Microstructure and mechanical properties of high-strength low alloy steel by wire and arc additive manufacturing

A high-building multi-directional pipe joint(HBMDPJ) was fabricated by wire and arc additive manufacturing using high-strength low-alloy(HSLA) steel. The microstructure characteristics and transformation were observed and analyzed. The results show that the forming part includes four regions. The solidification zone solidifies as typical columnar crystals from a molten pool. The complete austenitizing zone forms from the solidification zone heated to a temperature greater than 1100°C, and the typical columnar crystals in this zone are difficult to observe. The partial austenitizing zone forms from the completely austenite zone heated between Ac1(austenite transition temperature) and1100°C, which is mainly equiaxed grains. After several thermal cycles, the partial austenitizing zone transforms to the tempering zone, which consistes of fully equiaxed grains. From the solidification zone to the tempering zone, the average grain size decreases from 75 to20 μm. The mechanical properties of HBMDPJ satisfies the requirement for the intended application.     

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.     

Effect of heat treatment on the microstructure and mechanical properties of structural steel–mild steel composite plates fabricated by explosion welding

Two dissimilar steel plates, structural steel and mild steel, were joined by explosion welding to form a composite. The composite was then heat-treated by quenching at 840°C for 30 min followed by tempering at 200°C for 3 h. The microstructure was investigated under an optical microscope and a scanning electron microscope. The mechanical properties were measured using Vickers microhardness and Charpy impact tests. The results show a deformed interface with typical wave features at the welding zone, but no defects were observed. Moreover,the ferrite in the parent plate in the weld zone was elongated due to the strong plastic deformation from the explosion. After heat treatment, the hardness of the flyer plate(structural steel) was over HV0.2 800, while that of the parent plate(mild steel) was HV0.2 200. The increase in hardness was due to the presence of martensite. Moreover, the average impact energy was increased from 18.5 to 44.0 J following heat treatment;this is because of the formation of recrystallized grains at the weld interface, which is due to the dynamic recovery and local recrystallization,and the strong elemental diffusion that occurred between the two plates.     

Improving the solidification structure of commercially pure aluminium with electropulse acting on liquid metal

The effect of electropulse on the solidification structure of commercially pure aluminium was studied. The orthogonal array L9 was used to determine the effect of three process parameters of electropulse modification (EPM), which were pulse current intensity, pulse frequency, and treating time. For each factor, three levels were chosen to cover the experimental region. According to the experimental results, the solidification structure of commercially pure aluminium was modified from large grains with columnar crystals to finer grains with equiaxed crystals, by allowing the electropulse to act on liquid aluminium. However, the solidification structures could be refined differently at different EPM parameters. Certain EPM parameters should be selected to get the optimum solidification structure. Among the three parameters, pulse frequency was the most important factor influencing the solidification structure, the secondary factor was current intensity, and treating time was the third one. The optimum parameters were the pulse frequency of 5 Hz, the current intensity of 68 A, and the treating time of 10 s.     

Effect of arsenic content and quenching temperature on solidification microstructure and arsenic distribution in iron–arsenic alloys

The solidification microstructure, grain boundary segregation of soluble arsenic, and characteristics of arsenic-rich phases were systematically investigated in Fe–As alloys with different arsenic contents and quenching temperatures. The results show that the solidification microstructures of Fe–0.5wt%As alloys consist of irregular ferrite, while the solidification microstructures of Fe–4wt%As and Fe–10wt%As alloys present the typical dendritic morphology, which becomes finer with increasing arsenic content and quenching temperature. In Fe–0.5wt%As alloys quenched from 1600 and 1200°C, the grain boundary segregation of arsenic is detected by transmission electron microscopy. In Fe–4wt%As and Fe–10wt%As alloys quenched from 1600 and 1420°C, a fully divorced eutectic morphology is observed, and the eutectic Fe2 As phase distributes discontinuously in the interdendritic regions. In contrast, the eutectic morphology of Fe–10wt%As alloy quenched from 1200°C is fibrous and forms a continuous network structure. Furthermore, the area fraction of the eutectic Fe2 As phase in Fe–4wt%As and Fe–10wt%As alloys increases with increasing arsenic content and decreasing quenching temperature.     

Microstructure,mechanical properties,and wear resistance of VC_p-reinforced Fe-matrix composites treated by Q&P process

A quenching and partitioning(QP) process was applied to vanadium carbide particle(VCp)-reinforced Fe-matrix composites(VC-Fe-MCs) to obtain a multiphase microstructure comprising VC, V8 C7, M3 C, α-Fe, and γ-Fe. The effects of the austenitizing temperature and the quenching temperature on the microstructure, mechanical properties, and wear resistance of the VC-Fe-MCs were studied. The results show that the size of the carbide became coarse and that the shape of some particles began to transform from diffused graininess into a chrysanthemum-shaped structure with increasing austenitizing temperature. The microhardness decreased with increasing austenitizing temperature but substantially increased after wear testing compared with the microhardness before wear testing; the microhardness values improved by 20.0% ± 2.5%. Retained austenite enhanced the impact toughness and promoted the transformation-induced plasticity(TRIP) effect to improve wear resistance under certain load conditions.     

Precipitation and stability of reversed austenite in 9Ni steel

A new method was used to analyze the factors affecting the precipitation of reversed austenite during tempering. The samples were kept at various tempering temperatures for 10 min and their length changes were recorded. Then, the precipitation of reversed austenite which led to the length reduction was shown by thermal expansion curves. The results show that the effects of process parameters on the precipitation of reversed austenite can be determined more accurately by this method than by X-ray diffraction. When the quenching and tempering process is adopted, both the lower quenching temperature and higher tempering temperature can promote the precipitation of reversed austenite during tempering; and when the quenching, lamellarizing, and tempering process is used, intercritical quenching is considered beneficial to the precipitation of reversed austenite in the subsequent tempering because of Ni segregation during holding at the intercritical temperature.     

Numerical Modeling of Dendrite Growth in AI Alloys

Dendritic grains are the most often observed microstructure in metals and alloys. In the past decade, more and more attention has been paid to the modeling and simulation of dendritic microstructures.This paper describes a modified diffusion-limited aggregation model to simulate the complex shape of the dendrite grains during metal solidification. The fractal model was used to simulate equiaxed dendrite growth.The fractal dimensions of simulated AI alloy structures range from 1,63-1.88 which compares well with the experimentally-measured fractal dimension of 1.85; therefore, the model accurately predicts not only the dendritic structure morphology, but also the fractal dimension of the dendrite structure formed during solidification,     

Austenite grain growth behavior of Q1030 high strength welded steel

The austenite grain growth behavior of Q1030 steel was studied under different heating conditions. The austenite grain size increases with the heating temperature and holding time increasing. Austenite grains grow in an exponential manner with rising heating temperature and in a parabolic manner with prolonging holding time. A mathematical model for describing the austenite grain growth behavior of Q1030 steel was obtained on the basis of experimental results using regression analysis. When the heating temperatures lie between 1000 and 1100℃ at a certain holding time, abnormal grain growth appears, which causes mixed grains in Q1030 steel.     

Numerical Modeling of Dendrite Growth in Al Alloys

Dendritic grains are the most often observed microstructure in metals and alloys. In the past decade, more and more attention has been paid to the modeling and simulation of dendritic microstructures. This paper describes a modified diffusion-limited aggregation model to simulate the complex shape of the dendrite grains during metal solidification. The fractal model was used to simulate equiaxed dendrite growth. The fractal dimensions of simulated Al alloy structures range from 1.63-1.88 …     

Hot deformation characteristics of as-cast high-Cr ultra-super-critical rotor steel with columnar grains

Isothermal hot compression tests of as-cast high-Cr ultra-super-critical (USC) rotor steel with columnar grains perpendicular to the compression direction were carried out in the temperature range from 950 to 1250°C at strain rates ranging from 0.001 to 1 s-1. The softening mechanism was dynamic recovery (DRV) at 950°C and the strain rate of 1 s-1, whereas it was dynamic recrystallization (DRX) under the other conditions. A modified constitutive equation based on the Arrhenius model with strain compensation reasonably predicted the flow stress under various deformation conditions, and the activation energy was calculated to be 643.92 kJ·mol-1. The critical stresses of dynamic recrystallization under different conditions were determined from the work-hardening rate (θ)–flow stress (σ) and -?θ/?σ–σ curves. The optimum processing parameters via analysis of the processing map and the softening mechanism were determined to be a deformation temperature range from 1100 to 1200°C and a strain-rate range from 0.001 to 0.08 s-1, with a power dissipation efficiency η greater than 31%.     

Simulation of macrosegregation in a 36-t steel ingot using a multiphase model

Macrosegregation is the major defect in large steel ingots caused by solute partitioning and melt convection during casting. In this study,a three-phase(liquid, columnar dendrites, and equiaxed grains) model is proposed to simulate macrosegregation in a 36-t steel ingot. A supplementary set of conservation equations are employed in the model such that two types of equiaxed grains, either settling or adhering to the solid shell, are well simulated. The predicted concentration agrees quantitatively with the experimental value. A negative segregation cone was located at the bottom owing to the grain settlement and solute-enriched melt leaving from the mushy zone. The interdendritic liquid flow was carefully analyzed, and the formation of A-type segregations in the mid-height of the ingot is discussed. Negative segregation was observed near the riser neck due to the specific relationship between flow direction and temperature gradient. Additionally, the as-cast macrostructure of the ingot is presented, including the grain size distribution and columnar–equiaxed transition.     

Crystallization characteristics of iron-rich glass ceramics prepared from nickel slag and blast furnace slag

The crystallization process of iron-rich glass-ceramics prepared from the mixture of nickel slag (NS) and blast furnace slag (BFS) with a small amount of quartz sand was investigated. A modified melting method which was more energy-saving than the traditional methods was used to control the crystallization process. The results show that the iron-rich system has much lower melting temperature, glass transition temperature (T_g), and glass crystallization temperature (T_c), which can result in a further energy-saving process. The results also show that the system has a quick but controllable crystallization process with its peak crystallization temperature at 918℃. The crystallization of augite crystals begins from the edge of the sample and invades into the whole sample. The crystallization process can be completed in a few minutes. A distinct boundary between the crystallized part and the non-crystallized part exists during the process. In the non-crystallized part showing a black colour, some sphere-shaped augite crystals already exist in the glass matrix before samples are heated to T_c. In the crystallized part showing a khaki colour, a compact structure is formed by augite crystals.     

Microstructure,mechanical properties,and wear resistance of VC<Subscript>p</Subscript>-reinforced Fe-matrix composites treated by Q&amp;P process

A quenching and partitioning (Q&P) process was applied to vanadium carbide particle (VCp)-reinforced Fe-matrix composites (VC-Fe-MCs) to obtain a multiphase microstructure comprising VC, V₈C₇, M₃C, α-Fe, and γ-Fe. The effects of the austenitizing temperature and the quenching temperature on the microstructure, mechanical properties, and wear resistance of the VC-Fe-MCs were studied. The results show that the size of the carbide became coarse and that the shape of some particles began to transform from diffused graininess into a chrysanthemum-shaped structure with increasing austenitizing temperature. The microhardness decreased with increasing austenitizing temperature but substantially increased after wear testing compared with the microhardness before wear testing; the microhardness values improved by 20.0% ±2.5%. Retained austenite enhanced the impact toughness and promoted the transformation-induced plasticity (TRIP) effect to improve wear resistance under certain load conditions.     

Microstructural evolution of ternary Ag33Cu42Ge25 eutectic alloy inside ultrasonic field

Ultrasonic field with a frequency of 20 k Hz is introduced into the solidification process of ternary Ag33Cu42Ge25 eutectic alloy from the sample bottom to its top. The ultrasound stimulates the nucleation of alloy melt and prevents its bulk undercooling. At low ultrasound power of 250 W,the primary ε2phase in the whole alloy sample grows into non-faceted equiaxed grains, which differs to its faceted morphology of long strip under static condition. The pseudobinary(Ag t ε2) eutectic transits from dendrite shape grain composed of rod type eutectic to equiaxed chrysanthemus shape formed by lamellar structure. By contrast, the ultrasound produces no obvious variation in the morphology of ternary(Ag t Ge t ε2) eutectic except a coarsening effect. When ultrasound power rises to 500 W, divorced ternary(Ag t Ge t ε2) eutectic forms at the sample bottom. However, in the upper part, the ultrasonic energy weakens, and it only brings about prominent refining effect to primary ε2phase.The microstructural evolution mechanism is investigated on the cavitation, acoustic streaming and acoustic attenuation.     

GH3535/316H爆炸复合板的界面精细结构表征

An explosion-welded technology was induced to manufacture the GH3535/316H bimetallic plates to provide a more cost-effective structural material for ultrahigh temperature, molten salt thermal storage systems. The microstructure of the bonding interfaces were extensively investigated by scanning electron microscopy, energy dispersive spectrometry, and an electron probe microanalyzer. The bonding interface possessed a periodic, wavy morphology and was adorned by peninsula- or island-like transition zones. At higher magnification, a matrix recrystallization region, fine grain region, columnar grain region, equiaxed grain region, and shrinkage porosity were observed in the transition zones and surrounding area. Electron backscattered diffraction demonstrated that the strain in the recrystallization region of the GH3535 matrix and transition zone was less than the substrate. Strain concentration occurred at the interface and the solidification defects in the transition zone. The dislocation substructure in 316H near the interface was characterized by electron channeling contrast imaging. A dislocation network was formed in the grains of 316H. The microhardness decreased as the distance from the welding interface increased and the lowest hardness was inside the transition zone.     

Formation kinetics of austenite in pearlitic ductile iron

This work evaluated the isothermal transformation of austenite in unalloyed pearlitic ductile iron and drew the isothermal phase diagram of austenitization in the ductile iron. Austenite forms at grain boundaries and then grows up to graphite regions during austenitization. The formation kinetics of austenite complies with the Avrami equation, in which the parameter (n) ranges from 4.71 to 4.99. The start time and finish time of transformation can be calculated at each temperature using the Avrami equation.     

热机械循环淬火调质处理对AISI 1345钢组织、力学和电化学性能的影响

Thermomechanical cyclic quenching and tempering (TMCT) can strengthen steels through a grain size reduction mechanism. The effect of TMCT on microstructure, mechanical, and electrochemical properties of AISI 1345 steel was investigated. Steel samples heated to 1050°C, rolled, quenched to room temperature, and subjected to various cyclic quenching and tempering heat treatments were named TMCT-1, TMCT-2, and TMCT-3 samples, respectively. Microstructure analysis revealed that microstructures of all the treated samples contained packets and blocks of well-refined lath-shaped martensite and retained austenite phases with varying grain sizes (2.8–7.9 μm). Among all the tested samples, TMCT-3 sample offered an optimum combination of properties by showing an improvement of 40% in tensile strength and reduced 34% elongation compared with the non-treated sample. Nanoindentation results were in good agreement with mechanical tests as the TMCT-3 sample exhibited a 51% improvement in indentation hardness with almost identical reduced elastic modulus compared with the non-treated sample. The electrochemical properties were analyzed in 0.1 M NaHCO₃ solution by potentiodynamic polarization and electrochemical impedance spectroscopy. As a result of TMCT, the minimum corrosion rate was 0.272 mm/a, which was twenty times less than that of the non-treated sample. The impedance results showed the barrier film mechanism, which was confirmed by the polarization results as the current density decreased.     

Microstructure Characteristics and Apparent Viscosity of Hypereutectic Al-24%Si Alloy Melt During Semi-solid State Stirring

The microstructural evolution and apparent viscosity of hypereutectic Al-24%Si alloy during semi-solid state shearing were studied with a Searte type viscometer. When the alloy melt was continuously stirred from 720 ℃ to eutectic temperature, the primary Si crystals were gradually changed from elongated platelets to near-spherical shapes. It was found that some nondendritic α-phase formed when the melt was stirred below 585 ℃. The experiment showed that the semi-solid stirring had strong effect on inhibiting the anisotropic growth of Si crystals during solidification. The apparent viscosity of the alloy melt increased slowly with the decreasing of temperature before the formation of nondendritic α-phase, which caused the dramatic increase of apparent viscosity.     

Microstructure and texture evolution of Mg–3Zn–1Al magnesium alloy during large-strain electroplastic rolling

Large-strain deformation by single electroplastic rolling(EPR) was imposed on AZ31 magnesium alloy strips. During EPR at low temperature(150–250°C), numerous twins formed in the alloy. After EPR at a high temperature(350°C), the number of twins reduced and some dynamic recrystallization(DRX) grains formed at grain boundaries and twinned regions. The synergic thermal and athermal effects generated by electropulsing, which promoted dislocation motion, induced a few small DRX grains, and ductile bandings were mainly responsible for large-strain deformation during EPR. The inclination angle of the basal pole stemmed from the counterbalance of the inclination direction of the basal pole between the DRX grains and deformed coarse grains.