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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 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.

This study presents the fabrication of 14Cr Fe–Cr–Al oxide dispersion strengthened (ODS) alloy by a direct oxidation process. In order to explain how oxide nanoparticles are formed in the consolidation process, the powders after oxidation are subjected to vacuum thermal treatment at high temperatures. Differential scanning calorimeter, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy techniques are used to detect the generation, evolution of oxides on both the surface and interior of the powder, as well as the type of oxide nanoparticles in the fabricated ODS alloy. It is found that an iron oxide layer is formed on the surface of the powder during low temperature oxidation. And the iron oxide layer would be decomposed after thermal treatment at high temperature. In the consolidation process, the oxygen required by the reaction of alumina and yttrium oxide to produce nanoscale Y–Al–O particles mainly derives from the decomposition of iron oxide layer at elevated temperature and the inward diffusion of oxygen. Using the direct oxidation process, YAlO3 nanoparticles are dispersed in the grains and at the grain boundaries of Fe–Cr–Al ODS alloy.

In this study, tensile behavior of Nb-containing 25Cr–20Ni austenitic stainless steels composed of coarse or fine grains has been investigated at temperatures ranging from room temperature to 900 °C. Results show that the tensile strength of fine-grained specimens decreases faster than that of coarse-grained specimens, as the test temperature increases from 600 °C to 800 °C. The rapidly decreasing tensile strength is attributed to the enhanced dynamic recovery and recrystallization, because additional slip systems are activated, and cross-slipping is accelerated during deformation in fine-grained specimens. After tensile testing at 700–900 °C, sigma phases are formed concurrently with dynamic recrystallization in fine-grained specimens. The precipitation of sigma phases is induced by simultaneous recrystallization as the diffusion of alloying elements is accelerated during the recrystallization process. Additionally, the minimum ductility is observed in coarse-grained specimens at 800 °C, which is caused by the formation of M23C6 precipitates at the grain boundaries.

Heavy forgings are the core components of major equipment and even play an indispensable role in national security and the national economy. Traditionally, heavy forgings are usually manufactured with ingots of more than 100 tons. However, there are serious macroscopic segregation, shrinkage porosity and other defects in heavy forgings due to size effect of metal solidification process, which seriously affect the quality of heavy forgings and have become a worldwide problem. Avoiding the traditional concept in which a heavy forging must be created using a large steel ingot, Institute of Metal Research, Chinese Academy of Sciences first put forward a novel technology to manufacture heavy high-quality forgings by additive forging (AF). Small-sized slabs with excellent quality are used as the base elements. After surface cleaning, stacking, assembling, vacuum packaging, and deformation process characterized by pressure-forging and multi-directional forging at high temperature, the homogenized heavy forgings can be obtained with completely healed interface, which realizes the new manufacturing technology of “by making greatly small”. Through in-depth research on the microstructural evolution and bonding mechanism of the interface, we found the self-decomposition phenomenon of interface oxides. 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 MnCr x Al2– x O4 to Mn x Al3– x O4 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 should be of great importance to the engineering application of Additive Forging. Besides, we illustrated the dynamic recrystallization (DRX) mechnism of the bonding interface. The bonding of joints is related with interfacial grain boundary (IGB) bulging process, which is considered as a nucleation process of DRXed grain under different deformation environments. During recrystallization process, the bonded interface moved due to strain-induced boundary migration (SIBM) process. Stored energy difference (caused by accumulation of dislocations or difference of subgrain size at the bonding interface) was the dominant factor for SIBM during DRX. This technology has been applied to hydroelectric alloy steel spindles (110 tons in weight) and nuclear power stainless steel giant rings (15.6 m in diameter), which plays an important role in promoting the rapid development of high-end equipment in China and ensuring the independence and control of core materials for major equipment.

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.

The effects of different double ageing processes on the mechanical properties and microstructural evolution in the 1460 alloy were investigated. The corresponding microstructure and mechanical properties of the heat-treated specimens were studied by means of transmission electron microscopy, differential scanning calorimetry, hardness testing and tensile testing. The results show that the elongation of two-step aged (130 °C/84 h + 160 °C/24 h) samples can be increased to 8 % with a minor decrease in tensile strength (450 MPa). Throughout the two-step ageing process, the precipitation behaviour of the alloy exhibited the following characteristics: the δ′ phase formed at lower temperature dissolved into the matrix instead of coarsening and becoming over-aged at the beginning of the second ageing step; finely distributed θ′(Al2Cu) and δ′(Al3Li) at a steady state were obtained, resulting in a significant improvement in tensile strength; and the peak ductility occurred with the precipitation of T1(Al2CuLi) and consumption of δ′(Al3Li), as small δ′ particles caused a co-planar slip, resulting in lower ductility.

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 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 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.

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.

A novel heat treatment route consisting of a low-temperature solution followed by an over-aging treatment at 500 °C is proposed to develop a high-strength, high-cryogenic-toughness maraging steel by forming an ultrafine-grained martensite (α′) and austenite (γ) dual-phase microstructure. Compared to the same maraging steel subjected to the conventional heat treatment with a mostly martensitic microstructure, the present dual-phase microstructure offers a remarkable increase by 12 times of cryogenic impact energy (~140 J at 77 K), while the yield strength is not reduced obviously. A large amount of ultrafine-grained austenite (about 50% volume fraction) are formed in the present steel due to the reversed transformation of martensite to austenite during the over-aging process at 500 °C. It is surprised that the present steel with such a high fraction of austenite still possesses a high yield strength comparable to the conventional maraging steel with a mostly martensitic microstructure. Intensive nanoprecipitates are found not only in martensite but also in austenite phases, indicating both phases have high strength. This is confirmed by nanoindentation test, showing similar hardness values in both martensite and austenite phases. Such intensive nanoprecipitates in both phases ensure the high yield strength of the present steel. The excellent cryogenic toughness of the present steel mainly origins from: (i) the pronounced amount of reversed austenite that are intrinsically tough due to their face-centered cubic (fcc) structure; (ii) transformation-induced plasticity (TRIP) toughening as some austenite grains transforming to martensite during impact test; (iii) the ultrafine-grained structure of both martensite and austenite phases. The present heat treatment route offers a potential solution for processing large engineering components for cryogenic application that require a long heat treatment duration to achieving uniform mechanical properties in the components.

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.

The competitive growth of microstructures in the entire weld pool for both the Al–Cu alloy and the pure aluminum was simulated by the cellular automata method to comparatively investigate the micro-mechanisms for the morphological evolution of the axial structure and the curved columnar grain in the weld. The competitive mechanism of grains during the epitaxial growth and the morphological evolution of the grain structure in the weld with various welding speeds were studied. The results indicate that both the thermal conditions and the solidification characteristic of the weld metal exert an important influence on the grain competition and the resulting structure in the weld. For the Al–Cu alloy, the dendritic structure with a large S/L interface curvature appears during the epitaxial growth. The preferential orientation affects the competition result obviously. Owing to the anisotropic growth kinetics, the straight axial structure forms at low welding speeds. With the increase of the welding speed, the width of the axial region decreases and eventually disappears. For the pure aluminum, the S/L interface during the epitaxial growth is planar, and the grain competition is controlled by the thermal conditions completely. The columnar grains curve gradually to follow the highest temperature gradient direction at low welding speeds and become straight at high welding speeds.

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.

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.

Effects of the aging temperature on the hardening response, the tensile properties and the precipitate microstructure evolution of 1460 alloy were studied in this work. It was found that Al3 (Sc, Zr) and δ′ (Al3Li) phases were precipitated from the matrix at the very early aging stage, while the precipitation of T1 (Al2CuLi) and θ′ (Al2Cu) was much slower than that of the δ′ phase. When aging at higher temperature (160 and 190 °C), the δ′, T1 and θ′ phases tended to form simultaneously and grow up very quickly. Conversely, the δ′ and θ″ (Al2Cu) phases were precipitated separately and more dispersive at lower aging temperature (130 °C). Taken together, the alloy aged at 160 °C exhibited improved mechanical properties owing to the uniform dispersion of the fine T1 precipitates.

A double loop electrochemical potentiokinetic reactivation (DL-EPR) test was conducted to investigate the relationship between the evolution of delta-ferrite and the intergranular corrosion (IGC) of Ti-modified Super304H, which was aged at 650 °C for 4–500 h. Scanning electron microscopy and transmission electron microscopy were adopted to analyze the evolution of delta-ferrite. The results indicated that a higher fraction of delta-ferrite with poor stability increased the IGC sensitisation of Ti-modified Super304H. Moreover, the self-healing of the sensitisation of Ti-modified Super304H occurred after 48 h due to the diffusion of chromium atoms mainly from the adjacent primary austenite rather than the delta-ferrite.

The effects of boron on the microstructure and the pitting corrosion behavior, especially the metastable pitting, were investigated in Super304H austenitic stainless steels with 0-130 ppm boron solution treated at 1100 degrees C. The microstructures were analyzed using scanning electron microscopy, transmission electron microscopy with energy dispersive spectroscopy and secondary ion Mass spectroscopy. Meanwhile, the pitting corrosion behavior was evaluated by potentiodynamic and potentiostatic polarization tests in 3.5% NaCl solution at 25 degrees C. Microstructural characterizations showed that boron addition favored the precipitation of Cr-rich boride (M2B), which resulted in an asymmetric Cr-depleted zone around M2B and reduced the chromium content in the austenite matrix. The Cr-depleted zones could act as extra sites for pit initiation, along with the interfaces between the austenite matrix and non-metallic inclusions. The potentiodynamic polarization test results indicated that boron addition decreased the pitting potential. The potentiostatic test results revealed that boron addition promoted the initiation frequency and growth rate of metastable pits, and increased the probability of stable pitting. (C) 2015 The Electrochemical Society.

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.

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 effect of minor addition of Sc on microstructure, age hardening behavior, tensile properties and fracture morphology of 1460 alloy have been studied. It is found that Sc content increase from 0.11 wt% to 0.22 wt% is favorable for grain refinement in as-cast alloy but results in a coarsening of Cu-rich particles. The alloy with 0.11 wt% Sc exhibits enhanced mechanical properties and age hardening effect. Transmission electron microscopy (TEM) investigations on the alloy with 0.11 wt% Sc have suggested that a large amount of Al 3 (Sc, Zr) particles precipitated at the earlier aging may inhibit recrystallization effectively.

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 phase transformation from martensite to austenite during intercritical tempering with high heating rate in a low carbon martensitic stainless steel Fe-13%Cr-4%Ni-Mo has been investigated to clarify the microstructure evolution in some regions of the weld joint heat affected zone (HAZ). The experimental results indicate that the start and finish temperatures of the martensite to austenite transformation keep constant when the heating rate is higher than 10 K/s, and the transformation is much faster than nickel diffusion. The mechanism of the martensite to austenite transformation changes from diffusion to diffusionless during the intercritical tempering when the heating rate is higher than 10 K/s. The diffusionless transformation and higher A s temperature render it difficult for any austenite to remain at room temperature during the intercritical tempering with high heating rate that occurs in the HAZ. Adding a proper intercritical tempering with low heating rate can induce some reversed austenite in the rapid heated sample.

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 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.

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.

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.

The thermal stability of the reversed austenite in a low carbon Fe-13%Cr-4%Ni-Mo (in wt pct) martensitic stainless steel was investigated by X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometer. Different amount of the reversed austenite was obtained by controlling tempered temperature. It was found that the amount of the reversed austenite increased after keeping the specimens at liquid nitrogen temperature for 72 h. The mechanism of the anomalous phase transformation from martensite to austenite was discussed.

The microstructure and mechanical properties of a Nb bearing weld metal under different normalizing processes had been evaluated and analyzed. The results showed that there was a great difference between the microstructure and mechanical properties of the as-welded and the as-normalized weld metals, and that the normalizing process played an important role in determining the microstructure and mechanical properties of a Nb bearing weld metal. The microstructure of the weld metal was converted from a columnar grain structure in the as-welded state into equiaxed grain, and the degenerated pearlite and NbC precipitates were observed in the weld metal after a 920 degrees C normalizing treatment. Corresponding to the microstructure, the normalized weld metal had lower yield and tensile strengths, higher elongation and higher -20 degrees C impact energy than the as-welded weld metal. With the prolonging of the holding time at the normalizing temperature of 920 degrees C, the grain size in the weld metal remained almost constant, while the size of NbC precipitate increased. The mechanical properties of the weld metal showed no obvious change with the increasing holding time. With an increase of the normalizing temperature, the quantity of the NbC particles decreased and the proportion of Widmanstatten ferrite microstructure in the weld metal increased, which caused the yield and tensile strengths to increase obviously, while the elongation and impact toughness decreased significantly. When normalizing at 1 200 degrees C, the NbC particles in the weld metal disappeared due to dissolution and the twin subplates were formed in the Widmanstatten ferrite.

Single dendrite and multi-dendrite growth for Al-2 mol pct Si alloy during isothermal solidification are simulated by phase field method. In the case of single equiaxed dendrite growth, the secondary and the necking phenomenon can be observed. For multi-dendrite growth, there exists the competitive growth among the dendrites during solidification. As solidification proceeds, growing and coarsening of the primary arms occurs, together with the branching and coarsening of the secondary arms. When the diffusion fields of dendrite tips come into contact with those of the branches growing from the neighboring dendrites, the dendrites stop growing and being to ripen and thicken.

A modified cellular automaton modeling has been performed to investigate the dynamic strain-induced transformation (DSIT) from austenite (c) to ferrite (a) in a low carbon steel. In this modeling, the c–a transformation, ferrite dynamic recrystallization and the hot deformation were simulated simultaneously. The simulation provides an insight into the mechanism of the ferrite refinement during the DSIT. It is found that the refinement of ferrite grains derived from DSIT was the result of the increasing ferrite nuclei density by the ‘‘unsaturated” nucleation, the limited ferrite growth and the ferrite dynamic recrystallization. The effects of prior austenite grain size and strain rate on the microstructural evolution of the DSIT ferrite and the characteristics of the resultant microstructure are also discussed.

This study presents a two-dimensional cellular automaton (CA) simulation of austenite static recrystallization (SRX) following hot deformation by coupling with a crystal plasticity finite element (CPFEM) modeling. The initial deformed microstructure and the spatial distribution of the stored deformation energy quantified by CPFEM were incorporated in the CA model as the input for the simulation of the subsequent SRX nucleation and grain growth. This modeling provides an insight into the influence of heterogeneous deformation on the subsequent SRX at the mesoscale. The influence of pre-deformation on the SRX microstructure evolution and the transformation kinetics are discussed for three deformation levels of e = 0.3, 0.5, 0.8.

Temperature dependence of the tensile behavior of a Fe-based gamma' strengthened austenitic alloy was investigated from 25 to 900 degrees C. The stress-stain curves and the microstructures after deformation were studied. The results showed an abnormal tensile behavior with increasing temperature. Serrated stress-strain curves were observed in the samples deformed at temperatures from 300 to 700 degrees C. Microstructure observation on the deformed samples shows that twins formed during deformation at 300 to 700 degrees C and became profuse at 700-800 degrees C. Those twins had a predominant effect on the deformation mechanisms at high temperatures. (C) 2007 Elsevier B.V. All rights reserved.

The precipitation mechanism of cellular η phase at grain boundaries and its effect on the high temperature tensile properties in an Fe–Ni–Cr alloy were investigated by using TEM, SEM, EBSD and tensile testing. It was revealed that η phase nucleates at high energy grain boundaries and has a coherent orientation relationship with the austenite matrix: {0 0 1}η//{1 1 1}γ, η// γ. The cellular austenite in the vicinity of the η phase has the same orientation as the grain from which the η phase growth starts. The formation of cellular austenite is suggested to proceed by grain boundary migrating with precipitated η phase into the neighboring grain which is incoherent with the η phase. The precipitation of cellular η phase at grain boundary improves the high temperature ductility of the alloy dramatically.

Two-dimensional cellular automaton modeling has been performed to investigate the mechanism of ferrite refinement during the dynamic strain-induced transformation (DSIT) from austenite (gamma) to ferrite (alpha) in a low-carbon steel. The simulated results indicate that the refinement of ferrite grains derived from the DSIT could be interpreted in context of "un-saturated" nucleation and limited growth. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The mesoscale deterministic cellular automaton (CA) method and probabilistic Q-state Potts-based Monte Carlo (MC) model have been adopted to investigate independently the individual growth behavior of ferrite grain during the austenite (gamma)-ferrite (alpha) transformation. In these models, the gamma-alpha phase transformation and ferrite grain coarsening induced by alpha/alpha grain boundary migration could be simulated simultaneously. The simulations demonstrated that both the hard impingement (ferrite grain coarsening) and the soft impingement (overlapping carbon concentration field) have a great influence on the individual ferrite growth behavior. Generally, ferrite grains displayed six modes of growth behavior: parabolic growth, delayed nucleation and growth, temporary shrinkage, partial shrinkage, complete shrinkage and accelerated growth in the transformation. Some modes have been observed before by the synchrotron X-ray diffraction experiment. The mesoscopic simulation provides an alternative tool for investigating both the individual grain growth behavior and the overall transformation behavior simultaneously during transformation. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-temperature deformation-induced ferrite grains supersaturated with carbon offer superior properties such as higher elastic modulus and microhardness compared with proeutectoid ferrite because of they contain nanometer martensite flakes. After annealing at 700 °C, fine carbides precipitate due to the decomposition of these nanometer martensite flakes.

The shape-memory characteristics in the Ni41.3Ti38.7Nb20 alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni41.3Ti38.7Nb20 alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.

A metallurgical through-process model is presented which describes the microstructural evolution and predicts the final mechanical properties of low carbon steel during hot strip rolling. Process models concern the thermal and deformation phenomena, which take into account the strain, strain rate and temperature distribution along the length of the strip. And the metallurgical models cover five modules, which are (i) austenitization of cast slab in reheating furnace, (ii) recrystallization of austenite in hot rolling, (iii) phase transformation of austenite-ferrite in laminar cooling on the run-out-table, (iv) grain growth after coiling, and (v) final structure-mechanical properties of products. Temperature is the main parameter and has dominant influence on the microstrutural evolution and the mechanical properties. The related temperature variation in hot strip rolling concerns air cooling, scaling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. These complex factors are incorporated into the thermal models to simulate the temperature distribution along the length of the strip from the reheating furnace exit to the down-coiler. A self-learning algorithm is employed to improve the calculation accuracy and the computational temperatures are compared with the measured ones at typical locations. In the structure-property relationships, two key process parameters (e.g., finishing exit temperature (FT7) and coiling temperature (CT)) are introduced in the model to consider the influence of morphology of microstructure on mechanical properties.

Hot strip rolling process includes four main stages, which are reheating process, roughing and finishing process, laminar-cooling process, and coiling process respectively. Temperature is the most sensitive parameter and has direct effect on the microstructural evolution and further the mechanical properties, and the accurate control of temperature guarantees the quality of products and homogeneity of properties along the strip length. However, for the conventional hot strip rolling process, thermal history along the strip length is very complex, the related temperature variation concerns air cooling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. Based on the actual hot strip mill, the thermal models are established in this paper to simulate the temperature distribution along the whole strip length from the reheating furnace exit to the down coiler. Different interface heat transmission coefficients are selected for the scale breaking and spray water-cooling process, and a self-learning algorithm is thus employed to improve the calculation accuracy. This model is characterized as simple and fast, and convenient for on-line/off-line prediction of temperature. Finally the simulated results are verified by the on-line temperature detection at typical points such as roughing exit (RT2), finishing exit (FT7) and coiling position (CT).

A Q-state Potts model is adopted to simulate the austenite-ferrite diffusive transformation during continuous cooling in a low carbon steel on a mesoscale with Monte Carlo method. The effects of the interfacial energy and the carbon diffusion on the phase transformation are both taken into account. The consideration of the interfacial energy effect makes the simulated ferrite grains polygonal. To simulate the carbon diffusion during the process of phase transformation, a random jumping based mesocopic diffusion model is developed. In this paper, the evolutions of the microstructure and the carbon concentration field are simulated. The simulation results of the effects of the cooling rate on the volume fraction of the ferrite and the ferrite grain size agree well with the experimental results. The simulation results show that increasing the cooling rate is an effective way to refine the microstructure. This Monte Carlo simulation technique provides a novel way for investigating the diffusive transformation on a mesoscale. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The oxidation behavior of thin layers of two Cu-Fe alloys containing 25 and 50 wt.% Fe, respectively, prepared by magnetron sputtering deposition on cast alloys of the same composition (Cu-Fe coatings) and presenting grain sizes in the nanometer range, was studied at 600-800degreesC in air to examine the influence of the reduction in the grain size on the selective oxidation of the most reactive component in two-phase binary systems. A continuous Fe3O4 layer formed beneath an external region of copper oxide on the Cu-25Fe coating, whereas an external iron oxide scale mostly composed of Fe3O4 free from copper oxides formed on the Fe-50Cu coating. In both cases, an iron-depleted region was present in a subsurface alloy layer. These results differ remarkably from the oxidation behavior of cast Cu-Fe alloys of similar composition but with a large grain size, which formed mixed external scales of iron and copper oxides in air and simultaneous internal and external oxidation of Fe under both high and low oxygen pressures. Therefore, a grain size reduction can effectively promote the selective external oxidation of the more reactive component in binary two-phase alloys due to an increase in the mutual solubility of the two components associated with the method of alloy preparation as well as to the presence of a large density of grain boundaries in the coatings which may act as short-circuit diffusion paths, allowing a faster outward diffusion of iron during oxidation. (C) 2002 Elsevier Science Ltd. All rights reserved.