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The demand for higher speeds and heavier loads has emphasized the necessity to tackle the inherent trade-off between strength and toughness in pearlitic rail steels. Herein, the co-additions of Cr, Ni and Cu and accelerated cooling rate on the microstructure and mechanical properties of a medium carbon pearlitic steel were systematically investigated. In comparison with base alloy, the new steel exhibits smaller prior austenite grain size (PAGS) due to the drag effect of Cu and Ni solutes. The finer PAGS and lower C content increased the volume fraction of proeutectoid ferrite. The finer pearlitic nodule size (PNS) and pearlitic colony size (PCS) were attributed to the small PAGS and greater extent of undercooling due to the combined effect of multi-alloying and accelerated cooling. Additionally, the synergistic effects of multi-alloying and accelerated cooling rate decrease the pearlitic phase transformation temperature, accountable for the finer interlamellar spacing (IS). Compared with the base steel, the yield strength and ultimate tensile strength of the new steel increased from 587 MPa to 1069 MPa to 740 MPa and 1178 MPa, respectively, with a simultaneous increase of ductility from 12.4 % to 14.7 %. The strength increments are mainly attributed to the finer IS in the new steel. Meanwhile, the new steel possesses a higher impact toughness compared to the base steel due to the increased volume fraction of proeutectoid ferrite, and finer PNS and IS. [ABSTRACT FROM AUTHOR]

Dynamic ferrite transformation is a vital microstructure evolution mechanism for carbon alloy steels and an essential technology for manufacturing high-performance steel materials. In practice, carbon steel hot working processes are usually carried out at various temperatures, involving the evolutionary behavior of the single-phase austenite and the ferrite dynamic transformation process. Few models can simultaneously predict the microstructural state and deformation resistance in the single-phase austenitic interval and the dynamic ferrite transformation interval. The deformation mechanisms involve work-hardening, dynamic recovery, and dynamic recrystallization of austenite and ferrite phases, as well as the dynamic transformation of austenite to ferrite and coordinated deformation between the two phases. Therefore, this paper extends the application range of the model based on the viscoplastic unified constitutive model by considering the above deformation mechanisms. The constitutive model proposed in this paper applies to both the single-phase austenite interval and the dynamic ferrite transformation interval. Moreover, it can accurately predict carbon alloy steel's microstructure state and deformation resistance in different intervals. This paper can help to construct a hot deformation constitutive model in a wide range of deformation temperature intervals of steel materials. [ABSTRACT FROM AUTHOR]

This study examines the relationship between microstructure and low-temperature mechanical properties of newly designed medium manganese steel with ultra-low carbon, low-Ni content. The microstructural features crucial for impact toughness were identified. The study indicated that post-annealing at 650 °C for 30 min endowed the steel with superior low-temperature mechanical properties, namely tensile strength of 914 MPa, total elongation of 37.2 %, and impact energy of 187 J at −60 °C. With longer intercritical annealing durations, reverted austenite transitioned from a film-like to a blocky morphology, with an increase in both volume fraction and size, accompanied by a substantial decrease in stability. High-stability reverted austenite contributed to sustained work hardening by facilitating a continuous Transformation-Induced Plasticity (TRIP) effect during steel deformation, thereby significantly enhancing the low temperature tensile properties of the steel. Furthermore, enhanced stability of reverted austenite was identified as the pivotal microstructural factor influencing impact toughness. [ABSTRACT FROM AUTHOR]

Advanced (ultra)high-strength steels that utilise bcc-fcc microstructures are appealing solutions for producing a combination of high strength and deformability. However, they are also susceptible to hydrogen embrittlement (HE). As larger less stable retained austenite (RA) can impair mechanical performance, its size and morphology are critical factors for achieving and maintaining the desired properties. Here, we present a combined experimental–density functional theory (DFT) study on HE with medium-carbon direct-quenched and partitioned (DQ&P) martensitic steels with varying vol% and film-thickness of RA, showing significantly improved HE resistance as a function of bcc-enrichment and increasing RA film-thickness. DFT reveals low attraction of hydrogen in bcc-Fe with Al, implying a stronger push towards fcc. Furthermore, the solubility of hydrogen increases dramatically with the carbon-enrichment of fcc-Fe when hydrogen and carbon are second neighbours. The theoretical results explain the observed differences in hydrogen diffusion, trap density, and the consecutively lower HE in Al-DQ&P over Si-DQ&P. Our combined experimental and theoretical study thus highlights the important interplay of bcc and fcc phases and H-uptake within austenite-containing carbon steels. • Improving HE resistance as a function of RA film-thickness and bcc-alloying with Al. • Insights into the interplay of bcc/fcc and H-uptake for austenite-containing carbon-steels. • Outstanding performance of Al-DQ&P with thickest RA-films under in situ hydrogen-charged SSRT tests. • DFT reveals H-solubility in bcc-Fe lower with Al-alloying over Si-alloying. • H-solubility in fcc increases significantly when H and C are second neighbours. [ABSTRACT FROM AUTHOR]

In the face of increasingly demanding service conditions for high-end bearings, the need for bearing steels that demonstrate superior mechanical properties, specifically a robust balance of strength and toughness, is growing. The adoption of near-net-shaped cold rolling methods has emerged as a leading international technology to enhance bearing performance. In this study, the effect mechanisms of pre-cold deformation on the phase transformation kinetics, microstructure evolution and mechanical properties of nano-bainite bearing steels are systematically studied. Results reveal that pre-cold deformation treatment effectively reduces the A c1 temperature of the experimental steel, expedites nano-bainite transformation and promotes strong variant selection during bainite phase transformation. With the increase in pre-cold deformation from 0 % to 50 %, the size of the prior austenite grains decreases from 6.7 μm to 3.3 μm, and the size of carbides in specimens becomes more uniform, whilst the refinement of austenite grains can considerably reduce the thickness of bainitic ferrite plates. The decrease in grain size and the increase in V1–V2 variant pairs lead to an improvement in the toughness of the nano-bainite bearing steel from 64.9 J/cm2 to 114.3 J/cm2. A marginal increase in the hardness of the bearing steel is observed with the increase in pre-cold deformation. Furthermore, this study discusses the evolution mechanism of microstructure in the subsequent phase transformation by different microstructures in the early stage. Results show that pre-cold deformation leads to the transition of the martensite twin substructure in the specimens from {112} <111> twins to twinned variants. These insights offer critical understanding into the microstructural underpinning the enhanced performance of nano-bainite bearing steels subjected to pre-cold deformation treatment. • Finding pre-cold deformation leads a decrease in the degree of carbide dissolution during the austenitising process of high carbon bearing steel. Building models to prove that there are differences in the mechanism of carbide dissolution and precipitation in bearing steel with different grain sizes during austenitising. • Finding that grain refinement has both accelerating and delaying effects on bainite transformation kinetics. • Revealing the influence of the difference of crystallographic characteristics on the impact toughness of bearing steel. • Revealing a novel twinning mechanism caused by grain refinement in high carbon bearing steel exists. The increase of strain energy and mechanical stability of retained austenite lead to the transition of the martensite twin substructure in the specimens from {112} <111> twins to twinned variants. [ABSTRACT FROM AUTHOR]

This study explored the potential significance of controlled forging with finish forging temperature as low as ∼650 °C in improving the mechanical properties of a novel V + Nb-microalloyed medium-carbon steel 42CrNiMoVNb. The results show that compared with conventional quenching and tempering, the martensite microstructure of the controlled forged and direct quenched sample with high dislocation density was refined with accelerated precipitation of carbides when tempered either at 200 °C or 600 °C. After tempering at a low temperature of 200 °C, the controlled forged sample showed excellent tensile properties compared with conventional quenched sample, this is, a tensile strength of 2010 MPa, a yield strength of 1687 MPa and a considerably high ductility of total elongation of 10.5%. When the tempering temperature was increased to 600 °C, the strength of both the controlled forged and conventional quenched samples decreased significantly while the former still exhibited considerably higher tensile strength (1443 MPa) than the latter (1343 MPa). Therefore, controlled forging is more effective in improving the tensile properties of the tested steel tempered at a low temperature of 200 °C than at a high temperature of 600 °C, which is mainly due to significant dislocation strengthening for the 200 °C tempered sample. Enhanced precipitation strengthening and grain refining strengthening mainly account for the still considerably high strength for the 600 °C tempered sample. [ABSTRACT FROM AUTHOR]

The dynamic recrystallization (DRX) behavior and microstructure evolution of Nb-bearing and Nb-free high-carbon steels were analyzed in the present study through thermal simulation, theoretical calculation, and morphological analysis. The results indicate that 0.027 wt% Nb refined the austenite grains by approximately 57.1%. Based on the calculation of true stress-strain curves at deformation temperatures from 900 to 1150 °C and strain rates of 1 and 0.1/s, Nb addition resulted in the increment of critical strain, peak strain and peak stress of DRX at lower deformation temperatures. It shows that DRX process was hindered at lower deformation temperatures due to Nb addition, but the inhibition effect was progressively weakened with the increase of the deformation temperature. The starting time of DRX was delayed and the activation energy of DRX was increased by 7.2%. In addition, Nb hindered the transformation of the hot-deformed austenite to pearlite due to its solute drag effect. The higher the deformation temperature, the stronger the hindering effect. Moreover, the addition of Nb refined the pearlite nodules and this effect was enhanced by increasing the strain rate at the deformation temperature of 900 °C. It means that the effect of Nb on the pearlite nodules in the high-carbon rail steel was closely related to whether DRX occurred. Meanwhile, Nb addition led to the refinement of pearlite interlamellar spacing at the deformation temperature of 1000 °C, and the refinement effect was almost similar whether DRX occurred or not. [ABSTRACT FROM AUTHOR]

The impact of hot rolling temperature on the microstructure evolution and mechanical properties of hot/cold-rolled AA5083 alloy containing Sc and Zr was studied. The results revealed that low hot-rolling temperatures (LHRT: 400–425 oC) resulted in superior tensile properties compared to high hot-rolling temperatures (HHRT: 500–525 oC). The yield strength (YS) of the LHRT samples reached 450 MPa in the H18-temper and 291 MPa in the O-temper, which was respectively 20% and 39% higher relative to the HHRT samples. Microalloying with Sc/Zr and two-step homogenization promoted the formation of dispersed particles in the form of Mn-dispersoids and nanosized Al 3 (Sc,Zr) precipitates. The rolling temperature was found to have a profound impact on the characteristics of these dispersed particles and on the recrystallization resistance. The HHRT caused the coarsening of both dispersed particles during rolling. The LHRT preserved the deformed elongated grain structure due to a stronger retardation of recovery and recrystallization during hot rolling and post-annealing. The strengthening mechanisms of the Mn-dispersoids and Al 3 (Sc,Zr) precipitates were quantitatively analyzed based on particle characteristics. The predicted YS contributions in the O-temper were in good agreement with the measured YS difference between the LHRT and HHRT samples. [ABSTRACT FROM AUTHOR]

To achieve the objective of enhancing the yield strength (YS) of quenching and partitioning (Q&P) steel without compromising tensile strength and ductility, a stepped partitioning (SP) strategy was proposed for Q&P treatment on medium carbon silicon-rich steel. The optimized partitioning strategy involves a two-stage partitioning, the first stage is conducted at temperatures above the martensite start (M s) temperature, followed by the second stage carried out at a lower temperature for an extended period, involving isothermal partitioning. Experimental results demonstrate that the SP strategy yields a significant quantity of small carbides, with an average size of 59.02 nm, along with 15.71 vol% of retained austenite (RA) and a carbon concentration of 1.01 wt% in RA. Furthermore, the SP-treated specimen exhibits a remarkable balance of strength and ductility, showcasing YS, ultimate tensile strength (UTS), and total elongation (TE) values reaching 1732 MPa, 2263 MPa, and 14.9%, respectively. The microstructure evolution during tensile deformation demonstrates that RA in short-duration conventional partitioning specimens underwent premature martensitic transformation during tensile deformation due to early deformation, whereas RA in long time conventional partitioning specimens was overstabilized due to highest C concentration, which resulted in poor enhancement of strengthening and ductility by TRIP effect. In contrast, the SP strategy achieved a reasonable stability of RA, resulting in a simultaneous increase of tensile strength and ductility. Simultaneously, an analysis of precipitation strengthening indicates that the abundance of fine carbides in martensite contributes to an increase in YS due to precipitation strengthening. • A novel stepped partitioning strategy had been introduced to the Q&P process of a medium carbon silicon-rich steel. • An exceptional strength-ductility combination has been achieved through the novel partitioning strategy. • Investigated the mechanisms of stepped partitioning strategy to improve the strength and ductility simultaneously. [ABSTRACT FROM AUTHOR]

Three silicon-manganese steels with carbon content of 0.25, 0.33, and 0.44 wt% were subjected to quenching and partitioning (Q&P) treatment to obtain the microstructures consisting of primary martensite (PM) and significant fraction of retained austenite (RA). The microstructural changes during Q&P were systematically characterized by various techniques while tensile and impact tests were performed to investigate the mechanical response of the steel samples. The carbon partitioning between martensite and austenite at 350 °C was accompanied by the formation of carbide-free bainite and resulted in the stabilization of the irregular layers and thin films of RA. Moreover, the precipitation of carbides in PM before partitioning and their subsequent growth affected the carbon partitioning from PM to RA. The volume fraction and width of RA islands in the final microstructure increased with increasing the carbon content. The transformation of significant fraction of RA in 0.44C steel before the strain localization improved uniform elongation, resulting in superior combination of strength and ductility (σ 0.2 = 1225 MPa, UTS = 1580 MPa, δ = 16.8 %). On the other hand, the transformation of more stable RA in lower carbon steels occurred mainly in the neck portions of tensile specimens and its contribution to uniform elongation was negligibly small. The prevalence of the dimple pattern on the fracture surfaces of the specimens after tensile and impact tests indicated ductile fracture mode. The effect of carbon content on the precipitation and growth of carbides in PM and its influence on the impact toughness are discussed. • Effect of carbon on the phase characteristics in 0.25–0.44C steels after Q&P. • Completed bainitic transformation during partitioning at 350 °C for 600 s. • Transition η-carbides precipitate during salt bath quenching at 230 °C. • Austenite morphology and carbon content affect the carbon partitioning during Q&P. • Ductile fracture in the steels with stabilized retained austenite. [ABSTRACT FROM AUTHOR]

This work precisely investigated the compressive creep behaviors of Mg-4.5 wt%Y alloys with grain sizes ranging from d = 10 μm to d = 160 μm. The compressive creep tests were conducted on the hot-rolled sheet at 493 K under 100 MPa with the loading direction along the rolling direction (RD) and normal direction (ND). It was discovered that the compressive creep resistance of the RD sample was decreased with increasing grain size because the grain boundary strengthening and the pyramidal < c + a > dislocations hardening improved the compressive creep resistance of the samples with fine grains (d = 10 μm and d = 15 μm), while { 10 1 ‾ 2 } twinning and high-density dislocations decreased the compressive creep resistance of the samples with coarse grains (d = 25 μm to d = 160 μm). Differently, cross-slip was inhibited and dislocation climb was promoted with increasing grain size, resulting in the grain size independence of compressive creep resistance in the ND samples. As a result, the compressive creep anisotropy was continuously increased with increasing grain size. It is hoped that this work can provide important reference to the physic modeling of compressive creep behaviors in Mg-RE alloys. [ABSTRACT FROM AUTHOR]

Solid-state bonding is paramount in joining dissimilar materials, often combined with metal forming, such as forging, rolling and extrusion, to fabricate composites with a designed structure. Owing to its superior mechanical properties and low density, the carbon steel/Ti composite structure is attractive in automotive and aerospace industries but is difficult to fabricate due to the decarburization of carbon steel's faying surface. In this work, a carbon steel/Ti6Al4V composite structure was first fabricated via a forge-bonding method with the assistance of a Ni coating layer. Results show that the Ni layer on the faying surfaces precludes the decarburization of the steel and the growth of the TiC phase so as to improve the bonding quality. Meanwhile, the in-situ eutectic reaction between the Ni layer and β -Ti was observed. As a result, a seamless bonded interface with the total elimination of the Ni layer can be obtained. The bonding strength was examined to establish the relationship between bonding windows, microstructure and the resulting mechanical properties. An experimentally validated thermal-mechanical finite element model was also developed to understand the process-dependent interface evolution. This work opens a new avenue for fabricating carbon steel/Ti6Al4V composite, extending the flexibility of achieving complex structures by combining solid-state bonding with other metal-forming technologies. [ABSTRACT FROM AUTHOR]

Low-alloyed steels with body-centered cubic crystal structure are a material class that is widely used for sheet metal forming applications. When having an adequate crystallographic texture and microstructure, their mechanical behavior is characterized by an isotropic in-plane flow behavior in combination with a low yield strength. The decisive processing steps for obtaining these beneficial mechanical properties are cold rolling and subsequent annealing. While for the former the number of passes, the deformation rates, and the total thickness reduction are the main processing parameters, the latter is described mainly by the heating rate and the holding temperature and time. Primary static recrystallization during annealing subsequent to the cold rolling process alters mainly two aspects of the material state: It firstly replaces the elongated and heavily deformed grains of the cold rolled microstructure by small, globular grains with low dislocation density and secondly it changes the crystallographic texture insofar as it typically diminishes the α - and strengthens the γ -fiber texture components. In the present work, the recrystallization behavior of a commercial non-alloyed low carbon steel is studied. A quasi in situ setup that enables site-specific characterization is employed to gain a local picture of the nucleation and recrystallization process. From the Kernel Average Misorientation (KAM) values of the deformation structure, the tendency to be consumed by new grains can be predicted. Crystallographic analysis shows that the most deformed regions have either a γ -fiber orientation or belong to heavily fragmented regions. New grains nucleate especially in such highly deformed regions and inherit often the orientation from the deformation microstructure. [ABSTRACT FROM AUTHOR]

The effects of loading on spall strength of a mild steel are investigated systematically under the flat-top impact loading, including the effect of tensile stress history and the effect of shock-induced microstructure. Peak stress, strain rate, pulse duration, spall strength and tensile stress history are obtained directly or indirectly from free-surface velocity measurements, and the recovered samples are characterized with electron backscatter diffraction analysis. For different tensile stress histories, spall strength depends on peak stress or strain rate, depending whether spall occurs on the plateau or rising edge of a tensile pulse. Pulse duration has an effect on whether spall occurs (incipient spallation) but not spall strength. Low shock stresses induce small microstructure change (dislocations) which has weak effects on nucleation and propagation of brittle cleavage cracks, while high shock stresses lead to deformation twins (in addition to dislocations) which act as the source of crack nucleation. A model is proposed to predict the spall strength of the mild steel under arbitrary loading conditions (peak stress below the phase transition). [ABSTRACT FROM AUTHOR]

Quenching and partitioning (Q&P) steels have been attracted much attention due to excellent combination of strength and ductility. However, the actual Q&P microstructures may contain the carbon-depleted martensite (i.e., initial martensite, M 1), bainitic ferrite (BF), secondary martensite (M 2) and retained austenite (RA). Hence, it is challenging to build a quantitative model of the yield strength of Q&P microstructures, since it requires prior knowledge of the yield strength of each constituent phase that is not always in a clear state in Q&P microstructures. In the present work, based on the analysis with respect of microstructural heterogeneities inevitably existing during martensitic transformation, both as-quenched and Q&P microstructures are simplified and regarded as a mixture of α′ phase (containing M 1 , M 2 or BF) and γ phase (RA). The carbon content, dislocation density and substructure size of the α′ phase and γ phase were measured by SEM, EBSD and XRD. Then we build two generalized physical models to predict the yield strength of as-quenched and Q&P steels with the carbon content in range of 0.06–0.42 wt%. The physical models show the yield strength of α′ phase varies as the reciprocal of the square root of the block width (Hall-Petch relation), but varies as the inverse of the lath thickness (Langford-Cohen relation). The deviation analysis indicates that the assumption and simplification with regard of microstructure do not greatly affect the accuracy of the prediction of yield strength for both as-quenched and Q&P steels, with uncertainty less than 10%. The effect of carbon content and microstructural features on the yield strength of as-quenched and Q&P steels was discussed in terms of the developed physical models. It reveals that the residual carbon atom in α' phase is still the dominant factor controlling the yield strength of Q&P steels, despite of possible highly uneven carbon distribution. [ABSTRACT FROM AUTHOR]

Abstract Bainitic steel is applied in many cyclic loading projects due to its excellent comprehensive mechanical properties. In this study, the cyclic hardening/softening behavior and microstructure evolution of high-carbon nano-bainitic steel were investigated to analyze the cyclic deformation mechanism. Results showed that the optimal tempering temperature was determined to be at low temperature. During cyclic deformation, the retained austenite is transformed into martensite according to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) orientation relationships, and deformation twins are generated in the retained austenite. The cyclic deformation of high-carbon steels at a total strain amplitude of 0.6% manifests no obvious cyclic softening behavior after low-temperature tempering, mainly due to the high density of dislocation tangle around martensite and deformation twins induced by cyclic deformation. [ABSTRACT FROM AUTHOR]

Abstract The transient liquid phase bonding (TLPB) process is one of the selected joining technologies to replace threaded connections in solid expandable tubulars. In particular, for expandable hot-rolled seamless carbon steel tubular products using Ni-based amorphous metallic foils as filler material. In this work, a comprehensive mechanical properties and microstructural characterization was carried out in TLP-bonded bars for this base metal/filler material combination. Both the joint and the heat affected zone exhibited a strength which compares well with the base metal, and a ductility in accordance with that which is typically specified for steel arc-welded joints. A coalesced lath-like bainitic microstructure was found at the joint. In addition, and by means of orientation imaging, a parent austenite grain - which is shared by the joint and an adjacent ferrite grain from the base metal - was found, which demonstrates the epitaxial nature of the TLPB solidification process. Also, a cell-block-like structure at ferrite grains next to the joint was detected, due to the plastic deformation developed in tensile-tested samples at room temperature. [ABSTRACT FROM AUTHOR]

Abstract Dual-phase high carbon steels are widely used in mining industries for grinding applications and are thus subjected to repetitive loading for a very large number of cycles. This paper presents the high-cycle fatigue behaviour of austenitic-martensitic high carbon steel and establishes a concurrent structure-property relationship. Cantilever-type rotating bending fatigue test was carried out on hourglass specimens in the high cycle and very high cycle fatigue (VHCF) regime under ambient condition. The S-N curve exhibited a bilinear slope indicating a change in the mode of crack initiation and micromechanism of cyclic deformation. Based on the fracture surface examination, the initiation of the fatigue cracks was categorised into two groups: (a) surface-induced (SI) cracks in short life region and (b) non-inclusion (NI) induced subsurface cracks with a fish-eye formation for the long life region. The stress intensity factor at the periphery of the GBF (Δ K GBF) was observed in the range of 5.2–5.75 MPa m1/2 which is close to the crack growth effective threshold value. The Kernel average misorientation (KAM) maps revealed that intense localised deformation occurred at the interface of the austenite-martensite phases and then gradually decreased at the interior of the austenite grains. This observation was supported by the transmission electron microscopy (TEM) studies which showed the pile-up of dislocation at the phase boundaries. The three-dimensional surface roughness parameters were found to be increasing with an increase in the number of fatigue cycles thus indicating the increase in the dislocation activity of the material in the long life regime. [ABSTRACT FROM AUTHOR]

Abstract Shortening the bainite transformation time is one crucial issue on the application of high carbon nano-bainite steel. In this paper, an accurate black-box model that could predict the plate thickness of bainitic ferrite was constructed based on gradient boosting decision tree model, and a polynomial model was constructed to make predicting process easier. According to the constructed model, a two-steps process was proposed, which shortened the transformation time from over 60 h to nearly 25 h. After the new process treatment, the size distribution of bainitic ferrite in microstructure became more uniform, and the strength and toughness of steel were also improved simultaneously. [ABSTRACT FROM AUTHOR]

Abstract Microalloying behavior in medium carbon non-quenched and tempered steel was investigated in this paper. The mechanical properties and microstructures in steels with different V, Ti, Nb and V contents were carefully compared and analyzed. The results showed that V and N largely reduced the average size of network ferrite and increased the volume fraction of intragranular ferrite (IGF) and ferrite. Ti, and Nb had the same effect as V and N while the effect was weaker. Ti and Nb could further increase the IGF content by increasing the size of the IGF nuclei (V, Ti)(C, N) and (V, Ti, Nb)(C, N). The precipitation strengthening of V-N steel was the highest. The precipitation strengthening of other steels was weaker due to the larger size of the particles. The highest elongation of the microalloying elements free steel can be attributed to the lack of precipitation strengthening in the steel. The relatively lower elongation of V-N steel can be ascribed to the larger grain size of steel. The V(C, N) and (V, Ti)(C, N) particles decreased the impact energy of V-N steel and V-Ti-N steel. The highest impact energy of V-Ti-Nb-N steel resulted from the refinement of the grain size. The comprehensive mechanical properties can be obtained in V-Ti-Nb-N steel as: the yield strength was 509.7 MPa, the elongation was 19.2% and the impact energy AkU was 55.4 J. [ABSTRACT FROM AUTHOR]

Abstract The superior mechanical performance of ultra-high strength low carbon steel was attained through the co-precipitation strengthening and lath-martensitic microstructure refinement. The research results showed that the precipitates enriched with Cu, Ni, Mn and Al were observed at 525 °C aging by atom probe tomography (APT), the face-centered cubic (FCC) Ni 3 Al phase precipitated from matrix firstly, and then the body-centered cubic (BCC) Cu-rich precipitates nucleated on the surface of Ni 3 Al particles to form composite precipitation phase of nanoscale Ni 3 Al and Cu particles eventually. The co-precipitation strengthening increment of nanoscale Cu and Ni 3 Al particles was calculated as 413 MPa. With the addition of multiple micro-alloyed elements, such as Nb, Ti and Mo, a relatively high amount of (Nb, Ti, Mo)C nanoscale co-precipitates below 10 nm were obtained in the matrix aged at 525 °C. The substitution of Mo for Nb, Ti would decelerate the coarsening kinetics of (Nb,Ti)C and cause high amount of finer precipitates. While those fine (Nb, Ti) C particles could evidently impede the grain growth, the finer martensitic blocks were subsequently obtained at 820 °C quenching, and the grain size and effective grain size were 5.6 µm and 3.08 µm in this condition, respectively. In this research, the Ni-Al-Cu steel with best matching of strength and toughness has been achieved when it was aged at 600 °C after 820 °C quenching, the yield strength and −80 °C impact toughness could reach 1138 MPa and 74 J, respectively. [ABSTRACT FROM AUTHOR]

Abstract The aim of the present study is to evaluate the impact of heating rate on the microstructure and tensile properties of cold-rolled low and medium carbon steels. For this purpose, cold-rolled low and medium carbon steels were subjected to short peak-annealing experiments at 900 and 1100 °C under three heating rates (10, 450 and 1500 °C/s). The microstructure reveals a mixture of phases and microconstituents (ferrite, bainite, and as-quenched martensite) which are related to the carbon heterogeneities in austenite. The microstructural characterization suggests that the grain refinement achieved after ultrafast heating has a minor effect on the yield and ultimate tensile strength, compared to the relative microstructural distribution. It is suggested that the interplay of various strengthening mechanisms in samples subjected to ultrafast heating rates are responsible for the observed increase in strength and ductility. [ABSTRACT FROM AUTHOR]

Abstract High carbon low alloy steel with dual phase structures of austenite and martensite is widely used in mining industries for grinding. Such applications of this material undergo deformations due to varying strain rates as well as temperatures. This paper is aimed to understand the rate dependent flow behaviour of high carbon steel at different regimes of operating temperatures. Compression tests were performed over a wide range of strain rates (2.56 × 10−4 to 2.56 × 10−1/s) at different temperatures (25–175 °C) and the subsequent physical changes in the fracture surfaces were critically examined. Apart from the variation in yield strength and ductility, the effect of strengthening mechanisms on the strain hardening behaviour of the material was also investigated. The strain hardening rates were observed to be decreasing with an increase in the effective stress values for all the strain rates as well as temperature. Austenite to martensite transformation was observed at room temperature whereas formation of ferrite and cementite were noticed at elevated temperatures. Fracture surfaces revealed the presence of transgranular and intergranular cracks for the specimen deformed at low (2.56 × 10−4/s) and high (2.56 × 10−1/s) strain rates respectively under ambient conditions. At elevated temperatures, parabolic dimples and microvoids were observed indicating a plastic mode of deformation in addition to the formation of carbide precipitates. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Abstract By means of laser melting and following isothermal transformation (LMFIT), the shear band and α′-martensite in mid-carbon alloy steel layer induced by stress were analyzed. The residual stress, microstructures and nanohardness were analyzed by X-ray stress analyser, X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. The results indicate that the carbide-free baninte layers with high hardness are formed on the surface of mid-carbon and high-carbon alloy steels. By LMFIT, the hardness of mid-carbon alloy steel layer can be increased by the carbon supersaturation in bainite ferrite (BF). Meanwhile, shear band and α′-martensite can be induced by thermal stress in mid-carbon alloy steel layer to further improve the hardness. The gradient change of residual stress in the bainite layer leads to that of the nanohardness in bainite layer. [ABSTRACT FROM AUTHOR]

Abstract The influence of (Ti,V)N- and V(C,N)-type precipitates on the mechanical properties was studied using transmission electron microscopy, energy-filtered transmission electron microscopy, and electron energy-loss spectroscopy for four medium carbon 1Cr steels containing various amounts of vanadium and nitrogen in the hot-rolled and normalized states. (Ti,V)N-type nanoparticles were found only in the proeutectoid ferrite, while V(C,N)-type nanometer precipitates were distributed in ferrite and further promoted the transformation of pearlite. Addition of vanadium and nitrogen refined the size of the precipitates and increased the volume fraction of pearlite, leading to increased strength. A linear arrangement of (Ti,V)N nitrides and V(C,N) carbonitrides was observed after normalization and resulted in an increase of elongation-to-failure from 16% to 22% at the expense of a 103 MPa decrease in the strength. The precipitation mechanism of Ti-rich nitrides and V-rich carbonitrides and their effects on the mechanical properties of the ferrite-pearlite 1Cr steels were discussed. [ABSTRACT FROM AUTHOR]

Although studies on hypereutectic high-Cr white cast iron (HCWCI) castings have been conducted to improve the toughnesses and quantify the microstructures of these castings, to date, only few studies have been reported on the quantitative influences of solidification conditions on the microstructures and mechanical properties of these castings. Accordingly, herein, the effects of solidification conditions on microstructural morphologies and strengths of hypereutectic HCWCI castings were investigated. Microstructural observation implied positive correlations between the minimum Feret diameter of primary M 7 C 3 and length of M 7 C 3 calculated from a three-dimensional geometric relationship under the approximation of the cylindrical shape of M 7 C 3 and between experimental minimum Feret diameters of primary and eutectic M 7 C 3. Additionally, the experimental minimum Feret diameter of primary M 7 C 3 divided by the cube root of the volume fraction of M 7 C 3 corresponded to the solidification velocity and temperature gradient obtained via solidification simulation. These results indicate that the parameters demonstrating three-dimensional morphologies of primary and eutectic M 7 C 3 can be obtained by solidification simulation. Then, the strengths of the HCWCI castings were evaluated from the rule of mixtures considering the Stroh and Kelly–Tyson equations, where the volume fraction of M 7 C 3 was acquired via thermodynamic calculations according to the chemical compositions of the castings and the strength of the matrix was substituted by the standard strengths of carbon steels under the same C equivalents as that of the matrix. The calculated strength exhibited almost the same magnitude and tendency as those of the experimental strength. The abovementioned findings reveal that not only the microstructural morphologies but also the strengths of the HCWCI castings can be estimated via computer-aided engineering. The results of this study can be utilized to predict the distributions of microstructural morphologies and strengths of the HCWCI castings. [Display omitted] [ABSTRACT FROM AUTHOR]

Compact endless cast and rolling mill (CEM) is an innovative process to manufacture a hot-rolled steel strip by combining casting and hot-rolling processes. However, the yield point phenomenon (YPP) of low carbon steel strip-processed by CEM induces a negative effect in the steel product and an additional post-process is required to remove the YPP. In this study, the influence of the dislocation density and the grain interior solute atoms on the yield point elongation (YPE) of low carbon steels were quantitatively investigated using 3-dimensional atom probe tomography analysis and X-ray convolutional multiple whole profile fitting. The YPE of low carbon steel is suppressed as the dislocation density increases and carbon atom content decreases. Because the dislocation density of low carbon steel by the CEM process is increased by lowering the processing temperature, the yielding behavior of the CEM products can be eliminated without any additional post-processing. The quantitative study on the carbon/dislocation density and YPP of low carbon steel not only represents the theoretical basis of the role of carbon-dislocation interaction on YPP but also provides an effective solution to optimize the CEM process for superior products. [ABSTRACT FROM AUTHOR]

The effect of hot rolling temperature on microstructural evolution and mechanical properties of 0.05C-5Mn (wt%) steel was investigated. With the decrease of hot rolling temperature from 1150 °C to 900 °C, the average size of prior austenite grain was decreased from ~ 40 µm to ~20 µm, and average width of martensite lath was refined from ~ 400 nm to ~ 250 nm. The morphology of reversed austenite was changed from film-like shape to block-like shape, and the volume fraction of reversed austenite was increased from 14.75% to 16.28% because of the stimulated nucleation in refined prior austenite grains by rolling at lower temperature. The yield strength, tensile strength and the total elongation were simultaneously increased from 673 MPa, 805 MPa, and 27.5% to 704 MPa, 846 MPa, and 31%, respectively. The low temperature impact toughness was significantly enhanced from 133 J to 222 J at − 40 °C. The significant improvement in mechanical properties by lower hot rolling temperature were attributed to both the refined tempered martensite lath with higher density of high misorientation grain boundaries and the active TRIP effect of reversed austenite with suitable mechanical stability. [ABSTRACT FROM AUTHOR]

Interfaces have been found to play an important role in mechanical behaviors of metals and alloys with heterogeneous microstructures. In present study, low C steel/304 stainless steel (SS) laminates with varying fractions of interfacial zone were fabricated by hot-rolled bonding, annealing and pickling, then cold rolling, subsequent annealing followed immediately by water quenching to study the effect of fraction of interfacial zone on the tensile properties. Heterogeneous distributions in chemical composition, grain size, phase and hardness were observed in the interfacial zone. The yield strength was observed to be constant with varying fractions of interfacial zone, while the ultimate strength and the uniform elongation were found to increase with increasing fraction of interfacial zone. High density of geometrically necessary dislocations (GNDs) were found to distribute in the interfacial zone and show a peak at the interface due to the mechanical incompatibility across the interface. Moreover, the density of GNDs was found to increase with increasing tensile strain in the interfacial zone. Back stress hardening was found to play an important role in the laminates, especially at the elasto-plastic transition stage. Higher fraction of interfacial zone can induce stronger back stress hardening and higher density of GNDs in the samples, thus resulting in better tensile ductility. [ABSTRACT FROM AUTHOR]

In this paper, the grain size effect on martensite transformation and its mechanical properties were investigated in two high-carbon steels. The results show that grain refinement can induce a phase transformation of high carbon martensite substructure from twin to dislocations. When grains are refined to smaller than 4 µm, no twin could be observed, instead, full dislocation substructure is obtained. And the martensite substructure transition results in an enhancement in mechanical properties, an ultrahigh tensile strength of 2.28 GPa and a significant elongation of 6.8% are obtained in the 0.61 wt%C steel. A model based on the relationship between dislocation slip stress ( τ S ) and twin shear stress ( τ T ) with grain size ( d ) is developed to explain the above phase transformation phenomenon. The τ T is proportional to d − 1 and τ S is proportional to d −1/2 , both of which increase with reducing grain size, but τ T is more sensitive to grain size. Therefore, a critical grain size ( d c ) is obtained, at which the stresses for twining and for dislocation gliding are equal, lower than this value (i.e. grain is finer), the stress for twin is higher than that for dislocation gliding, so dislocation slip becomes main deformation process in martensite transformation. The theoretically calculated d c is about 2.7–7.6 µm, which is in good agreement with the experimental results. These findings may provide a new way to design ultra-high strength and high ductility steels. [ABSTRACT FROM AUTHOR]

The distribution pattern of solute atoms, interaction between C atoms and dislocations, and bake hardening behaviors of the P-added and P-free ultra-low carbon bake hardening (ULC-BH) steels were investigated, which were annealed at 810 °C for 2 min and subsequently treated with prestrains and baking. The three-dimensional atomic probe (3DAP) and internal friction results revealed that C atoms were homogeneously distributed in the matrix of the P-added ULC-BH steel, but they were strongly segregated in the P-free ULC-BH steel after annealing at 810 °C for 2 min. The solute C concentration of the P-added ULC-BH steel was higher than that of the P-free ULC-BH steel. The addition of P greatly strengthened the BH behavior of the ULC-BH steel. The variation in the BH behavior with the prestrain was apparently different between the P-added and P-free ULC-BH steels. The BH value of the P-added ULC-BH steel gradually increased with an increase in the prestrain ranging from 1% to 10%. However, the value of the P-free ULC-BH steel increased by increasing the prestrain up to 8% and decreased subsequently. The BH behavior showed higher sensitivity to the solute C concentration for the ULC-BH steel than for the low carbon (LC) steel. The BH value either increased or decreased with an increase in the prestrain, which mainly depended on the solute C concentration, i.e., whether the solute C concentration was sufficient or not to saturate the dislocation generated by the prestrain during the baking process. [ABSTRACT FROM AUTHOR]

The microstructural characteristics of three medium carbon steels, namely MnCrB, NiCrSi and NiCrMoV containing steels, have been investigated when the steels were hardened by quenching in water or oil from different austenitisation temperatures (i.e. 850, 900 and 950 °C). The microstructure was characterised using optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, and X-Ray diffraction technique, whereas the mechanical properties were measured by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The microscopy observations suggested a fully martensitic microstructure, whereas martensite was considerably finer in NiCrSi and NiCrMoV steels compared to MnCrB steel. Moreover, the NiCrSi and NiCrMoV steels showed significantly higher strengths and lower ductility than MnCrB steel. The results suggested that the small additions of alloying elements and different prior austenite grain sizes were mainly responsible for the observed microstructural and mechanical properties variations. [ABSTRACT FROM AUTHOR]

The effect of different thermomechanical treatments, i.e., ausforming and tempforming, on the microstructure and mechanical properties including impact fracture behavior of high-strength low-carbon S700MC-type steel was investigated. The tempforming resulted in the development of ultrafine grained elongated grain microstructure with the transverse grain size of 530 nm, whereas the ausforming led to the formation of typical tempered martensite structure with a distance between high-angle boundaries of 1.5 µm. The tempered microstructures involved the formation of carbides with an average particle size about 50 nm and M(C, N)-type carbonitrides with average size of 10 nm. The tempformed steel exhibited the ultimate tensile strength of 1110 MPa and the impact toughness, KCV, above 450 J/cm 2 at a temperature of 293 K, and KCV of 109 J/cm 2 at liquid nitrogen temperature for impact direction perpendicular to rolling plane. In contrast, KCV of the ausformed steel was 180 and 10 J/cm 2 at 293 and 77 K, respectively. The enhancement of toughness was associated with delamination cracks which occurred in the ultrafine elongated grain structure with anisotropic properties. [ABSTRACT FROM AUTHOR]

A low carbon steel microalloyed with Nb-V-Ti were heat treated by means of two-step quenching and partitioning (Q&P) process after partial austenitization. Combined use of SEM equipped with EBSD, XRD and TEM showed that the microstructure was composed of intercritical polygonal ferrite, tempered lath martensite, secondary twin martensite and carbon-enriched retained austenite, as well as microalloyed carbides (mainly VC) precipitated inside ferrite and martensite. When increasing partitioning time from 0 s to 3600 s, the volume fraction of retained austenite increased up to 6.5% with mean carbon content in austenite keeping a constant level. The retained austenite kept K-S orientation relationship with adjacent tempered lath martensite and neighboring secondary twin martensite in different martensite packet. As partitioning time increased up to 3600 s, the initial lath martensite tempered gradually to ferrite without any cementite formation. [ABSTRACT FROM AUTHOR]

The effect of temperature gradient field on microstructure of Fe-0.88C-1.36Si-1.01Cr-0.43Mn (wt%) is investigated. The distribution of bainite is related to cooling temperature gradient field. The oriented bainitic ferrite with subunit structure is formed due to the high transformation driving force formed by large cooling temperature gradient, which displays large hardness. [ABSTRACT FROM AUTHOR]

The mechanical stability of retained austenite is explored in martensitic bearing steels under cyclic compressive stresses up to ∼ 10 6 cycles at 3 GPa, combining X-ray diffraction and repetitive push testing. Finite element analysis and hardness testing were adopted to interpret the stress distribution across the specimen, and the stress-strain response was revealed. Austenite decomposition was observed for all samples regardless of the difference in their chemical composition and volume percentage. The decomposition is partial and a significant amount of austenite could be retained even after ∼ 10 6 stress cycles. A scenario revealing different stages of retained austenite behaviour under compressive stresses has been established. It is observed that retained austenite first decomposes during the first tens of cycles and at 10 3 cycles, whilst it remains stable at cycles ranging 10 2 – 10 3 and after 10 4 . More importantly, results show the potential TRIP effect of retained austenite decomposition on dynamic hardening of bearing steels. [ABSTRACT FROM AUTHOR]

Thermal stability of retained austenite in 1C-1.5Cr steels with two Si and Mn contents is studied. Time-resolved high resolution synchrotron X-ray radiation and dilatometry are employed. The threshold transformation temperatures, decomposition kinetics, associated transformation strain, as well as the influence of Si and Mn were investigated. The coefficients of linear thermal expansion for both the bulk materials and individual phases are also obtained. The results indicate that an increase in the Mn and Si contents show little influence on the onset of retained austenite decomposition, but result in more thermally stable austenite. The decomposition is accompanied by a simultaneous increase in ferrite content which causes an expansive strain in the order of 10 − 4 , and subsequent cementite development from 300 to 350 °C which causes a contraction that helps to neutralise the expansive strain. During decomposition, a continuous increase in the carbon content of austenite, and a reduction in that of the tempered-martensite/ferrite phase was observed. This process continued at elevated temperatures until full decomposition was reached, which could take less than an hour at a heating rate of 0.05 ° C /s. Additionally, the observation of austenite peak splitting on samples with high Mn and Si contents suggests the existence of austenite of different stabilities in such matrix. [ABSTRACT FROM AUTHOR]

Fracture toughness studies of line-pipe steels are extremely crucial as line-pipes are exposed to extreme conditions in application. The effects of crystallographic texture and constraint on fracture toughness of API X70 line pipe steel were investigated in this study. The J-integral method was considered to quantify the fracture toughness across orientations and thicknesses of the investigated steel. The difference in the crack initiation toughness clearly demonstrated the presence of strong anisotropy. Further the counteracting effects of thickness and delamination on constraint were found to govern fracture mechanism as well as the fracture toughness of the specimens. The anisotropy in toughness was investigated with macro-texture measurements of the specimens. The major crystallographic textures and their influences on fracture toughness were analyzed. Amongst the observed texture components, strong intensities of {113}<110>component of α-fiber was identified as the major reason for anisotropy in fracture toughness. The distribution of volume fractions of {112}<110> component also showed a strong correlation with the fracture toughness across orientations. [ABSTRACT FROM AUTHOR]

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (H IT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The H IT values displayed a distinct variation in strength between different zones. The H IT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity. [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]

High-carbon carbide-free nano-bainitic steels show extremely high strength but relatively poor fracture toughness and welding prospects. Therefore, an attempt has been made to develop a strong and damage tolerant low-carbon bainitic steel in the current study by taking recourse to two-step austempering at 350 °C for 20 min and 250 °C for 7 h (20min-TS) and the properties have been compared with another block prepared using a single step transformation for 3 h at 350 °C (3hr-SS). Two stage heat-treatment leads to lath thickness refinement in addition to block refinement as compared to a single step transformation. Moreover, a NW misorientation developed in 20min-TS as compared to KS in a single-stage transformation. An average bainitic lath thickness below 100 nm in a steel having 0.26 wt% carbon has been obtained in 20min-TS which in turn results in an ultimate tensile strength of 1.5 GPa, percentage elongation of 18.5, impact energy of 31 J and plane-strain fracture toughness of 82 MPam−1/2. This is an around two and a half times increase in toughness without significantly compromising on the strength when compared to nanostructured bainitic steels produced in a similar duration of austempering for high carbon steels. • A novel low carbon nano-bainite is produced in 2 stages of austempering. • A combination of high toughness, ductility and strength is achieved. • Ductile features are observed in the tensile test. • Stages of austempering affects the size and orientation of crystallographic variants. [ABSTRACT FROM AUTHOR]

Aging treatments under three temperatures were performed to investigate the aging hardening response of high carbon high manganese austenitic Fe-30Mn-10Al-2C steel. The dramatic differences in hardness and microstructure were observed after aging under different temperatures. The aging treated at 500 ℃ had a typical aging hardness curve and the hardening response originated from the precipitation of κ-carbide, which plays an important role in performance improvement. When aging was conducted at 700 ℃, there was no significant change in hardness because a balance between aging hardening and high-temperature softening was built. On the contrary, the aging under 600 ℃ exhibited novel continuous increase in hardness and dramatic secondary hardening due to the formation of β-Mn which had an intrinsic high hardness. In addition, the β-Mn was also found after aging for long time under other temperatures. Unfortunately, the formation of β-Mn distributed along γ grain boundaries led to serious brittle fracture. For further investigation and application, the β-Mn transformation behavior was discussed based on the diffusion and redistribution of elements. Considering the dramatic difference in the formation kinetics of β-Mn depending on aging temperature, the diagram of β-Mn transformation was achieved to obtain optimized aging conditions. [ABSTRACT FROM AUTHOR]

Many researchers have investigated the relationship between hardness and crystallographic texture, including the Hall–Petch effect, crystal anisotropic-plasticity, dislocation strengthening, and strengthening by solutions. However, few studies have considered these effects simultaneously. This paper reports a method for estimating the impact of these multiple effects on the microscopic hardness of ferritic steel solely via electron backscatter diffraction (EBSD). Additionally, the correlation between the estimated hardness and fatigue damage is evaluated. The estimation method is based on the experimental correlation between the crystallographic texture obtained via EBSD and the hardness measured using nanoindenters. Three types of extra-low-carbon steel with different grain sizes were evaluated. First, a micro-Vickers test was conducted for each grain and the hardness distributions in the steel were evaluated. Second, EBSD measurements and nanoindentation tests were conducted at same locations to evaluate the relationship between the hardness obtained via nanoindentation and individual factors such as grain size, Taylor factor in the loading direction of indentation, kernel average misorientation (KAM), and indentation load. Each crystallographic factor had a rather low correlation with hardness, even though these low correlations were analogous to those of previous studies. Thus, we presume that the low correlations are due to the multiple effects of these factors, and a multiple-effect relational equation based on the strengthening mechanism is proposed with constants based on fitted data. The predicted and experimental hardness distributions were similar. Moreover, the grains with lower predicted hardness tended to be damaged after the fatigue test. These results suggest that the proposed equation can be used to predict the location of the lowest hardness in a material. [ABSTRACT FROM AUTHOR]

In this study, dynamic tensile tests were conducted on TWinning Induced Plasticity (TWIP) and low-carbon (LC) steel sheets at a strain rate of 1500–2000/s by using a split Hopkinson tensile bar, and deformation mechanisms related with improvement of dynamic tensile properties were investigated by a digital image correlation (DIC) technique. The dynamic tensile strength was higher than the quasi-static tensile strength in both TWIP and LC sheets, while the dynamic elongation was same to the quasi-static elongation in the TWIP sheet and was much lower than the quasi-static elongation in the LC sheet. According to the DIC results of the dynamically tensioned TWIP sheet, the homogeneous deformation occurred before the necking at the strain of 47.4%. This indicated that the dynamic deformation processes were almost similar to the quasi-static ones as the TWIP sheet was homogeneously deformed in the initial and intermediate deformation stages. This could be explained by deformation mechanisms including twinning, in consideration of favorable effect of increased twinning on tensile properties under the dynamic loading. On the other hand, the dynamically tensioned LC sheet was rapidly deformed and fractured as the necking was intensified in a narrow strain-concentrated region. The present DIC technique is an outstanding method for detailed dynamic deformation analyses, and provides an important idea for practical safety analyses of automotive steel sheets. [ABSTRACT FROM AUTHOR]

The relationship between the mechanical properties and the fatigue limit (FL) of medium-carbon steels with various microstructures and tensile properties was investigated through measurement of their hardness, tensile properties, and high-cycle fatigue resistance after subjecting them to heat treatment or prestraining. Carbon steels with 0.30 wt% C and 0.55 wt% C subjected to austempering and quenching-tempering treatments underwent microstructure variation from ferrite-pearlite to binate and to tempered martensite, respectively. Cold rolling of austenitic high-Mn steels with 0.57 wt% C caused an increase in their tensile strength owing to strain hardening. Fatigue tests of these materials showed that the FL increased linearly with increasing hardness (HV) of the material irrespective of the microstructure; this relationship can be expressed as FL=1.54·HV+189. In addition, the relationship between the FL and the ultimate tensile strength (UTS) can be expressed as FL=0.55·UTS+134. Application of additional fatigue test results of high-carbon (0.86 wt%) steel and 151 data points extracted from the Fatigue Data Handbook to these HV- or UTS-based FL prediction models confirmed the high reliability of these models, with good agreement between the experimental and predicted FL values. [ABSTRACT FROM AUTHOR]

Shock loading tests have been carried out on low carbon steel specimens using a single stage gas gun with projectile velocities ranging from 200 to 800 m/sec. In addition to dislocation slip, twinning was observed to be a contributing deformation mode. Similar impact tests were performed on specimens that have been previously subjected to cold rolling and it was shown that pre-straining increases the threshold stress for twin formation under shock loading. Efforts are made to determine the critical stress and strain rate required for twinning in cold rolled specimens. For this purpose a set of compression tests were performed on as-received and pre-strained specimens using a split Hopkinson pressure bar at various strain rates. These tests were carried out at liquid nitrogen temperatures where the thermal activation of dislocations is assumed to be higher than that for twin nucleation. Results of these tests are combined with FE analysis to provide knowledge of the twin stress as a function of pre-strain and strain rate. In addition, post-impact specimens were examined to identify characteristics of the deformation induced twins. It is shown that more than one {112} <111> type twin system is active at high stress levels as evident by the non-parallel nature of twins formed at the higher impact pressures. This observation was incorporated into a constitutive model based on one dimensional wave and conservation equations for predicting twin volume fraction by resolving the shear stress in multiple directions and applying twin growth equations for each plane. Results of this model show similar trends when compared with experimental outcomes for as-received and pre-strained steel. [ABSTRACT FROM AUTHOR]

The temperature dependence of the effective stress indicated that both Ni and Mn induce solid solution softening at low temperatures. The activation energy for dislocation glide was obtained from the temperature dependence of the activation volume and effective shear stress. Either Ni or Mn decreases the activation energy for dislocation glide, which suggests that both Ni and Mn decrease the brittle-to-ductile transition (BDT) temperature. However, the temperature dependence of the absorbed energy for fracture showed that the transition temperature decreases with Ni but increases with Mn. Fracture surfaces tested at 100 K indicated transgranular fracture at 2 mass% Ni and intergranular fracture at 2 mass% Mn, which suggests a decrease in energy for grain boundary fracture with Mn. The mechanism behind the opposite effects of Ni and Mn on the transition temperature of ultralow-carbon steels was examined on the basis of dislocation shielding theory. [ABSTRACT FROM AUTHOR]

The influence of magnetic field on the mechanical properties and microstructure of 0.56%C plain carbon steel specimens is studied experimentally. Specimens have been subjected to magnetic field intensities: 400, 600, and 800 Oe. The results reveal that the modulus of elasticity and the ductility are decreased. While, no significant change in the hardness and the microstructure is observed. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) of 410L ferritic/martensitic stainless steel via laser metal deposition (LMD) process is investigated. The carbon content of 410L powder used is low at 0.004 wt%. Heat treatment is utilized to study the microstructure and mechanical properties evolution of the deposited material. The microstructure of as-built low carbon 410L includes equiaxed ferrite phase, widmanstatten ferrite, martensite, and (Fe,Cr) 23 C 6 nanoprecipitates. After heat treatment at 1000 °C for 10 min, refined equiaxed grains and high fraction of martensitic phase are formed. The yield strength is 601.9 MPa for as-built condition and 930.6 MPa for heat-treated condition (1000 °C/10 min); the ultimate tensile strength (UTS) is 923.0 MPa for as-built condition and 1101.5 MPa for the heat-treated condition (1000 °C/10 min); the elongation is 17.7% for as-built condition and 15.1% for the heat-treated condition (1000 °C/10 min). The low carbon 410L fabricated using LMD shows a better combination of high strength and good ductility compared with 12Cr stainless steels with higher carbon contents fabricated using conventional processes. This study provides a benchmark of 410L stainless steel fabricated using fusion-based metal AM process. [ABSTRACT FROM AUTHOR]

Liquid-solid (L-S) bonded stainless steel/carbon steel (SS/CS) laminated composites have strong metallurgical bonding interface, while the solidification structure of SS deteriorates the mechanical properties and corrosion resistance. In this study, hot rolling and annealing were proposed to regulate the microstructures and balance the performance of L-S bonded SS/CS clad plates. SS cladding CS plates were prepared by horizontal continuous liquid-solid composite casting and hot rolled at 1200 °C with a reduction rate of 90%, and annealed at 1000 and 1050 °C for 30 min. The microstructural evolution was studied by scanning electron microscope and electron back-scatter diffraction, elemental distribution was investigated by electron probe microanalysis. Microhardness, tensile test and anodic-cathodic polarization test in a 3.5% NaCl solution were carried out on hot-rolled and annealed SS cladding CS plates, and the mechanism for the simultaneous enhancement of strength-ductility synergy and corrosion resistance by hot rolling and annealing was analyzed. Annealing at 1050 °C resulted in excellent properties with tensile strength of 550 MPa and elongation of 50%, corrosion potential of −0.21 V and corrosion current density of 0.13 μA cm−2. The strength and ductility of the SS cladding CS plate were synergistically improved by a strong metallurgical bonding cladding interface, high frequency of low-Σ CSLs and refinement of grains in 304 SS. The low-Σ CSLs in 304 SS restricted the formation of carbides in the carburized layer and broke down the connectivity of random boundaries, improving the corrosion resistance. [ABSTRACT FROM AUTHOR]