Search Results (438)

This study investigated the relationship between the surface microstructure of pearlite steel wheels and the formation of fatigue cracks during the braking process by using a full-size wheel braking test rig. After fatigue failure, the surface microstructural evolution and fatigue crack initiation and propagation of the wheel sample were systematically analyzed by optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results showed that after braking of 1572 cycles, a large number of fatigue cracks formed at the wheel tread, which caused the wheel to break. After fatigue failure, some dark areas formed at the wheel tread, which were composed of Fe₃O₄ compounds. This indicates that severe oxidation was produced at the wheel tread during braking due to the high temperature. After fatigue failure, a continuous thermal white etching layer (T-WEL) was formed in some areas of the wheel tread, while crescent-shaped T-WEL was found in other areas. The microstructure of the T-WEL was composed of martensite phase. The rapid increase and decrease in temperature at the wheel tread during the braking process caused martensitic transformation at the wheel tread. The hardness of the sample reached to about 900 HV in WEL and it reduced with the increase in distance from the surface. The cracks were initiated from the surface and gradually propagated into the matrix. However, the crack propagation mode in the continuous T-WEL and crescent-shaped T-WEL was different. In the continuous T-WEL, the continuous T-WEL of the wheel can be peeled off during the braking wear process, and then the crack was gradually propagated into the matrix in the T-WEL peeled area. As for the crescent-shaped T-WEL, due to the large hardness difference between T-WEL and pearlite, the crack initiated at the interface between the T-WEL and pearlite and gradually propagated into the matrix. Full article

The pearlitic microstructure, comprising alternating lamellae of ferrite and cementite, provides a favorable combination of strength, toughness, and wear resistance. Consequently, pearlitic steels have been widely utilized in pipeline systems due to their advantageous mechanical properties and cost-effectiveness. These characteristics also render pearlitic steel pipelines promising candidates for hydrogen transport infrastructure, particularly in the context of repurposing existing natural gas networks. However, interactions between hydrogen and the pearlitic microstructure raise significant concerns regarding hydrogen embrittlement, a phenomenon that can substantially degrade mechanical performance and compromise long-term structural integrity. Experimental observations indicate that pearlitic microstructures are particularly susceptible to hydrogen embrittlement, largely due to the high density of ferrite–cementite interfaces, which act as effective hydrogen trapping sites. These detrimental effects motivate the present study, which aims to develop a deeper understanding of nanoscale mechanisms of hydrogen-assisted crack initiation and propagation in pearlitic microstructures. In this work, molecular dynamics simulations are employed to investigate the initiation and propagation of hydrogen-affected cracks in pearlitic microstructures, considering lamellar orientations both parallel and perpendicular to the applied tensile loading direction. The analysis focuses on the synergistic interaction between hydrogen-enhanced decohesion (HEDE), which promotes interfacial separation due to hydrogen segregation, and hydrogen-enhanced localized plasticity (HELP). Full article

The present study employed molecular dynamics (MD) simulations to investigate the deformation behavior of a ferrite/cementite (α-Fe/Fe₃C) dual-phase system under ultra-high strain rate conditions. A systematic examination was conducted to investigate the effects of deformation temperature and loading direction (LD-I: loading on the ferrite side; LD-II: loading on the cementite side) on the mechanical properties, microstructural evolution, and strain transfer of the system. The results show that both the tensile strength and the maximum uniform plastic strain (plasticity) exhibit a non-monotonic variation with increasing temperature, first increasing and then decreasing, with the optimal strength-ductility synergy achieved. At the same deformation temperature, the strength and plasticity of the system under LD-I are significantly superior to those under LD-II conditions. The observed differences in mechanical properties are attributed to alterations in strain-transfer uniformity, atomic rearrangement activity, and crack-nucleation sites induced by temperature and loading direction. This study provides mechanistic insights into the dynamic failure of pearlitic steels under extreme conditions. These atomistic mechanisms can serve as a reference for multiscale modeling or for interpreting experiments under comparable dynamic loading. Full article

This study investigates pulsed gas metal arc welding (pGMAW) of API 5L X65 pipeline steel for sour service applications where H₂S exposure is anticipated. Mechanized pGMAW in the 5G downhill position was employed to fabricate girth welds using ER70S-6 filler wire with Ar-20%CO₂ shielding. Comprehensive characterization, including optical microscopy, tensile testing, fractography, EBSD, and fracture toughness evaluation via SENT specimens, was conducted on specimens tested in both air and H₂S-precharged sour conditions. Microstructural analysis revealed ferritic–pearlitic base metal, weld metal with acicular ferrite and bainitic constituents, and a transformed HAZ gradient. Tensile testing demonstrated severe hydrogen embrittlement in sour conditions, with elongation dropping from 22% in air to 4% after H₂S exposure, accompanied by a transition from ductile cup–cone fracture to quasi-cleavage morphology. EBSD showed texture sharpening toward ⟨101⟩ fiber post-deformation, with a broader orientation spread under sour conditions, indicating heterogeneous strain localization. Fracture toughness testing revealed approximately a 50% reduction in CTOD values under sour exposure, with the weld centerline exhibiting greater degradation (0.50 mm to 0.27 mm) compared to the HAZ (0.92 mm to 0.47 mm). Fractography confirmed hydrogen-assisted cracking features, including shallow dimples, cleavage facets, and secondary cracking. These findings establish critical baseline data for engineering a critical assessment of pGMAW-welded X65 pipelines in sour service. Full article

A transverse fracture occurred in U75V pearlitic rail after 5 months of service on a sharp radius curved track of mixed passenger-freight railway. Systematic tests including chemical composition analysis, mechanical properties testing, macroscopic fracture inspection, metallographic observation and microscopic morphology characterization were conducted on the failed rail sample. The results indicate that the rail base metal has qualified metallurgical quality. Its chemical composition, fundamental mechanical properties and microstructure fully meet the requirements of Chinese railway standard TB/T 2344.1-2020. The failure mode is identified as instantaneous brittle fracture. Severe mechanical extrusion and impact cause prominent plastic deformation on the rail foot, leading to surface plastic flow and further triggering micro-crack initiation. Under continuous cyclic stress induced by train loads, the micro-crack tips undergo repeated tearing and closing. Severe stress concentration accelerates the formation of transgranular cracks, which propagate rapidly and unstably toward the rail interior, eventually resulting in catastrophic transverse fracture. Standardized procedures in rail transportation, hoisting and laying are essential to avoid mechanical damage, while regular line inspection and timely replacement of damaged rails should be strictly enforced. Full article

This study investigates hydrogen embrittlement in welded joints of X52 (L360) pipeline steel obtained from an operating natural gas transmission network after 31 years of service, with particular emphasis on production (longitudinal) and girth (circumferential) welds. The aim is to elucidate the influence of microstructural heterogeneity across the pipe wall and within different welded joint types on hydrogen transport, trapping behavior, and fracture mechanisms. The investigation combines X-ray diffraction, electrochemical hydrogen permeation testing, fractographic analysis, and transmission electron microscopy. X-ray diffraction results show that the base metal and girth weld consist predominantly of body-centered cubic ferrite, whereas the production weld additionally contains retained austenite associated with an elevated manganese content. These phase-related differences are consistent with transmission electron microscopy observations of martensite–austenite constituents within the weld microstructure. Electrochemical hydrogen permeation measurements reveal pronounced microstructure-dependent hydrogen transport behavior. The production weld exhibits a significantly lower apparent diffusion coefficient and a markedly higher hydrogen trap density, approximately five times greater than those of the base metal and girth weld, providing a mechanistic explanation for the observed differences in hydrogen uptake behavior. Fractographic analysis demonstrates a transition from ductile microvoid coalescence in the uncharged condition to predominantly brittle fracture following hydrogen charging. This transition is accompanied by a substantial increase in the fraction of brittle fracture zones, reaching approximately 53% in hydrogen-charged specimens. A pronounced gradient in hydrogen embrittlement susceptibility is observed across the pipe wall thickness, with outer-wall specimens consistently exhibiting greater susceptibility than inner-wall specimens. This behavior reflects the combined influence of long-term soil corrosion and hydrogen-assisted degradation. Transmission electron microscopy reveals that plastic deformation governs dislocation generation, while hydrogen significantly modifies dislocation behavior by promoting dislocation pile-ups near martensite–austenite constituents and non-metallic inclusions. These observations indicate strong interactions between hydrogen, dislocations, and microstructural heterogeneities. A clear size-dependent role of non-metallic inclusions is identified. Sub-micron inclusions act primarily as irreversible hydrogen trapping sites that contribute to hydrogen redistribution within the microstructure, whereas larger inclusions serve as preferential crack initiation sites under hydrogen charging conditions. Overall, the results demonstrate that hydrogen embrittlement behavior is governed by the combined effects of microstructural state, welded joint type, and long-term service-induced degradation, resulting in distinct hydrogen transport characteristics and fracture responses across the pipe wall. Full article

The main aim of this study is to investigate the microstructure and microhardness of Wire Arc Additive Manufactured (WAAM) samples produced under different layer deposition strategies and corresponding interlayer temperature conditions. Experimental samples were produced using the WAAM process with X45CrSi9-3 steel. During the experiments, both the number of layers and the thermal conditions (heating and cooling) were systematically varied. This was achieved by fabricating samples consisting of five layers with three beads per layer. The layer deposition procedure was implemented in two different ways: (i) with a waiting period after each layer to allow cooling to room temperature, and (ii) without such a waiting period. Thermal cycles at selected locations within the samples were calculated using simulation modeling. By combining these thermal cycles with the continuous cooling transformation (CCT) diagram, the expected microstructures in the vicinity of these locations were determined. These predictions were supplemented by microstructural analysis and hardness measurements. Particular emphasis was placed on the influence of interlayer temperature and repeated heating and cooling cycles. The analyses enabled the identification of process parameters that facilitate control over microstructure, microhardness, and property gradients. It can be concluded that the interlayer holding time provides an effective means of controlling the microstructure of the workpiece, ranging from predominantly austenitic to predominantly martensitic. Depending on the thermal cycles, the measured microhardness varied within the range of 360–900 HV. Metallographic examination revealed a wide spectrum of non-equilibrium microstructures, including martensite with varying degrees of tempering, retained austenite, pearlite, and bainite. The application of a thermal model to the conducted experiments, combined with the CCT diagram, indicated that the expected microstructures consist predominantly of martensite with varying degrees of tempering, retained austenite, carbides, and, in some cases, up to 5% pearlite. Full article

The utilization of hydrogen as a clean energy carrier requires an assessment of existing natural gas pipelines with respect to hydrogen embrittlement (HE). In this study, the structural integrity and hydrogen sensitivity of X52 (L360) pipeline steel from the Bulgarian gas transmission network after 31 years of service were investigated, focusing on production (longitudinal) and girth (circumferential) welded joints. Hydrogen content was measured in the base metal, production weld and girth weld before and after electrochemical charging, while in situ hydrogen charging during tensile testing was applied to simulate service conditions. Mechanical behavior was evaluated by tensile tests, and microstructural and fracture characteristics were analyzed by SEM and TEM. The results show significant spatial variations in hydrogen concentration, related to local microstructural heterogeneity and hydrogen trapping. In the as-operated state, fracture was localized mainly in the heat-affected zone. Hydrogen charging led to a pronounced reduction in ductility (approximately twofold), whereas yield and tensile strengths were only slightly affected. Failure analyses indicate a transition toward more brittle fracture mechanisms, dominated by quasi-cleavage and intergranular cracking in the as-charged state, with hydrogen embrittlement susceptibility indices demonstrating higher hydrogen sensitivity of the girth-welded joints. Full article

The objective of this article is to optimise the deposition modes and the content of exothermic additions (EAs) in the core filler in Fe-C-Cr-Ti with Cu additions hardfacing. To achieve this, JMatPro Release 7.0, Sente Software Ltd., 2016 material characterisation software was used to simulate and calculate the equilibrium phase structure and composition of the Fe-C-Cr-Ti-Cu alloy during the welding thermal cycle. A synergistic approach combining the Taguchi–Analysis of Variance (ANOVA)–Factorial design (FD) method with the standard hybrid Taguchi–ANOVA–Principal Component Analysis (PCA)–Grey Relational Analysis (GRA) is used and justified to optimise factors and develop mathematical models for parameters in the L9 orthogonal experimental design. The study examines how the transfers of deoxidisers depend on the content of exothermic additions in the cored wire filler (EA) and the contact tip-to-work distance (CTWD), while the behaviour of carbide formers is influenced by wire feed speed (WFS) and present arc voltage at the power source (U_set). The research specifically investigates the Fe-C-Cr-Ti-Cu system and the role of copper in stabilising austenite. Findings show that high Cu concentrations (7 wt.%) enhance hardenability by 13%, effectively suppressing pearlite transformation and expanding the bainite region. The desired chemical composition of the deposited metal is determined by the distribution of selected factors, as measured by the transfer coefficients of each element. Full article

Fused Filament Fabrication (FFF) is a low-cost additive manufacturing process that produces metallic parts from printing with a metal-polymer filament, followed by a debinding–sintering process. It presents an opportunity for the tooling sector to improve performance by geometrical optimization while keeping costs low. This study investigates the possibility of producing a molding core for plastic injection by FFF technology. This research aimed to characterize 17-4 PH stainless steel and H13 hot work tool steels produced through this process. Their heat treatment behavior was investigated using dilatometry, which led to the obtention of a Continuous Cooling Transformation (CCT) diagram. Results show that for as-sintered materials, martensitic steel with some residual austenite is present in 17-4 PH, and a pearlitic microstructure is observed in H13. Porosity (around 4%) falls within the reported range in the literature and can be removed by hot isostatic pressing. CCT diagrams do not show significant differences with conventional materials. The low hardness of as-sintered H13 (around 175 HV1) is improved (>500 HV1) by suitable heat treatment. Finally, both materials meet the requirements for this specific industrial application, and demonstrators were produced. Full article

The microstructural characteristics and fracture behavior of a magnetic pole screw were investigated here. The screw threads were produced by cold thread rolling. Microstructural analysis (OM, SEM, EBSD), mechanical testing (tensile, hardness, fastening), and fracture morphology observation were conducted. The results indicate that work hardening and microstructural deformation were introduced by the gradient plastic deformation in the screw thread. The elongated microstructure of ferrite and pearlite was obtained in the deformation zones, resulting in increased hardness and decreased plasticity. The thread root subsurface experienced severe localized indentation deformation and exhibited the highest hardness. The distinct forming stress states led to a notable difference in the hardened layer depth between the thread crest and root. The torsional overload fracture was initiated at the stress-concentrated thread root, where the work-hardened microstructure exhibited a limited capacity to accommodate large plastic deformation. The crack propagation was influenced by the gradient microstructure, following three primary propagation paths: transgranular through ferrite, along the ferrite–pearlite phase interface, and cracking through lamellar pearlite. The results provide theoretical support for material design and process optimization to achieve the production of high-performance screws with high strength and hardness at the thread surface and high plasticity in the center. Full article

To meet the mechanical property requirements of gray cast iron for the shells of coal mine explosion-proof equipment and investigate the effect of austenitizing temperature on the microstructure and mechanical properties of gray cast iron, isothermal quenching was conducted at four austenitizing temperatures (890 °C, 910 °C, 930 °C, and 950 °C), with cast samples as the control group. The microstructure was using a scanning electron microscope, and the mechanical properties were tested using a universal tensile testing machine, a drop-weight impact testing machine and a hardness tester. The results show that the matrix microstructure of gray cast iron transforms from ferrite + pearlite to ausferrite after isothermal quenching, and the proportion of ausferrite increases gradually with the rise of austenitizing temperature. At an austenitizing temperature of 930 °C, the hardness of the sample reaches a maximum value of 247.6 HBW, which is 31.9% higher than that of the cast sample. At 910 °C, the impact energy and tensile strength achieve the optimal values of 9.59 J and 219 MPa, respectively, with an increase of 6.43 J and 51 MPa compared with the cast sample. Comprehensive analysis indicates that the austenitizing temperature of 910 °C can improve the strength while maintaining good toughness, which makes it more suitable for application scenarios requiring both strength and toughness such as coal mine explosion-proof equipment. Full article

Q460 steel with varying Ni contents was produced via the hot continuous rolling process. The microstructure and properties were examined using scanning electron microscopy (SEM) and universal testing machines. The results show that with increasing Ni content, the number of inclusions rises, and the microstructure gradually evolves from ferrite and pearlite to a mixture of ferrite, pearlite, and bainite. Due to the formation of bainite and refined grains, both the yield strength and tensile strength are significantly improved. However, with the increase in inclusions, the elongation and impact energy of the material do not exhibit substantial enhancement. Considering both cost and performance, the Ni content in Q460 steel is preferably controlled at 0.3%. Full article

Coating a sword’s surface with clay before quenching in water not only produces distinctive patterns but also modifies its hardness and corrosion resistance. This study investigated two steel swords with differing carbon contents (L01 containing 0.69% C and L02 containing 0.98% C) subjected to the clay-coated quenching process to assess its impact on the blades’ microstructure, hardness, and corrosion characteristics. Samples from each sword underwent analysis through metallography, microhardness tests, electrochemical tests, and scanning electron microscopy. The investigation revealed that L02 comprising martensite, pearlite, retained austenite and carbides, exhibited a greater diversity of microconstituents than L01 containing martensite and pearlite. In addition, the hardness range of L02 (425~1050 HV) showed a broader hardness spectrum than that of L01 (HV 550~846), further illustrating that L02 possessed a higher degree of microstructural gradation and better balance of hardness and toughness. However, the electrochemical tests showed that each test area of L01 exhibited consistently lower corrosion rates than their counterparts on L02. The i_corr values for L01 ranged from 5.12 to 8.29 μA·cm⁻², while L02 had i_corr values between 21.17 and 25.23 μA·cm⁻². Importantly, the calculated R_p values across the different zones of L01 (ranging from 2338 to 4129 Ω·cm²) exceeded those of the corresponding zones of L02 (ranging from 502 to 816 Ω·cm²). The electrochemical impedance spectroscopy (EIS) data revealed that the R_ct values for L01 (ranging from 2016 to 2837 Ω·cm²) were also greater than the corresponding values for L02 (range: 424~571 Ω·cm²). The data indicated that L02 exhibited inferior corrosion resistance compared to L01, attributable to its higher carbon content. This increased carbon content facilitated the development of a more heterogeneous and diversified microstructure during clay quenching, resulting in a greater electrochemical potential difference and subsequently accelerating corrosion. These insights delineate a distinct microstructure–corrosion relationship in gradient steel blades processed by clay-coated quenching and offer practical guidance for selecting carbon content to enhance both mechanical properties and corrosion resistance in traditionally crafted blades. Full article

This study employed a Gleeble-3800TM thermal simulator to conduct thermal cycle experiments on the coarse-grained heat-affected zone (CGHAZ) of Nb-Ti-La microalloyed steel under welding heat inputs of 50, 80, 100, and 120 kJ/cm. A systematic analysis was carried out to investigate the influence of heat input on the microstructure and impact toughness of the CGHAZ. The results indicate that the microstructure of the CGHAZ across different heat inputs consists of acicular ferrite (AF), granular bainite ferrite (GBF), polygonal ferrite (PF), as well as hard phases such as M/A constituents and degenerated pearlite (DP). With increasing heat input, the content of GBF decreases monotonically, while the content of PF increases monotonically, and the amount of hard phases rises continuously. In contrast, the content of AF initially increases and then decreases, reaching its peak at 100 kJ/cm. The microstructural changes induced by higher heat input lead to increased inhomogeneity in the local microstrain, thereby causing a monotonic reduction in crack initiation energy. Regarding crack propagation energy, the optimal performance is achieved at 100 kJ/cm due to the formation of a high proportion of AF, which heterogeneously nucleates on La-rich inclusions. This structure provides a high density of high-angle grain boundaries that effectively hinder crack propagation. Consequently, under the combined influence of crack initiation and propagation behaviors, the CGHAZ exhibits the best impact toughness at a heat input of 100 kJ/cm. Full article