been developed and widely used in structural applications. The third-generation of low alloy steels is expected to achieve high strength and improved ductility and toughness, while satisfying the new demands for weight reduction, greenness, and safety. This paper reviews recent progress in the development of third-generation low alloy steels with an M3 microstructure, namely, microstructures with multi-phase, meta-stable austenite, and multi-scale precipitates. The review summarizes the alloy designs and processing routes of microstructure control, and the mechanical properties of the alloys. The stabilization of retained austenite in low alloy steels is especially emphasized. Multi-scale nano-precipitates, including carbides of microal-loying elements and Cu-rich precipitates obtained in third-generation low alloy steels, are then introduced. The structure–property relationships of third-generation alloys are also discussed. Finally, the promises and challenges to future applications are explored.

Quantitative phase analysis of martensite-bainite steel using EBSD and its microstructure, tensile and high-cycle fatigue behaviors

Abstract: This study conducted a quantitative analysis of the martensite/bainite (M/B; bainitic structure region) fraction of martensite-bainite steel using electron back-scatter diffraction (EBSD) analysis. The M/B fraction analyzed using the EBSD analysis method was then compared with phase fraction measurement results with an optical microscope (OM), field emission scanning electron microscope (FE-SEM), and field-emission transmission electron microscopy (FE-TEM). In addition, microstructure, tensile and high-cycle fatigue behaviors according to M/B phase fraction were investigated. Initial microstructural observation measured a prior austenite grain size (PAGS) of 24 μm (alloy A) and 11 μm (alloy B). Both alloys were observed to have martensite and bainite structures. XRD phase analysis of the two alloys identified an α-Fe peak expected to be martensite or bainite. Quantitative phase fraction of M/B using EBSD analysis measured M: 40.37% and B: 59.63% for alloy A, and M: 53.03% and B: 46.97% for alloy B. Tensile tests of the above materials confirmed that alloy B, which had finer PAGS and a higher martensite fraction, had greater yield strength (1423 MPa) and tensile strength (1826 MPa) that were approximately 200 MPa higher than alloy A. The yield strength was calculated based on the M/B phase fraction using EBSD and the measured microstructure factors, with a consideration of the prediction model. The calculation value was similar to the actual tested strength one. In the high-cycle fatigue test, alloy B, with its greater strength, had an approximately 200 MPa higher fatigue limit (1075 MPa) than that of alloy A. EBSD analysis of the fatigue crack initiation area confirmed that the M/B interface can act as a fatigue crack initiation site. Based on the above findings, tensile and fracture surface analyses were performed, and attempts were made to identify the tensile and deformation mechanism according to the M/B phase fraction.

Effects of process parameters on the microstructure and mechanical properties of 24CrNiMo steel fabricated by selective laser melting

Abstract: In this paper, the effects of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of 24CrNiMo high-strength low-alloy steel were investigated. The optimal parameter combinations and corresponding action rules were obtained by relative density index method which directly affects the microstructure and mechanical properties of materials. As a result, the relative densities of as-received 24CrNiMo steel increased rapidly with the growth of energy densities in the range of 38–50 J/mm3, then increased slowly between 50 and 100 J/mm3 and ultimately stabilized at maximum 99.6%. The hardness and tensile strength showed an initial upward trend and afterward a fall with increasing the energy densities. In addition, high energy densities led to an increase in the sub-micron cellular and columnar grain size produced by the rapid solidification during SLM, while the grain evolved from columnar grains into cellular grains as the ratio of temperature gradient to solidification rate decreased from the bottom to the top of the molten pool. The granular bainite and the meta bainite including carbon-rich retained austenite films were obtained in as-fabricated samples. The quantity of retained austenite phase decreased due to austenite decomposition with increasing energy densities. Thus, 24CrNiMo steel exhibited a high microhardness of 374.4 HV10 and a high tensile strength of 1249.5 MPa respectively at the most optimized energy density of 85 J/mm3 because of low porosity, fine grain size, and an appropriate fraction of stable retained austenite.

Development of continuously cooled low-carbon, low-alloy, high strength carbide-free bainitic rail steels

Abstract: In the present work, attempts have been made to design and develop low-carbon, low-alloy, high strength carbide-free bainitic steels for application in heavy-haul rail tracks. The study starts with an analytical approach for understanding the effects of different alloying elements on the thermodynamic and kinetic aspects of bainite transformation and the resultant microstructure and mechanical properties. Based on the analysis, two steel compositions were designed for further investigation. The alloys were prepared by melting and casting and processed by hot-rolling and air cooling, which is the usual route for the industrial production of rail sections. Microstructures of the developed steels primarily consisted of plates of carbide-free bainitic ferrite, interspersed with fine films of retained austenite. These continuously cooled steels offer ultra-high strength levels with very good ductility which are far superior to existing pearlitic grade rail steels. Detailed characterization and quantification of microstructural constituents have been carried out and they are correlated with the tensile proprieties using semi-empirical formulations available in literature.

Effect of vanadium on microstructure and mechanical properties of bainitic forging steel

Abstract: This investigation attempts to thoroughly explore the effect of microalloying element V on the microstructural characteristics and mechanical properties of 20Mn2SiCrS bainitic forging steel. The results show that the cooling rate range of bainitic transformation was broadened with a 0.13% V addition and mainly a granular bainite microstructure was obtained after post-forging continuous cooling. The addition of V could refine the martensite/austenite (M/A) constituent and inhibit the formation of pro-eutectoid ferrite. Physical-chemical phase analysis revealed that only ~8.5% of the added V was in the V(C,N) precipitate in the as-forged condition, which resulted in relatively small precipitation strengthening (~40 MPa) compared with that in ferritic-pearlitic medium carbon forging steel. The addition of V can enhance the overall mechanical properties of the steel and it is concluded that the primary role of V in the as-forged bainitic forging steel is mainly to promote the formation of bainite and to refine the microstructure especially the M/A constituents as well as to resist softening of the M/A during the post-forging continuous cooling or during subsequent tempering when tempering treatment is necessary.

The effect of laser scanning speed on microstructural evolution during direct laser deposition 12CrNi2 alloy steel

Abstract: This paper focused on the effect of different laser scanning speed (4 mm/s, 5 mm/s, 6 mm/s and 7 mm/s) on the microstructural evolution of direct laser deposition (DLD) 12CrNi2 alloy steel, and analyzed the relationship between microstructure and performance of DLD-processed samples. The results showed that the microstructure in the middle of as-deposited samples consisted of a large amount of bainite, a small amount of martensite (M) and ferrite (F). With the increase of laser scanning speed, the fraction of ferrite decreased from 55.6% to 14.7%, while that of martensite increased from nearly 0% to 4.9%. Besides, as increasing the laser scanning speed, granular bainite (GB) transformed into lath bainite (LB) due to the increase of cooling rate, and the fraction of LB reached the maximum of 29.9% when the scanning speed was 7 mm/s. In addition, the functions about the relationship between laser scanning speed and phase fractions were fitted in order to provide a theoretical basis for the design of DLD process parameters. EBSD maps of as-deposited samples exhibited anisotropy due to the complex heat flux direction during the multi-layer laser deposition process. With the increase of laser scanning speed, the grain size showed a downward trend from 5.89 µm2 to 3.44 µm2. The sample fabricated at 7 mm/s contained more LB and M, leading to the highest mean microhardness of 355 ± 6 HV0.2. The sample fabricated at 6 mm/s exhibited the best wear resistance due to its optimum combination of hardness and toughness. Because of a large amount of ferrite with optimal toughness, the sample fabricated at 4 mm/s had the best impact toughness of aku = 80 J/cm2.

Evolution of microstructure and mechanical properties during tempering of M50 steel with Bainite/Martensite duplex structure

Abstract: The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.

Effect of matrix structures on TRIP effect and mechanical properties of low-C low-Si Al-added hot-rolled TRIP steels

Abstract: We applied different hot-rolling direct quenching and partitioning (HDQ&P) processes to a low-C low-Si Al-added steel and obtained eight TRIP-assisted steels with different matrix structures, viz, martensite, ferrite/bainite, ferrite/martensite and ferrite/bainite/martensite. The microstructures were characterized using SEM, TEM and XRD. The mechanical properties were investigated by means of uniaxial tensile tests. Quasi in-situ tensile tests in combination with EBSD and microscopic digital image correlation (μ-DIC) analyses were performed on microstructure regions containing ferrite, martensite and retained austenite to investigate the TRIP effect and composite effect. Considering both the chemical and strain partitioning among the structure components, we analyzed the specific influence of the matrix structures on the austenite stabilization, martensitic transformation of retained austenite and ductility of the material. The results show that the TRIP steel with a martensitic matrix exhibits high ultimate tensile strength (UTS) and yield ratio reaching up to 1200 MPa and 0.87, with the product of strength and elongation (PSE) of about 18000 MPa%. The introduction of 8%–25% (in area fraction) of ferrite leads to a decrease of the yield strength in 100–200 MPa, but without significant reduction of the UTS. The increase of the ferrite fraction to 30%–35% results in an obvious decrease of the UTS and yield ratio to about 950 MPa and 0.6. Retained austenite, with the amount of 14 vol %, was stabilized in the TRIP steel with a martensitic matrix. The introduction of ferrite (8%–35% in area fraction) and granular bainite can promote the carbon partitioning, thus enhancing the stabilization of retained austenite. The TRIP steel with a martensitic matrix exhibits a slight martensitic transformation of retained austenite because of the low deformability of the martensite. For TRIP steel with a matrix composed of ferrite and martensite, the low deformation compatibility of the soft and hard structure components also leads to a week martensitic transformation of retained austenite. The introduction of granular bainite can effectively improve the deformation uniformity and enhance the martensitic transformation during deformation. The TRIP effect and the composite effect of matrix structures jointly control the ductility of the TRIP steels. To optimize the ductility, we not only need to enhance the TRIP effect but also to improve the deformation compatibility of the matrix structures by tuning the structure components and their strength differences.

Acquiring a low yield ratio well synchronized with enhanced strength of HSLA pipeline steels through adjusting multiple-phase microstructures

Abstract: Intercritical treatments within the dual-phase (α+γ) region were applied on HSLA pipeline steels, for acquiring a low yield ratio (YR) well balanced with desirable strength. Intercritical cooling treatment (ICT), step cooling treatment (SCT) as well as direct cooling treatment (DCT) after full austenization were designed to obtain an optima multiphase microstructure. Effects of cooling rate in DCT routine and intercritical temperatures in ICT and SCT routines on microstructural evolution and corresponding mechanical properties were also investigated. Especially, the SCT treatment applied with an intercritical temperature of 750 °C produces a microstructure composited of “soft” coarse polygonal ferrite, and “hard” acicular bainite and lath martensite containing large amounts of dislocation tangles or networks generated by deformation. Such multiple phase constituents guarantee the high strength and remarkable ductility on deformation, meanwhile cleavages propagation is hindered by the high-angle boundaries of bainite and martensite sheaves, which leads to the lowest YR ~0.61 combined with highest tensile strength among all. In addition, by using the Swift equation to elucidate the relationship between the phase component and yield ratio, it is found that simply increasing the fraction of low-temperature transformed phases, like high-strength acicular bainite and lathed martensite, or the percentage of soft polygonal ferrite for good ductility, can hardly solve the problem how to achieve ultralow-YR pipeline steels balanced with enhanced strength. The present result proves that, through utilizing the proposed SCT heat treatment on pipeline steels, an ultralow yield ratio ~0.61 achieved is synchronized with a desirable strength, which efficiently overcomes the trade-off limit between the strength and yield ratio when applying conventional heat-treatment routines. The fact indicates that, rationally adjusting the content of multi-phase microstructure through optimizing the intercritical treatment conditions, can enable us of realizing the synchronous improvement of the YR and strength in HSLA pipeline steels for real engineering.

Phase change materials as quenching media for heat treatment of 42CrMo4 steels

Abstract: In the present work, paraffin phase change material is used as quenchant for the heat treatment of 42CrMo4 alloy and compared with water, air, and CuO doped paraffin. The samples were prepared based on ASTM E 8M-98 standard for tensile test and then heated up to 830 °C, kept for 4 h in an electric resistance furnace and then quenched in the mentioned media. Elastic modulus, yield strength, ultimate tensile strength, elongation, and modulus of toughness were determined according to the obtained stress-strain curves. Moreover, the hardness and microstructural evolution were investigated after the heat treatment at different media. The samples quenched in paraffin and CuO-doped paraffin are higher in ultimate tensile strength (1439 and 1306 MPa, respectively) than those quenched in water (1190 MPa) and air (1010 MPa). The highest hardness, with a value of HV 552, belonged to the sample quenched in CuO-doped paraffin. The microstructural studies revealed that the non-tempered steel had a ferrite/pearlite microstructure, while by quenching in water, paraffin and CuO-doped paraffin, ferrite/martensite microstructures were achieved. It is also observed that using the air as quenchant resulted in a three-phase bainite/martensite/ferrite microstructure.

On the Post-Printing Heat Treatment of a Wire Arc Additively Manufactured ER70S Part.

Abstract: Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, ie, acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample; however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component

Tensile deformation damage behavior of a high deformability pipeline steel with a ferrite and bainite microstructure

Abstract: The work-hardening characteristics and deformation behavior of high deformability pipeline steel with a polygonal ferrite and bainite microstructure were studied in this paper. The high work-hardening ability of polygonal ferrite with 35% bainite (PF+35%B) dual phase sample contributed to its high tensile properties. Electron backscattered diffraction was conducted to analyze the mechanism of the micro-damage behavior. The results of kernel average misorientation and grain reference orientation deviation of the deformation microstructure revealed that the main deformation mechanism of the polygonal ferrite grain was slip with a number of slip systems. Dislocation rearrangement and a local orientation gradient caused by crystal rotation during sliding to cause the formation of micro-voids. The strain dispersion ability of the PF+35%B sample was better than that of the polygonal ferrite with 27% bainite (PF+27%B) dual phase sample. The risk of micro-voids nucleation and crack propagation were reduced to an improved plastic deformation ability for PF+35%B sample. The average Taylor factor (M) of the PF+35%B sample was higher than that of the PF+27%B sample, which indicated the grains of the PF+27%B sample was more liable to slip during deformation and yield under low stress. The evolution characterization of the micro-texture showed that the γ-fiber was more stable compared with the α-fiber, and the stronger γ-fiber may contribute to the improved plasticity.

Effects of ausforming temperature on bainite transformation kinetics, microstructures and mechanical properties in ultra-fine bainitic steel

Abstract: The effects of ausforming temperature on bainite transformation kinetic and plastic deformation mechanism were evaluated by thermal simulation method and warm rolling process. Results showed that entire process of bainite transformation austempered at 300 °C was notably accelerated by ausforming due to the increased nucleation sites, and by diminishing ausforming temperature as well. However, ausforming would increase undercooled austenite stability, leading to the decrease of maximum attainable volume fraction of bainitic ferrite. Compared with traditional isothermal transformation, ausforming process could effectively refine microstructure, as well as improve mechanical properties. And with decreasing ausforming temperature, strength, hardness and ductility were all increased, which attributed to the thinner thickness of bainite lath and smaller dimension and proportion of blocky retained austenite, by contrast, the change trend of impact toughness was virtually opposite resulted from variant selection. When ausforming at 300 °C, the ultra-fine bainitic steel exhibited the best comprehensive mechanical performance, which was almost 1850 MPa and with up to 23% for ultimate tensile strength and total elongation, respectively.