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gives the austenite grain feature of the super bainitic steel tested at two soaking temperatures (1000 and 1100 @BULLET C) and the corresponding bainite morphology after 1-h bainitic transformations at 330 @BULLET C. It is apparent that the bainite morphology depends upon the austenite grain size before the transformations. The growth of bainite sheaves is restrained by austenite grain boundaries and
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In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100°C for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite...
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... Metallurgists designing and producing advanced steel defined carbide-free bainite (CFB) as the bainitic microstructure acquired without carbide precipitation during the austempering of the steels [11][12][13][14][15]. Because of the combination of superior technological possessions (excellent wear resistance, high tensile properties, good weldability, and enhancing corrosion properties) with the truncated production cost, carbide-free bainite (CFB) steels are counted as a promising class of HSS (high-strength steels) [11][12][13][14][15]. Carbide-free bainitic (CFB) steels are in development because of their excellent technological properties, which are demanded in different appliances, e.g., building construction, marine concrete, and highway (rails track). ...
... Metallurgists designing and producing advanced steel defined carbide-free bainite (CFB) as the bainitic microstructure acquired without carbide precipitation during the austempering of the steels [11][12][13][14][15]. Because of the combination of superior technological possessions (excellent wear resistance, high tensile properties, good weldability, and enhancing corrosion properties) with the truncated production cost, carbide-free bainite (CFB) steels are counted as a promising class of HSS (high-strength steels) [11][12][13][14][15]. Carbide-free bainitic (CFB) steels are in development because of their excellent technological properties, which are demanded in different appliances, e.g., building construction, marine concrete, and highway (rails track). ...
In this investigation, an attempt has been made to identify the metallurgical properties with the change of silicon contents, either theoretically by utilizing Thermo-Calc (the Scheil–Gulliver model implemented in the Thermo-Calc software) or JMatPro software, as a computational prediction technique, in addition to experimental examinations by dilatometer. Microstructures of investigated steels were evaluated optically at low magnification using optical microscopy. Scanning electron microscopy was used to study the observed microstructure at high magnification . ASTM standard specification E-8 was utilized for measuring the tensile property values, while fracture surfaces of the tensile samples were inspected by EDS (point-analyzer) employed in scanning electron microscopy. The investigated steel’s as-polished surface was studied using the experimental results, which revealed that the steel microstructure ranged from full-bainitic to full-pearlitic structures according to the variation of silicon contents. The count of non-metallic inclusions decreased and vice versa by the area occupied by non-metallic inclusions with the rising silicon content. The steel containing silicon of 0.87 wt.% has the best toughness combined with high tensile strength and hardness incomparable with conventional steel. Elongation (16.2%) combined with an ultimate tensile strength (1113 MPa) was achieved for the steel containing 0.87 wt.% Si.
... *Corresponding author: e-mail address: vgefremenko@gmail.com Beginning from the pioneering research of H. Bhadeshia et al. in the 90s and to the present, the approach to develop the high-strength steels with nanostructured carbide-free bainite (nanobainite) has been actively studied [4][5][6]. The main peculiarity of this structure is nanoscaled (30-60 nm width) laths of bainitic ferrite with interlayers of carbon-enriched retained austenite (RA) with no cementite carbides [7,8]. ...
Tensile/impact behaviour of lower bainite obtained in high-Si steel 55Si3Mn2CrMoVNb was studied using SEM, TEM, and XRD. Specimens were austenitized at 900 • C and isother-mally treated at 250, 270, and 300 • C with holding up to 600 min. The heat treatment results in the formation of cementite-free lower bainite/retained austenite structure, where retained austenite was found as blocky "islands" and interlaths "films". The width of bainitic ferrite laths decreases from 170-240 µm to 45-80 µm with holding temperature decreasing. This results in increasing UTS (to 1700 MPa) and hardness (to 52 HRC). The optimal combination of mechanical properties (UTS 1397-1522 MPa, hardness 45-47 HRC, total elongation 18-21 %, U-notched impact toughness 105-139 J cm −2) refers to holding at 300 • C to be associated with higher amount of retained austenite (30-33 %). With prolonging the bainitizing duration the hardness and ductility decreases while impact toughness increases. Prolonged holding at 300 • C leads to a continuation of bainite transformation and precipitation of transitional carbides within ferrite laths. K e y w o r d s : carbide-free lower bainite, retained austenite, phase transformations, mi-crostructure, mechanical properties
... It Fig.3(c). The average grain sizes of ferrite including polygonal ferrite (PF) and irregular ferrite (IF) were measured by the linear intercept method [17] . The average ferrite grain sizes in the steels with the WAC mode and AWC mode are 2.9 and 3.8 μm, respectively. ...
The industrial trials of two cooling modes, i e, water cooling in forepart + air cooling in later part (WAC) and air cooling in forepart + water cooling in later part (AWC), were carried out for a Ti-Nb microalloyed steel. The average cooling rates and coiling temperature were the same for two modes. The continuous cooling transformation (CCT) curve of the tested steel was drawn. The effects of the cooling mode on the microstructure, precipitates, and properties of the steels were investigated. Results show that the strength of the steel in the WAC mode is significantly larger than that in the AWC mode, mainly because the smaller the grain size, the more and finer the grain precipitates. Therefore, when the average cooling rate is constant, the fast cooling in the forepart is an effective method to increase the strength of steels. However, the increase in the strength is accompanied by the decrease in toughness, so that the toughness of the steel should be considered when changing the cooling mode.
... And the machine was introduced elaborately in Ref. [11]. In addition, LSCM has been gradually used recent years, for instance, Liu et al. studied the austenite growth in Fe-C-Mn-Si super bainitic steel [12], Kolmskog et al. directly observed bainite formation below the martensite start temperature (Ms) by LSCM [13] and Terasaki et al. poured attention into morphology and crystallography of bainite transformation in a single prior-austenite grain of low carbon steel with the aid of LSCM [14], and so on. In a word, the LSCM will play as accurate a role to verify the effective grain boundary. ...
Grain size dependence of microhardness has been addressed in the bainitic reheated weld metals by in situ observation of morphological evolution and characterization of microstructural development. A higher cooling rate promotes the boundary of smaller prior austenite grains to provide more effective sites for primary bainitic ferrite nucleation, yet a lower cooling rate is increasingly beneficial to sympathetic nucleation as well as impingement of secondary bainitic ferrite. The microstructures, obtained by cooling at a higher rate and composed of abundant lath bainite, are closer to the microstructures in the raw weld metal than those cooled at a lower rate, including lath bainite, acicular ferrite and intercritical ferrite. Microhardness is decisive by prior austenite grain size mainly, as well as microstructures. Smaller grains contribute notably to microhardness, and it is worth stressing that the sizes of smaller grains lie on prior austenite grain boundaries rather than the subboundaries generated by intragranular acicular ferrite and intercritical ferrite.
... The bainite morphology depends apparently upon the austenite grain size before the transformation. The growth of bainite sheaves is restrained by austenite grain boundaries [23]. The lath-like or plate-like AF grains can divide large austenite grain into smaller separate regions [24]. ...
1. Introduction There have been ongoing developments in high-strength steel weld metal (WM) to increase strength while maintain acceptable toughness since 1960s [1]. The mechanical properties of welds are determined by the microstructure developed during welding process [2]. The chemical composition, austenite grain size and cooling rate are the main factors that determine the microstructure of a weld [3]. In order to increase the mechanical properties of low-carbon bainite welds, the selection of an appropriate flux composition plays quite an important role to obtain a fine lath bainite (LB) and acicular ferrite (AF), which can improve the properties of welds [4]. It is crucial to add suitable alloy elements in WM to obtain the desired properties with appropriate microstructures. The addition of Mo to Nb microalloyed steels promotes the formation of a low angle misorientation substructure, which at the same time enhances the strengthening effect arising from grain size reduction and dislocation density [5-7]. Mn and Cr have a large effect on the hardenability that all promote bainite formation at the expense of AF. However, the combined effect of these elements together may also lead to the formation of low carbon martensite [8]. Among the alloying elements of low carbon martensite/bainite steels, an increase in Ni content was found to be an effective way to improve both the strength and the fracture toughness [9]. As one of vital causes, grain refinement, nevertheless, can increase both strength and toughness of WM. Since alloying elements in WM play vital roles in microstructure evolution, including grain refinement, an appropriate alloying strategy becomes critical to promote a desirable microstructural distribution and to achieve specification requirements [10]. The decrease of the cooling temperature and the increment of alloying content can reduce the mean unit size. In addition, small columnar grains were associated with a Nieq between 3.4 and 6.2% [11], however, few researches report the effect on grain size of Ni addition in WM. The tensile properties, and more specifically the yield strength (YS), are controlled by combining different strengthening mechanisms such as microstructural refinement, solid solution hardening, precipitation strengthening, and dislocation hardening related to the modification of the final microstructures, from the conventional polygonal ferrite to non-polygonal phases or bainitic structures [12]. Some authors have reported an effect of the presence of coarse grains on the ultra-tensile strength (UTS), but their effect on the YS is reported to be more limited [13]. These studies have emphasized the effects of some alloying elements, rather less attention has been paid to effects of Ni element on properties of low carbon bainite WMs. The objective of this work is to detail the effect of Ni content in metal powder-cored wire on the microstructure and mechanical properties of WMs obtained by the multi-pass welding process. 2. Experiments The Q345 HSLA steels were automatic Gas Metal Arc welded using a single consumable with various Ni contents in the metal power flux-cored wires. Three WM specimens with different levels of Ni in WMs were made under the same welding conditions. All of the mentioned assignments were carried out in the Atlantic China Welding Concumables, Inc, Zigong. The chemical compositions of WMs are given in Tables 1. The physical conditions for the welding process are listed in Table 2. The schematic illustration of the Y-type joint by multi-pass welding is shown in Fig. 1(a), and cross-sectional views of multi-pass welding shows that the former pass is the back sealing welding in Fig. 1(b). As the deposited WM specimens were obtained by multi-pass welding, the microstructure in each pass differs. In this work the microstructure were divided into two typical zones i.e. the as-deposited zone (Zone I in Fig. 1(b)) and reheated WM zone (Zone II in Fig. 1(b)), respectively.
... Kolmskog et al. directly observed bainite formation below the martensite start temperature (M s ) by LSCM [6]. Liu et al. studied austenite growth in Fe-C-Mn-Si super bainitic steel [7]. Terasaki and Komizo devoted attention to the morphology and crystallography of bainite transformation in a single prior-austenite grain of lowcarbon steel with the aid of LSCM [8]. ...
A combination of laser scanning confocal microscopy (LSCM) and dilatometry was utilised to simultaneously qualitatively analyse the morphological evolution of bainite by examination of in situ observed micrographs and to quantitatively investigate the amount of bainite transformation by studying dilatometry data. Shorter bainite structures form for smaller prior austenite grain sizes and lower cooling rates, which causes greater bainite transformation to occur in the latter stages of the transformation process. On the other hand, the amount of lath-shape sub-structure increases due to a higher cooling rate. In addition, the surface relief presents greater height and the peak and valley values stay farther away from the horizontal line for specimens at a higher cooling rate. In addition, a lower strain energy per unit volume gives rise to greater bainite transformation compared to that from abundant driving forces.
... where L is the length of line, N the number of grains intercepted, and M the magnification on the micrograph [26]. The average grain size of the specimen annealed at 550°C for 30 min is 330 nm, while it is 310 nm in the specimen annealed at 650°C for 2 min. ...
The microstructures and properties of the ultrafine-grained low-carbon steel were investigated. Martensite microstructure was obtained by quenching a low-carbon steel, followed by 50% strain cold rolling and then annealing at 500–650 °C for 2 and 30 min, respectively. Microstructures were observed, and tensile properties were measured for the specimens treated with cold rolling and annealing. The effects of annealing parameters on the microstructure and mechanical properties were analyzed. It shows that the microstructure of specimen annealed at 550 °C for 30 min consists of ferrite grains with an average size of 330 nm. The ultrafine-grained low-carbon steel exhibits not only high tensile strength (867 MPa), but also good elongation (16.7%).
... So the amount of bainitic transformation decreases significantly with Nb addition, indicating that Nb inhibits bainitic transformation. It has been reported that smaller austenite retards bainite transformation [21][22][23] because smaller grains hinder the growth of bainite. Nb addition decreases the grain size of austenite (Fig. 3) and thereby hinders the bainitic transformation. ...
To investigate the effects of Nb addition on transformation kinetics and microstructure properties in low-carbon bainitic steels, two C–Si–Mn bainitic steels were designed, one of which was added with 0.025 (wt%) Nb and the other was C–Si–Mn steel. Heat treatment experiments were carried out on thermal simulator. The microstructures were observed by scanning electron microscope. The results show that lath-like bainite is obtained in both two steels. Film-like retained austenite distributes between bainite laths. In addition, Nb retards bainite transformation owing to smaller parent austenite grains in steel with Nb addition. However, Nb improves the strength of the low-carbon bainitic steel by grain refinement. Moreover, transformation kinetics equations for two tested steels are established based on the experimental data. The experimental results are useful to clarify the function of Nb in low-carbon bainitic steels and provide the theoretical reference for the composition design of low-carbon bainitic steels.
... The investigations were conducted on a VL2000DXSVF17SP laser scanning confocal microscope. The specimen chamber was initially evacuated to 6 × 10 −3 Pa before heating and argon was used to protect specimens from surface oxidation [16,17]. The experimental routes for LSCM were the same as those for thermal simulation experiments without stress ( Figure 2). ...
... The investigations were conducted on a VL2000DXSVF17SP laser scanning confocal microscope. The specimen chamber was initially evacuated to 6ˆ10´3 Pa before heating and argon was used to protect specimens from surface oxidation [16,17]. The experimental routes for LSCM were the same as those for thermal simulation experiments without stress ( Figure 2). ...
In this study, thermal simulation experiments under different austenitization temperatures and different stress states were conducted. High-temperature laser scanning confocal microscopy (LSCM), thermal dilatometry, and scanning electron microscope (SEM) were used to quantitatively investigate the effects of the uniaxial compressive stress on bainitic transformation at 330 _C following different austenitization temperatures. The transformation plasticity was also analyzed. It was found that the promotion degree of stress on bainitic transformation increases with the austenitization temperature due to larger prior austenite grain size as well as stronger promoting effect of mechanical driving force on selected variant growth at higher austenitization temperatures. The grain size and the yield strength of prior austenite are other important factors which influence the promotion degree of stress on bainitic transformation, besides the mechanical driving force provided by the stress. Moreover, the transformation plasticity increases with the austenitization temperature.
Two series of high-carbon pearlitic steels (68C-xNb and 75C-xNb) with different C and Nb contents were produced by hot-rolling. The effects of Nb on the microstructure and impact toughness of high-carbon pearlitic steels were investigated using scanning electron microscope, transmission electron microscope, electron backscattering diffraction and impact toughness tests. In particular, the nucleation and growth of pearlite were investigated through in-situ observations. It was found that the average pearlitic nodule was markedly refined from 10.2 μm to 6.0 μm with the addition of 0.026 wt% Nb in 68C steels, and it was refined from 13.7 μm to 6.7 μm with the addition of 0.014 wt% Nb in 75C steels. In situ observations determined that the refinement of the pearlitic nodule was due to the refinement of prior austenite grains and retardation of pearlite growth by Nb addition. The impact toughness of high-carbon steels was significantly improved by Nb addition without a decrease in strength because a finer pearlitic nodule was obtained. The maximum improvement ratios for 68C and 75C steels were 81% and 26%, respectively. In addition, according to the experimental results and theoretical calculations of the dissolution of Nb-containing precipitates, the refinement of prior austenite was mainly caused by the solute-dragging effect of Nb during isothermal holding at 1180 °C. Moreover, there was an optimum addition of Nb to 75C steels (less than 0.015 wt%); above this threshold, neither the prior austenite grains or pearlitic nodules were further refined, nor was the impact toughness further improved.
