Search Results (265)

This paper assesses experimentally and theoretically the hydrogen-assisted cracking sensitivity of an ultrahigh-strength lath martensitic steel, recently used to manufacture tendon rods for structural engineering. The experimental values of the J-integral were obtained by tensile testing up to failure precracked SENT specimens in air, as an inert environment and in a thiocyanate aqueous solution, as a hydrogen-promoter medium. In parallel, the theoretical resources necessary to apply the Dugdale cohesive model to the SENT specimen were developed from the Green function in order to predict the J-integral dependency on the applied load and the crack size, with the cohesive resistance being the only material constant concerning fracture. The comparison of theoretical and experimental results strongly supports the premise that the cohesive crack accurately models the effect of the mechanisms by which the examined steel opposes crack propagation, both when in hydrogen-free and -embrittled conditions. The identification of experimental and theoretical limit values respectively involving a post-small-scale-yielding regime and unstable extension of the cohesive zone allowed for the value of the cohesive resistance to be determined, its condition as a material constant in hydrogen-free medium confirmed, and its strong decrease with hydrogen exposure revealed. Full article

This study systematically explores the synergistic effects of direct aging treatment (480 °C for 6 h) combined with cryogenic treatment (−196 °C for 8 h) on the mechanical properties and microstructural evolution of 18Ni300 maraging steel fabricated via selective laser melting (SLM). Three conditions were investigated: as-built, direct aging (AT6), and direct aging plus cryogenic treatment (AT6C8). Microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), while the mechanical properties were evaluated via microhardness and tensile testing. The results show that the AT6C8 sample achieved the highest microhardness (635 HV_0.5) and tensile strength (2180 MPa), significantly exceeding the as-built (311 HV_0.5, 1182 MPa) and AT6 (580 HV_0.5, 2012 MPa) samples. Cryogenic treatment induced a notable phase transformation from retained austenite (γ phase) to martensite (α phase), with the peak relative intensity ratio ranging from 1.42 (AT6) to 1.58 (AT6C8) in the XRD results. TEM observations revealed that cryogenic treatment refined lath martensite from 0.75 μm (AT6) to 0.24 μm (AT6C8) and transformed reversed austenite into thin linear structures at the martensite boundaries. The combination of direct aging and cryogenic treatment effectively enhances the mechanical properties of SLM-fabricated 18Ni300 maraging steel through martensite transformation, microstructural refinement, and increased dislocation density. This approach addresses the challenge of balancing strength improvement and residual stress relaxation. Full article

In this study, we delve into the intricate microstructural features of Ti-6Al-4V alloy additively manufactured and heat-treated at 800 °C for 4 h. Our in-depth analysis will enable us to gain a better understanding of the β-Ti precipitation process, its dependence on temperature, and its ultimate effect on the overall mechanical properties. As well as α-Ti martensite grains and β-Ti particles interspersed in the α-Ti grain boundaries, there is a third microstructural feature, overlooked by many researchers. This feature is observed as nano-sized particles homogeneously embedded inside the α-Ti laths. Using high-resolution transmission electron microscopy, we reveal that these nano-sized features do not constitute a different phase. Instead, they define isolated regions of α-Ti in its relaxed form, surrounded by the heavily strained form of the α-Ti phase. This phenomenon is a result of the “incomplete” precipitation of the β-Ti phase following the heat treatment stage. The straining of the α-Ti phase appears as a shift in the equilibrium atomic position. Full article

The direct energy deposition (DED) metal additive manufacturing process enables rapid deposition and repair, providing an efficient approach to producing durable tool steel components. Here, 5 wt% Cr cold-work tool steel (Caldie) was developed by reducing carbon and chromium to suppress coarse carbide formation and by increasing molybdenum and vanadium to enhance dimensional stability. In this study, Caldie tool steel was fabricated via DED for the first time, and the effects of post-heat treatment on its hierarchical microstructure and mechanical properties were investigated and compared with those of wrought (reference) material. The as-built sample exhibited a mixed microstructure comprising lath martensite, retained austenite, polygonal ferrite, and carbide networks, which transformed into full martensite with fine carbides after heat treatment (DED-HT). The tensile strength of the DED Caldie material increased from 1340 MPa to 1949 MPa after heat treatment, demonstrating superior strength compared to other heat-treated, DED-processed high-carbon tool steels. Compared to DED-HT, the wrought material exhibited finer martensite, a more uniform Bain group distribution, and finer carbides, resulting in higher strength. This study provides insights into the effects of heat treatment on the hierarchical microstructure and mechanical behavior of Caldie tool steel manufactured by DED. Full article

Low-carbon lath martensite is highly susceptible to hydrogen embrittlement due to the presence of a high density of lath/block boundaries. In this study, I employ a continuum decohesion model to investigate the effects of varying hydrogen concentrations and tensile loads on the cohesive strength of low- and high-angle block boundaries. The thermodynamic properties of the cohesive zone are described using excess variables, which establish a link between atomistic energy calculations and the continuum model for gradual decohesion at a grain boundary. The aim of this study is to develop an in-depth understanding of how hydrogen affects the cohesive strength of block boundaries in a lath martensitic structure by integrating continuum and atomistic computational modeling and to apply the resulting insights to investigate the effects of varying hydrogen concentrations and tensile loads on interface decohesion. I incorporate hydrogen mobility and segregation at low- and high-angle twist boundaries in body-centered cubic (bcc) Fe to quantify the hydrogen-induced effects on progressive decohesion under tensile stress. A constant hydrogen flux through the free surfaces of a bicrystal containing a block boundary is imposed to simulate realistic boundary conditions. The results of the simulations show that, in the presence of hydrogen flux, separation across the block boundaries occurs at a tensile load significantly lower than the critical stress required for rupture in a hydrogen-free lath martensitic structure. Full article

The martensitic transformation mechanism in the heat-affected zone of DP1180 steel plays a decisive role in the strength of welded joints. In this work, the nucleation and growth kinetics of martensite laths in the coarse grain heat-affected zone (CGHAZ) are analyzed by a high-temperature laser scanning confocal microscope (LSCM). The grain distribution and stress distribution of the samples after in situ observation are analyzed by electron backscatter diffraction (EBSD). The results reveal that austenite grain growth is realized by continuous grain boundary annexation and grain boundary migration of small grains by large grains during the heating process. Seven growth modes of CGHAZ martensitic laths under laser welding conditions are proposed. Additionally, the end growth of martensitic laths is mostly attributed to the collision with grain boundaries or other laths to form a complex interlocking structure. The results of this study could provide important data support for the development of dual-phase steel materials and improvement of welding quality. Full article

To address the issue of surface grain coarsening in laser-induction hybrid phase transformation of 42CrMo steel, this study investigated the effects of four pretreatment processes (quenching–tempering (QT), laser-induction quenching (LIQ), laser-induction normalizing (LIN), and laser-induction annealing (LIA)) on the austenite grain size and wear resistance after laser-induction hybrid phase transformation. The results showed that QT resulted in a tempered sorbite structure, resulting in coarse austenite grains (139.8 μm) due to sparse nucleation sites. LIQ generated lath martensite, and its high dislocation density and large-angle grain boundaries led to even larger grains (145.5 μm). In contrast, LIN and LIA formed bainite and granular pearlite, respectively, which refined austenite grains (78.8 μm and 75.5 μm) through dense nucleation and grain boundary pinning. After laser-induction hybrid phase transformation, all specimens achieved hardened layer depths exceeding 6.9 mm. When the pretreatment was LIN or LIA, the specimens after laser-induction hybrid phase transformation exhibited surface microhardness values of 760.3 HV0.3 and 765.2 HV0.3, respectively, which were 12 to 15% higher than those of the QT- and LIQ-pretreated specimens, primarily due to fine-grain strengthening. The friction coefficient decreased from 0.52 in specimens pretreated by QT and LIQ to 0.45 in those pretreated by LIN and LIA, representing a reduction of approximately 20%. The results confirm that regulating the initial microstructure via pretreatment effectively inhibits austenite grain coarsening, thereby enhancing the microhardness and wear resistance after transformation. Full article

This study systematically investigates the influence mechanism of the element Cr on the mechanical properties of the heat-affected zone in pipeline steels for supercritical CO₂ transportation. Microstructural evolution in the heat affected-zone was characterized through thermal simulation tests, Charpy impact testing (−10 °C), and microhardness measurements, complemented by multiscale microscopic analyses (optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy). The results demonstrate that Cr addition enhances the base metal’s resistance to supercritical CO₂ corrosion but reduces its low-temperature impact toughness from 277 J to 235 J at −10 °C. Notably, the intercritical heat-affected zone exhibits severe embrittlement, with impact energy plummeting from 235 J (base metal) to 77 J. Microstructural analysis reveals that Cr interacts with carbon to form stable carbonitride particles, which reduce the free carbon concentration and diffusion coefficient in austenite, thereby inducing heterogeneous austenitization. Undissolved carbonitrides pin grain boundaries, creating carbon concentration gradients. During rapid cooling, these localized carbon-enriched microregions preferentially transform into core–shell-structured M-A constituent, characterized by a micro-twin containing retained austenite core encapsulated by high hardness lath martensite. The synergistic interaction between micro-twins and interfacial thermal mismatch stress induces localized stress concentration, triggering microcrack nucleation and subsequent toughness degradation. Full article

In this study, the phase transformation mechanism during the decomposition of undercooled austenite and its effect on the deformation behavior of a high-strength medium Mn steel were studied. The results indicate that the austenite formation during heating (α → γ) is a relatively fast reaction. However, the transformation of undercooled prior austenite above the martensite start (M_s) temperature (γ → α) is difficult due to its high thermal stability. Only martensite transformation occurred during the final air-cooling stage following a 120-h isothermal treatment at 360 °C (slightly above M_s). The growth of martensite laths was limited by the boundaries of prior austenite grains and martensite packets. High-strength tensile properties were achieved, with a yield strength of 955 MPa, ultimate tensile strength of 1228 MPa, and total elongation of 11.6%. These properties result from the synergistic hardening effects of grain refinement, high-density lattice distortion, and an increased boundary length per unit area. The composition design with medium Mn content increased the processing window for high-strength martensite transformation, providing a theoretical basis for an energy-saving approach that depends on the decomposition transformation of undercooled austenite. Full article

This study investigated the high-temperature tensile durability of G115 steel after aging at 650 °C for different periods of time using a high-temperature tensile testing system. The results show that with the increase in aging time, the microstructure of G115 steel changes to a certain extent, which shows that as the martensite lath width and HAGB increases, the dislocation density decreases and the second precipitate shows obvious growth. As the tensile test temperature increases, the tensile strength decreases from ~750 MPa to ~350 MPa. The effect of aging time on the high-temperature tensile durability of steel after a 3000 h aging process is not significant, which should be related to the relatively high microstructure thermal stability of G115 steel during the aging process. Full article

Hot stamping is a promising method to manufacture ultra-high-strength automotive steel components with high dimension accuracy. In this work, two actual industrial production flows (with and without Al-Si hot dipping) were investigated to reveal their microstructural evolution and hydrogen content at different production steps. Meanwhile, the variations in composition and phase structures of the Al-Si coating layer were studied in terms of energy-dispersive spectrometry and electron backscattering diffraction techniques. The results showed that the microstructure at the steel substrate changed from the pancake-shaped pearlite and ferrite, degenerated pearlite and annealed ferrite, lath martensite, and then tempered martensite with the progress of the production steps, which was not affected by the Al-Si hot dipping. The final coating layer exhibited a multi-sublayer structure with the alternative distribution of FeAl and Fe₂Al₅, which contained many microcracks on the brittle phase Fe₂Al₅. The Al-Si-coated specimens always had higher hydrogen content than the bare steel specimens because of the hydrogen generation at the hot stamping stage and hydrogen absorption during the hot-dip aluminizing stage. 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

Martensitic stainless steel (MSS) is widely used in several parts of automobiles where high strength, hardness, and corrosion resistance are required. However, the metastability of retained austenite can transform into martensite under severe deformation, adversely affecting material properties. Cryogenic treatments (CTs) have been extensively employed in iron-based alloys for fastener application due to their advantageous effect. This study explores the heat treatment processes applied to 13Cr-2Ni-2Mo martensitic stainless steel (MSS), including austenitizing, cryogenic treatment, and tempering cycles. Cryogenic treatment at (−150 °C) for varying durations, followed by tempering at 200 °C for 2 h, and the impact of post-cryogenic tempering at 200 °C for different tempering duration on the microstructure and mechanical properties were evaluated. Experimental results indicate that the sample quenched at 1040 °C for 2 h (CHT) contains lath martensite, retained austenite, δ-ferrite, and undissolved carbide precipitation. Compared to as-quenched samples, hardness decreased by 5.04%, 7.24%, and 7.32% after tempering for 2 h, 5 h, and 10 h, respectively. Extending cryogenic durations to 2 h, 12 h, and 20 h promoted nucleation of a mixture of M₃C and M₂₃C₆ small globular carbides (SGCs) and grain refinement but resulted in hardness reductions of 5.04%, 5.32%, and 8.36%, respectively. The reduction in hardness is primarily attributed to a decrease in solid solution strengthening and promoted carbide coarsening. Full article

An efficient extension of conventional rate-dependent crystal plasticity is formulated to include the influence of block boundaries in lath martensite. It is shown that this can be achieved by minor modifications to a conventional crystal plasticity model, providing an efficient modeling approach with a negligible additional computational cost. With this modification, the parameter related to lattice friction is made dependent on a Hall–Petch-type block size dependency, resulting in block strengthening. The boundaries are modeled as obstacles which restrict the motion of dislocations, resulting in lower slip activity and promoting dislocation accumulation close to the boundaries. The model is calibrated and validated using experimental data from the literature, and its capabilities are demonstrated in a series of simulation examples. Full article

Low-cost and low-alloy dual-phase (DP) steel with a tensile strength (TS) above 1000 MPa and high ductility is in great demand in the automobile industry. An approach to using a medium-carbon and fibrous DP structure for developing such new DP steel has been proposed. The microstructure and mechanical performance of fibrous DP steel obtained via partial reversion from martensite in Fe-C-Mn-Si low-alloy steel have been investigated. The TS of the as-quenched DP steel is above 1300 MPa, while the total elongation is less than 6%. The total elongation was increased to above 13%, with an acceptable loss in TS by performing additional tempering. The fibrous tempered-martensite/ferrite DP steel exhibits an excellent balance of strength and ductility, surpassing the current low-alloy DP steels with the same strength grade. Plate-like or quasi-spherical fine carbides were precipitated, and the relatively high-density dislocations were maintained due to the delay of lath recovery by the enrichment of Mn and C in martensite (austenite before quenching), contributing to the tempering softening resistance. In addition, nanotwins and a very small amount of retained austenite were present due to the martensite chemistry. High-density dislocations, fine carbide precipitation, and partially twinned structures strengthened the tempered martensite while maintaining relatively high ductility. Quantitative strengthening models and calculations were not included in the present work, which is an interesting topic and will be studied in the future. Full article