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In this study, the microstructure and mechanical properties of 55SiCrVNb ultra-high strength spring steel with 0.7 and 1.5 wt.% Mn were systematically investigated. The results show that 0.7Mn steel exhibited higher strength and toughness. The morphology and content of retained austenite, content of element in solid solution and the type and quantity of precipitates were basically the same in 0.7Mn and 1.5Mn steels. Hence, the variant reconstruction, as a unique perspective, was used to unravel the strengthening and toughening mechanisms governing the evolution of variants across varying Mn contents. Compared with the 1.5Mn steel, the decrease in Mn content deteriorated the variant selectivity (decreased variant frequency over 50 %), which led to a more uniform distribution of close-packed plane (CP) and Bain groups in 0.7Mn steel, thus refining the block size and improving the grain refinement strengthening (σ p). The 0.7Mn steel with a higher density of high angle grain boundaries (HAGBs) effectively constrained dislocation slip, thereby enhancing dislocation strengthening (σ d). As the Mn content decreased, the block size decreased from 0.24 to 0.14 μm, and the contribution of σ p and σ d increased by about 107 and 96 MPa, thus improving the yield strength. In addition, the increased HAGBs, which were dominated by the highly selective V1/V2, V1/V3(V5) and V1/V6 variant pairs, and the uniform CP and Bain group also contributed to inhibiting crack deflection and enhancing the impact toughness. Moreover, the higher Schmid factor with high geometrically necessary dislocation of 0.7Mn steel improved the deformation resistance of variants and strength. This work provided a new mechanism of strengthening and toughening dominated by variants. [ABSTRACT FROM AUTHOR]

A self-developed high strength steel (14Ni3Cr3Mo1MnTiAl HSS) was used to manufacture a thin-walled structure by wire + arc additive manufacturing, the microstructural evolution and the anisotropy of mechanical properties were investigated. The tensile properties and impact toughness at room temperature, −20 °C and −40 °C were compared and discussed in detail. According to the results of OM and SEM, the main microstructure of WAAM-ed 14Ni3Cr3Mo1MnTiAl HSS was martensite and ferrite, and numerous Ni 3 Ti IMCs existed in the matrix. Reversed austenite only existed in the bottom and middle samples, and there was a K-S orientation relationship in reversed austenite and martensite/ferrite. The formation of reverted austenite could be attributed to repeated thermal cycles and high content of Ni, providing beneficial environment and condition for the formation of reverted austenite. Low temperature could increase the strength of vertical samples, while it had a negative effect on plastic and had little effect on the strength of horizontal samples. Differently, the impact toughness hardly decreased at −20 °C, while it had significant decrease at −40 °C. The anisotropy of tensile properties was attributed to the weak bonding of interlayer, phase content, the size of martensite packet/lath, and the size of Ni 3 Ti IMCs. Columnar crystals were one of the key factors leading to the low impact toughness of horizontal samples. The impact direction was parallel to the growth direction of columnar crystals, and the crack was prone to propagate along the boundaries of columnar crystals. [ABSTRACT FROM AUTHOR]

As a powder metallurgy (PM) defect, existence of prior particle boundaries (PPBs) often makes mechanical properties visibly deteriorated. Hot rolling has been found to be an effective method of destroying PPBs. In present study, five different rolling reductions are applied to explore the effects of rolling reduction on evolution of PPBs in PM 30CrMnSiNi2A steels. Microstructure, mechanical properties and fracture modes of five samples are systematically analyzed and compared. Increase of rolling reduction not only makes Al-rich PPBs refined but also makes continuous PPBs structure gradually transform to dispersed oxides and short chain fragments, which can be attributed to deformation of powder particles. Change on distribution of PPBs leads to the variation of fracture modes. A strength-ductility-toughness balance is achieved when the rolling reduction scales up to 60 %. Although increase of rolling reduction causes grain refinement, work hardening and improvement of PPBs, it promotes formation of edge cracks. Occurrence of edge cracks is strongly associated with PPBs inclusions and makes ductility and toughness deteriorated. • High performance PM 30CrMnSiNi2A steel was prepared by HIPing combined with hot rolling. • An excellent strength-ductility-toughness balance was achieved when reduction increased to 60 %. • Effects of rolling reduction on evolution of PPBs were analyzed. • Change of fracture modes showed a strong association with improvement of PPBs. • Formation of edge cracks was closely related with PPBs inclusiosns. [ABSTRACT FROM AUTHOR]

In this work, the reverse transformation of ε -martensite located inside nanotwin lamellae has been introduced to improve the ductility of a high-strength Fe–Mn–C steel with a high density of nanotwins. The steel was designed to have a low stacking-fault energy (SFE, 17.8 mJ/m2) to introduce high density of nanotwins through pre-deformation. Meanwhile, the fine ε -martensite was also obtained in nanotwin lamellae. The steel was strengthened significantly, with an increase in yield strength from 357 to 1275 MPa by the introduction of nanotwins. A subsequent intermediate annealing was applied to control the reverse phase transformation from ε -martensite to austenite to enhance the deformation ability of the nanotwinned steel (rising tensile elongation from 5 to 16%). The nanotwins with a width of 32 nm on average were stable during the heat treatment. The high strength was retained because the size of the austenite transformed from ε -martensite was restrained by the stable nanotwins. This strategy holds promise and can be widely applied to alloys with low SFE. [ABSTRACT FROM AUTHOR]

This study addresses one of the most crucial ongoing discussions concerning the mitigation of Zinc-induced Liquid Metal Embrittlement (LME) in third-generation Advanced High-Strength Steels (AHSS). The role of Silicon (Si) and Aluminum (Al), both common alloying elements in AHSS, in triggering LME-induced cracking is thoroughly investigated. Two chemical compositions, 0.2C–3Mn-0.8Si and 0.2C–3Mn–1Al, featuring martensitic microstructures, were selected. Hot tensile tests were conducted in the temperature range of 600–900 °C to simulate the thermomechanical cycle during spot welding. This resulted in a significant reduction in ductility for both Si and Al-alloyed steels in the coated samples compared to the uncoated ones. The loss of ductility was notably more pronounced in Si-alloyed steel than in Al-alloyed steel. These findings were also evident in a specialized LME-triggering spot-welding test, suggesting that Si, as an alloy element in steel, leads to higher LME susceptibility compared to Al. This can be attributed, as indicated by thermodynamic equilibrium calculations and investigations of the Zn–Fe layer, to an increased destabilization and retardation of the protective intermetallic Zn–Fe phases by Si. The insolubility of Si in intermetallic phases and liquid Zn is considered a thermodynamic constraint. In contrast, the theoretically possible full solubility of 1% Al in intermetallic phases as well as in liquid Zn was experimentally demonstrated and supported by thermodynamic simulation. Furthermore, Al, by increasing the Ac3 temperature, results in lower amounts of austenite in the microstructure. It is known that austenite plays a detrimental role in triggering LME. Based on this comprehensive study, it is reasonable to conclude that the formation of intermetallic phases plays a critical role in controlling the contact time between steel and liquid Zn as an embrittling agent, and this is strongly influenced by the Si and Al content of the steel. • The crucial role of intermetallic phases, particularly Γ, as a barrier between liquid Zn and the steel to reduce LME sensitivity could be clearly demonstrated in this work. • A full substitution of 0.8 % Si by 1 % Al can significantly mitigate the Zn-induced liquid metal embrittlement of AHSS during hot tensile and spot-welding tests. • Thermodynamic simulation and experimental work showed that Al has a significant higher solubility in liquid Zn and the intermetallic Zn–Fe phases in contrast to Si. • Al vs. Si enhances LME resistance with faster Zn–Fe reaction, stable Zn–Fe phases at high temp., and reduced austenite in microstructure. [ABSTRACT FROM AUTHOR]

The effect of the Cu additives (0.8 wt%) on strain-induced coarsening of the Laves phase was investigated via the comparison of the Laves phase behavior in the Re-containing steels with the different W and Cu contents during creep and aging at a temperature of 650 °C. The formation of the "Cu"-clusters/precipitates with an average size of 3 nm randomly distributed within the ferritic matrix was revealed in the steel with 0.8% Cu, which gave +(20–50) MPa to the room yield strength compared to the steels with 0.2% Cu. The strengthening mechanism due to the "Cu"-clusters/precipitates was revealed as a linear sum of the chemical effect induced by the creation of a new interface area during the cutting of the dislocation through the particle and the pinning of the glide dislocation by an array of obstacles. During creep and aging, the fine "Cu"-clusters/precipitates were dissolved with the precipitation of the fcc "Cu"-rich phase along the boundaries. The nucleation of the Laves phase on the fcc "Cu"-rich phase along the boundaries was detected, which provided the increase in particle number density during both aging and creep conditions. During aging, the coarsening rate constant of the Laves phase particles in the steel with 0.8% Cu was minimal among all studied steels that was caused by the Cu atoms occupying the favorable positions for segregation along the boundaries. However, during creep, the Laves phase also nucleated on the "Cu"-clusters/precipitates, which led to abnormal precipitation of the Laves phase within the martensitic laths and the accelerated growth of the grain boundary Laves phase particles. The grain boundary Laves phase demonstrated intensive strain-induced coarsening during creep, increasing the value of the coarsening rate constant by one order of magnitude compared to the aged state. This is related to the initiation of the additional diffusion way of W atoms from the Laves phase located within the lath interiors to the grain boundary particles. [ABSTRACT FROM AUTHOR]

The effect of ultrasonic surface rolling process on tensile properties of a novel Fe-0.6C-2.0Al-1.5Mn-0.04 Nb quenching-partitioning-tempering steel was investigated. Optical microscope, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were brought out to characterize the microstructure. Compared with quenching-partitioning-tempering steel, the strength and plasticity of the ultrasonic rolling specimen were increased simultaneously, reaching 1329.1 MPa and 23.6%, respectively, and the product of strength and elongation reached the range of the third generation Advanced High Strength Steels. The plasticity improvement mechanism of the USRP1specimen is mainly stemmed from two aspects: First, obvious strain gradients during the tensile process was found in the USRP specimens, which not only avoids the strain localization, but also generates a biaxial stress state. The geometrically necessary dislocations induced by the strain gradient and the dislocations induced by the biaxial stress state promoted extra work hardening, thus ensuring high plasticity of the material. Second, there was a moderate content and small size of twinned martensite in the USRP1 specimen. The improvement in yield strength of the USRP specimens primarily results from grain refinement strengthening, dislocation strengthening, phase transformation strengthening, and back stress strengthening. Among them, phase transformation strengthening contributed the most. [ABSTRACT FROM AUTHOR]

This paper presents an in-situ observation, using neutron imaging, of delayed crack propagation in a high-strength martensitic steel specimen. Delayed cracking is believed to be caused by hydrogen embrittlement occurring due to the slow diffusion and accumulation of hydrogen ahead of a crack front, causing decreased ductility and eventual cracking under constant load. The experiment involved mechanical loading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H 2 O + 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Intermittent crack propagation was observed for 12 h after the environment had been removed. The stress state at each crack configuration was extracted from a three-dimensional elastic–plastic finite element simulation, which was tailored to match the quantitative information acquired from the neutron radiographs of the fracture process. To gain insight into the evolution of hydrogen concentration with crack propagation, a modeling scheme for stress-assisted hydrogen diffusion was also employed. [ABSTRACT FROM AUTHOR]

We present here a new pathway of combining hot rolling, coiling and tempering for multiphase heterostructure to achieve a balanced combination of strength and ductility in low alloy steels. The multiphase heterostructure was composed of bainite/martensite and retained austenite (RA), which was tuned by coiling and tempering. The steel coiled at 450 °C exhibited superior mechanical properties compared with the steels coiled at 350 and 550 °C. After tempering at 300 °C, the steel achieved excellent combination of strength and ductility, with twice the elongation and a corresponding product of strength and elongation (PSE) of 25.1 GPa·%. After coiling, 17 vol% of RA was retained. The RA fraction decreased slightly to 14 vol% by tempering, the steel still sustained considerable austenite. Tempering further enabled carbon partitioning and improved the mechanical stability of RA. The continuous transformation induced plasticity (TRIP) effect from stable RA over the entire strain range resulted in good ductility of the steel. [ABSTRACT FROM AUTHOR]

The prior particle boundaries (PPBs) are a typical powder metallurgy (PM) defect, which usually leads to the deterioration of mechanical properties in PM materials. In this study, two different hot rolling modes (unidirectional rolling and cross rolling) are designed to improve PPBs in hot isostatic pressed (HIPed) 30CrMnSiNi2A ultrahigh strength steel. The microstructure, mechanical properties and fracture features of the HIPed compact, the hot unidirectional rolled (HURed) and hot cross rolled (HCRed) sheets are systematically compared. The results manifests that hot rolling has a significant influence on size and distribution of PPBs. PPBs in HIPed compact are comprised of Al-rich oxides which is distributed along particle boundaries. After hot rolling, large PPBs inclusions are crushed and the size of PPBs are mostly concentrated in 0–0.20 μm. Unidirectional rolling makes continuous PPBs structure broken into short-chain fragments and dispersed oxides completely, while cross rolling makes most of continuous PPBs structure maintained and only a few oxides are dispersed. Excellent mechanical properties are obtained by hot rolling, reflecting on an ultimate tensile strength (UTS) of about 2.0 GPa, a total elongation (TE) of 17.9 %, a Charpy impact energy of 28.6J along rolling direction and a UTS of over 2.1 GPa, a TE of 16.1 %, a Charpy impact energy of 18.1J along transverse direction, respectively. High performances can be attributed to formation of martensite, grain refinement, improvement of PPBs and work hardening. Furthermore, hot-rolled samples manifests an obvious anisotropy in mechanical properties. The HCRed sample reveals a lower anisotropy compared to the HURed sample, mainly owing to differences of PPBs distribution and rolling textures. • High mechanical properties are obtained by HIPing combined with hot rolling in PM 30CrMnSiNi2A steels. • Rolling makes continuous prior particle boundaries (PPBs) broken. • Influences of different rolling modes on PPBs are discussed. • Great performances are due to fine martensite grains, rolling textures, improvement of PPBs and work hardening. [ABSTRACT FROM AUTHOR]

In this study, we designed a lightweight Fe 47 Mn 25 Al 13 Cr 7 Ni 5 C 3 medium-entropy alloy (MEA) (calculated density of 6.803 g/cm3) with enhanced mechanical properties. The MEA samples were produced in three conditions with varying microstructures via thermomechanical processing. They achieved excellent tensile properties through strengthening mechanisms such as partial recrystallization, ultrafine grains, and M 23 C 6 and B2 precipitates. Different strengthening mechanisms were applied depending on the annealing heat treatment conditions, resulting in three different strength-elongation combinations. Furthermore, the MEA, under all designed conditions, exhibited superior specific yield strength-uniform elongation and specific ultimate tensile strength-uniform elongation combinations compared to previously studied lightweight high-entropy alloys (HEAs) and lightweight steel. This was primarily attributed to the combination of benefits obtained from the low proportion of iron (47 at%) as a principal element and the large amount of aluminum addition (13 at%). The proposed MEA and its design strategy can satisfy the requirements for lightweight, cost-effective, strong, and ductile metallic materials, making a great contribution to the automotive industry in terms of crash resistance and fuel efficiency. • Lightweight, inexpensive, strong, and ductile medium-entropy alloy was designed. • Three different tensile properties were derived via annealing heat treatment. • This alloy achieved superior specific strength and uniform elongation. [ABSTRACT FROM AUTHOR]

High strength martensitic strip steels used in the yellow goods industry require good ductility and high strength. This paper examines how local compositional inhomogeneity of alloying elements (Mn, Mo, and Cr), caused by interdendritic microsegregation during casting and subsequent deformation, affect local straining behaviour in a >900 MPa autotempered martensite hot rolled strip steel. The alloy rich and alloy depleted bands have been characterised using micro-XRF and SEM-EDX through thickness for a 10 mm thick strip. Tensile samples were selectively machined from areas with a range of microsegregation severity in the longitudinal and transverse orientations. Digital Image Correlation (DIC) during tensile testing was used to examine local straining behaviour and shear band formation. Differential straining of up to 2.5x between solute enriched (lower strain) and solute depleted (high strain) bands was found in the strongly microsegregated transverse samples due to the lower hardness of the solute depleted bands (reduced solid solution strengthening). The high local strain in solute depleted bands in the strongly segregated sample led to premature failure (at lower global applied strain) compared to the less segregated samples. No significant strain partitioning was observed for the longitudinal oriented samples regardless of segregation level. [ABSTRACT FROM AUTHOR]

The microstructure evolution of cold-rolled microalloyed steel during annealing at different soaking temperatures was investigated, with emphasis on the influence of the starting microstructure prior to annealing. The starting microstructure of cold-rolled martensite exhibited the fastest recrystallization kinetics, followed by the cold-rolled ferrite-pearlite initial microstructure, while the recrystallization was nearly suppressed in the cold-rolled ferrite-cementite microstructure. The different recrystallization kinetics appear to be influenced by the competing effects of the driving pressure for recrystallization and the solute drag of microalloying elements, which leads to dissimilar post-annealing microstructures even with comparable fractions of new ferrite originating from the reversely transformed austenite. The yield strength following annealing was found to be affected by the starting microstructure. In addition to the dislocation strengthening associated with recrystallization retardation, contributions from other strengthening mechanisms, such as grain refinement and precipitation, varied depending on the starting microstructure. Consequently, the strengthening effects are subject to interplay between the starting microstructure and microalloying elements during annealing, which requires quantitative evaluation for a comprehensive understanding of changes in yield strength. [ABSTRACT FROM AUTHOR]

The non-fusion weld-brazing process has attracted considerable attention in the automotive industry due to its lower susceptibility to welding-related defects and high capability to join similar and dissimilar materials. However, the mechanical performance of the brazed joint is limited to the strength of the filler material, which restricts the widespread adoption of this technique in the automotive industry. The objective of the present study is to investigate a viable and effective solution for enhancing the local mechanical properties of the braze bead through the in-situ formation of precipitates during weld-brazing of ZnAlMg coated steel. The results showed that increasing heat input led to the dilution of molten filler material (i.e., CuSi 3 Mn alloy) by the steel substrate, resulting in the formation of a high number density of FeSi(Cu) precipitates with irregular morphologies within the braze bead. Analysis of cyclic load-depth indentation measurements within the braze bead revealed that the presence of FeSi(Cu) precipitates led to a considerable improvement of mechanical properties in the local region of the braze bead. Ultimately, this resulted in higher resistance of the braze bead to cracking as well as greater structural integrity when subjected to non-uniform deformation (such as rotation) during shear-tensile loading. The results also showed that the increased heat input led to optimized joint geometry and improved bonding of joint interfaces, further contributing to the improving mechanical properties of the joint. This study introduces a novel approach to the development of highly robust braze joints through the use of a precipitation-based strengthening mechanism. This approach offers an effective solution to extend the scope of the brazing process for joining high/ultra-high-strength materials used in the automotive industry. [ABSTRACT FROM AUTHOR]

This study highlights the strengthening mechanisms observed during the metal joining of high-strength grade steels (Al-TRIP and Si-TRIP) by providing a concise investigation of microstructural features, mechanical strength evaluation, and employing Finite Element Method (FEM) analysis to understand the deformation behaviour in the joint. The base metals (BMs), Al-TRIP and Si-TRIP are cold-rolled sheets with thicknesses of 0.9 mm and 1.3 mm, respectively. Al-TRIP contains 2.4 wt% Al, while Si-TRIP contains 1.5 wt% Si. The Al-/Si-TRIP joint was processed by laser welding at low energy input 24 J/mm. Electron backscattering diffraction and transmission electron microscopy extensively characterized the microstructural features in the fusion zone (FZ) and heat-affected zone (HAZ) to study strengthening mechanisms induced by welding. Uniaxial tensile tests examined joint mechanical strength, while microindentation hardness (H IT) measurements evaluated mechanical response in the weld zones. The FZ showed a fully martensitic structure, while the HAZs displayed refined grains. Ultrafine-grained structures with an average size of 1 μm were observed in the HAZs, resulting in higher H IT hardness values (∼6.7 GPa) compared to the FZ (∼6.3 GPa). Interestingly, the mechanical tensile properties of the joint were unaffected as failure occurred in the thinner Al-TRIP steel. Finite Element Method (FEM) analysis simulated the tensile testing, revealing localized plasticity in the thinner Al-TRIP and explaining the observed fracture. [ABSTRACT FROM AUTHOR]

Fracture toughness of intercritically-reheated coarse-grained HAZ (ICRCGHAZ), i.e., local brittle zone, is severely reduced at low temperatures by the presence of undesirable microstructural features such as coarsened prior austenite grains, bainitic microstructures including bainitic ferrite (BF), granular bainite (GB), and acicular ferrite (AF), and martensite-austenite constituent (MA). In this study, effects of the microstructural features on critical CTOD and crack initiation and propagation process were investigated in the ICRCGHAZ of three Mo-added high-strength low-alloy (HSLA) steels. The Mo addition caused the increased volume fractions of GB, BF, and MA and the resultant deterioration of critical CTOD. According to the observation of cracking process, most of hard MAs, particularly MAs located along prior austenite grain boundaries (PAGBs), provided initiation sites of {100} cleavage cracks. In addition, a large hardness difference between PAGB-MA and matrix led to an easy MA/matrix interfacial separation and an increase in crack initiation sites, thereby resulting in very low critical CTOD in the high-Mo-containing steel having the highest fraction of MA. The detailed microscopic observation of cracking process at MAs and macroscopic MA/matrix hardness difference provide plausible ideas for reliably evaluating critical CTOD and for designing HSLA steels in consideration of ICRCGHAZ microstructures. [ABSTRACT FROM AUTHOR]

The stress corrosion cracking (SCC) behavior and mechanism of the simulated heat-affected zone (HAZ) of high-strength low-alloy (HSLA) steel in a sulfurated marine atmosphere were surveyed in detail using electrochemical measurements and slow strain rate tensile (SSRT) tests combined with microstructure analysis. The SCC of the simulated HAZs is controlled by both anodic dissolution (AD) and hydrogen embrittlement (HE), which are attributed to the synergistic effect of Cl− and SO 4 2−, as Cl−-induced localized dissolution causes microcrack initiation, and SO 4 2--catalyzed acid regeneration facilitates microcrack propagation. The intercritical HAZ and fine-grained HAZ present high crack numbers because of the high amount of prior austenite grain boundaries (PAGBs), lath bainite boundaries (LBBs), and martensite/austenite (M/A) constituents, which act as preferential sites for hydrogen trapping and crack initiation. However, coarse-grained HAZ exhibits the highest SCC susceptibility because of the coarse PAGBs, wide lath bainites (LBs), and high local dislocation density, which promote crack propagation. [ABSTRACT FROM AUTHOR]

An age-hardening cold-rolled ultrahigh-strength Fe-0.2C-7Mn-3Al-3Ni (wt.%) steel with large ductility was developed via intercritical annealing and followed by aging. It was observed that aging treatment not only resulted in high density nanosized NiAl intermetallic precipitate but it also strongly affected ferrite-austenite duplex microstructure especially the content and stability of austenite due to the partition of elements. Besides, it was found that aging treatment led to a simultaneous increase in both strength and total elongation, which was attributed to high-density NiAl nanoprecipitate, strong dislocation hardening, much stronger transformation-induced plasticity (TRIP) effect and the involvement of twinning-induced plasticity (TWIP) effect. [ABSTRACT FROM AUTHOR]

The advanced high strength steels (AHSS) have gained major attention from research groups across the world especially in the last two decades. The exact description of the forming behavior of these steels has several challenges associated with it. The origin of these challenges is related to complex forming behavior and failure characteristics of AHSS sheets at macro and micro scale. In the present study, the fracture characteristics of two important AHSSs, i.e. DP980 and TRIP1180, were investigated using uniaxial tension and shear tests. The critical strains were calculated considering both necking and fracture using digital image correlation (DIC) technique. The deformed specimens were examined at macro and micro scale to understand the fracture mechanisms of both steels. Numerical simulations were performed for identification and validation of constitutive parameters. A simple fracture law was used to predict the fracture and the results were compared with the experiments. Reasonable fracture prediction was achieved for both DP980 and TRIP1180. [ABSTRACT FROM AUTHOR]

Abstract Retained austenite plays an important role in alloy steel to obtain high strength and excellent ductility. In this work, the pre-tempering process was developed to refine the microstructure of H13 steel. The mechanical properties were effectively improved with the tensile strength ∼1921 MPa, yield strength ∼1534 MPa, impact toughness ∼13.8 J‧cm-2, tensile elongation ∼11.8% and hardness ∼53.2 HRC. What's more, the tensile strength and hardness of the samples maintained above 83% after holding in 550 °C for 50 h, which were higher than the tensile strength (1569 MPa) and hardness (48.3 HRC) of samples treated by traditional 600 °C tempering treatment. It implied that the strength was improved by tempered martensite with high density dislocation and dispersed carbides in H13 steel, while the ductility was improved by the stable lath retained austenite. It is expected that the pre-tempering process can be applied widely in other die steel for superior performance. [ABSTRACT FROM AUTHOR]

Abstract Effect of boron addition on microstructure and tensile properties of a copper modified ultra high strength low alloy NiSiCrCoMo steel is investigated in the present study. Base, Cu and Cu + B modified steels were austenitised and quenched, followed by tempering in the temperature range of 200–750 °C. The effect of tempering temperature on tensile properties along with detailed microstructural changes has been evaluated for these steels at tempering temperatures of 200 °C, 650 °C and 750 °C. Boron addition results in delayed recovery of laths and finer lath structure at higher tempering than that due to only Cu addition in base steel. This contributes for higher strain hardening in boron modified steel at high tempering temperature. A compensating effect in strength has been noticed in Cu modified steel; increase in strength due to presence of precipitates is nullified by the formation of equiaxed polygonal ferrite in this steel. [ABSTRACT FROM AUTHOR]

Abstract Three models for predicting the gigacycle fatigue strength of high-strength steels were investigated in this study: the Tanaka-Akiniwa model, the Murakami equation and a newly-formulated model. The Tanaka-Akiniwa model proved to have the drawback of overestimating the inclusion size effects. In the Murakami equation, the calculated ODA growth curve was imaginary. Our results demonstrated that the new model generated the most accurate predictions. Predictions were then derived, with each constant calculated by fitting the experimental data to each steel grade. The derived predictions were therefore not universal but different for each steel grade. The accuracy of the derived predictions varied according to the grade of steel. The worst accuracy was for JIS-S40C, the second worst was for JIS-SUJ2 and the third worst was for JIS-SCM440 under R = –1. Other than these three predictions, though, the accuracy was excellent. During the fitting of the experimental data, the existence of new fatigue limits was suggested in the gigacycle regions: these were revealed by fatigue testing up to 1011 cycles. The derived predictions indicated only minor differences between steel grades for gigacycle fatigue strength. With increasing inclusion size, the drop in gigacycle fatigue strength became less steep, and looked at in terms of the mean stress effect, a modified Goodman line could safely be used for high-strength steel. [ABSTRACT FROM AUTHOR]

Abstract A simple approach for tailoring the microstructure to meet the property requirements of low carbon steels is studied with the material constants of the Swift equation. Experimental data of the specimens subjected to various chemical compositions and rolling conditions were used to elucidate the relationship among the microstructures, the tensile properties, and the material constants of the Swift equation. This relationship reveals that the material constants of the Swift equation are closely related to the microstructural features of low carbon steels, and in particular constant b can be expressed as a function of the volume fraction of the constituent phases. The approach proposed in this study includes (i) the determination of appropriate range of constant b to meet the required properties and (ii) the selection of the optimum microstructures corresponding to the determined b values. Since this simple approach can provide a guidance in how to control the microstructure to acquire target properties and link microstructural features with macroscopic constitutive behavior, it is very useful for the development of new advanced steels and to save time and production cost. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen embrittlement behaviors of PH17-4 steel and PH13-8Mo steel used as steam turbine last stage blades were investigated by means of slow strain rate tensile tests and hydrogen permeation tests. The results indicate that PH13-8Mo steel exhibits higher strength level mainly due to higher precipitate strengthening and lath strengthening compared with PH17-4 steel in the absence of hydrogen. Moreover, pre-hydrogen charging does not affect tensile strength of PH17-4 steel, but decreases tensile strength of PH13-8Mo steel. The plasticity of PH17-4 steel and PH13-8Mo steel is degraded after hydrogen charging and the deterioration is accompanied with considerable change in fracture mode. The loss of plasticity of PH13-8Mo steel is higher than that of PH17-4 steel, indicating that PH13-8Mo steel exhibits lower resistance to hydrogen embrittlement, which is mainly related to higher hydrogen diffusion behaviors and higher strength level of PH13-8Mo steel. Additionally, PH17-4 steel exhibits lower apparent hydrogen diffusion coefficient and higher apparent hydrogen solubility in comparison with those of PH13-8Mo steel, which implies that incoherent Cu-rich precipitates in PH17-4 steel possess higher capability to trap hydrogen atoms than that of coherent NiAl precipitates in PH13-8Mo steel. [ABSTRACT FROM AUTHOR]

Abstract Austenitic high - Mn TWinning - and Transformation - Induced Plasticity (TWIP and TRIP) steels are strong candidates for GPa - grade cold - rolled steel sheets. The reduction in C or Mn content from high - Mn TWIP steels help generate a TRIP mechanism and prevent serration. However, these high - Mn TRIP steels show low yield strength because of the inherent characteristics of austenite, and often contain a band-shaped segregation of solutes, making the steels acts as hetero - structural materials. Therefore, in this study, we investigate the effects of compositionally - segregated microstructures on tensile properties and serration behavior in precipitation - hardened high - Mn TRIP steels. The present TRIP steels showed high yield strength (778–824 MPa) and an excellent strength-ductility balance, along with serration in their stress-strain curves which could not be explained by existing theories of dynamic strain aging. A considerable amount of martensite was formed step by step as localized deformation bands passed through the specimen gage section, which implied that the serration occurred only when the transformation rate increased substantially. In microstructural aspects, the martensitic transformation occurred sequentially along Mn - segregated bands due to differences in austenite stability and Mn content between high - and low - Mn bands, thereby leading to discontinuous transformation and consequently the serrated flow. [ABSTRACT FROM AUTHOR]

Abstract Dissimilar metal joining is challenging because of intermetallic compound (IMC) formation. Solid state welding techniques provide an opportunity. 5083-H116 aluminum alloy and HSLA-65 steel sheets were butt welded by friction stir welding. Joint strength and failure position were determined by IMC thickness and stress concentration at welded interface. [ABSTRACT FROM AUTHOR]

Abstract High-performance magnesium alloy rods with mechanical isotropy were fabricated by a designed compression-extrusion process to better adapt for industrial massive manufacture. To be specific, yield strengths over 240 MPa in both tension and compression were obtained, highlighting the improvement of tension/compression asymmetry. After investigation, tension deformation in the later compression-extrusion was dominated by slip similar to that in the as-received rod. A slight decrease in yield strength was observed in spite of remarkable grain refinement, mainly caused by the weakening of {0002} fiber texture that enhanced the activation of basal slip. On the contrary, compression deformation was initially induced by {10−12} twinning, responsible for the rather low yield strength, but it gradually shifted to slip particularly after twinning exhausted. The twinning-induced deformation was significantly suppressed by both texture weakening and grain refining, resulting in a significantly increased tendency in yield strength. Both the yield strengths could be well estimated by texture modified Hall-Petch relationships. Additionally, excellent ductility of ~35% in elongation was simultaneously achieved due to the combined contribution of texture weakening and grain refinement, indicating good secondary formability. Moreover, the addition of rare earth element could further enhance the uptrending tendency of yield strengths, which was mainly due to dispersed reinforcement. [ABSTRACT FROM AUTHOR]

This study focuses on enhancing the strength of low-carbon martensitic steel through the refinement of the prior austenite grain size. After subjecting the steel to heavy cold deformation, ultra-rapid heating in a salt bath was employed. The steel was then quenched in water. The results showed that the ultra-rapid heating for 5 s achieved an impressive tensile strength exceeding 2 GPa, accompanied by a higher hardness of over 500 kgf/mm2. By introducing more nucleation sites through heavy cold rolling, it was possible to achieve an ultrafine-grain structure in the steel, with the prior austenite grain size ranging from 1∼3 μm. In some regions, even smaller grain sizes of around 1 μm or less were observed. The boundaries between martensite packets or blocks disappeared, replaced by two or three lath variations. The study also investigated the effect of high-temperature holding time on the austenite phase. It was found that increasing the holding time resulted in the growth of the austenite grains and a subsequent decrease in tensile strength. Considering carbide dissolution, the optimal temperature holding time was approximately 5 s at 900 °C. A shorter holding time allowed more carbides to remain, which weakened the strengthening effect of the carbon solid solution. The relationship between strength and grain size was analyzed by fitting experimental data. The Hall-Petch slopes were calculated to be about 862 MPa mm1/2 and 1022 MPa mm1/2 for the prior austenite grain size and the effective grain size, respectively. Moreover, excluding the influence of martensite packet or block boundaries, the strength contributions from the carbon solid solution and dislocation strengthening were determined to be approximately 583 MPa and 268 MPa, respectively. [ABSTRACT FROM AUTHOR]

Achieving a reliable bond between advanced ceramics and high-temperature application alloys for high-temperature applications can greatly enhance the operating temperature range of vital components like engines and turbines. In this work, we have achieved success in bonding Yttria-stabilized zirconia (YSZ) ceramics with TiAl-base alloys through the use of a novel ceramic-metal joining method called thermoplastic bonding (TPB). Compared with traditional brazing methods, TPB requires a lower bonding temperature that is below the melting point of the filler. This advantage enables TPB to use fillers with higher melting points, resulting in improved high-temperature performance of the joint. Furthermore, TPB substitutes the conventional liquid phase bonding method in brazing with a solid phase bonding process. This not only diminishes residual stress on the joint but also prevents the generation of fragile oxides, which effectively enhances the strength of the joint. Shear tests have demonstrated that TPB joints possess excellent room temperature strength, reaching up to 141 MPa. Remarkably, the TPB joint retains remarkable strength, with a value of approximately 84 MPa even when exposed to a high temperature of 600 °C. These test results proved that TPB was an effective strategy for successfully realizing high-strength and high-temperature resistant metal-ceramic joins. • A novel ceramic-metal bonding process called Thermoplastic bonding was used to join YSZ ceramics and TiAl alloys. • The shear strength of YSZ-TiAl joints at room temperature and 600 °C can reach 141 MPa and 84 MPa, respectively. • The microstructure of the joint was characterized in detail and the joint microstructure evolution mechanism was analyzed. [ABSTRACT FROM AUTHOR]

The effects of phase constituents on the deformation behavior and the correlated mechanical properties at different solution temperatures were investigated in medium Mn high Al lightweight steels with different Al contents (Fe-12Mn-(6,8)-Al-1C). This study indicates that the mechanical properties of medium Mn high Al lightweight steels can be significantly enhanced though changing the content of Al and solution treatment. 6Al steel shows lower yield strength (YS) and tensile elongation (TEL) values than those of 8Al steel at the same temperature due to the unitary austenite phase. When Al is added to 8 wt%, both the precipitation of intergranular and intragranular κ-carbide are promoted and the constituents of multiple phases (austenite + ferrite +κ-carbide) result in ultra-high strengthening. The apparent strain partitioning causes by the different characteristics between different phases leads to the high initial strain hardening. During the deformation process, the sufficient formation of slip bands in austenite grains keeps the high strain hardening exponent and postponed the plastic stability, which also contribute to a pronounced ductility. The existence of multiple phases owing to the addition of Al is the key feature of excellent properties in the investigated lightweight steel. [ABSTRACT FROM AUTHOR]

Precipitation-hardened high/medium entropy alloys (PH-HEAs/MEAs) have attracted extensive interest due to their excellent mechanical properties at both room and high temperatures. Strengthening mechanism models can be used to guide the discovery and design of new alloys, however, the parameters of the models are often lacking, limiting their use for compositional screening from a large space. We proposed a strategy combining machine learning with precipitation strengthening mechanism for high-performance PH-HEAs/MEAs. Using this strategy, we successfully synthesized an alloy Al 5 Ti 8 Co 40 Fe 7 Ni 40 with a yield strength approaching 1.15 GPa, 10% higher than the best value of the existing AlTiCoCrFeNi HEAs/MEAs strengthened by L1 2 -type nanoprecipitates. The contribution of precipitation strengthening from the L1 2 phase is approximately 717 MPa. Our strategy that unites the strengthening mechanism and machine learning may inspire the rapid design and discovery of precipitation-hardened alloys. [Display omitted] •A strategy combining machine learning with precipitation strengthening mechanism was proposed. •The volume fraction and the antiphase boundary energy of the precipitated phase were improved by Machine Learning. •One precipitation-hardened alloy with a yield strength about 1.15 GPa was synthesized, which outperform the existed alloys. •The contribution of different strengthening mechanism was discussed. [ABSTRACT FROM AUTHOR]

In this paper, two high-manganese steels with comparable grain sizes and manganese contents, namely 34Mn0.1C and 32Mn0.6C, were investigated to explore the effect of carbon content on the mechanical behavior and deformation mechanisms at both room temperature (RT) and liquid nitrogen temperature (LNT). The results indicate that increasing the carbon content promotes the transition of deformation mechanism from dislocation slip to deformation twinning, which is similar to the effect of reducing the deformation temperature. At RT, the strength and elongation are effectively improved by increasing the carbon content, which is attributed to the transformation of deformation mode. Specifically, the yield strength, the ultimate tensile strength and the total elongation increase from 262 MPa, 595 MPa and 50.4% for the 34Mn0.1C steel to 360 MPa, 846 MPa and 87.4% for the 32Mn0.6C steel, respectively. However, at LNT, the 32Mn0.6C steel exhibites higher strength but lower elongation than the 34Mn0.1C steel. The total elongation decreases from 70.6% to 52.3%, which is because the high carbon content and cryogenic temperature induce the rapid activation of deformation twins, leading to premature fracture. [ABSTRACT FROM AUTHOR]

The high strength of martensitic steel can be attributed to its fine hierarchical structure and high dislocation density. To further improve the mechanical properties of low-carbon martensitic steel, a nano-lamellar structure in size of 28 nm was first produced by warm rolling on the dual-phase structure. Subsequent annealing at 870 °C for different times was performed to tune the hierarchical structures of the martensitic steels with different prior austenite grain (PAG) sizes. The WRQ-15 steel sample, which underwent a 15-min annealing process, exhibits an outstanding combination of strength and elongation with a yield strength (YS) of 1.55 GPa, ultimate tensile strength (UTS) of 2.0 GPa, and elongation to failure (ETF) of 8.7%. It shows higher strength than the martensitic sample annealed from undeformed steel (UQ-15 steel), which has a YS of 1.31 GPa and UTS of 1.68 GPa. The WRQ-15 sample has a finer PAG size (7.2 μm) than the UQ-15 sample (9.5 μm). The results reveal that the refined PAGs lead to further refinement of packets, blocks, and laths, as well as increased dislocation density, contributing to the superior mechanical properties of the WRQ-15 sample. Moreover, the refined hierarchical structures provide high-density boundaries, impeding the propagation of cleavage cracks to improve the toughness. However, the excessive refinement of the hierarchical structures, increase of dislocation densities, and the undissolved carbides in the WRQ-1 sample are harmful for the strength-toughness balance. [ABSTRACT FROM AUTHOR]

In the current work, the relationship between the microstructural evolution and strengthening mechanisms in a vanadium micro-alloyed high strength low alloy (HSLA) martensitic steel has been systematically investigated by multi-scale characterizations and modeling. Two heat treatment routes, i.e., solution quenching plus tempering (SQT) and solution quenching plus isothermal annealing (QIA) are applied to produce a martensitic matrix combined with considerable nano-scale MC-type carbides (M = V, Mo, Cr) in the experimental steels. Theoretic calculations of the strengthening contributions are conducted based on quantitative characterizations of microstructural parameters, the overall modeling results agree quite well with the experimental results for estimating yield strength of the specimens. Precipitation and dislocation strengthening are found to be the main factors causing strength evolution with different isothermal holding time (IHT) at 600 °C. In both SQT and QIA specimens, the prolonged IHT contributes to an increased particle size and volume fraction of MC precipitates but a decreased dislocation density, and the most excellent mechanical properties are achieved with IHT of 2 h. The precipitation strengthening of SQT specimens is higher than that of QIA specimens because of its finer particle size and higher volume fraction of the MC precipitates, resulting in the higher yield strength of the SQT specimens than that of the QIA specimens. This can be explained by the hierarchy lath martensite that is highly dislocated with numerous interfaces, providing considerable nucleation points and high-speed atomic diffusion paths for precipitation, leading to the significantly refined MC particle size in the SQT specimens. Moreover, the MC precipitates exhibit no significantly coarsening during isothermal holding at 600 °C, which is a result of the presence of Mo/Cr atoms in MC precipitates that yields precipitate size stabilization due to its higher chemical potentials than that of V, thus maintaining a stable precipitation strengthening after IHT exceeding 2 h. [ABSTRACT FROM AUTHOR]

Aluminum-steel dissimilar joints have been widely applied attributing to their advantages to meet the requirements for weight reduction and energy savings. The aluminum alloy 5754 and galvanized CRRA1000 advanced high strength steel (AHSS) are overlap-welded by the laser welding-brazing technique. The tensile test results show that the laser welded joints fabricated with the spot location containing 2/3 steel and 1/3 aluminum are almost failed in the weld metal (WM). The average tensile strength and elongation are 344 MPa and 10%. Larger proportion of laser spot on steel and heat input triggers the formation of the coarsened intermetallic compounds (IMCs), which are rich in elements Fe, Al and Si, in the region near the notch. By decreasing the proportion of laser spot on steel and heat input, uniform IMC layer successfully forms in the interface between the WM and steel. All the welded joints fabricated with the spot location containing 1/2 steel and 1/2 aluminum rupture in the base metal of Al alloy (Al-BM). The average tensile strength and elongation are 345 MPa and 15%. During the tensile test, the cracks are initiated at the notch. The cracks start turning into the WM in the region with coarsened IMC, and the crack propagation direction is mainly along the coarsened IMCs eventually failure in the WM. For the cracks in the uniform IMC, the crack propagation in the uniform IMC is hindered, contributing to the failure transition from the WM to the base metal (BM) of aluminum alloy. • The welding-brazing of Al to steel with ideal tensile property is obtained. • The uniform IMC layer makes the interface be effectively bonded. • The coarsened IMC triggers the change of the crack propagation direction. [ABSTRACT FROM AUTHOR]

This paper presents a study on the thermo-mechanical behavior of an ultra-high strength CrNiMoV steel at high temperatures and medium strain rates through hot tensile tests. The material was examined in two conditions: as-cast/heat-treated (AC/HT) and as-rolled (AR). Tensile tests were conducted at temperatures of 800 , 900 , 1000 , 1100 , and 1200 ° C , and strain rates of 0. 1 , 1 , and 10 s − 1 . Inclusion and porosity analysis was also performed on the tensile specimens. The results revealed that the flow stress decreased by approximately 70% on average from 800 ° C to 1200 ° C , while increasing by approximately 32% on average from 0. 1 s − 1 to 10 s − 1 in strain rate. The elongation exhibited an increase from 16.5% at 800 ° C to 33% at 1200 ° C. However, the ductility transition was slower than expected, particularly for the AC/HT conditions. The AC/HT samples exhibited higher levels of inclusions and porosity compared to the AR samples, with porosity significantly affecting the elongation to failure and ultimate tensile strength (UTS) at a strain rate of 1 s − 1 . Furthermore, calibrated Johnson–Cook parameters were reported and compared, demonstrating excellent agreement between predicted and experimental values with less than 20% variation. The calibrated Johnson–Cook model can be effectively employed for modeling purposes within the studied temperature range, and its application can even be extrapolated for higher strain rates. [ABSTRACT FROM AUTHOR]

Abstract Multiphase bainitic steels, as a third generation of advanced high strength steels, show promising properties for automotive applications. Understanding the micro-mechanisms of damage initiation during plastic deformation is a key to further mechanical properties enhancement. The topological effect of martensite as a second phase constituent on local damage nucleation activity is significant. This effect has been studied in two different martensite banded microstructures produced by two various annealing cycles. The postmortem damage analysis by a scanning electron microscope on uniaxial loaded samples, revealed more damage nucleation along the dispersed and fragmented martensite phase within martensite banded regions. More pronounced strain partitioning was observed in coarse bainitic grains between adjacent martensite blocks. It is shown that the fracture strain is not controlled by local damage activities, implying that earlier damage initiation or an increased volume fraction of voids does not give rise to a reduced ductility. [ABSTRACT FROM AUTHOR]

Abstract The formability of newly developed steel grades is a key parameter for their successful implementation in the car body. In the present work, the influence of the microstructure on formability during air bending is studied. First- and third-generation advanced high strength steels with tensile strengths above 1000 MPa are investigated, which exhibit a broad range of microstructural constituents (ferritic bainite, bainite, martensite, and retained austenite). Interrupted bending experiments are performed on two grades of dual phase and complex phase steels, respectively, in order to reveal the microscopic deformation and failure processes. The formability in hole expansion tests is determined and used as a measure for the resistance of the materials against crack formation. The macroscopic deformation behavior of the materials is taken into account by measuring the differential strain hardening exponent and performing finite element simulations of the air bending process. Advantages and disadvantages of various microstructural features are worked out, and it is explained why the first-generation dual phase steel exhibits the lowest and the third-generation complex phase steel the best formability during air bending. [ABSTRACT FROM AUTHOR]

Ultra-high strength Al-Zn-Mg-Cu alloy extrusion bars were processed by equal channel angular pressing (ECAP) at room temperature (RT). Post aging treatment was used to determine the optimum time of aging. ECAPed sample was divided into two types: near the upper channel (P1) and lower channel (P2). Micro-hardness and tensile tests showed that ECAPed sample reached peak aging state at 8 h, which greatly shortened the time to T6 state. The mechanical properties of ECAPed samples were also better than traditional T6 samples. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to study the phase transformation from extrusion bars to ECAPed samples. AlZnMgCu phase and Al 2 CuMg phase containing Zn can be found in ECAPed sample. Meanwhile, the coarsening phase Al 7 Cu 2 Fe decreased significantly. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the microstructure evolution such as shear bands and grain size during ECAP process. In P1, the shear bands with width ranging from 90 to 300 µm can be found. These shear bands were divided into many thinner shear bands and became intragranular deformation bands gradually in P2. Compared with P2, the average grain size in P1 decreased greatly and the precipitate η′ phase became finer and denser. Grain boundary strengthening and precipitation strengthening in P1 were much better than P2. The diameter of the channel should not exceed 20 mm when it used to press ultra-high strength Al-Zn-Mg-Cu alloy and the 3/4 parts near the top channel of product should be used to avoid unrefined grains. [ABSTRACT FROM AUTHOR]

The tempering of re-austenized, quenched and tempered (RAQT) martensitic steels is an extensively studied and well understood field of metallurgy. However, a similar understanding of the effect of tempering on direct-quenched (DQ) high-strength steels has been lacking. Now, for the first time, the effect of tempering in the range of 250–650 °C on the strength, toughness, bendability, microstructure, crystallography and dislocation density of a DQ steel is reported. In the case of tempering at 570 °C, the effects of having a RAQ or DQ starting condition are compared. For the composition and thermal cycles studied, it was found that a peak tempering temperature in the range of 570–600 °C resulted in a DQT steel with an optimal balance of strength, bendability and toughness, i.e. a yield strength greater than 960 MPa, a minimum usable bending radius of 2 times the sheet thickness and T28J of − 50 to − 75 °C depending on the test direction. Crystallographic texture, dislocation density and the distribution of carbides are important factors affecting the bendability of DQT strip. Tempering had no effect on texture, but strongly influenced the size and distribution of carbides thereby resulting in differences in bendability and impact toughness transition temperature. [ABSTRACT FROM AUTHOR]

This work describes here the synergistic effect of niobium micro-alloying in combination with boron addition on the development of a 900 MPa grade ultra-high strength low carbon cost-effective bainitic steel processed by direct-quenching and induction tempering. A mixed microstructure consisting of acicular ferrite and lath bainite associated with high density of high angle grain boundaries was obtained by controlled rolling and direct-quenching. Nano-sized precipitation behavior during controlled rolling and induction tempering was studied by transmission electron microscopy (TEM). A number of nano-sized precipitates were observed in the matrix after controlled rolling and direct-quenching. These precipitates were identified by electron energy loss spectrometry (EELS) in scanning TEM (STEM) to be TiN-Nb(C,N) or TiN-NbC composite precipitates, and were associated with mean austenite grain size of ~ 34 ± 6 µm in steel before finishing rolling. A finish rolling reduction of 67% below non-austenite recrystallization temperature pancaked the austenite grains to 10–15 µm in thickness. Nano-sized NbC formed during induction tempering at 670 °C had an average diameter of ~ 4.3 nm and 9.5 nm for tempering duration of 5 min and 30 min, respectively. It is noted that while nano-sized NbC precipitates smaller than 5 nm provide significant precipitation hardening effect to increase the mechanical strength, acicular ferrite in the mixed microstructure helps in retaining high elongation of low carbon direct-quenched and tempered steel. Ultra-high yield strength of 944 MPa with high ductility (uniform elongation of 6.3% and total elongation of 20%) was obtained after induction tempering at 670 °C for 5 min. [ABSTRACT FROM AUTHOR]

Laser hatch style and part placement strategy are acknowledged to have a considerable effect on processing quality and mechanical performance of selective laser melting (SLM) products. In an effort to obtain 316L stainless steel products with favorable properties, hatch style integration and part placement optimization were adopted in this study. The tensile property of SLM parts at different placement angle ψ was identified and the corresponding deformation behavior and failure mechanism were investigated. Interestingly, the part prepared by double stagger melt (DSM) style presented a strong <001> texture along the transverse direction (TD) rather than the growth orientation of the columnar grains (building direction, BD). This crystallographic variation imposed an immense effect on yield strength of SLM-processed parts by modifying the orientation relationship between texture and growth direction of grains. As ψ was set to 60°, the DSM-fabricated parts exhibited a lower Schmid factor, hence achieving a higher average yield strength σ s of 536.6 MPa. The static fracture toughness U T was closely associated with the normal component force F n imposed on preferential loading plane. By means of hatch style integration and part placement optimization, the dangerous “opening or tensile” loading mode was effectively avoided and reasonable compromise between the strength and ductility was achieved elaborately, thus obtaining an ultrahigh U T (3.478 × 10 8 J/m 3 ) of SLM-processed parts. This study scientifically showed the great potentials of the SLM process to prepare 316L stainless steel products demanding challenging high-strength and high-toughness performance. [ABSTRACT FROM AUTHOR]

A computationally efficient rate independent crystal plasticity finite element (CPFE) model was used to predict the forming limit curve (FLC) for a multiphase advanced high strength steel (AHSS). The CPFE model accounts for mechanical properties of the steel phases based on their individual plastic deformation and slip systems. The macroscopic behavior of the polycrystalline aggregate was predicted based on the volume-averaged response of the representative phases, and their volume fraction in the steel sheet. In addition to the random texture distribution assumption for each grain, the volume fractions of various phases were also assumed to be randomly distributed at each integration point. The microstructural inhomogeneity of the material, as well as a geometrical inhomogeneity in the form of a groove region in the specimen, based on the Marciniak-Kuczynski (MK) theory, were considered in the calculation of the FLC using the CPFE model (MK-CPFE). The validity of predicted FLC for quenched and partitioned QP980 steel was confirmed by comparing the results with experimental measurements. The FLC calculated by CPFE model showed that the presence of microstructural inhomogeneity allows for a more realistic prediction of the localized necking phenomenon. Subsequently, the FLC was used to compute the limit strains for the T-shape part stamping process. The limit strains and the punch force-stroke relationship for the T-shape part predicted by the multiphase CPFE model were in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

High-strength S135 drill pipe is an important component in the oil and gas exploration and development of deep and ultradeep wells under complicated environments. It is of great significance to the study on the corrosion fatigue fracture of high-strength steel S135 in complex environments. In this paper, two samples of smooth and unilateral notch were used to conduct fatigue in air, pre-corrosion fatigue and corrosion fatigue at different temperatures, respectively, to study the corrosion fatigue behavior of high-strength steel S135 under the complex environments of the notch, corrosion medium, temperature and alternating stress. The experimental results indicated that the influences of coupling multiple factors (notch, temperature, corrosion medium and alternating stress) on the corrosion fatigue of S135 steel were greater than that of single factor, and the more factors indicated the greater influence on fatigue. The influence of three kinds of coupling factors, such as the notch, corrosion medium and temperature, was the most significant (80.8%). In single factor, the impact of notch on fatigue was the largest (49.9%), followed by corrosion (41.6%) and temperature (34.4%). Obviously, the notch was the dominant factor of fatigue fracture, and the temperature had a significant influence on corrosion fatigue. It was found that the influence of coupling-factor on fatigue was less than of the superposition of single factor, so the relationship among multiple factors was not superposed but competitive and synergistic relationship. This can be attributed to the fact that the crack was always initiated from the corrosion pits at the notch root under coupling multi-factor in complex environments and the initiation was the combined action of stress concentration, anodic dissolution and temperature. The stress concentration at the notch was the main factor of crack initiation, and the electrochemical activity of at the notch root was the highest under the action of cyclic stress and this place gave priority to corrosive pitting, made the increase of local stress concentration and promoted the initiation of crack. And the temperature provided the energy to the corrosion electrochemical reaction and accelerated the evolution of the corrosion pits, so crack initiation was accelerated. Besides, article established the testing method of rotating bending fatigue in complex environments, revealed the relationship of fatigue fracture as well as the mechanisms of crack initiation and propagation of high-strength S135 steel under coupling multiple factors in complex environment. [ABSTRACT FROM AUTHOR]

Microstructural characterization of deformation-induced martensite in its early stage of formation has been performed in an ultrafine-grained (UFG) medium Mn advanced high strength steel (AHSS). By using field emission scanning electron microscope (FESEM) and electron backscattered diffraction (EBSD), the nucleation site of deformation-induced martensite has been studied. Grain sizes, Schmid factors and aspect ratios of the parent austenite grains of deformation-induced martensite were measured in order to know their influence on the formation of deformation-induced martensite. Orientation relationship between deformation-induced martensite and its parent austenite has also been analyzed. Discussion on the effect of microstructural parameters on the formation of deformation-induced martensite in AHSS is given. [ABSTRACT FROM AUTHOR]

Advanced high strength steels for the auto industry are in high demand regarding the development of new type of steels, reliable and economic welding techniques, etc. In this study, the third generation advanced high strength steels, cold-rolled quenching and partitioning steels (QP980), were butt-welded using fiber laser to evaluate the microstructure, mechanical properties and formability of joint. The fusion zone composed of lath martensite with high microhardness (499 HV). Martensite tempering and carbide precipitation were found at the sub-critical heat affected zone, resulting in a slight decrease of microhardness (~ 37 HV drop). The presence of joint had no effect on tensile strength and all welded samples failed at the base metal with 98.8% joint efficiency. The formability of welded blanks in Erichsen cupping test was dependent on the weld line position. The formability ratio was only 68.08% of base metal when weld line offset was 0 mm, and increased as the weld line moving away from the blank center because more deformation was transferred to base metal. Stretch-flange-formability of the joint in hole expanding test was inferior to that of base metal due to the high volume fraction of coarse martensite in fusion zone. [ABSTRACT FROM AUTHOR]

The quenching and partitioning (QP) steels have received much attention due to its high strength and good ductility, which results from deformation induced martensitic transformation (DIMT). However, due to the stress state, temperature and strain rate dependent DIMT behavior, the DIMT related flow behavior of QP steels might be more complex than other advanced high strength steels (AHSS) without martensitic transformation. During forming process of such materials (non-linear loading process), effects of DIMT behavior and DIMT related flow behavior on springback prediction are still unknown. In this paper, we focused on the martensitic transformation behavior of QP steels under complex loading (linear loading and cyclic loading). The X-ray diffraction and digital image correlation techniques were performed to monitor the retained austenite evolution and strain history of the specimen, respectively. Then, flow behaviors of QP sheets steels subjected to complex loading are analyzed and compared with DP980 sheets steels (without DIMT). Finally, modified Yoshida and Uemori model calibrated by two different cyclic loading tests, i.e., tension-compression test and cyclic shear test, are adopted in this study to investigate the stress state dependent DIMT effect on springback prediction. The results show, for QP steels, the DIMT is stress state dependent. Accordingly, compared with DP steels, its flow behavior exhibits remarkable stress state sensitivity. Due to these special DIMT behavior and flow behavior of QP steels, its springback prediction should be treated differently compared with DP steels. [ABSTRACT FROM AUTHOR]