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Rare earth (RE) addition to steels to produce RE steels has been widely applied when aiming to improve steel properties. However, RE steels have exhibited extremely variable mechanical performances, which has become a bottleneck in the past few decades for their production, utilization and related study. Here in this work, we discovered that the property variation of RE steels stems from the presence of oxygen-based inclusions. We proposed a dual low-oxygen technology, and keeping low levels of oxygen content in steel melts and particularly in the raw RE materials, which have long been ignored, to achieve impressively stable and favourable RE effects. The fatigue life is greatly improved by only parts-per-million-level RE addition, with a 40-fold improvement for the tension-compression fatigue life and a 40% enhancement of the rolling contact fatigue life. We find that RE appears to act by lowering the carbon diffusion rate and by retarding ferrite nucleation at the austenite grain boundaries. Our study reveals that only under very low-oxygen conditions can RE perform a vital role in purifying, modifying and micro-alloying steels, to improve the performance of RE steels.

The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal. The statistical results revealed that intergranular cracks could occur within 2000 h under 650 °C when the residual stress was applied with greater than 18 KN pre-compression force. Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior, and the intergranular Nb(C, N) carbides were one of the causes to crack under short-term aging time. For long-term aging time conditions, the intergranular M23C6 carbides were more susceptible to crack than intergranular Nb(C, N) carbides. Finally, the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging, respectively.

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo hot-compression bonding joints was systematically investigated. It is found that for the bonding interface of rough surfaces, elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area. As the surface roughness increases, the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area. Meanwhile, high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation. A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces. In contrast to that of rough surfaces, bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions. While with the increase of deformation strain, small grain nuclei form along the bonding interface, which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries. Moreover, the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces. This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces, leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAl alloy. The forged alloy exhibits a high density that exceeds 97 % of the theoretical density. The ODS alloy was investigated in terms of the residual porosity, morphology and phase structure of oxide nanoparticles, impact toughness and tensile properties. It was found that refined grains were obtained during powder forging. A residual porosity less than 1.1 % has no impact on the precipitation of oxide nanoparticles. The average diameter of the oxide particles is 7.99 nm, with a number density of 2.75 × 1022 m−3. Almost all of the oxides are identified as orthorhombic YAlO3 particles. The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700 °C, and enable the ductile-to-brittle transition temperature to be close to room temperature. However, a slight decrease in strength at 1000 °C and the Charpy upper shelf energy has been suggested to be due to the residual porosity. These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.

The heterogeneous deformation behavior of austenite and ferrite in the 2205 duplex stainless steel was subjected to multiscale analysis based on the in situ synchrotron-based high energy X-ray diffraction, microscopic digital image correlation, electron backscatter diffraction, and transmission electron microscopy. It is found that the heterogeneous deformation triggers from the yielding of austenite. During this deformation stage, austenite experiences greater strain in the area near the phase boundaries because of the impeded function of the phase boundaries to dislocations. Owing to the relatively small difference in hardness between the constituent phases, the strain in austenite grains extends into the adjacent ferrite grains when entering into the ferrite yielding stage. In addition, the strain distribution of the austenite grains is more homogeneous than that of the ferrite grains because of the lower stacking fault energy of austenite, which results in a planar slip, and higher stacking fault energy in case of ferrite, causing cross slip. The interaction between austenite and ferrite becomes considerably obvious when the strain further increases after both constituent phases yielding because of the back stress and forward stress in austenite and ferrite, respectively, which are generated by the pile-up of the geometrically necessary dislocations.

The evolution of heterogeneous microstructure and mechanical properties of a 12Cr–10Ni–Mo–Ti maraging steel was investigated at different aging temperatures. As the aging temperature increases, more reversed austenite forms with the recovery of martensite matrix. When aging temperature is up to 560 °C, more lath-like reversed austenite coalesces together and leads to the formation of martensite and austenite dual-phase microstructure. Fine η-Ni3(Ti, Al) particles initially precipitate at 440 °C and grow up with increased aging temperature. Notably, it was found that η-precipitates can be encompassed in reversed austenite at 500 °C. Thermodynamic calculations further verify the austenite reversion, and nanoprecipitation can occur independently of each other though there is competition of Ni element. However, it leads to the coarsening and dissolution of η-precipitates as the aging temperature further increases. The desirable heterogeneous microstructure of the aged martensite matrix, soft reversed austenite and stable η-precipitates at 500 °C contributes to a high yield strength (~ 1 GPa, 25 °C; ~ 1.4 GPa, -196 °C) and a reasonable cryogenic impact toughness (~ 60 J, − 196 °C), which is mainly ascribed to the precipitation strengthening of η-precipitates, transformation-induced plasticity (TRIP) toughening effect from the lath-like reversed austenite and the increased misorientation toward high-angle grain boundary.

The effects of rare earth (RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite (GB) composed primarily of bainitic ferrite and martensite and/or austenite (M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700 °C for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature (DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.

Here, we report the leading manufacture of the large-scale integral weldless stainless steel forging ring (φ = 15.6 m) by the multilayer additive hot-compression bonding technology. Moreover, the detailed interface healing mechanism involving interfacial oxide evolution is elucidated, which validates the feasibility and reliability of the technique we proposed.

High-carbon chromium bearing steels with different rare earth (RE) contents were prepared to investigate the effects of RE on inclusions and impact toughness by different techniques. The results showed that RE addition could modify irregular Al2O3 and MnS into regular RE inclusions. With the increase of RE content, the reaction sequence of RE and potential inclusion forming elements should be O, S, As, P and C successively. RE inclusions containing C might precipitate in molten steel and solid state, but the precipitation temperature was significantly higher than that of carbides in high-carbon chromium bearing steel. For experimental bearing steels, the volume fraction of inclusions increased steadily with the increase of RE content, but smaller and more dispersed inclusions could be obtained by 0.018% RE content compared with bearing steel without RE, whereas the continuous increase of RE content led to an increasing trend for inclusion size and a gradual deterioration for inclusion distribution. RE addition could improve the transverse impact toughness and isotropy of bearing steel, and for modified high-carbon chromium bearing steel by RE alloying, the increase of RE content continuously increased both transverse and longitudinal impact toughness until excessive RE addition.

To understand the effects of nitrogen on precipitation and tensile behaviors at elevated temperatures, 25Cr 20Ni austenitic stainless steels with different nitrogen contents (0.065 wt %, 0.13 wt % and 0.17 wt %) were tensioned from room temperature to 1000 °C. Results show that the elongation and area reduction change differently with temperature increasing from 800 °C to 1000 °C, which are caused by different fracture modes. The necking concentrates in a small zone in the tensile samples of 0.17 wt % N steel, while the necking deformation occurs in all gauge length in specimens of other two steels. The local necking in 0.17 wt % N steel is induced by the enhanced recrystallization softening effect during tensile deformation. As the dislocation recovery are retarded by more solute nitrogen atoms and nitrides in 0.17 wt % N steel, the dislocation density difference between deformed grains and recrystallized grains turns larger, which results in a higher recrystallization rate and enhanced softening effect in further deformation. Additionally, the content of M23C6 precipitates unexpectedly increases with the increasing nitrogen after tensile test at 800 °C. This is because the increasing nitrogen promotes the formation of NbCrN in higher nitrogen steels during solution heat treatment, instead of Nb(C, N) in lower nitrogen steels. The precipitation of NbCrN releases carbon into austenitic matrix, leading to the acceleration of M23C6 precipitation during tensile test at 800 °C.

The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700 °C. M23C6, Nb(C, N), α-Cr phase and Nb-nitride phase (Z phase) were observed in the microstructures of the aged weld metals. The results showed that the α-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to 700 °C for 500 h and the element Nb accelerated the precipitation of the α-Cr phase significantly. The density of the α-Cr phase decreased with the increase of the distance away from the primary Nb(C, N). Additionally, the α-Cr phase showed a crystallographic relationship with the austenitic matrix, [ 1 1 ¯ 1 ¯ ] α-Cr // [ 1 1 ¯ 0 ] γ and ( 01 1 ¯ ) α-Cr // ( 1 ¯ 1 ¯ 1 ) γ. It was observed that the Z phase precipitated in the periphery of the Nb(C, N) and may replace the Nb(C, N) after long term exposure to high temperature. The transformation of the Nb(C, N) into Z phase suggested that the Z phase had a higher stability than the Nb(C, N) particles at 700 °C for long term aging. The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time. However, the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.

The dispersion behaviors of wave propagation in waveguides of piezoelectric helical structures are investigated. By using the tensor analysis in the helical curve coordinate, the general strain − displacement relationship of piezoelectric helix is firstly considered. This paper's formulation is based on the spectral finite element which just requires the discretization of the cross-section with high-order spectral elements. The eigenvalue matrix of the dispersion relationship between wavenumbers and frequencies is obtained. Numerical examples on PZT5A and Ba2NaNb5O15 helical waveguides of a wide range of lay angles are presented. The effects of the piezoelectric on the dispersive properties and the variation tendency of dispersion curves on helix angles are shown. The mechanism of mode separation in piezoelectric helical waveguides is further analyzed through studying waves structures of the flexural modes.

The deformation behaviors of 310s stainless steel weld metals (WMs) with different Nb contents are investigated under uniaxial tension at temperatures ranging from ambient temperature to 1000 °C. The results show that Nb effectively improves the strength of the WMs in the whole test temperatures range. The addition of Nb causes a change in the fractures of the WMs from intergranular brittle fractures to intragranular ductile fractures at elevated temperature. Hence, elemental Nb enhances the elevated temperature plasticity of the WMs. However, as the eutectic Nb(C, N) promotes the nucleation and propagation of the cracks, Nb decreases the elongation of the WMs at temperature below 700 °C. With the increase in the deformation temperature, the strength of the WMs decreases monotonously. However, the elongation of the WMs shows a nonmonotonic relationship with the temperatures. The deformation twins can occur at room temperature and improve the strength and elongation of the WMs. Meanwhile, the cracks nucleate around the eutectic Nb(C, N) at 1000 °C, leading to the minimum elongation of the Nb-bearing WMs in the whole test temperature range. The differential scanning calorimetry result suggests that the melting of the eutectic Nb(C, N) may be the dominant reason for the nucleation of the cracks and result in the reduction in the elongation of the Nb-bearing WM at 1000 °C.

The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 °C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180° to 650°C before the standard tempering at 700 °C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 °C tempering. The experimental results show that 455 °C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 °C tempering. Microstructural observations reveal that during the 455 °C pre-tempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 °C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.

The mechanical properties of solid state bonding joints have always been greatly deteriorated due to surface oxide scales. Here, we report a method to completely heal solid state bonding joints with recovered mechanical properties by hot compression bonding and post-holding treatment. Using scanning electron microscopy and transmission electron microscopy, we discovered that the recovery of mechanical properties of the bonding joint is attributed to the dissolution and evolution of the interfacial oxides. While holding the joint at 1200 °C, the interfacial oxides (MnCr2O4) gradually decomposed. With the decomposed oxygen ions diffusing toward the matrix, oxide particles precipitated around both sides of the interface, forming the particle precipitation zone (PPZ). As the holding time increased, the width of the PPZ increased and the oxide precipitates in the PPZ transformed from MnCrxAl2−xO4 to MnxAl3−xO4 and finally to γ-Al2O3, depending on the local oxygen activity. After holding for 24 h, the interfacial oxides completely decomposed and only a few nano-scale γ-Al2O3 oxide precipitates remained dispersed far away from the bonding interface, leading to the recovery of the mechanical properties of the bonding joints. This recovery mechanism may be of great importance to the design and manufacture of high-quality heavy bonding joints. Keywords: Hot compression bonding, Interfacial oxides, Mechanical properties, TEM, Interface healing

This study aims to clarify the effects of aluminum and sulfur content on fatigue property together with corresponding crack initiation and propagation behavior of bearing steel in the very high cycle fatigue (VHCF) regime. For these purposes, ultrasonic tensile-compression fatigue tests were carried out on fatigue specimens with different directions extracted from bearing steel bars containing different aluminum and sulfur content. As a result, high aluminum content in bearing steel leads to worse fatigue property by forming a large collection area of Al2O3 particles. MnS inclusions can cause fatigue failure of low sulfur bearing steel at the 45-degree angle and contribute to fatigue anisotropy. However, there are no MnS inclusions to be found at crack initiation region of failed specimens with ultralow sulfur content. For the VHCF fracture of bearing steel, fracture surface can be simply divided into four different areas according to crack propagation path except fatigue source. The stress intensity factor at the periphery of fine granular area (FGA) ΔKFGA can be regarded as the critical driving force for crack propagating through martensite laths. In addition, crack initiation and propagation are the result of mutual coordination and matching of the driving force ΔK, crack propagation speed and microstructure. Dislocations and precipitated carbides play an important role in the formation of FGA by helping form the corresponding nanoscale grain boundary.

The high-temperature tensile behaviors of two 25Cr-20Ni austenitic stainless steels with different V concentration (0 wt% V and 0.3 wt% V, respectively), have been studied at temperature between 200 ℃ and 900 ℃ . The ultimate tensile strength of both steels is strong temperature dependent, which decreases slowly first at 200–300 ℃ , keeps platform then at 300–500 ℃ and decreases rapidly afterwards from 600 ℃ to 900 ℃ . It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallization at different temperatures. At higher than 800 ℃ , the elongation of both steels increases markedly due to the dynamic recovery and dynamic recrystallization. However, because of the deteriorated effects of M23C6 precipitates at grain boundary, the elongation of both steels at 700 ℃ does not increase despite decreasing strength. Additionally, the addition of 0.3 wt% V decreases the ductility of the material in the temperature range of 800 ℃ to 900 ℃ , which is induced by the impeding effects of solute vanadium on dynamic recovery and recrystallization.

By means of the first-principles calculations, the lattice parameters, electronic structures, phonon dispersions, and mechanical properties of the rare earth metal borocarbide YB 2 C have been theoretically investigated. The dynamically stability of the layered tetragonal YB 2 C has been evidenced based on the frozen phonon method. We have found that the covalent bonding between B-2 p , C-2 p and Y-5 d orbitals are responsible for the strong interlayer interactions based on the calculated electronic structures and ELF images. The estimated hardness of P 4 2 / mbc -YB 2 C is around 23.46 GPa which is comparable with the well-known ultra-incompressible oP 6-OsB 2 . Additionally, the analysis of the ideal shear and tensile strength of YB 2 C reveals the importance of covalent bonds between Y and B/C layer which help to enhance the resistance under deformation.

Stress relaxation cracking failure is widely reported in the materials used for thermal power plants, which seriously affected the safety of thermal power units under long-term service. In our study, the influence of intergranular precipitation behavior on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal was investigated based on the pre-compressed CT technique. The 45° misorientation grain boundary has the lowest critical interfacial nucleation energy, followed by the grain boundaries away from the 45° misorientation. As a result, the coarse intergranular M23C6 carbides were more prone to precipitate at 45° misorientation grain boundary, which led to the lowest resistance to stress relaxation cracking susceptibility at 35 to 45° grain boundary. Further analysis indicated that intergranular cavities were easily initiated at the non-coherent M23C6/γ interface. Since the non-coherent interface with a low Cr concentration zone and high specific interfacial energy had a lower strength, the lattice distortion at the non-coherent interface could lead to higher strain concentration.

The evolutions of microstructure and mechanical properties during tempering at 700 °C, of normalized and oil-quenched 2.25Cr-1Mo-0.25V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated. It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure. After 0.5 h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite. However, when the tempering time is increased from 0.5 to 128 h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample. This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample.

The effects of tempering on the microstructure and mechanical properties of a low alloy Mn-Ni-Mo steel were investigated. The results indicate that the steel is characterized by granular bainite consisting of bainitic ferrite, blocky islands of martensite and/or austenite (M-A islands) and film-type retained austenite (AR) before tempering. The volume fraction of AR decreases with an increase in the tempering temperature: slowly before 280°C, rapidly at 350°C, and approaching zero at 450°C. When tempered at 280°C, the steel has optimum mechanical properties because of the internal stress relaxation and the decomposition of blocky M-A islands, especially twinned martensite islands. Additionally, the enhancement of AR stability is to some extent responsible for improving the impact toughness. When tempered at 350−450°C, tempering brittleness occurs, which results from the decomposition of the AR films into discrete flake-type carbides along the interfaces. Further increase in the tempering temperature results in high toughness and low strength of the steel, mainly due to the recovery and recrystallization of bainitic ferrite, and the precipitation of globular M3C carbides.

'Hundred Talents Project' of the Chinese Academy of Sciences; Key Research Program of Chinese Academy of Sciences [KGZD-EW-T06]; National Natural Science Foundation of China [51474202, 51174188]; Beijing Supercomputing Center of CAS (Shenyang branch); high-performance computational cluster in the Shenyang National University Science and Technology Park; Major Research Plan [91226204]

The distribution of boron and the microstructure of grain boundary (GB) precipitates (M23(C, B)6 and M2B) have been analyzed with their effects on the susceptibility of ductility-dip-cracking (DDC) and tensile properties for NiCrFe-7 weld metal, using optical microscopy (OM), secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that boron segregates at GBs in NiCrFe-7 weld metal during the welding process. The segregation of boron at GBs promotes the formation of continuous M23(C, B)6 carbide chains and M2B borides along GBs. The addition of boron aggravates GB embrittlement and causes more DDC in the weld metal, by its segregation at GBs presenting as an impurity, and promoting the formation of larger and continuous M23(C, B)6 carbides, and M2B borides along GBs. DDC in the weld metal deteriorates the ductility and tensile strength of the weld metal simultaneously.

This study aims to clarify the effect of rare earth (RE) addition on the very high cycle fatigue (VHCF) behavior of high-carbon chromium bearing steel. For this purpose, ultrasonic tension-compression fatigue tests were carried out on specimens extracted from bearing steels with and without RE addition. As a result, RE addition can prolong the fatigue life of bearing steel over 10 times and improve the fatigue limit at 109 cycles by 9.4% from 720 MPa to 788 MPa, which mainly result from the modification of RE elements to common CaO-Al2O3-MgO-SiO2-CaS inclusions, forming complex RE inclusions and leading to the decrease of inclusion size and the volume fraction of inclusions in bearing steel with RE addition. On the VHCF fracture of bearing steels with and without RE addition, both internal cracking mode and interface cracking mode exist at inclusions. Compared with bearing steel with RE addition, bearing steel without RE addition exhibits a longer crack initiation life due to stronger internal binding force of complex inclusions and interfacial binding force between complex inclusions and the matrix. However, smaller complex RE inclusions make crack propagation life of the former significantly longer than that of the latter. In addition, fine granular area (FGA) can be used to estimate the fatigue limit of different fatigue life in the VHCF regime and the fatigue limit at 109 cycles evaluated based on FGA indicates an error of around 2% with respect to the fatigue limit measured by staircase method.

The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M23C6 in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering. Keywords: Retained austenite, Intercritical tempering, Fe–13%Cr–4%Ni steel, Thermal stability, Mechanical constraint

The structure, interdendritic, and intergranular segregation, and precipitates of an Alloy 690 electro-slag remelting (ESR) ingot in commercial scale (3t) were investigated by the optical microscopy, electroprobe microanalysis, scanning electron microscopy, and transmission electron microscopy (TEM) techniques. The results indicate that the central longitudinal section of the ESR ingot comprised the ramp-up, steady-state, and hot-top regions, which could be easily distinguished from each other through the macrostructures of them. In the interdendritic area, Cr and Ti were enriched, while Ni and Fe were depleted, and the nominal segregation indexes (ζ i = C 0 i /C interdendritic i ) of Ti, Cr, and Ni were 0.40, 0.91, and 1.04, respectively, in the hot-top region where suffered the severest segregation. Nitrides, principally precipitated between dendrites, were identified as TiN by TEM and EDS. The morphology, size distribution, and volume fraction of them were determined as well. In terms of the intergranular area, Cr and C coexisted, while Ni and Fe were depleted. And the dendrite-like carbides continuously distributed on the interface between grains, which were identified as M23C6 by the selected area diffraction pattern.

EP-823 steel is one of the candidate materials for accelerator-driven systems/lead-cooled fast reactors (ADS/LFR). Its weldability was investigated by mechanical property tests and microstructure analysis on the enlarged heat-affected zones (HAZs) made by numerical and physical simulation. The finite element numerical simulation could simulate the welding thermal cycle of the characteristic regions in HAZs with extremely high accuracy. The physical simulation performed on a Gleeble simulator could enlarge the characteristic regions to easily investigate the relationship between the microstructure evolution and the mechanical properties of the HAZs. The results showed that the simulated partially normalized zone comprising tempered martensite, newly formed martensite and more tiny carbides has the highest impact energy. The fully normalized zone exhibits the highest hardness because of the quenched martensite and large carbides. The ductile property of the overheated zone is poor for the residual delta-ferrite phases and the quenched martensite.

Deformation-induced phase transformation (DIPT) of metastable austenite in Fe–13% Cr–4% Ni steel occurs during the macroscopic elastic stage and is accompanied by the yielding of metastable austenite at microscopic level. The DIPT rate is accelerated by yielding the martensite matrix, which increases the available nucleation sites for DIPT by enhancing the plastic deformation in austenite.

The effect of Cr-enriched M23C6 carbides distributed at lath boundaries on the oxidation behavior of a high-silicon 12% Cr ferritic/martensitic (HSiF/M) steel has been investigated. The M23C6 carbides were oxidized during the exposure of HSiF/M in air at 823 K. The oxidation of M23C6 carbides promotes the external oxidation of Si at lath boundaries due to the third-element effect, which induces inhomogeneous distribution of elements at the microscale in the oxidation products.

A macro–micro coupled model for epitaxial nucleation and the subsequent competitive dendrite growth was developed to study the morphological evolution of both dendrite and grain structures in molten pool of the gas tungsten arc welding (GTAW) for Fe–C alloy. The simulation of heat and mass transfer in molten pool was conducted by the three-dimensional finite element (FE) model to obtain the transient solidification conditions. The process of epitaxial nucleation and the competitive dendrite growth was simulated by a two-dimensional cellular automata (CA) model. The size and random preferential orientations of substrate grains were considered in this model. The transient thermal conditions used in the CA model were obtained from the results of FE model through the interpolation method. In addition, the effects of the substrate grain size and the welding speed on the morphologies of both dendrite and grain structures were investigated. The simulated results indicate that dendrites with the preferential orientations parallel to the direction of the highest temperature gradient are more competitive during the competitive dendrite growth, and the morphology of resulting columnar grains is determined by the competition between different dendritic arrays. Under the same welding conditions, with the increase of substrate grain size, the average width of resulting columnar grains becomes larger, and the characteristics of dendrite structure within the columnar grains do not change obviously. Without considering the new nucleation in the melt, with the increase of welding speed, the dendrite structure in weld seam becomes much finer, and the average columnar grain width within the calculation domain of the CA model does not change obviously. The trend of the simulated results of dendrite arm spacing under various welding conditions are consistent with the analytical and experimental data.

In order to guide the design of 9–12% Cr ferritic/martensitic (F/M) heat resistant steels appropriate for use in accelerator-driven system, the impact toughness and behavior of precipitation of a 12% Cr F/M heat resistant steel containing high C and Si concentrations have been investigated. Particular focus has been given to the interaction of carbon and carbonitride-forming element. Because of the presence of primary NbC and a large amount of M 23 C 6 , the impact toughness of the investigated steel was much lower than that of the commercial 9–12% Cr F/M heat resistant steels. The primary NbC crystallizes directly from liquid metal when the Nb concentration is higher than a critical value. The critical value of Nb decreases with increase in the carbon concentration. In addition, the higher content and faster precipitation kinetics of M 23 C 6 in the investigated steel during the tempering heat treatment also lowers the impact toughness.

The effects of double shielded TIG (tungsten inert gas) torch's structural parameters, including the flow rate ratio between the inner and outer layers of gas and the extended length of the electrode (abbreviated as ELE in this work), on the fusion zone profile have been investigated for 0Cr13Ni5Mo martensitic stainless steel. Results show that the double shielded TIG process yields relatively high penetration of the weld pool in a broad range of the structural parameters. ELE over 3 mm is too large and causes adverse reactions on the protection of electrode. The outer gas with relatively high flow rate or the outer layer with high oxygen content is conducive to the oxygen dissolved into the arc, which results in the oxidation of the weld pool surface and the electrode tip. The double shielded TIG welded metal was tested and presented good impact property.

The effects of M 23C6 (M = Cr, Fe) on the high-temperature performance of the NiCrFe-7 welding rods and weld metals were studied by high-temperature tensile tests and microstructure analysis. M 23C6 at the grain boundaries (GBs) has a cube-on-cube coherence with one grain in the NiCrFe-7 weld metals, and the adjacent M 23C6 has the coherence relationship with the same grain. The grain with a coherent M 23C6 has a Cr-depletion region. The number and size of M 23C6 particles can be adjusted by heat treatment and alloying. There are two temperatures [T E1: 923 K to 1083 K (650 °C to 810 °C) and T E2: 1143 K to 1203 K (870 °C to 930 °C)] at which the GBs and grains of the NiCrFe-7 welding rod have equal strength during the high-temperature tensile test. When the temperatures are between T E1 and T E2, the strength of the GBs is lower than that of the grains, and the tensile fractures are intergranular. When the temperatures are below T E1 or over T E2, the strength of the GBs is higher than that of the grains, and the tensile fractures are dimples. M 23C6 precipitates at the GBs, which deteriorates the ductility of the welding rods at temperature between T E1 and T E2. M 23C6 aggravates ductility-dip-cracking (DDC) in the weld metals. The addition of Nb and Ti can form MX (M = Ti, Nb, X = C, N), fix C in grain, decrease the initial precipitation temperature of M 23C6, and mitigate the precipitation of M 23C6, which is helpful for minimizing DDC in the weld.

A new welding method named double shielded tungsten inert gas (TIG) has been developed to improve the TIG weld penetration. The main principles to increase the weld depth have been discussed. Results show that the critical oxygen content in the weld pool is around 100 × 10−6 as the temperature coefficient of surface tension changes from negative to positive. The tracer test using pure silver shows that the direction of Marangoni convection changes as the oxygen content increases in the weld pool. The effect of arc constriction on the weld depth has been evaluated on a water-cooled copper plate, and the result indicates that the torch of double shielded can give a more powerful arc. Heavy oxide on the pool surface has undesirable impacts on the increasing of weld depth as the oxygen excessively accumulates in weld pool. It is possible to form chromium oxide in the weld process, while the iron oxide may form as the weld surface exposes to the air after the shielded gas moving away.

The mechanical stability of reversed austenite at room temperature in two 13%Cr–4%Ni low carbon martensitic stainless steel samples after different heat treatments has been investigated. The uniaxial tensile tests indicate that the reversed austenite resulting from the one-stage and two-stage intercritical tempering heat treatment have different mechanical stability, which induces distinct strength-ductility balance of the material. Experiments and crystal plasticity finite method simulations reveal that the special grain orientation relationship between the reversed austenite and martensite matrix, besides chemical composition, plays important roles on the mechanical stability of the reversed austenite. It is found that in both samples the Nishiyama–Wassermann or Kurdjumov–Sachs relationship between the reversed austenite and the martensite matrix provides an easy way for the active slip systems in austenite to penetrate the phase boundary to the adjacent martensite. This results in a high mechanical stability of the reversed austenite. In addition, the larger austenite grains in the second-stage tempering sample have higher mechanical stability in the martensitic steel due to the favored austenite stabilizing elements distribution behavior, and the favored stress distribution originating from the intrinsic strengths of the austenite and martensite matrix.

The effects of the minor elements Ti and Nb on the microstructure and mechanical properties for multi-pass weldments from the alloy ER NiCrFe-7 were studied using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe micro analysis (EPMA), as well as tensile and bend tests. The results show that grain size does not change significantly by increasing the Ti content from 0.28 wt% to 0.87 wt% in the weld metal (WM), whereas the grain boundaries become tortuous. The intragranular precipitate in the WM with Ti are AlO and Ti(C, N), whereas the intragranular precipitate in the WM with Nb are AlO and MX (M=Nb, Ti, X=C, N). As the Ti and Nb content increased in the WM, more MX was produced. Furthermore, the majority of C was fixed in the grain, not segregated to the grain boundaries; and less M 23 C 6 (M=Cr, Fe) formed at the grain boundaries. Fewer ductility-dip-cracking (DDC) was observed for WM with higher levels of Ti and Nb. The tensile strength and elongation simultaneously increased with an increase in Ti and Nb in WM. The number and length of the cracks in the bend specimens decreased upon adding Ti and Nb.

The effects of filler metal (FM) composition on inclusions and inclusion defects for ER NiCrFe-7 weldments have been investigated and analyzed. Results show that as Al, Ti content in FM increases from 0.14 wt% Al, 0.30 wt% Ti to 0.42 wt% Al, 0.92 wt% Ti, the Al, Ti reduction will increase during welding. Inclusion defects (point-like defects named by welding workers) are prone to form in the high Al, Ti content weldments. Inclusion defects with Mg, Ca, Al, and Ti as major metallic elements have been found on the surface and interior of the weldments, as Al, Ti content in FM is over 0.29 wt% Al, 0.62 wt% Ti. Less Ti content in FM cannot prevent ductility-dip-cracking (DDC) through producing enough intragranular precipitates and lessening intergranular M23C6 precipitates. Nb can be used to replace Ti to reduce the sensitivity of the DDC in the NiCrFe-7 alloy weldments.

The influences of different normalizing heat treatments on microstructure, non-metallic inclusions and impact toughness in MAG Ti-bearing weld metal of HSLA steel has been studied. It has been shown that for the Ti-bearing weld metal the impact toughness after normalizing treatment decreases significantly against prolonging holding time and increasing normalizing temperature. The Mn-depleted zone forms around the Ti-bearing phase (MnTiO3) precipitated on Mn-Si oxide. Proeutectoid ferrite preferentially nucleates at the Mn-depleted zone and the interface between austenite and proeutectoid ferrite becomes the nucleation sites for pearlite thereafter. Mn-depleted zone formation increases the pearlite nucleation sites, and makes the pearlite fine. The dissolve of Ti-bearing precipitate causes disappear of Mn-depleted zone at strong normalizing processes (longer normalizing time and higher normalizing temperature), and the number of ferrite nucleation sites decreases, then the pearlite become coarser, which causes the deterioration of impact toughness.

Using first-principles local density functional approach, we have calculated the ground-state structural phase stabilities and enthalpies of formation of thirty-six binary transition-metal refractory TM and TM3 compounds formed by Group IV elements T(T = Ti, Zr, Hf) and platinum group elements M (M = Ru, Rh, Pd, Os, Ir, Pt). We compared our results with the available experimental data and found good agreement between theory and experiment in both the trends of structural stabilities and the magnitudes of formation enthalpies. Moreover, based on our calculated results, an empirical relationship between cohesive energies (Delta E) and melting temperatures (T-m) was derived as T-m = 0.0292 Delta E/k(B) (where k(B) is the Boltzmann constant) for both TM and TM3 compounds. (C) 2012 Elsevier Ltd. All rights reserved.

The industrial manufacture processes of three kinds of roll core used ductile irons have been investigated via systematical experiments. Effects of the ratio of C/Si, pig iron, nodularizer and alloying method on the microstructure and mechanical properties of the heavy section ductile iron have been analyzed. It has been found that when treated with RE-Mg plus Sb, high quality nodular castings can be produced even if much anti spheroidizing alloy elements are included in the pig iron. The alloy element Sb played an important role in the control of graphite morphology.

Though extensively studied, hardness, defined as the resistance of a material to deformation, still remains a challenging issue for a formal theoretical description due to its inherent mechanical complexity. The widely applied Teter's empirical correlation between hardness and shear modulus has been considered to be not always valid for a large variety of materials. The main reason is that shear modulus only responses to elastic deformation whereas the hardness links both elastic and permanent plastic properties. We found that the intrinsic correlation between hardness and elasticity of materials correctly predicts Vickers hardness for a wide variety of crystalline materials as well as bulk metallic glasses (BMGs). Our results suggest that, if a material is intrinsically brittle (such as BMGs that fail in the elastic regime), its Vickers hardness linearly correlates with the shear modulus (H(v) = 0.151G). This correlation also provides a robust theoretical evidence on the famous empirical correlation observed by Teter in 1998. On the other hand, our results demonstrate that the hardness of polycrystalline materials can be correlated with the product of the squared Pugh's modulus ratio and the shear modulus (H(v) = 2(k(2)G)(0.585) - 3 where k =G/B is Pugh's modulus ratio). Our work combines those aspects that were previously argued strongly, and, most importantly, is capable to correctly predict the hardness of all hard compounds known included in several pervious models. (C) 2011 Elsevier Ltd. All rights reserved.

The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 degrees C. It is found that at the temperatures below 680 degrees C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined I:IN the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 degrees C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

For oxygen evolution electrocatalysis of water splitting, unique urchin-shaped Co3O4 spheres were successfully grown on nickel foam by hydrothermal synthesis of Co(OH)F precursor and subsequent annealing method. The formation process was investigated by the evolution of phase structure and morphology with hydrothermal reaction time. And it can be explained by a 'disks-flowers-urchins' mechanism. Moreover, the Co3O4 urchins/NF exhibits considerable catalytic properties. It shows a low overpotential of 308 mV at a current density of 20 mA cm-2 in alkaline solution. In the meantime, such material has a small Tafel slope of 82.1 mV dec-1, large electrochemical active surface area and good long-term stability. The obvious promotion of oxygen evolution reaction performance can be attributed to the special morphology and the direct attachment to the substrate, which improve the exposed active sites, lower the internal resistance and accelerate the charge transport. Thus, the Co3O4 urchins/NF not only has a great potential promising behavior, but also provides the basis for subsequent performance improvement.

The effects of oxygen on the microstructure of Ti-47Al-0.7B (at. pct) alloy for as-cast automotive valves were investigated. Six alloys with oxygen content from 0.4 to 1.4 at. pct were prepared by induction melting and centrifugal casting in CaO crucible under protective atmosphere. The microstructures were observed by optical microscopy (OM) and scanning electron microscope (SEM). The results show that the increase of oxygen content led to grain refinement and enhanced the microhardness as well as the alpha(2) volume fraction in the TiAl-based alloys.

The mechanical properties and microstructure were evaluated and analyzed by optical microscopy (OM) and transmission electron microscopy (TEM) for micro-alloy carbon steel weld metal with and without Nb addition, respectively, under different heat treatment processes including stress relief annealing, normalizing, and no treatment after welding. The strength and elongation of the weld metal without treatment after welding were improved with the addition of Nb element, and the impact toughness was not affected obviously with the Nb addition. After stress relief annealing, the strength decreased for the Nb-free weld metal, while the elongation and impact toughness increased. However, for the Nb-bearing weld metal, stress relief annealing improved the strength of the weld metal significantly, and deteriorated the elongation and impact toughness. In the case of normalizing treatment to the weld metal, it was shown that with the increase of the holding time at the normalizing temperature of 920 °C, for both the weld metals with and without Nb addition, the microstructure of the columnar grain zone (CGZ) was transformed from one of columnar grain into one of equiaxed grain. The grain size of the equiaxed grain zone (EGZ) increased initially, then remained almost unchanged with the prolonging of the holding time. The mechanical properties of the weld metal with and without Nb addition showed no obvious change with the increasing holding time. With the increase of the normalizing temperature, the strength of the Nb-bearing weld metal increased, while the elongation and impact toughness decreased significantly. OM and TEM analysis found that the fine NbC particles were precipitated at the normalizing temperature of 920 °C, which refined the grains of the weld metal and increased the impact toughness. With the increase of the normalizing temperature, the content of widmanstatten ferrite (WF) in the Nb-bearing weld metal increased, whereas the quantity of the NbC particles decreased, which improved the strength and lowered the impact toughness.