Search

The microstructure and hydrogen embrittlement sensitivity of two X65 pipeline steels at different t 8/5 (the time required to cool from 800 °C to 500 °C) were investigated employing thermal simulation. With the increase of t 8/5 for both steels, the martensite transferred to acicular ferrite (AF) and bainite, and further converts to bainite and martensite/austenite (M/A) constituents. Elongated polygonal Mn–Nb–S inclusions appear in steel A, which are easy to fall off and the hydrogen embrittlement sensitivity is thus increased. However, spherical Al–Ca–Nb–N–O–S inclusions appear in steel B, which can promote nucleation of AF, thus obtaining lower hydrogen embrittlement sensitivity under lower t 8/5. • The HE sensitivity of two X65 pipeline steels at different t 8/5 were investigated. • The elimination of M and the nucleation of AF can reduce HE sensitivity. • Elongated polygonal Mn–Nb–S inclusions easily shed and increase HE sensitivity. • Spherical Al–Ca–Nb–O–S inclusions promote AF nucleation and reduce HE sensitivity. [ABSTRACT FROM AUTHOR]

By means of microstructure observation, phase analysis, and mechanical‐property tests, the effect of martensite–bainite (M–B) duplex microstructure on carbide precipitation and mechanical properties of M50 steel is studied. In that results, it is shown that the distribution of secondary carbides in specimens with M–B duplex microstructure is more uniform and finer, and the stability of retained austenite (RA) in the steel is also improved, so that the content of RA in specimens with M–B duplex microstructure is 2.34%, which is higher than the 0.94% of the specimens with full martensite microstructure. The M–B duplex microstructure leads to the reduction of tempering hardness of M50 steel to 60.9 unit of Rockwell hardness (HRC), compared to the 61.6 HRC of the specimens with full martensite microstructure, but the wear resistance is slightly enhanced. Moreover, the M–B duplex microstructure effectively improves the impact toughness and fatigue properties by refining the microstructure and carbides in the steel, and the increase amplitude is 47.4% and 41.0%, respectively. [ABSTRACT FROM AUTHOR]

By means of salt‐bath austempering treatment, microstructure observation, X‐Ray diffraction analysis, microhardness measurement, and friction and wear performance tests for 8Cr4Mo4V steels with standard composition and with trace addition of 0.005% boron, the effect of boron on the phase composition and mechanical properties of the steel is studied. The results show that trace element boron greatly increases the content of needle‐like lower bainite in the quenched microstructure, promotes the dissolution of undissolved carbides, reduces the content of M/A island, but coarsens grains. After tempering, trace element B increases the amount of bainite generation, refines the lath martensite, and significantly promotes the precipitation of carbides, whose size is finer, and the distribution is more dispersed. In terms of mechanical properties, trace element B improves the hardness by promoting the precipitation of the second phase and refining the lath martensite, and the increase in the bainite content changes the wear mechanism and enhances the wear resistance. [ABSTRACT FROM AUTHOR]

Hot stamping is a well‐established process in the automotive industry for structural parts to decrease the weight of a vehicle without reducing the safety and stiffness of components. In this work, a bainitic approach during the hot stamping process is studied by anisothermal annealing. The effect of different strategies on the microstructure and mechanical properties of the hot‐stamped parts is investigated. The microstructure is analyzed using light microscopy (LM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) techniques. Tensile and hardness tests are applied to analyze mechanical properties. A microstructure consisting of a carbide‐free bainite matrix containing martensite, fine‐grained ferrite, and retained austenite is obtained. SEM analysis of fracture surfaces from tensile samples reveals that the bainitic microstructure showed mainly a ductile fracture by dimples and voids along with some quasi‐cleavage fracture. [ABSTRACT FROM AUTHOR]

Overcoming the trade-off relationship between strength and ductility has always been a challenge. In this article, a neatly arranged ultrafine bainite that is composed of multi-scale retained austenite was obtained by introducing weak chemical heterogeneity in low-alloy steel. The optimized microstructure makes transformation-induced plasticity (TRIP) behavior and coordinating deformation behavior between each phase to be smoother. This results in better comprehensive properties, with 28.5% and 9% increases in uniform elongation and toughness, respectively, and a similar strength. This study provides a new way to improve the properties of low-alloy steels. A strategy of weak chemical heterogeneity has been proposed in low-alloy steel, which effectively optimizes the microstructure and enhances the plasticity and toughness of steel. [ABSTRACT FROM AUTHOR]

SA516 Gr.70 steel exhibits an excellent combination of toughness and plasticity, which is widely used in the cryogenic field. However, the presence of coarse grain in fusion-welded joints degrades the toughness and ductility of the joint. In this work, SA516 Gr.70 steel was welded using friction stir welding at rotation rates of 400–800 rpm, and the microstructural evolution and mechanical properties of joints were investigated. At 800 rpm, coarse martensite was found in the nugget zone (NZ), and with the decrease of rotation rate, some bainite was also observed besides martensite. The yield strength of joints at different rotation rates was significantly higher than that of base metal (BM). With decreasing rotation rate, the elongation of the joint increases significantly, and the highest elongation of 13.6% is obtained at 400 rpm since the joint with low hardness difference has more areas participating in coordinated deformation, alleviating the concentration of strain at BM. In addition, with the decrease of rotation rate, the toughness of NZ is improved. Finally, the maximum impact energy of 109 J/cm2 was obtained in the NZ at 400 rpm, depending on the finer grains and the presence of bainite. [ABSTRACT FROM AUTHOR]

The impact of silicon and aluminum on phase transformation behavior, particularly bainite, and microstructure evolution in Fe–0.2C–2.5Mn steel are presented. Continuous–cooling–transformation (CCT) and time–temperature–transformation (TTT) diagrams are determined experimentally. An aluminum extended empirical formula is introduced to estimate the martensite start temperature (Ms) with a thorough assessment of existing formulae. Results show that aluminum significantly increases Ms and has a stronger influence on promoting ferritic microstructures than silicon. During continuous cooling, alongside bainite, formation of Widmanstätten structures is induced in aluminum‐alloyed steel at higher cooling rates due to increased prior austenite grain size. Silicon decelerates bainite transformation kinetics by enhancing austenite's chemical stability through carbon enrichment via preventing carbide precipitation and by strengthening austenite against displacive phase transformation via solid solution hardening. Although aluminum has similar effects, incubation time is shortened during isothermal treatment because of the increased driving force, which overcompensates for the retardation effects. A finer carbide‐free bainitic microstructure is achieved in aluminum‐alloyed steel with more pronounced film‐like retained austenite (RA) formation and superior carbon enrichment, improving RA stability and suppressing martensite–austenite island formation. Finally, with the proposed formula, an accurate approximation to experimental Ms is accomplished. [ABSTRACT FROM AUTHOR]

The welding thermal simulation experiment of Q1100 low alloy high-strength steel was conducted using high-temperature laser scanning confocal microscope(HT-LSCM) and Gleeble-3500 thermal simulation tester with different welding heat inputs. In situ observation showed that prior austenite grains merged and grew instantly when the temperature reached the peak temperature, and then continued to migrate and grow during the subsequent cooling process. The prior austenite grain sizes and the CGHAZ microstructure were similar for the different welding heat inputs between high-temperature laser scanning confocal microscope and Gleeble-3500 thermal simulation. The microstructure consisted of granular bainite and proeutectoid ferrite, and a small amount of martensite/austenite (M–A) constituents when at t8/5 of 300 s. With increasing cooling rate, the microstructure transformed into lath bainite and lath martensite. When t8/5 was reduced to 15 s, the microstructure became lath martensite. Microhardness increased gradually as the welding heat inputs decreased, and impact toughness first increased and then decreased. By in situ observation of the microstructure and impact toughness testing, it is feasible to rapidly determine appropriate welding parameters and evaluate the weldability of materials. [ABSTRACT FROM AUTHOR]

Effects of transformation temperature and carbon content on the effective grain bounded by high‐angle boundary, i.e., "Bain size", in bainite structure are studied in Fe–2Mn–(0.05–0.75)C (mass%) alloys transformed isothermally between 773 and 673 K. Bainitic ferrite (BF) nucleates at austenite (γ) grain boundary with strong variant selection for carbon content less than 0.2 mass%, resulting in formation of coarse BF regions consisting of the Kurdjumov–Sachs (K–S) variants belonging to the same Bain variant. As temperature is lowered, K–S variant pairs with large misorientation are frequently formed. Variant selection at γ grain boundary becomes weaker for higher carbon content due to frequent intragranular nucleation particularly at 673 K. The Bain size decreases generally as transformation temperature is lowered, which is ≈15 μm at 773 K but down to ≈5 μm at 673 K, and also slightly decreases at 773 K with increasing carbon content. At lower temperatures, it decreases with the increase of carbon content in low carbon range less than 0.2 mass%C but inversely increases in higher carbon range. The observed changes in the variant paring and the Bain size are well understood in terms of the driving force of BF formation and self‐accommodation of transformation strain. [ABSTRACT FROM AUTHOR]

Based on recent research, it is known that the strength of pearlitic rail steels has reached its limit. On the basis of the literature analysis, the steels used in the world practice for production of bainite railway rails have been identified and their comparison with the chemical composition of steels used in various areas in the world has been carried out. Regularities of austenite decomposition kinetics of 30HGS steel grade with 0.28% C, 1.49% Si, 0.92% Mn, 0.99% Cr have been investigated by the method of mathematical modeling. Analytically, using the developed model, the intervals of cooling rates, within which changes in the mechanism of structure formation during the decay of austenite are observed, are determined. Chemical composition of experimental steels for railway rails was developed and ingots were melted in laboratory conditions with fixing of cooling rate with subsequent study of microstructures. [ABSTRACT FROM AUTHOR]

Low-carbon Cr-Ni-Mo steels widely used in mechanical engineering are studied: 18Kh2N4MA, 25Kh2N4MA, 25KhN3MA. Thermokinetic diagrams are plotted by a dilatometric method. Temperature-time ranges for microstructure constituent formation are established. It is shown that bainite within the steels studied may be formed both above and below the Ms temperature. Features of isothermal bainite transformation are investigated, kinetics of bainite transformation are determined, as well as the quantitative ratio of microstructure constituents formed as a result of austempering. It is established that the largest amount of bainite within the steel structures studied (80–95%) is achieved at a temperature near Ms. Mechanical properties (strength, ductility, impact strength) of the steels being studied are analyzed after various heat treatment methods: various cooling intensities, upper and lower bainite austempering. It is shown that formation of upper bainite has an ambiguous effect on steel impact strength. [ABSTRACT FROM AUTHOR]

To measure textures of hot rolled martensitic and bainitic steels, electron backscatter diffraction (EBSD) is applied to large areas, each covering about a thousand of prior grains. A scanning step is selected to keep reasonable time of the data acquisition still providing in any grain a significant number of analyzed points. Contributions to the transformation texture appearing from deformation and recrystallization components of the parent texture are estimated with the Bain relationship. To assess the parent austenite state, their proportion is then expressed by a scalar parameter. The results are verified by independent metallographic data. Avoiding the reconstruction of prior grains, this statistical approach will facilitate evaluation of complex austenite structures with comparable fractions of dissimilar material states. [ABSTRACT FROM AUTHOR]

The mechanical properties of ausferritic ductile iron are strongly influenced by the ability of the austenite to undergo martensite formation. Strain-induced martensite formation occurring under the right circumstances results in transformation induced plasticity (TRIP) that improves ductility and strength. TRIP has been shown to depend on the austenite's stacking fault energy (SFE), which describes the crystals micromechanical behaviour. In austenitic FeMnAlSi-TRIP steels, TRIP as a reaction to mechanical load only occurs for SFE <20 mJ/m2. For carbon-stabilised austenite as in ADI, the relationship between stacking faults, mechanical properties and martensitic transformation has not yet been established. To investigate the TRIP-effect in ADI, unalloyed ADI with 3.43 wt% C, 2.52 wt% Si and 0.21 wt% Mn was ausferritised and subjected to tensile tests at temperatures between −180 °C and 200 °C. The amount of martensite produced by thermal and mechanical activation, crystalline microstrain and stacking fault density were measured on deformed and undeformed regions of the specimen by XRD and the resulting SFE calculated. Between −70 and 20 °C, the elongation at fracture exceeded 10 % compared to below 2 % in the temperature range of −130 to −180 °C. At temperatures above 80 °C, elongation at fracture gradually decreased to 4.5 % at 200 °C. High sample ductility was associated with lower SFEs as low as 35 mJ/m2, indicating a correlation of stacking fault formation and strain-induced martensite formation in ADI. [ABSTRACT FROM AUTHOR]

Silicon carbide was employed as a source of silicon and carbon to produce sintered silicon steels, offering the advantages of silicon addition. This work explored the effects of low SiC contents of 1.0 and 2.0 wt.% on as-sintered and as-tempered microstructures and mechanical properties of sintered silicon steels. The experimental sintered steels were produced from mixtures of pre-alloyed Fe-1.50Mo and SiC powder. The mixtures were compacted to tensile bars, sintered at 1553 K for 2.7 ks, and cooled at 9.0 K/s with N2. Sintered specimens were tempered at 873 K and held for different times. The microstructures and mechanical properties of the sintered and tempered specimens were characterized. It was revealed that the sintered silicon-bearing steels showed low temperature phase transformations under cooling at 9.0 K/s. The sintered steel produced by 1.0 wt.% SiC addition had degenerate upper bainite microstructure and its tempered microstructure was upper bainite. The sintered steel produced by 2.0 wt.% SiC addition had a microstructure consisting of bainitic-ferrite plates, martensite plates, and austenite blocks. Its tempered microstructure was upper bainite. The tempered microstructures resembled those of sintered Si-free Fe-Mo-C steels. Tempered specimens exhibited tensile strength higher but elongation lower than sintered ones attributed to carbide precipitation strengthening. [ABSTRACT FROM AUTHOR]

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200–650 °C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases. Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role. The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600–650 °C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate. This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis. This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels. It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.

Recently carbide free bainitic steels, with microstructure consisting in nanoscaled bainitic ferrite plates and high carbon-enriched austenite, have been developed. These steels have found commercial use in manufacturing engineering components, nevertheless, their high carbon concentration poses a limitation as they cannot be welded, restricting their potential applications, therefore the actual research has shifted to the investigation to medium carbon steels. In this work, Butt weld – V groove weldment have been realized with a novel medium carbon high silicon carbide-free bainitic steel by means GMAW and GTAW. Optical microscopy, scanning and transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of both the fusion and heat affected zone. Dilatometry was utilized to study the phase transformations during post-welding heat treatment. Tensile and micro-hardness tests were carried out to evaluate the mechanical properties in the as-weld state after post-welding heat treatment. The analysis shows the possibility to obtain high strength weldments with high integrity and high strength (>1 GPa) coupled with reasonable ductility and the absence of cold or hot cracking phenomena.

Simultaneously enhancing the strength, plasticity, and toughness of metal materials has been a prominent research objective. In this study, the precipitation of nano-scale vanadium carbide particles at a lower tempering temperature is controlled to achieve three key benefits: reducing lattice distortion in bainitic ferrite, strengthening through precipitation, and enhancing the toughness of bainitic ferrite, while also maintaining the content and stability of retained austenite. The adopted procedure increases the comprehensive mechanical properties of steel by 17%. In particular, the room-temperature impact toughness is increased by 41%. This comprehensive microstructure optimization endows bainitic steel with an excellent combination of mechanical properties. [ABSTRACT FROM AUTHOR]

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

The paper studies the influence of accelerated cooling after helical rolling on the structure and mechanical properties of X70 steel in different sections of the rod. Accelerated cooling is carried out in two modes: cooling and holding at a temperature of 530°C (mode I) and cooling to a temperature of 200°C without holding (mode II). It is found that conditions of cooling affect the fraction and morphology of the bainitic phase. This in turn determines different microhardness and strength in different sections of the rods. [ABSTRACT FROM AUTHOR]

Steel with a combination of strength and plasticity is prevalently demanded for lightweight design and emission reductions in manufacturing. In this study, a high-strength Cr-Ni-Mo martensitic steel treated by quenching and partitioning (Q&P) and ultrasonic surface rolling (USR) processes was studied for both strength and plasticity enhancement. Specimens were austenitized at 850 °C and then quenched to 240 °C via cooling by water, oil, and normalization in quenching. This was followed by partitioning, in which two groups of specimens were heated to 370 °C and 350 °C for 45 min, respectively. At last, all the specimens were quenched to room temperature with the same methods of quenching. The highest tensile strength increased from 681.73 MPa to 1389.76 MPa when compared to as-received (AR) steel after the Q&P process. The USR process with a static force of 800 N further improved the tensile strength of specimens with high tensile strength after the Q&P process, which improved from 1389.76 MPa to 1586.62 MPa and the product's strength and elongation (PSE) increased from 15.76 GPa% to 15.9 GPa%, while the total elongation showed a mitigatory decrease from 11.34% to 10.02%. Tensile fractures were also studied and verified using a combination of strength and plasticity after a combined process of Q&P and USR. [ABSTRACT FROM AUTHOR]

It is frequently found in the literature that an acicular ferrite (AF) dominant microstructure has unsteady or even low‐impact toughness. It is necessary to analyze whether they are qualified for real AF. Herein, a Ti‐killed high strength low alloy steel is prepared and subjected to heat‐affected zones simulation tests to induce various AF‐like microstructures. The morphological differences of these microstructures are compared, and their crystallographic characteristics are analyzed. It is found that even within a single prior austenite grain, the transformed microstructure can be heterogeneous. Local areas having clear lath boundaries and a chaotic or interlocking morphology should be classified as AF. Adjacent laths in these areas are from different close‐packed plane and Bain groups. In contrast, local areas having vague lath boundaries and a granular‐bainite‐like morphology should be classified as granular bainite. Neighboring laths in these areas are from the same Bain group, and a relatively low density of high‐angle grain boundaries can be found. The heterogeneous microstructure consisting of granular bainite and AF has a wider grain size distribution and a higher area fraction occupied by large grains, which can lead to the scatter of Charpy impact energy. Criteria for the identification of AF are also proposed in this research. [ABSTRACT FROM AUTHOR]

Nanostructured bainitic steel obtained through austempering or bainitic transformation of high Si steels consists of bainitic ferrite and carbon‐enriched retained austenite. Secondary tempering of those microstructures tailors their mechanical properties through its unique nanostructure and austenite stability. Herein, fatigue and tensile properties of a nanobainitic steel in different secondary tempering conditions are presented and the results are discussed based on the deformation mechanisms taking place due to the microstructure evolution during secondary tempering. Influence of secondary tempering and elevated testing temperature on tensile and fatigue properties at room temperature and 250 °C is presented. Based on the present deformation, mechanism results are discussed supported by microstructure evolution results obtained through dilatometry, X‐ray diffraction, and microscopy. Depending on secondary tempering temperature, main influencing mechanisms are martensite formation after secondary tempering during cooling, transformation‐induced plasticity effect, and precipitation hardening obtained by secondary tempering. [ABSTRACT FROM AUTHOR]

Micro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel are investigated through experimental simulation and microstructural characterization. The results show that the deformation strengthening effect is fully exerted by thermo‐mechanical control process in steel containing 0.015 % lanthanum. Finally a kind of multi‐layer bainite ferrite microstructure featured by 3.5 μm blocks, 0.41 μm plates and 108 nm sub‐plates with ultrafine sub‐subunits and 55 nm θ‐M3C inside is achieved, which enhances the strength and toughness synergistically. And the nanoscale refinement mechanism of bainite ferrite plates lies in the formation of massive ultrafine sub‐subunits with the average size of 20 nm ×32 nm. Besides, a large number of twinning martensite with the size of 2 nm to 20 nm and high‐density entangled dislocations can be found on the boundaries of ultrafine sub‐subunits. Further, the density of dislocation is increased by 2.92×1014 cm−2 and its contribution to the strength is calculated to be 18 MPa. Moreover, micro‐alloying effects of lanthanum in thermo‐mechanical control process are explored to be that lanthanum enhances the interaction between bainite ferrite plates and dislocations, strengthens the entanglement of carbon and θ‐M3C with dislocations, and promotes the pinning effect of θ‐M3C on ultrafine subunits. [ABSTRACT FROM AUTHOR]

The structure and mechanical properties of rolled sheets with a thickness of 40 mm made of low-carbon low-alloyed steel after thermomechanical treatment of different modes are investigated. The structural factors that affect obtaining high mechanical properties are established. It is shown that a complex treatment including quenching from the rolling temperature and high-temperature tempering with a subsequent additional cycle of quenching and tempering leads to formation of rolled sheet uniform over the cross section with a fragmented bainite structure with disperse niobium carbides and carbides of cementite type. Such structure provides a yield strength of no less than 630 MPa in combination with high values of the impact energy at negative temperatures and a level of plasticity of δ ≈ 20%. [ABSTRACT FROM AUTHOR]

A hot rolled low carbon (0.17%) steel strip with 1.72% Mn, 1.35% Si and micro-alloyed with 0.04% Ti, 0.02% Nb was subjected to single stage quench partitioning followed by air cooling resulted in ultra high strength levels between 1039 and 1178 MPa with a total elongation between 21.6 and 29.9% with the product of strength and elongation exceeding 30 GPa at selected conditions. The yield ratio varied between 0.39 and 0.51. To achieve this range of properties, the steel was subjected to austenitization at 925 °C for 2 min followed by quenching in salt bath. The partitioning was carried out between 291 and 391 °C for 5 min in the salt bath followed by air cooling. The steel microstructure showed ferrite-bainite as the major phase with small amount of martensite and retained austenite. [ABSTRACT FROM AUTHOR]

This study evaluates the feasibility of producing high-strength low-alloy (HSLA) steel using advanced wire-fed laser additive manufacturing (LAM-W) and wire arc additive manufacturing (WAAM) technologies. Optimized parameters were independently developed for each heat source, utilizing a self-designed HSLA steel wire as the feedstock. Microstructural features and mechanical properties of the fabricated steels were characterized and compared, taking into account differences in heat input and cooling rates. LAM-W samples exhibited a finer columnar grain microstructure, while both LAM-W- and WAAM-produced steels predominantly showed lower bainite and granular bainite microstructures. LAM-W demonstrated higher strength and hardness, lower ductility, and comparable low-temperature toughness compared to WAAM. Both processes demonstrated an excellent balance between strength and ductility, with absorbed energy exceeding 100 J at −40 °C. The study confirms the feasibility of producing high-strength and tough HSLA steel parts using LAM-W and WAAM technologies, and compares the advantages and disadvantages of each method. These findings assist in selecting the most suitable wire-fed AM process for HSLA steel fabrication at high deposition rates. [ABSTRACT FROM AUTHOR]

In this study, we investigate the impact of prior ferrite on the bainite transformation kinetics and microstructure of medium-carbon steel interrupted by an intercritical annealing (IAA) process. It was found that the incubation time and completion time decreased from 687 s and 6018 s to 20 s and 4680 s, with the volume fraction of ferrite increasing from 9.5% to 28.6%, while the maximum transformation rate increased from 00271 μm/s to 0.0436 μm/s. The ferrite/austenite interface is introduced, and the nucleation sites are increased to accelerate the subsequent bainite transformation due to the formation of prior ferrite. However, there is a competitive relationship between the number and activation energy of bainite nucleation. According to the experimental results and theoretical calculations, the activation energy of the bainite transformation in the medium-carbon bainite steel decreases gradually with an increase in the volume fraction of prior ferrite. [ABSTRACT FROM AUTHOR]

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C. [ABSTRACT FROM AUTHOR]

X80 pipeline steel has played a vital role in oil and gas transportation in recent years. However, hydrogen-related issues frequently lead to pipeline failures during service, resulting in significant losses of properties and lives. Three heat treatment processes (furnace cooling (FC), air cooling (AC), and water cooling (WC)) were carried out to investigate the effect of different microstructures on hydrogen-induced cracking (HIC) susceptibility of X80 pipeline steel. The WC sample demonstrated the highest hydrogen embrittlement index, registering at 21.9%, while the AC and FC samples exhibited progressively lower values of 15.45% and 10.98%, respectively. Under equivalent hydrogen charging durations, crack dimensions with a maximum length exceeding 30 μm in the WC sample generally exceed those in the FC sample and AC sample. The variation is attributed to the difference in microstructures of the samples, predominantly lath bainite (LB) in water-cooled samples, granular bainite (GB) in air-cooled samples, and ferrite/pearlite (F/P) in FC samples. The research results demonstrate that the sensitivity of lath bainite (LB) to HIC is significantly higher than that of pearlite, ferrite, and granular bainite (GB). The presence of a large amount of martensite/austenite (M/A) constituents within bainite results in a multitude of hydrogen trap sites. HIC cracks in bainite generally propagate along the profiles of M/A constituents, showing both intergranular and transgranular cracking modes. [ABSTRACT FROM AUTHOR]

The experimental measurements of ${2D}$ 2 D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature. [ABSTRACT FROM AUTHOR]

In order to promote the understanding of bainitic transformation in nitrogen alloyed steels, the microstructure, transformation kinetics, and diffusion/precipitation characteristics of an isothermally heat-treated carbon-free system Fe–5Cr–N were investigated. Quenching dilatometry was used to perform heat treatments at temperatures between 400 °C and 500 °C and simultaneously receive information about the transformation kinetics. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) investigations indicate a plate- to lenticular-like, fully ferritic microstructure with nanometer scale CrN. In order to analyze the diffusion and precipitation behavior of nitrogen, atom probe tomography measurements were performed. On the one hand, nitrogen segregation was found, and on the other hand, the presence of CrN precipitates and preliminary stages of CrN nitrides were validated. [ABSTRACT FROM AUTHOR]

AISI 52100 is a high carbon alloy steel typically used in bearings. One hardening heat treatment method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), held at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperatures and holding times will develop different bainite percentages in the steel and result in a variation in mechanical properties. In the present work, the bainitic transformation kinetics of AISI 52100 were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The microstructure of austempered samples were characterized with optical microscope and x-ray diffraction. The bainitic transformation kinetics were analyzed by Avrami equations using measured hardness data. The CHTE quench probe was used to measure the cooling curves in the molten salt from austenitizing temperature to the selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with the measured cooling rates and used to calculate the bainitic transformation kinetics via DANTE software. The experimental results were compared with the calculated results and displayed good agreement. [ABSTRACT FROM AUTHOR]

The mechanical properties of welded joints have a crucial impact on the safety of pipelines. This article focuses on examining the correlation between the microstructure and mechanical properties of welded joints of X65MS pipeline steel for sour service. The results indicate that the welding thermal cycle forms the microstructure consisting of acicular ferrite, granular bainite, and polygonal ferrite. The hardness across the joint ranges from 210 and 270 HV, peaking within the welded zone and decreasing towards the base metal (BM). At −40 °C, the welded zone shows the lowest impact energy of 53 J, while heat‐affected zones exhibit superior impact toughness exceeding 270 J with the highest impact energy of 326 J. This can be attributed to the exceptionally fine‐grained microstructure, a small quantity of small‐sized martensite‐austenite (M/A) constituents in the heat affected zone. As for the low toughness of the weld, the main factors can be attributed to the presence of large‐sized elongated M/A constituents and inclusions. [ABSTRACT FROM AUTHOR]

Herein, the microstructure and mechanical properties of a high‐carbon bainitic steel treated by long‐time bainitic austempering and short‐time austempering plus tempering processes are compared. The multiphase microstructures are characterized by dilatometry, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy to correlate with mechanical properties. Results show that although long‐time austempering treatment can reduce the volume fraction of brittle martensite, no significant improvement is observed in fracture damage resistance. Besides, the cementite is prone to precipitation from the austenite at the later period of the long‐time austempering process. The cementite precipitation in austenite decreases the carbon content in retained austenite (RA) and consequently reduces the mechanical stability of RA. In contrast, the cementite has not been able to precipitate from austenite after short‐time austempering treatment, whereas the martensite is softened and the stability of RA is improved during subsequent tempering. Therefore, excellent mechanical properties are obtained in the samples treated by short‐time austempering plus tempering process: ultimate tensile strength, 1489 MPa, yield strength, 1014 MPa, total elongation, 33.2%, and the product of strength and elongation (PSE) of 48.4 GPa%, where PSE is increased by 27% compared with the sample after long‐time bainitic austempering. [ABSTRACT FROM AUTHOR]

This study describes how microstructural constituents affected the hydrogen embrittlement resistance of high-strength pipeline steels. The American Petroleum Institute (API) X60, X70, and X80 pipeline steels demonstrated complicated microstructure comprising polygonal ferrite (PF), acicular ferrite, granular bainite (GB), bainitic ferrite (BF), and secondary phases, e.g., the martensite-austenite (MA) constituent, and the volume fraction of the microstructures was dependent on alloying elements and processing conditions. To evaluate the hydrogen embrittlement resistance, a slow strain rate test (SSRT) was performed after electrochemical hydrogen charging. The SSRT results indicated that the X80 steel with the highest volume fraction of the MA constituent demonstrated relatively high yield strength but exhibited the lowest hydrogen embrittlement resistance because the MA constituent acted as a reversible hydrogen trap site. [ABSTRACT FROM AUTHOR]

The microstructures throughout a 696 × 1360 mm cross-section of an ISO 1.2738 prehardened steel block for a plastic mold were characterized via optical and electron microscopy and electron backscatter diffraction. The hardness, strength, and polishing performance of the steel block were also tested. The results showed that the microstructure of the steel bloom from the edge to the core consisted of tempered sorbite, tempered bainite, and pearlite microstructures. Abnormal upper bainite and coarse carbides were also found. The bloom sections with hardness values of 37.4 to 39.3 HRC comprised tempered sorbite and bainite. The hardness of the core was approximately 36.5 HRC due to the presence of pearlite. The tensile and yield strengths were the same in the edge and middle areas mainly owing to tempered sorbite. The polishing performance was affected by the microstructure. Tempered sorbite produced the best polishing performance due to its fine and uniform microstructure, whereas that of tempered bainite and pearlite, which contained large carbide particles and mixed phases, was worse. [ABSTRACT FROM AUTHOR]

High-carbon nano bainitic steel is currently a hot research topic. The effect of the matrix's carbon content and carbon atom distribution on the toughness of high-silicon, high-carbon bainitic steel is studied. The microstructure under an incomplete austenitization process consists of undissolved carbides, bainitic ferrite, and retained austenite. Using this process, the carbon content in bainitic ferrite is relatively low. Under the complete austenitization process, the carbon content in the bainite ferrite in the sample is high, and there is more retained austenite in the blocky type. The sample exhibits high impact toughness under an incomplete austenitization process, which is mainly affected by the low carbon content of bainite ferrite, high coordination ability of retained austenite, and high interface density of microstructure. The EBSD results show that the crack easily propagates between parallel bainite laths with low interface density compared with the high interface density perpendicular to the laths. [ABSTRACT FROM AUTHOR]

ABSTRACTIn this work, we provide with an overview of the main theories, metallurgical and microstructural concepts used for the design of carbide free bainitic steels adapted to industrial hot rolling and coiling practices. The selected alloys were cast and tested at pilot plant scale and the obtained good combinations of properties tend to validate the thorough and careful approach undertaken to predict the microstructures during the alloy design process.

The processes of formation of carbide-free bainite and the effect of the temperature-time parameters of isothermal hardening in the range of bainitic transformation on the structure and impact toughness of low-alloy structural steels are studied. A method for nondestructive testing of the impact toughness of medium-carbon steels heat-treated by isothermal quenching is developed on the basis of identified patterns of changes in the properties of retained austenite and in the impact toughens of the steels. [ABSTRACT FROM AUTHOR]

An air‐cooled low‐carbon Mn–Si–Cr alloyed steel with multiphase microstructure of carbide‐free bainite/martensite (CFB/M) is developed. The kinetics of bainitic transformation during air‐cooling process is discussed via considering the transformation latent heat with help of the measured temperature‐cooling curves. Results show that the CFB formed during air cooling can effectively refine the multiphase microstructure and stabilize the film‐like retained austenite that contributes to transformation‐induced plasticity effect upon deformation. An optimum combination of strength, ductility, and toughness of the air‐cooled bainitic steel is achieved (tensile strength: 1418 MPa, total elongation: 15.8%, and Charpy V‐notch impact energy: ≈71 J). The newly developed Mn–Si–Cr alloyed air‐cooled bainitic steel exhibits superior mechanical properties to the commercial 23CrNi3Mo steel but has a much lower alloy cost. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties were investigated in a 17.48-mm-thick electron beam-welded (EBW) API 5L X65 steel sheet. Electron beam welding has the advantage of joining thick steel parts in a single pass with a high energy conversion efficiency, low heat input and low warpage. The base metal has elongated bands of ferrite interspersed with pearlite colonies aligned along the rolling direction. After welding, a narrow and fine-grained heat affected zone (HAZ) consists of degenerate pearlite and martensite–austenite (M–A) islands. The microstructure of the fusion zone is coarser than the HAZ and more complex, consisting of lower bainite, Widmanstätten ferrite, acicular ferrite, grain boundary ferrite, and small islands of the M–A constituent. These microstructure changes are minimized when EBW is used and do not significantly affect the mechanical properties of the weld. Results of Charpy V-notch tests for weld beads parallel and perpendicular to the rolling direction met the requirements of the API 5L standard. EBW has proven to be a viable alternative for welding thick plates of advanced steels applied in pipeline construction and assembly. [ABSTRACT FROM AUTHOR]

In this research, the failure of a metallic fastener was surveyed. The main reason for the failure was oxide inclusions in the raw material of the fastener. This brittleness problem was observed only in coated products and not in austempered products. Due to the existence of inclusions in raw material, there was a sharp reduction in their toughness and consequently, the effect of TME occurred in the coating process became visible and made fasteners brittle during the assembly process. [ABSTRACT FROM AUTHOR]

In this study, the microstructure evolution of laser solid forming (LSF) 34CrNiMo6 steel with different deposition thickness was investigated. The microstructure of 10 layers (1.35 mm) sample from bottom to top presented the martensite with a large amount of upper bainite, the mixture of martensite and lower bainite, and the mixture of martensite and retained austenite. When deposition thickness reached 20 layers (3.12 mm), the microstructure of the deposition zone consisted of a large amount of bainite (needle-like lower bainite and feather-like upper bainite) and a small amount of martensite. Besides, the number and size of the feather-like upper bainite gradually increased from the top to the bottom of the deposition, and the morphology of martensite changed from lath-shaped to island-shaped. Two types of carbides morphology existed in lower bainite at the top of 20 layers (3.12 mm) sample: a short rod-shaped with an angle of 55-60° from the main axis and a strip-shaped approximately parallel to the main axis. The carbides in lower bainite can be identified as Fe3C. [ABSTRACT FROM AUTHOR]

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150–300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite. [ABSTRACT FROM AUTHOR]

Herein, the microstructure and mechanical properties of a low‐alloy Cu‐bearing transformation‐induced plasticity (TRIP) steel and their dependence on the processing history are investigated. The distributions of the retained austenite and bainite are found to be dependent on the initial cold‐rolled microstructure, which can be tailored by the preceding hot‐rolling route (unidirectional/cross rolling) and the subsequent cooling rate (furnace/air cooling). Mechanical properties and TRIP effect strongly depend on the initial microstructure, where the air‐cooled sheet shows remarkable mechanical properties; while the cross‐rolled sheet shows isotropic properties. The addition of copper results in an increase in the amount of the retained austenite, enhancement of strength‐ductility balance, and improvement of the hardening behavior of TRIP‐assisted high‐performance steel. [ABSTRACT FROM AUTHOR]

The microstructure and impact toughness in an ultra-high-strength casing were investigated, attempting to reveal the reason for toughness fluctuations along the casing in terms of crystallography. The morphological structures at the head of the casing are lath bainite, while those at the end are granular bainite and lath bainite. The head exhibits a higher density of high-angle grain boundaries dominated by block boundaries and a higher impact toughness than the end, showing an inhomogeneous microstructure and causing toughness fluctuations in the casing. The higher density of block boundaries in the head resulted from weaker variant selection because of its higher cooling rate than the end. Therefore, the underlying reason for the toughness fluctuations lies in the varying densities of the block boundaries along the casing triggered by uneven cooling. [ABSTRACT FROM AUTHOR]

Four types of multiphase Fe–V alloys were examined, comprising ferrite, pearlite, and bainite microstructures. Electrochemical impedance spectroscopy and potentiodynamic polarization were employed to study the effect of microstructure on electrochemical properties of the alloys in 3.5% NaCl solution. The cathode reaction was found to be the main step controlling the corrosion dissolution process. The lowest corrosion current density (2.104 μA/cm2) for the Fe–V alloy with bainite indicated that it had outstanding corrosion resistance. Although an increase in pearlite content had a negative effect on corrosion resistance of the alloy, refinement of the lamellar spacing had the opposite effect. Surface corrosion morphologies were observed by scanning electron microscopy. The results indicate that fine cementite uniformly distributed in the bainite promoted slight uniform dissolution of ferrite. Corrosion and dissolution that occurred at the junctions of the pearlite colonies with different directions was more severe. The increase of content of V effectively improved the corrosion resistance of Fe–V alloy. Compared with the matrix, the VC precipitated phase was cathodically protected in the electrochemical corrosion process by virtue of its positive corrosion potential. [ABSTRACT FROM AUTHOR]