Search Results (156)

Characterising and quantifying complex multiphase steels is a challenging and time-consuming process, which is often open to subjectivity when based on image analysis of optical metallographic or SEM images. The properties of multiphase steels are highly sensitive to their individual phase properties and fractions, necessitating the development of robust characterisation tools. This paper presents a method for classifying nanoindentation maps into proportional fractions of up to five distinct microstructural regions in dual-phase and complex-phase steels. The phases/regions considered are ferrite, ferrite containing mobile dislocations, bainite, tempered martensite, and untempered martensite. A range of microstructures with varying fractions of phases were evaluated using both SEM/EBSD and nanoindentation. A machine learning (ML) approach applied to EBSD data showed good consistency in characterising a two-phase system. However, as the microstructural system complexity increased, variations were observed between different analysts and the sensitivity to the ML training data increased when four phases were present (reaching up to ~11% difference in the ferrite phase fraction determined). The proposed nanoindentation mapping technique does not show operator sensitivity and enables the quantification of additional microstructural features, such as identifying and quantifying ferrite regions with a high density of mobile dislocations and the degree of martensite tempering. Full article

The lattice parameters of both the product phase and the matrix phase have determined using in situ X-ray and neutron diffraction measurements during forward and reverse transformations in steels. The lattice parameters are well known to be influenced by various factors, including temperature, internal stresses induced by transformation strains, partitioning of alloying elements, crystal defects, and magnetic strains. Therefore, it is crucial to accurately disentangle the contributions of these factors to the observed changes in lattice parameters. This review examines the evaluation of internal strain (stress) associated with ferrite, pearlite, bainite, martensite, and reverse austenite transformations, with a particular emphasis on the distinction between diffusional and displacive transformations. Additionally, the effects of plastic deformation of austenite on the bainite or martensite transformation are discussed. In this context, the roles of dislocations and vacancies are highlighted as key areas for further investigation. Full article

This study investigated the control of porosity during gas metal arc welding with pulsed arc (GMAW-P) of complex-phase 780 (CP780) galvanized steel. Due to the Zn coating on this type of steel, porosity forms during welding as a result of Zn vaporization. The objective was to optimize the welding parameters to minimize porosity with a design of experiments using an L9 orthogonal array to analyze the effects of peak current (I_p), pulse time (t_p), and pulse frequency (f) in high-speed welding conditions. The results showed that porosity was significantly reduced with a peak current of 313 A, a frequency of 10 Hz, and a pulse time of 10 ms, achieving ~0% porosity in the validation welding trials. A microstructural analysis identified allotriomorphic ferrite, Widmanstätten ferrite, acicular ferrite, bainite, and martensite in the heat-affected zone (HAZ). A predictive model to anticipate the percentage of porosity with an R² of 99.97% was developed, and an ANOVA determined the peak current as the most critical factor in porosity formation. Full article

The isothermal bainitic transformation kinetics, microstructure, and mechanical properties of the quenched low-carbon high-strength steel have been investigated via dilatometric measurements, microstructural characterization, and mechanical tests. The results show that the pre-transformed isothermal bainite promotes martensitic transformation, increasing the martensitic transformation temperature, and enhancing the transformation rate. The microstructure of the 400 °C isothermal steel consists predominantly of lath bainite ferrite with dot/slender M-A constituents, whereas the steel treated at 450 °C contains a combination of martensite/lath bainite and granular bainite. The presence of massive M-A constituents contributes to brittle fracture as these constituents tend to promote crack initiation. Hence, the 450 °C treatment, which leads to the formation of massive M-A constituents, induces brittleness, while the finer M-A constituents formed at 400 °C exert minimal influence on the toughness and result in a more stable microstructure owing to their small size and the surrounding fine lath microstructure. The differences in microstructure and properties between the steels treated at 400 °C and 450 °C illustrate the importance of controlling the quenching cooling rate in the high-temperature bainitic transformation region during thick plate quenching processes. Full article

In order to find the optimal heat input for simulating the welding of the coarse-grained heat-affected zone (CGHAZ) of a novel Q690 MPa V-N microalloyed medium and heavy plate, the study investigated the precipitation of V (C, N), microstructural changes, and impact toughness under five different heat inputs (E). The results show that in the CGHAZ, as the heat input increases, the dominant microstructure changes from intragranular acicular ferrite (IGAF) and lath bainitic ferrite (LBF) to polygonal ferrite (PF) and a small amount of IGAF. At the same time, the area fraction of the brittle phase martensite/austenite (M/A) constituents increased from 4.96% to 7.95% as heat input increased, and the microhardness difference between the M/A constituents and the matrix significantly increased. In addition, with the E increases, the fraction of high-angle grain boundaries (HAGBs), which can hinder crack propagation, increases from 59.2% to 62.2% and then decreases from 62.2% to 49.3%. Moreover, the impact toughness of the simulated CGHAZ of the Q690 MPa V-N microalloyed medium and heavy plate first increases from 62 J to 100 J and then decrease to 20 J. Full article

With the rapid development of long-distance transmission pipelines for oil and natural gas, pipeline steel is continuously evolving towards higher pressure, larger diameter, and thicker wall thickness. Many extensive studies and research have been conducted on X70 pipeline steel produced through traditional processing routes. This study focuses on Nb-V-Ti-N microalloyed steel with different nitrogen contents, systematically investigating the variations in microstructure and tensile properties after quenching and tempering processes. The results indicate that after quenching treatment, when the nitrogen content of the tested steel is 0.0020 wt%, its primary microstructure consists of granular bainitic ferrite (GBF), acicular ferrite (AF), and residual M/A (martensite/austenite) components. As the nitrogen content increases, the contents of acicular ferrite and M/A constituents gradually rise, while granular bainitic ferrite correspondingly decreases. After tempering treatment, the microstructure of the tested steel transforms into granular bainitic ferrite, acicular ferrite, and carbonitrides. Notably, with the elevation of nitrogen content, the number of high-angle grain boundaries in the microstructure significantly increases. Meanwhile, the mean equivalent diameter (MED) defined by the misorientation angle (MTA) ranging from 2 to 15° and the dislocation density (ρ) exhibit an overall decreasing trend. Both of these factors contribute significantly to yield strength, resulting in a gradual increase in yield strength (YS) as the nitrogen content rises. Additionally, the study finds that as the nitrogen content increases, the size of precipitated particles continuously enlarges, and their proportion in the microstructure gradually increases. This discovery provides important theoretical basis and practical guidance for further optimizing the microstructure and mechanical properties of X70 pipeline steel. Full article

Fusion welding easily causes microstructural coarsening and tempering softening in the heat-affected zone (HAZ) of high-strength pipeline steel joints, which considerably deteriorates the strength and toughness. Here, X80 pipeline steel was subjected to friction stir welding (FSW), and external cooling was used to tailor the microstructure to optimize the strength–ductility combination of the nugget zone (NZ). Coarse granular bainite (GB) appeared at air cooling, whereas a fine ferrite/martensite microstructure was achieved at solid CO₂ cooling. The highest ratio of high-angle boundaries was obtained at solid CO₂ cooling because the variants were evenly distributed within the four close-packed (CP) groups. The low yield strength (YS) of 595 MPa was obtained in the NZ under air cooling, whereas a high YS of 755 MPa was achieved in the NZ under solid CO₂ cooling due to dislocation strengthening and fine-grain strengthening. Furthermore, an ultra-high tensile strength of 910 MPa and utilizable elongation of 15% were obtained in the NZ under solid CO₂ cooling, which was attributed to the fine effective grains and ferrite/martensite microstructure facilitating a ductile fracture. Full article

In order to gain insight into the changes of the organization and hardness of 500 MPa steel-grade low-temperature-resistant steel bars (HRB500DW) for liquefied nature gas (LNG) storage tanks during the continuous cooling phase transformation process, the effects of different rolling temperatures and cooling speeds on the organization of the phase change law, microstructure and hardness were studied. The results show that the critical phase transformation points A_C1 and A_C3 of the test steel were 702 and 880 °C, respectively. The organization of the test steel was polygonal ferrite and pearlite when the cooling rate was 1–2 °C/s. At a cooling speed of 5 °C/s, a small amount of bainite started to be produced in the region of a large deformation of rolling, and at 15 °C/s, some slate martensite started to be produced. At a cooling speed of 10 to 25 °C/s, the organization was mainly bainite. At a cooling rate of 40 °C/s, continuous pre-eutectic reticulated ferrite was formed at the austenite grain boundaries, reducing material properties. As the cooling speed increased, the hardness of the matrix organization of the test bars increased. The lower initial rolling temperature led to the expansion of the martensitic transformation zone. For rebar producers, the initial rolling temperature of 1050 °C was better than the initial rolling temperature of 1000 °C. Full article

Understanding the volume fractions of microstructure constituents such as ferrite, pearlite, bainite, and martensite in low-alloy steels is critical for tailoring mechanical properties to specific engineering applications. To address the complexity of these relationships, this study explores the use of artificial neural networks (ANNs) as a robust tool for predicting these microstructure constituents based on alloy composition, specific Jominy distance, and heat treatment parameters. Unlike previous ANN-based predictions that rely on the hardness after quenching as an input parameter, this study excludes hardness. The developed model relies on readily available input parameters, enabling accurate estimation of microstructure composition prior to heat treatment, which significantly improves its practicality for process planning, optimization, and reducing trial-and-error on industrial applications. Three different input configurations were tested to evaluate the predictive capabilities of ANNs, with results showing that the use of specific Jominy distance as an input variable enhances model performance. Furthermore, the findings suggest that specific Jominy distance could serve as a practical alternative to detailed chemical composition data in industrial applications. The predictions for ferrite, pearlite, and martensite were more accurate than those for bainite, which can be attributed to the complex nature of bainite formation. Full article

This study addresses the characterization of the particular microstructural constituents of multiphase transformation-induced plasticity (TRIP)-aided steels belonging to the first and third generations of Advanced High Strength Steels (AHSS) to explore the possibilities of the EBSD method. Complex microstructures composed of ferrite, bainite, retained austenite and martensite were qualitatively and quantitatively assessed. Microstructural constituents with the same crystal structure were distinguished using characteristic EBSD parameters like confidence index (CI), image quality (IQ), kernel average misorientation (KAM) and specific crystallographic orientation relationships. A detailed linear analysis of the IQ parameter and misorientation angles was also performed. These tools are very helpful in linking different symmetric or asymmetric features of metallic alloys with a type of their structure and morphology details. Two types of samples were investigated: thermomechanically processed and subjected to 10% tensile strain to study the microstructural changes caused by plastic deformation. Full article

This paper comprehensively examines three structural steel grades’ microstructural features and mechanical properties, evaluating their suitability for shipbuilding applications. The steels analyzed include quench and tempered (Q and T) steel, thermomechanical controlled processed (TMCP) steel, and hot rolled (HR) steel. A microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The analysis was complemented by extensive mechanical testing including assessments of hardness, tensile, and Charpy impact tests across a range of temperatures. Additionally, corrosion behavior was evaluated using the potentiodynamic polarization test. The findings revealed that Q and T grade steel exhibited the most refined microstructure, characterized by a complex mixture of ferrite, tempered martensite, upper bainite, and Fe₃C phases. In contrast, the TMCP grade steel demonstrated a balanced microstructure of polygonal ferrite and pearlite. Meanwhile, the HR grade steel contained polygonal ferrite and aligned pearlite. The tensile testing results demonstrated that the Q and T grade steel had superior hardness, yield strength (YS), and ultimate tensile strength (UTS), although it exhibited the lowest elongation % (El %). The TMCP grade steel met all ABS standards for marine steels, displaying optimal YS, UTS, and El %. Despite the superior YS of the HR grade steel, it did not meet the necessary criteria for UTS. Charpy impact tests revealed that the TMCP grade steel exhibited the highest impact energy absorption across a range of temperatures. As a result, the TMCP grade steel emerged as the optimal choice for ship construction, fulfilling all ABS requirements with a balanced combination of strength, ductility, and impact energy absorption. Additionally, the potentiodynamic polarization results revealed that the Q and T grade steel demonstrated the highest corrosion resistance. Following Q and T steel, the HR grade steel ranked second in corrosion resistance, with TMCP steel closely behind, showing only a slight difference. Full article

High-strength steels are widely used in various mechanical production and construction industries for their low cost, high strength and high toughness. Among these, bainitic steels have better comprehensive performance relative to martensite and ferrite. In this paper, from the point of view of its microscopic fine structure and mechanical properties, the high-carbon silicon-containing steel Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni was austenitized at high temperature after a brief isothermal treatment at 280 °C and is briefly reviewed. We have used EBSD, TEM and 3D-APT to observe a unique transformation in which high-carbon silicon-containing steels form nanostructured bainite with nanometer widths. Intriguingly, as the isothermal duration decreases, the beam bainite width becomes increasingly finer. When the beam bainite width falls below 50 nm, there is a sudden shift in defect type from the conventional edge-type dislocations to a defect characterized by the insertion of a semi-atomic surface in the opposite direction, which leads to different degrees of reduction in the micro- and macro-mechanical properties of high-carbon silicon-containing steels from 1754 MPa to 1667 MPa. This sudden change in the sub-structural properties is typical of the reverse Hall–Petch effect. Full article

This study looks at how welding intensity, speed, voltage, and stick-out affect the structural and mechanical characteristics of metal inert gas (MIG) welding on SUS 201 stainless steel and C20 steel. The Taguchi method is used to optimize the study’s experiment findings. The results show that the welding current has a more significant effect on the tensile test than the welding voltage, stick-out, and welding speed. Welding voltage has the lowest influence. In addition to the base metals’ ferrite, pearlite, and austenite phases, the weld bead area contains martensite and bainite microstructures. The optimal parameters for the ultimate tensile strength (UTS), yield strength, and elongation values are a 110 amp welding current, 15 V of voltage, a 500 mm.min⁻¹ welding speed, and a 10 mm stick-out. The confirmed UTS, yield strength, and elongation values are 452.78 MPa, 374.65 MPa, and 38.55%, respectively, comparable with the expected value derived using the Taguchi method. In the flexural test, the welding current is the most critical element affecting flexural strength. A welding current of 110 amp, an arc voltage of 15 V, a welding speed of 500 mm.min⁻¹, and a stick-out of 12 mm are the ideal values for flexural strength. The flexural strength, confirmed at 1756.78 MPa, is more than that of the other samples. The study’s conclusions can offer more details regarding the dissimilar welding industry. Full article

To ensure consistent performance of additively manufactured metal parts, it is advantageous to identify alloys that are robust to process variations. This paper investigates the effect of steel alloy composition on mechanical property robustness in laser-directed energy deposition (L-DED). In situ blending of ultra-high-strength low-alloy steel (UHSLA) and pure iron powders produced 10 compositions containing 10–100 wt% UHSLA. Samples were deposited using a novel configuration that enabled rapid collection of hardness data. The Vickers hardness sensitivity of each alloy was evaluated with respect to laser power and interlayer delay time. Yield strength (YS) and ultimate tensile strength (UTS) sensitivities of five select alloys were investigated in a subsequent experiment. Microstructure analysis revealed that cooling rate-driven phase fluctuations between lath martensite and upper bainite were a key factor leading to high hardness sensitivity. By keeping the UHSLA content ≤20% or ≥70%, the microstructure transformed primarily to ferrite or martensite, respectively, which generally corresponded to improved robustness. Above 70% UHSLA, the YS sensitivity remained low while the UTS sensitivity increased. This finding, coupled with the observation of auto-tempered martensite at lower cooling rates, may suggest a strong response of the work hardening capability to auto-tempering at higher alloy contents. This work demonstrates a methodology for incorporating robust design into the development of alloys for additive manufacturing. Full article

This study employs the finite element method to investigate the microstructural evolution and deformation behavior of a 40Cr steel automobile front axle during the quenching process. By establishing a multi-physics field coupling model, the study elucidates the variation patterns of the microstructure field in the quenching process of the front axle under different immersion orientations. It is found that along the length direction, the bainite and martensite structures decrease from the center to the edge region, while the ferrite structure shows an increasing trend. Additionally, the influence of immersion orientation on the hardness of the front axle’s microstructure and deformation behavior is thoroughly discussed. The results indicate that, firstly, when quenched horizontally, the hardness difference among different regions of the front axle is approximately 8.2 HRC, whereas it reaches 10.3 HRC when quenched vertically. Considering the uniformity of the microstructure, the horizontal immersion method is preferable. Secondly, due to the different immersion sequences in different regions of the front axle leading to varying heat transfer rates, as well as the different amounts of martensite structures obtained in different regions, the deformation decreases along the length direction from the center to the edge region. Horizontal immersion quenching, compared to vertical immersion, results in a reduction of approximately 56.2% and 48.9% in deformation on the representative central cross-section (A-A) and the total length of the front axle, respectively. Therefore, considering aspects such as microstructure uniformity and deformation, the horizontal immersion quenching orientation is more favorable. Full article