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Showing papers on "Bainite published in 2017"


Journal ArticleDOI
TL;DR: In this paper, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite, and the thermal stability during heating of retained Austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements.
Abstract: The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

135 citations


Journal ArticleDOI
TL;DR: In this article, the structural and morphological features of the product phases obtained in isothermal treatments below the martensite start temperature in a low-carbon high-silicon steel were characterized.

102 citations


Journal ArticleDOI
TL;DR: In this paper, the hybrid deposition and micro-rolling (HDMR) process has been used to eliminate the ubiquitous anisotropy in the WAAM bainitic steel samples, and an initially optimized micro rolling morphology based on decreasing the depth of penetration and increasing the ratio of width to reinforcement was used.

100 citations


Journal ArticleDOI
TL;DR: In this paper, the authors tried to establish a direct correlation between crystallographic variants and impact toughness, and provided some novel insights into the mechanism of cooling rate on the impact toughness of coarse grained heat affected zone (CGHAZ) of offshore engineering steel by means of electron back-scattering diffraction (EBSD) analysis.
Abstract: In this article, the authors tried to establish a direct correlation between crystallographic variants and impact toughness, and provided some novel insights into the mechanism of cooling rate on the impact toughness of coarse grained heat affected zone (CGHAZ) of offshore engineering steel by means of electron back-scattering diffraction (EBSD) analysis. The results showed that variant selection becomes stronger with an decrease in the cooling rate, resulting in the decline of high angle grain boundaries (HAGBs) and thus lower the impact toughness. Moreover, the variation in impact toughness is mainly correlated to the crystallographic block size. The larger the block size, the lower the impact toughness. By visualizing the crystallographic features, it has been clarified that the transition from Bain zone grouping to close-packed plane grouping with the increase of cooling rate, while the corresponding microstructure changes from granular bainite to lath bainite. Furthermore, it has been found that ~ 25% (number fraction) reconstructed prior austenite grains in simulated CGHAZ present a twin-related structure (austenitic twin), which can enhance the variant selection and displays a negative effect on the formation of HAGBs.

93 citations


Journal ArticleDOI
TL;DR: In this paper, a C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization, and the discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated.
Abstract: A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280 MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.

76 citations


Journal ArticleDOI
TL;DR: In this article, the authors quantified the impact of the transformation of the metastable austenite phase embedded in a ferrite-bainite-martensite matrix on the work hardening of a high-strength industrial TRIP-assisted dual-phase steel.

71 citations


Journal ArticleDOI
TL;DR: The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability and an analytical model for void evolution and coalescence was developed and applied.
Abstract: The role of microstructural damage in controlling the edge stretchability of Complex-Phase (CP) and Dual-Phase (DP) steels was evaluated using hole tension experiments. The experiments considered a tensile specimen with a hole at the center of specimen that is either sheared (sheared edge condition) or drilled and then reamed (reamed edge condition). The damage mechanism and accumulation in the CP and DP steels were systematically characterized by interrupting the hole tension tests at different strain levels using scanning electron microscope (SEM) analysis and optical microscopy. Martensite cracking and decohesion of ferrite-martensite interfaces are the dominant nucleation mechanisms in the DP780. The primary source of void nucleation in the CP800 is nucleation at TiN particles, with secondary void formation at martensite/bainite interfaces near the failure strain. The rate of damage evolution is considerably higher for the sheared edge in contrast with the reamed edge since the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and initial damage behind the sheared edge. The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability. Microstructural damage in the CP and DP steels was characterized to understand the interaction between microstructure, damage evolution and edge formability during edge stretching. An analytical model for void evolution and coalescence was developed and applied to predict the damage rate in these rather diverse microstructures.

70 citations


Journal ArticleDOI
TL;DR: In this paper, microscopic analysis methods were achieved for identification and quantification of microstructures in the HAZs of three HSLA steels, including coarse-grained HAZ (CGHAZ) consisting of acicular ferrite, granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent.
Abstract: In order to understand and improve fracture toughness of heat affected zones (HAZs) of high-strength low alloy (HSLA) steels, complex microstructures including quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent should be identified, quantified, and then correlated with critical crack tip opening displacement (CTOD). In this study, microscopic analysis methods were achieved for identification and quantitation of microstructures in the HAZs of three HSLA steels. The coarse-grained HAZ (CGHAZ) consisted of AF, GB, and BF together with a small amount of MA, while the inter-critically heated HAZ (ICHAZ) consisted of QPF, GB, and MA. In the CGHAZ, Ni promoted the formation of AF, while it prevented the formation of GB, and the addition of Ni resulted in very high critical CTOD. In the CGHAZ, both Ni and Mn promoted the formation of AF and prevented the formation of GB, while Ni was more effective than Mn. Thus, the addition of Ni resulted in very high critical CTOD. In the ICHAZ, both Ni and Mn promoted the formation MA. However, in the high-Ni-containing steel, a number of MAs were densified along Ni-segregated bands, and thus readily provided void initiation sites. This played an important role in reducing the mean free path for coalescence of voids and crack propagation, which easily led to the serious deterioration of critical CTOD.

68 citations


Journal ArticleDOI
TL;DR: In this article, a low carbon bainitic steel with ultra high strength of ∼1650 MPa and elongation and toughness of ∼ 16% and ∼72 J/cm2 respectively, was obtained through austempering.

62 citations


Journal ArticleDOI
TL;DR: In this paper, a quantitative theory to explain bainite formation kinetics has been proposed based on the nucleation kinetics of bainitic sub-units, which shows acceptable correlation with experimental results, but it is observed that the kinetic models show a certain degree of discrepancy with actual kinetics.

60 citations


Journal ArticleDOI
TL;DR: In this article, martensite properties were varied through heat treatment in a low carbon Q&P steel consisting of retained austenite and martensites and additional conditions were produced by reheating the steel to 450°C for 30min or to 700°C followed immediately by quenching.

Journal ArticleDOI
TL;DR: In this paper, atomistic simulations are used to create an fcc-bcc iron interface having a structure and motion that match the major experimental observations on dislocated lath martensite.

Journal ArticleDOI
TL;DR: In this article, the effects of transformation temperature and carbon content on variant selection of bainite structure in Fe-2mass%Mn-C alloys with carbon content ranging from 0.2 to 0.75 mass% were investigated at transformation temperatures between 673 and 773 K.

Journal ArticleDOI
TL;DR: In this article, the mass balance for carbon is successfully achieved by considering all the transformation products together with an estimation of the carbon segregated to linear defects and precipitates in low temperature bainite.

Journal ArticleDOI
TL;DR: The evolutions of microstructure and mechanical properties during tempering at 700°C, of normalized and oil-quenched 225Cr-1Mo-025V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated.
Abstract: The evolutions of microstructure and mechanical properties during tempering at 700 °C, of normalized and oil-quenched 225Cr-1Mo-025V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure After 05 h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite However, when the tempering time is increased from 05 to 128 h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample

Journal ArticleDOI
TL;DR: In this paper, the effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses.
Abstract: The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 °C followed by tempering within 240–450 °C. Results show that the sample tempered at 340 °C occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 °C and 360 °C with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 °C. When the temperature is increased to 450 °C, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 °C, followed by decreasing with further increase in the temperature. This behavior is related to the competition between retained austenite further transforming into bainite and decomposing into carbide during tempering.

Journal ArticleDOI
TL;DR: In this paper, the authors present a comprehensive study on the strain-induced martensitic transformation and reversion transformation in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis.
Abstract: This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

Journal ArticleDOI
TL;DR: In this paper, structural changes and flow stress of advanced high-strength steels (AHSS) subject to deformation at high strain rates, taking into account the transformation of retained austenite into martensite, were investigated.

Journal ArticleDOI
TL;DR: In this article, the role of silicon in the stability of carbon-enriched austenite is evaluated in a series of quenching-and-partitioning (Q&P) heat treatments, with particular focus on the partitioning stage.

Journal ArticleDOI
TL;DR: In this paper, the carbon content of retained austenite (RA) with different neighboring phases is investigated by correlating electron backscattering diffraction, transmission electron microscopy and atom probe tomography.

Journal ArticleDOI
TL;DR: In this article, slow strain rate tension (SSRT) tests in high-pressure hydrogen gas, combined with hydrogen permeation tests and microstructure analysis were conducted on X80 steel, intercritical heated-affected zone (ICHAZ), fine-grained heat-affected zones (FGHAZ) and coarse-general heataffected zone(CGHAZ).

Journal ArticleDOI
TL;DR: In this paper, the crystallographic characteristics of bainite transformed in a temperature range of 200-350°C, where a nanobainitic structure is formed, were investigated.

Journal ArticleDOI
TL;DR: In this paper, a tool made of tungsten-rhenium alloy was used to weld 3-mm-thick HSLA plates using FSW and the relationship between microstructure and tensile strength was studied under various welding conditions.
Abstract: Friction stir welding (FSW) is a promising technique to join HSLA steels without the problems encountered during fusion based welding processes. In the present work, 3 mm thick HSLA plates were successfully welded using FSW. A tool made of tungsten-rhenium alloy was used in this work. The relationship between microstructure and tensile strength was studied under various welding conditions i.e. change in traverse speed (57–97 mm/min). The microstructure of the weld nugget revealed the presence of upper bainite and fine ferrite phases. The amount of upper bainite reduced with increase in traverse speed. EBSD images showed a reducing trend for grain size. The details of hardness, tensile strength and bending test were reported.

Journal ArticleDOI
TL;DR: A medium carbon Mn-Si-Cr alloyed steel was treated by a novel bainite-based quenching and partitioning (BQP uniform elongation and total elongation: 26.2% and 31.8%; the reduction of area: 47.9%).
Abstract: A medium carbon Mn-Si-Cr alloyed steel was treated by a novel bainite-based quenching and partitioning (BQP uniform elongation and total elongation: 26.2% and 31.8%; the reduction of area: 47.9%). Besides the transformation-induced plasticity effect of the retained austenite and the composite effect of the multiphase after BQ&P treatment, the formation of carbide free bainite also plays a significant role on the enhanced mechanical properties. The carbide-free bainite could improve the damage resistance of the multiphase due to the additional strain-hardening capacity within the local plasticity deformation zone near the tip of micro-cracks. In this case, the fraction and distribution of CFB should be controlled properly and the macrosegregation should be avoided.

Journal ArticleDOI
TL;DR: In this article, a quench dilatometer was used to estimate the fraction of martensite formed at a given temperature below the start temperature of the martensitic transformation profile, and the measured austenite fractions after quench treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions.
Abstract: In the present work, Quenching and Partitioning (Q&P) heat treatments were carried out in a quench dilatometer on a 0.2 wt% carbon steel. The microstructure evolution of the Q&P steels was characterized using dilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate the fraction of martensite formed at a given temperature below the martensite start temperature Ms. Q&P was shown to be an effective way to stabilize retained austenite at room temperature. However, the measured austenite fractions after Q&P treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitive to the quench temperature and were never larger than the ideal predicted maximum fraction. Competitive reactions such as austenite decomposition into bainite and carbide precipitation were found to occur in the present work. Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperatures and partitioning times. The direct contributions between Q&P microstructural constituents -such as retained austenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This was critically discussed and compared to quenching and austempering (QAT) which is a more conventional processing route of stabilizing retained austenite at room temperature. Finally, Q&P steels were shown to exhibit an interesting balance between strength and ductility. The achievement of this interesting combination of mechanical properties was reached for much shorter processing times compared to QAT steels.

Journal ArticleDOI
TL;DR: In this paper, the performance of a high strength pipeline steel with three different microstructures, granular bainite & lath bainites (GB, LB, and acicular ferrite), and quasi-polygonal ferrite (QF), was studied by using corrosion experiment based on standard NACE TM 0284.

Journal ArticleDOI
TL;DR: In this article, 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 bainitic-partitioning-parting-tempering- tempering, and the strength, ductility and toughness were enhanced concurrently after BQ-P-T treatment (i.e., ultimate tensile strength: 1416 MPa, the PSE: ~ 25.5 GPa and the CVN impact energy at
Abstract: 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: 1416 MPa, the PSE: ~ 25.5 GPa%, the CVN impact energy at 20 °C and − 40 °C: ~ 95 J cm −2 and ~ 45 J cm −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.

Journal ArticleDOI
TL;DR: The microstructure and hardness of Custom 465® precipitation hardened stainless steel were characterized following 4 hours aging at temperatures ranging from 482°C to 648°C as discussed by the authors.

Journal ArticleDOI
TL;DR: A combinatorial approach using diffusion couples and TEM analyses was carried out to investigate the composition-dependent martensitic transformation in NiMnGa alloys in this article, where the compositions cover a large portion of the off-stoichiometric Ni 2 MnGa compositions and some Mn-rich compositions.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the strength-ductility- impact toughness combinations in nano-bainite after two-step austempering process comparing with those obtained after conventional isothermal bainite transformation.
Abstract: It is crucial to eliminate the thermally and mechanically less stable austenite blocks as much as possible if enhanced mechanical properties are demanded in high performance nanostructured bainitic steels. Step-austempering would be an efficient procedure in this regard. This article aims to investigate the strength-ductility- impact toughness combinations in nano-bainite after two-step austempering process comparing with those obtained after conventional isothermal bainite transformation. It has been shown that large austenite blocks further decomposed to bainite after step-austempering process which in turn decreased the average volume fraction of austenite, increased its mechanical stability and generally refined the final microstructure. Step-austempering increased the hardness value and enhanced the yield strength and ultimate tensile strength properties. Additionally, the higher mechanical stability of retained austenite in step-austempered samples increased the elongation level and improved the impact toughness at earlier stages of the second step of transformation. The important point is, it has been found that ductility and toughness properties were influenced by transformation time at the second stage of austempering which approved the idea that not only the mechanical stability and morphology of retained austenite but also its volume fraction must be taken into account when applying a multi-step austempering heat treatment in nanobain steels.