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G115 steel is a novel martensitic heat-resistant steel, primarily utilized in the main steam pipelines and collectors of ultra-supercritical thermal power units. However, the oxidation resistance of martensitic steels in the high-temperature steam environment is usually suboptimal, significantly affecting the efficiency of power plants. In this paper, shot peening (SP) is employed as a surface treatment method for G115 steel, and the oxidation kinetics, oxide layer thickness, and microstructure of shot-peened G115 samples are compared with those of G115 steel. The results indicate that in the 650 °C steam environment, the oxidation kinetics of the shot-peened samples follow the parabolic law and that the oxidation weight gain is significantly smaller than that of the non-shot-peened samples. The higher the SP intensity, the smaller the oxidation weight gain and the better the oxidation resistance. This can be attributed to the fragmentation of the grains in the surface layer caused by external stress during SP, which creates a multitude of grain boundaries that can provide rapid diffusion pathways for corrosion-resistant Cr atoms, resulting in the accelerated outward diffusion of Cr atoms from the substrate. Simultaneously, a continuous and dense FeCr2O4 protective layer is produced at the interface between the SP layer and the substrate, obstructing the inward diffusion of oxygen and enhancing the oxidation resistance of G115 steel. [ABSTRACT FROM AUTHOR]

In this study, transient liquid-phase (TLP) bonding was adopted to obtain a reliable reduced-activation ferritic/martensitic (RAFM) steel joint with Fe-Si-B amorphous foil. The aging tests and creep tests of the TLP joints were carried out at 550 °C to study the microstructural evolution in the service process. The effect of stress loading on the microstructural evolution of the TLP joint was investigated. The results show that creep fractures in the TLP joints occur in the base material. The main factors affecting the creep performance of TLP joints are the recovery of substructures and the coarsening and deformation of martensitic laths. In addition, the M23C6 carbides in the base material were coarser than in the weld zone. Compared with aging samples and creep samples undergoing the same test temperature, the dislocation density in the isothermal solidification zone (ISZ) increased significantly with increases in the stress level. Furthermore, it is worth noting that the microstructure of the weld zone changed from large-sized ferrite to a mixed, fine microstructure of ferrite and martensite, which increases the heat resistance of the TLP joints, and thus results in creep fractures in the base metal. [ABSTRACT FROM AUTHOR]

In the present work, Tungsten (W)/oxide dispersion strengthened (ODS) steel joints were fabricated by the direct solid state diffusion bonding (SSDB) technology with a multistage cooling process, and the microstructure and tensile strength of the bonded interfaces and parent materials were experimentally investigated. The results show that W and ODS steel can be successfully bonded at the temperature ranging from 900 °C to 1050 °C, without severe macroscopic deformation or obvious microscopic defects. Reaction layers generated at the bonded interfaces are evolutive with the bonding temperature, result in different fracture locations of the bonded joints. In the joint bonded at 950 °C, a higher interfacial strength of ~ 234.2 MPa is achieved, due to the formation of nano-scale intermetallic compound FeW. Microstructure of W remains stable after all the SSDB processes, while the lath structure of ODS steel is completely broken and transformed into the equiaxed grains, which should be responsible for the deterioration of strength. When the bonding temperature is higher than 950 °C, the pinning effect of precipitates M23C6 and nano-oxide particles on the movement of dislocations is observed. [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to study the effect of flow rate (0.42∼2.09 m/s) on the corrosion behavior of WB36CN1 steel pipe in the simulated secondary circuit water environment (170°C, 6 mg/L ethanolamine + 100 µg/L NaCl), for which an autoclave was used to simulate the secondary circuit environment for carrying out related experiments. Design/methodology/approach: The corrosion behaviors were studied by electrochemical methods, morphological observations and elemental analysis. Findings: As flow rate increases, the amplitude of the current noise fluctuates increased, noise resistance Rn and spectral noise resistance Rsn decreased, the shear stress on the surface of WB36CN1 steel increases, the oxygen content on the surface decreases, the roughness becomes smaller. Meanwhile, the energy of energy distribution plot is concentrated at high frequencies under the three flow conditions, the slopes of current power spectral density curve approach 0 db/decade. This means that the oxide on the surface becomes less and corrosion rate increases with increasing flow rate. The corrosion type of WB36CN1 steel was uniform corrosion; the degree of uniform corrosion is higher at high flow rate. Originality/value: The effect of flow rate on the corrosion behavior of WB36CN1 steel pipe in the secondary circuit water environment was studied by using electrochemical methods in the laboratory. The effect mechanism of flow rate for corrosion behavior was obtained. [ABSTRACT FROM AUTHOR]

Kinetics of multiply ferrite/bainite phase transformation of HSLA steels is investigated by experiments and cellular automaton (CA) simulation. Peak-differentiation method to elucidate the sequential ferrite and bainite phase transformation individually, which is verified by the CA simulation. Such CA modelling executed using classic JMAK theory, but also gives an insight of microstructure evolution of the multi-phase transformation routine on different cooling rate. From that, it enables classic JMAK modelling to capture the detached phase transformation with different growth models and interface-migration mechanisms. Also, we find that the final phase constitution is sensitive to the cooling rate. With increasing the cooling rate, bainite sheaves nucleated at prior austenite boundaries and ferrite/austenite interfaces are significantly facilitated, which seriously inhibits the growth of prior ferrites. The scenario can be interpreted by the CA simulation and the influence of the cooling rate on sequential multi-phase transformation can be also obtained. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) steels with different contents of zirconium (denoted as 16Cr ODS, 16Cr-0.3Zr ODS and 16Cr-0.6Zr ODS) were fabricated to investigate the effects of Zr on strengthening mechanism of Al-alloyed 16Cr ODS steel. Electron backscatter diffraction (EBSD) results show that the mean grain size of ODS steels could be decreased by Zr addition. Transmission electron microscope (TEM) results indicate that Zr addition could increase the number density but decrease the mean diameter and inter-particle spacing of oxide particles. Furthermore, it is also found that in addition to Y-Al-O nanoparticles, Y-Zr-O oxides with finer size were observed in 16Cr-0.3Zr ODS and 16Cr-0.6Zr ODS steels. These changes in microstructure significantly increase the yield strength (YS) and ultimate tensile strength (UTS) of ODS steels through mechanisms of grain boundary strengthening and dispersion strengthening. [ABSTRACT FROM AUTHOR]

Microstructural observation and high-resolution dilatometry were employed to investigate acicular ferrite formation during isothermal holding in the HSLA steel. A decrease in isothermal temperature suppresses formation of polygonal ferrite and promotes formation of acicular ferrite. Island-like martensite/austenite constituents are dispersed inside of the acicular ferrite grain. The displacive model assuming autocatalytic nucleation was developed to describe the incomplete transformation phenomenon well. Decrease of isothermal temperature lowers the activation energy, and thus enhances the formation of acicular ferrite. Increase of amount of polygonal ferrite and acicular ferrite both results in decrease of M. Formation of polygonal ferrite causes diffusion of solute alloying atoms into the untransformed austenite, which lowers M. Acicular ferrite transformation suppresses martensitic transformation by diffusion of solution atoms during isothermal holding, and introduction of internal stress resulting from volume expansion during FCC → BCC transformation. [ABSTRACT FROM AUTHOR]

A reduced activation ferritic/martensitic (RAFM) steel with perfect creep property at elevated temperature (600 °C) was designed by implementing adding trace amount of Ta, Zr elements. In order to reveal the mechanism of creep property enhancing through adding Ta, Zr elements, the microstructural and mechanical evolution behaviors of RAFM steels, with and without containing Ta, Zr elements, were analyzed. It was found that Ta, Zr can not only refine the M 23 C 6 carbide particles, but also can increase the mean size of MX particles in RAFM steels, especially for Zr element. Furthermore, both of Ta and Zr can contribute to increasing the amount of MX particles. Through combining with experimental results and excluding the influence behavior of parent austenite grain (PAG) size on the mechanical property of RAFM steel, it is not the M 23 C 6 particles, but is the precipitation amount of MX carbide particles that promoted by adding Ta, Zr, which can have a significant effect on enhancing the strength and decreasing the creep rate of RAFM steel at 600 °C condition. In addition, the synergistic effect of Ta and Zr elements can significantly increase the creep-resistance breaking time of RAFM steel under the same conditions comparing with other RAFM steels, and then the mechanism of it was revealed in this work. • A reduced activation ferritic/martensitic (RAFM) steel with excellent creep property by changing Ta, Zr elements was developed. • Based on containing Ta element, adding Zr can slightly increase the size and amount of MX carbide particles in RAFM steel, which enhances the microstructure stability of RAFM steel at elevated temperatures. • M 23 C 6 particles have limited influences on increasing the tensile strength and creep properties of RAFM steels at 600 °C, but MX particles can do well with it. [ABSTRACT FROM AUTHOR]

Step quenching and tempering (SQT) for HSLA multi-phases steel was designed to obtain the desirable multi-phases microstructure with the superior mechanical properties. Quenching and tempering (QT) and intercritical quenching and tempering (IQT) was also conducted as controls. Besides, Effects of austenitizing temperature for the different treatment routes on microstructural characteristics and mechanical properties were also investigated. Due to the cooperation of the “soft” ferrite/pearlite and “hard” martensite, the samples undergoing SQT possess the relatively high tensile strength and impact toughness, and the lowest yield ratio. Increase of austenitizing temperature results in the coarse austenite grains, and thus significantly decrease of tensile strength and impact energy, in spite of the heat treatment paths. Higher austenitizing temperature also leads to lower density of dislocations and higher grain size, which reduces the yield ratio for all samples. [ABSTRACT FROM AUTHOR]

To reveal the effects of Al addition on the oxidation mechanisms of 16Cr ODS steel, high temperature oxidation at 1000 °C of 16Cr and 16Cr-3Al ODS steels with the same Cr content were investigated. After oxidation, the weight gain of the 16Cr ODS steel was relatively higher than that of the 16Cr-3Al ODS steel. The 16Cr ODS steel had triple oxide layers of an outer FeCr 2 O 4 layer, an intermediate Cr 2 O 3 layer, and an inner SiO 2 layer, while the 16Cr-3Al ODS steel had double oxide layers of an outer (Cr, Al) 2 O 3 layer and an inner Al 2 O 3 layer. The oxidation mechanisms of both ODS steels were explored through thermodynamic calculations, and the significant improvement in oxidation resistance of the 16Cr-3Al ODS steel compared to 16Cr ODS steel was attributed to the lower diffusion rate of O in Al 2 O 3 than in Cr 2 O 3. The existence form of oxide particles in the oxide layers was illustrated. These findings offer new strategies for enhancement in oxidation resistance of structural materials used in high temperature environment. • The 16Cr-3Al ODS steel exhibited better oxidation resistance compared to the 16Cr ODS steel. • Double oxide layers formed on 16Cr-3Al ODS steel, while triple oxide layers formed on 16Cr ODS steel. • No segregation of Y element observed in the oxide grain boundaries of 16Cr-3Al ODS steel. [ABSTRACT FROM AUTHOR]

In this work, reactive-inspired ball-milling (RBM) and spark plasma sintering (SPS) were employed to produce an ODS-316 L steel. The evolution of Y 2 O 3 particles during RBM and SPS processes was investigated, and the powder milled for different times was characterized for exploring the precipitation and coarsening behaviors of Y 2 O 3 particles. It was found that Y 2 O 3 particles with a metastable hexagonal structure precipitated in the early stage of ball-milling, and gradually transformed to a stable body centered cubic (BCC) structure during subsequent growth process. The size of Y 2 O 3 particles maintained at about 3–4 nm and no longer increased significantly in the RBM stage, but their average size increased rapidly to about 12 nm after SPS. The HRTEM results of the alloy after SPS showed that Y 2 O 3 particles inside the grains had a semi-coherent interface with the matrix. Compared with other methods, the united technology of RBM and SPS could significantly improve the tensile properties of the ODS-316 L steel. It was considered that the successful introduction of uniform and fine Y 2 O 3 particles by RBM and the small size of austenite grains due to rapid sintering of SPS were responsible for the improved tensile properties. [Display omitted] • An ODS-316 L steel was prepared by reactive-inspired ball-milling and SPS. • The size of oxides tended to be stable when it grew to 3–4 nm. • Hexagonal-to-cubic structural transformation of Y 2 O 3 precipitates occurred. • The ultimate tensile strength was 865 MPa and the elongation was 20.5%. [ABSTRACT FROM AUTHOR]

The creep properties, microstructural evolution, and fracture mechanism of 16Cr-3Al oxide dispersion strengthened (ODS) steel were systematically investigated at 600 °C under different uniaxial tensile stresses in the range of 160–200 MPa. The minimum creep rate had a linear relationship with the mean creep rate, which followed the modified Monkman-Grant relationship with a slope of 0.57. The threshold stress was determined as 145.09 MPa based on the modified Bird-Mukherjee-Dorn (BMD) equation. Both Laves phase and Cr-rich phase were identified after creep deformation. The evolution of Cr-rich phase during creep deformation included the dissolution of W-rich σ phase and the precipitation of Si-rich σ phase. Cross-sectional cracks could be observed near the rupture surface, and these cracks mainly formed near the Si-rich σ phase and the boundaries between coarse and fine grains. It was considered that the formation of Si-rich σ phase in fine grains was the main fracture mechanism of 16Cr-3Al ODS steel during creep deformation. [ABSTRACT FROM AUTHOR]

Abstract Corrosion behavior in the supercritical water (SCW) of an Al added high-Cr oxide dispersion strengthened (ODS) steel was investigated in this study. The steel was exposed to SCW at 600 °C and 25 MPa for various exposure times up to 1000 h, and the resultant oxide layers were characterized by using weight change measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopies (XPS), scanning/transmission electron microscopy (SEM/TEM), and energy dispersive spectroscopy (EDS). The weight gain after exposure for 1000 h was 6.24 mg / dm 2 , indicating the superior corrosion resistance of 16Cr-3Al ODS steel to SCW. Chemical analysis of the surface oxides showed that the content of Cr-rich oxides increased with the increase of exposure time in SCW. Cross-sectional observation of the oxide scales revealed that an outer layer composed of (Cr, Fe) 2 O 3 and an inner layer composed of Al 2 O 3 formed on the sample surface. The growth of both outer and inner oxide layers followed the parabolic law with the increase of exposure times. The corrosion mechanism was discussed in detail based on the microstructure evolution process with exposure time in SCW. Graphical abstract Unlabelled Image Highlights • The weight gain of 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h was 6.24 mg / dm 2. • The thickness of the oxide layer after exposure for 1000 h was 400 nm. • Duplex oxide layer formed on the sample's surface. • The formation of compact Al 2 O 3 inner oxide layer significantly enhances the corrosion resistance of the steel to SCW. [ABSTRACT FROM AUTHOR]

Manipulating chemical heterogeneity, which could substantially alter austenite decomposition behavior upon cooling, has recently proven to be very effective in tailoring retained austenite and architecting novel microstructures in steels. However, the mechanistic role of chemical heterogeneity in austenite decomposition and stabilization has not been fully understood, and its potential in microstructural architecting in commercial advanced high strength steels (AHSSs) has not been explored. In this study, we investigate the potential effects of Mn heterogeneity, which originates from the starting microstructure of Mn-lean ferrite and Mn-rich cementite, on austenite decomposition and stabilization in a commercial Quenching and Partitioning (Q&P980) steel. It is interestingly found that there is a strong kinetic interaction between the Mn enriched region in austenite and the migrating austenite/ferrite interface upon cooling, and phase field model predicts that the austenite/ferrite interface could migrate through the Mn-rich region via either a cutting through or by-passing manner, depending on the absolute Mn content of the Mn-rich region and the degree of Mn heterogeneity. The by-passing interaction could substantially promote C partitioning into the Mn-rich region, which further enhances the region's stability. The current study demonstrates austenite decomposition and C partitioning behavior can even be altered by a weak Mn heterogeneity, which needs to be carefully considered in the future design of AHSSs. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) steel is one of the most promising candidate structural materials for the high-temperature nuclear reactor application. In this study, two compositions of ODS steels (14Cr-ODS and 14Cr-Zr-ODS) were prepared to investigate the influence of Zr addition on the microstructures and mechanical properties of ODS steels. The microstructures, including dispersion morphology and crystal structures of oxide particles, particle-matrix interface coherency and particle-dislocation interactions, were characterized using TEM, HRTEM, and SEM, and the mechanical properties at room and high temperatures were measured using uniaxial tensile tests. Results show that Zr addition leads to the formation of finer precipitated particles, which was identified as rhombohedral Y 4 Zr 3 O 12 , with denser dispersion in the matrix. The calculation results reveal that the lattice misfit, δ , at the interface between particle and matrix increases as the particle size increases. In addition, the strength and elongation of ODS steels are improved with Zr addition due to the stronger interface bonding force between fine particles and matrix as well as the larger pinning effect of small particles to dislocation movements. [ABSTRACT FROM AUTHOR]

The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M 3 C, M 2 C, M 23 C 6 and M 7 C 3 . M 3 C precipitated when tempering at 200–400 °C, and it was replaced partially by M 2 C when the temperature is elevated to 500 °C, at which M 23 C 6 also precipitated. For a higher temperature, above 600 °C, M 3 C disappeared, and some M 2 C transformed into M 7 C 3 . The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained. [ABSTRACT FROM AUTHOR]

A new thermo-mechanical treatment strategy is proposed to improve the creep resistance of a recently developed G115 steel. The goal of this strategy is to construct a favourable microstructure with superior thermal stability at high temperatures up to 650 °C. The improved creep strength can be ascribed to the reinforced interactions between dislocations and nano-sized MX and Cu-rich precipitates. Creep analysis suggests that these MX and Cu-rich particles exhibit more important contribution to the creep resistance than the M 23 C 6 carbides. Following this strategy, the creep rupture life of the G115 steel can be significantly prolonged by more than two times. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, effects of Cu-rich phases on the microstructure evolution and creep deformation behavior of G115 steels with 1 (1Cu steel) and 2 wt% Cu (2Cu steel) were systematically investigated under 160–220 MPa at 650 °C. The results showed that dispersedly distributed Cu-rich phases played a vital role in retarding the microstructural degeneration and improving the creep strength. The Cu-rich phases inside grains could induce dislocation tangle and prohibit dislocation annihilation during creep, but they could be dissolved into the matrix due to the cutting effect of mobile dislocation lines. The Cu-rich phases at grain boundaries could hinder the migration of martensitic lath boundaries and thus delayed the creep failure behavior. Moreover, the independent Cu-rich phases at grain boundaries could provide heterogeneous nucleation sites for Laves phases, which strengthening pinning effect. As a result, compared with the 1Cu steel, the creep threshold stress of the 2Cu steel was enhanced from 103.36 to 117.93 MPa, and the creep lifetime was also prolonged markedly. [ABSTRACT FROM AUTHOR]

In this paper, the influences of prior cold deformation on creep behavior, microstructure evolution, and fracture mechanism of the novel G115 steel were systematically investigated. It was found that the creep rupture life of the G115 steel could be significantly prolonged by moderate deformation reduction, which was a synergetic effect of deformation-induced dislocations and dispersed strengthening particles. The optimal deformation reduction was approximately 20%, at which the precipitation of Laves phases could be obviously promoted, and then restricted the coarsening of M 23 C 6 carbides due to the competitive growth relationship. Besides, more nano-sized Cu-rich particles precipitated within lath interior, and obstructed the free dislocation motions. As a feedback, the dislocation recovery and the subgrain formation could be impeded, thus extending the creep rupture life. With the further increase of deformation reduction to 30% or higher, the coarsening of precipitates and the recovery of dislocations were intense, which could facilitate the formation of subgrains and then shorten the creep life. For the G115 steel with a deformation reduction within 30%, the creep damage was identified as microstructural degradation with limited contribution from necking. And, for the steel with a deformation reduction of 45%, the creep damage was cavity growth controlled by power law and diffusion creep. [ABSTRACT FROM AUTHOR]

By performing a two-step uniaxial diffusion bonding, the reliable joining between CLAM/CLAM steels has been attained. The microstructures at the vicinity of the joint region and in base material were respectively investigated through OM, SEM and TEM. The joint interface was integrated, and no microstructural defects were observed. In the base material, small amount of austenite is retained as thin films between martensite laths, which was suggested to be related to the compressive deformation in diffusion bonding. As a candidate structural material for the first wall in fusion energy systems, the radiation resistance of CLAM steel would be deteriorated by the retained austenite. Tensile and impact tests were carried out to assess the reliability of the joints subjected to post bond heat treatment. All the tensile specimens fractured in the base CLAM steel, meaning the good joining between CLAM steels. However, due to the low impact absorbed energy of the joints, efforts should still be made to optimize the bonding technology and the post bond heat treatment further. [ABSTRACT FROM AUTHOR]

In this paper, post bonding heat treatments to transient liquid phase bond reduced activation ferrite/martensitic steel were conducted, with a Fe–Si–B amorphous foil as the interlayer. The results show that, the post bonding heat treatments decreased the strength difference between different regions of the transient liquid phase bonded joint and altered the shape and the amount of Cr 2 B along the interface between the diffusion affected zone and base metal. In particular, it is found that the tensile fracture position of the as-bonded joint has been altered by post bonding heat treatment, from the interface between the diffusion affected zone and the base metal to where inside the base metal. In order to unveil such deformation differences between transient liquid phase bonded joints with and without post bonding heat treatment, digital image correlation method was adopted to study the influence of post bonding heat treatment on the local strain evolution of transient liquid phase bonded joint from both macro- and micro-scale. The non-uniform strain distribution upon the nominal surface of the tensile samples during deformation showed that, the strain of the as-bonded joint was always localized at the interface of the diffusion affected zone, resulting brittle fracture eventually. While for the joint after post bonding heat treatment, the strain localization gradually transferred from the interface to the base metal during the tensile test, which is attributed to the changing deformation resistance of these regions during straining. [ABSTRACT FROM AUTHOR]

[Display omitted] Bonding of tungsten (W) with oxide dispersion strengthened (ODS) steel is an urgent challenge during the practical manufacturing process in many applications, including advanced nuclear reactor systems, military equipment, etc. To eliminate the generation of brittle intermetallic compounds Fe-W and relieve the residual thermal stresses, a novel composite interlayer Zr/Cu, for bonding W with ODS steel using the transient liquid phase (TLP) diffusion technology, was designed in this work. The bonding materials were prepared as a sandwiched structure of W/Zr-Cu/ODS steel in a vacuum diffusion bonding device. Interfacial structure and tensile strength of the bonded joints were investigated, and the processing parameters were optimized. After the bonding process, a reliable Cu/ODS steel interface could be built at all applied temperatures or holding times. Nevertheless, only as the bonding temperature was 1000 °C or even higher, the inserted Zr foil could be completely consumed and a stable W/Cu interface free from intermetallic compounds and micro-cracks was built. Additionally, the bonding ratio was gradually improved with increasing the bonding temperature or holding time. As a result, a high-strength W/Cu/ODS steel joint of ∼430.4 MPa could be fabricated at 1000 °C for a holding time of 60 min, and the bonding ratio was as high as 98 %, which provided a significant guide for the assembly of plasma facing components (PFCs). [ABSTRACT FROM AUTHOR]

The dynamic recrystallization (DRX) behavior of austenite in reduced activation ferritic/martensitic steel was investigated by isothermal compression tests and constitutive modelling. The flow stress decreases with deformation temperature increasing and strain rate decreasing. The constitutive equations were constructed based on the analysis by Arrhenius equations. Based on the dynamic material model, the hot processing map was constructed. In the stability region, the power dissipation efficiency (η) increases with increasing strain, and the softening is dominated by DRX when η grows to a specific value. However, η decreases as strain increases further after the complete evolution of DRX, indicating coarsening of DRXed grains. As the strain increases, the processing stability region shifts to the low temperature and high strain rate region. A novel recrystallization kinetics model was developed by considering the characteristics of the recrystallization velocity with strain increasing. It can reflect the "slow-rapid-slow" growth trend of recrystallization volume fraction (X DRX). The relationship between X DRX and D ¯ (average grain size of austenite) was constructed, which indicates that D ¯ decreases with strain increasing after the onset of DRX. When X DRX is greater than 50%, the decrease rate of D ¯ slows down until the steady-state is reached. Although strain increase further, the grain size remains constant. This suggests that the expected D ¯ can be obtained by controlling X DRX during hot deformation. [ABSTRACT FROM AUTHOR]

In this paper, the effects of Cr and Mo microalloying on the microstructure evolution, mechanical properties, and austenite stability of 0.2C-7Mn steel were investigated at different intercritical annealing temperatures plus the same tempering treatment. Both the yield strength and ultimate tensile strength can be enhanced by nano-sized precipitates and solution hardening effect owing to Cr-Mo addition, while the ductility is well retained. The precipitates are mainly distributed on the ferrite matrix of the (Cr, Mo)-added steel, and directly produce a precipitation hardening effect. Due to the precipitation hardening and solution hardening effects of ferrite, the austenite in the (Cr, Mo)-added steel exhibits a lower mechanical stability, and more strain partitioning among the austenite and ferrite can be borne by the austenite than that in the reference steel, resulting in an accelerating transformation of austenite and a prevailed transformation-induced plasticity effect in the (Cr, Mo)-added steel. [ABSTRACT FROM AUTHOR]

The effect of carbonitrides on the growth of austenite grains in high-strength low-alloy (HSLA) steel was investigated through microstructure analysis and high-resolution dilatometry. The austenite grain growth at various temperatures was measured in situ using high temperature laser scanning confocal microscopy. Two kinds of precipitates with different morphologies containing large amounts of Ti and Nb were observed in the austenitized samples at different temperatures. The smaller Nb-rich spherical carbonitrides dissolved at the higher temperature, while the lager Ti-rich rectangular carbonitrides have a higher thermal stability and can pin at the austenite grain boundary. This study reveals that the dissolution of Nb in austenite causes the rapid growth of austenite grains and slows down the ferrite phase transition, which is beneficial to the formation of bainite. A kinetic model of austenite growth considering the dynamic dissolution of different types of carbonitrides is proposed. The results of the model show the same overall trend as the experimental results, and the individual differences can be explained. The proposed model can be used to predict the austenite grain size during the austenitizing process of HSLA steel and can reflect the austenite grain change throughout the process more continuously. • Two carbonitrides with different thermal stability at the high temperature were found. • The dissolution of Nb element affect the austenite grain size and also affect the phase transition in the cooling process. • Providing a new method to predict the pinning pressure and the austenite grain size with increasing time from dynamics. [ABSTRACT FROM AUTHOR]

Sulfide stress corrosion (SSC) in high strength low alloy (HSLA) pipelines was investigated in the work. Ultrasonic inspection and in-situ electrochemical noise (EN) measurements were carried out during stress corrosion experiments to monitor the progress of sulfide stress cracking in H 2 S environment. Moreover, the interrupt experiments were employed to reveal the evolution of corrosion morphology during SSC through the analysis for the EN signals. Microstructural characterization was also performed to identify the morphological evolution owing to the stress corrosion. It is found that the process of SSC is divided into three stages: uniform corrosion stage, pitting or local corrosion stage and cracks propagation stage. The period of time that the uniform corrosion transforms to the local/pitting corrosion is in the range of 12-24 h. The EN analysis results, such as time-domain curves, power spectral density (PSD) diagrams, energy distribution plots (EDPs) and current kurtosis, correspond to the microstructure observation and ultrasonic inspection. Especially, the recognition of crack initiation by current kurtosis curves correlate well with that by the ultrasonic inspection, which suggests that the current kurtosis analysis could provide the information for crack initiation in real time • SSC on HSLA steels were investigated via the combination of ultrasonic inspection and in-situ electrochemical noise analysis. • Interrupt experiments were carried out to examine the microstructural evolution on SSC. • The process of SSC is divided into the uniform corrosion, the local corrosion and the cracks propagation stage. • The period of time that the uniform corrosion transforms to the local corrosion is recognized in the range of 12-24 h. • The work well proposed an efficient method to monitor SSC process on steels. [ABSTRACT FROM AUTHOR]

Intercritical treatments within the dual-phase (α+γ) region were applied on HSLA pipeline steels, for acquiring a low yield ratio (YR) well balanced with desirable strength. Intercritical cooling treatment (ICT), step cooling treatment (SCT) as well as direct cooling treatment (DCT) after full austenization were designed to obtain an optima multiphase microstructure. Effects of cooling rate in DCT routine and intercritical temperatures in ICT and SCT routines on microstructural evolution and corresponding mechanical properties were also investigated. Especially, the SCT treatment applied with an intercritical temperature of 750 °C produces a microstructure composited of "soft" coarse polygonal ferrite, and "hard" acicular bainite and lath martensite containing large amounts of dislocation tangles or networks generated by deformation. Such multiple phase constituents guarantee the high strength and remarkable ductility on deformation, meanwhile cleavages propagation is hindered by the high-angle boundaries of bainite and martensite sheaves, which leads to the lowest YR ~0.61 combined with highest tensile strength among all. In addition, by using the Swift equation to elucidate the relationship between the phase component and yield ratio, it is found that simply increasing the fraction of low-temperature transformed phases, like high-strength acicular bainite and lathed martensite, or the percentage of soft polygonal ferrite for good ductility, can hardly solve the problem how to achieve ultralow- YR pipeline steels balanced with enhanced strength. The present result proves that, through utilizing the proposed SCT heat treatment on pipeline steels, an ultralow yield ratio ~0.61 achieved is synchronized with a desirable strength, which efficiently overcomes the trade-off limit between the strength and yield ratio when applying conventional heat-treatment routines. The fact indicates that, rationally adjusting the content of multi-phase microstructure through optimizing the intercritical treatment conditions, can enable us of realizing the synchronous improvement of the YR and strength in HSLA pipeline steels for real engineering. [ABSTRACT FROM AUTHOR]

In this paper, microstructure evolution and mechanical properties of transient liquid phase (TLP) bonding joints of reduced activation ferritic/martensitic (RAFM) steel using Fe–Si–B amorphous foil during homogenization stage were investigated. The results show that, numbers of block-like M 23 C 6 carbides and ~500 nm-size elliptical Fe 2 B phase are precipitated in the isothermal solidification zone (ISZ), while the larger-size rod-like Cr 2 B phase with size of ~3 μm is distributed in the diffusion affected zone (DAZ). When the homogenization time extends to 30 min, the amount of Cr 2 B phase reduces dramatically in the DAZ due to the interdiffusion of the alloying elements. The distribution of Cr atoms along the bonding joint was estimated through a model. The model indicates that the dissolution of the Cr 2 B phase works dynamically during the homogenization stage. The maximum shear strength was obtained about 558 MPa for bonding 30 min. With extending the homogenization time, the fracture transitions from brittle mode to ductile mode. The precipitation of coarse Cr 2 B phase is the main factor of the shear failure for short homogenization time. Hence, the bonding quality is able to be significantly improved with adjusting the geometric characteristics of such Cr 2 B via changing the homogenization time rationally. [ABSTRACT FROM AUTHOR]

Step quenching and tempering (SQT) treatment was applied on HSLA steels to achieve a multi-phase microstructure with a superior deformation performance. Intercritical quenching and tempering (IQT) as well as direct quenching and tempering (DQT) were also processed for comparison. Moreover, effects of intercritical temperatures before quench for different treatment methods on microstructure evolution and mechanical property were also investigated. Compared with IQT and DQT, SQT treatment produces a fine microstructure composited of soft ferrite/pearlite and hard martensite, with precipitates distributed along boundaries to maintain a large quantity of dislocations on deformation. Experiments involving tension and impact tests show that SQT samples with such multi-phase microstructure exhibit a desirable combination of mechanical properties. With increasing the intercritical temperature, the phase fraction of martensite is increased, which improves the strength but weakens the impact toughness in regardless of heat treatment type. Meanwhile, the yield ratio (YR) is found to be increased by higher intercritical temperature in all samples, due to the larger phase fraction of martensite with a large dislocation density and a finer grain structure. Such scenario is elucidated by using a model proposed through Swift's equation. That indicates that the content of multi-phase microstructure can be efficiently controlled via adjusting the intercritical temperature using SQT, which enables the optimization of phase composition according to the requirements of actual productions in engineering. [ABSTRACT FROM AUTHOR]

In this study, an austenitic oxide dispersion strengthened (ODS) heat resistance steel with added Y 2 O 3 is fabricated by process of mechanical alloying (MA) and hot isostatic pressing (HIP). The proceeded steel exhibits an ultrafine-grained (UFG) microstructure with a bimodal grain size distribution. Numerous nano particles located inside the grains are precipitated with an average size of 8.77 nm. Most particles exhibit a gathering structure with the Y-rich phase wrapped around the Cu-rich phase. The Y-rich phase is identified as Y 2 O 3 that is semi-coherent with the austenitic matrix. The existence of the Cu-rich phase is favorable for the Y 2 O 3 precipitation. Mechanical properties tests at RT and 650 °C reveal a great improvement in strengths over the commercial Super304H austenitic heat resistance steel due to the presence of the ultrafine grains and numerous oxide nano particles. The higher tensile strength makes this austenitic ODS heat resistance steel as an excellent candidate material for high-temperature applications. [ABSTRACT FROM AUTHOR]