Macro/semi-macro carbon segregation plays a key role for improving the steel product quality. Based on macrostructure qualitative rating comparison and element macro content analysis, the segregation extent has been controlled at different levels by the existing technologies, but there is an obvious shortcoming on segregation morphology description. Nowadays, delicacy control is demanded for higher quality requirement, especially for the production of high-quality H13 die steel by electro-slag remelting (ESR) technique. In this work, as to segregation point morphology, fractal dimension is introduced, and segregation characteristics of different locations in the ESR billet are quantitatively investigated in terms of area, number and outline morphology. The size of the billet is 160 mm×160 mm, and the sampling location in the central plane of billet. Two melting rates (350 and 400 kg/h) are considered for studying essential characteristics of segregation. Firstly, it is shown that the whole segregation extent in the billet is mostly influenced by the large segregation point (e.g., the area is larger than 0.1 mm2). The segregation ratio will be increased when increasing the number or area of the large segregation point. Secondly, it is found that fractal is a very important characteristic of the segregation point morphology in the billet. Moreover, fractal dimension can be used as a criterion for measuring the dispersion degree of the segregation. The dispersion degree will be increased when increasing the corresponding fractal dimension, and the large segregation point will be disintegrated by the small segregation point. Finally, the fractal dimensions in the columnar-equiaxed transition area and the solidifying end equiaxed area are less than the value of other locations. In addition, more researches are needed for accurately obtaining the influence factors of fractal dimensions of segregation point in the future.
The improvement of steam parameters in fossil power plants requires the development of new kinds of 9% Cr martensitic heat-resistant steels, among which FB2 steel is a 100×10-6 (mass fraction) boron-containing steel and mainly used for manufacturing components with thick walls operating at high temperatures above 600 ℃. In the alloy system of martensitic heat-resistant steels, boron plays an important role in suppressing type IV crack of weld joints by the formation of heat affected zone (HAZ) with no fine grains in the normalized and intercritical zones, where there exhibit fine grains in conventional 9%Cr heat-resistant steels with no boron such as P91 steel. In this work, the formation process of HAZ in FB2 steel was investigated. The microstructures before and after thermal simulation were compared using OM and SEM. It was concluded that the austenization of FB2 steel at rapid heating rates (≥100 ℃/s) took place by shear mechanism, demonstrating austenite memory effect; while at slow heating rates (≤5 ℃/s), the austenization was by atom short range diffusion mechanism, without austenite memory effect. The special phase transformation of austenization is the main cause for the formation of HAZ with no coarsened grain in the overheated zone. Based on the previous results reported by other researchers, a preliminary model was proposed to describe how boron atoms change the austenite transformation type of FB2 steel during heating process, which developed the previous ideas about the phenomenon.
It is significant to reduce the negative effects of non-metallic inclusion on steel and to improve steel mechanical properties through controlling the morphology of the secondary phase particle including non-metallic inclusion, nitride and carbide. Compared with particles with irregular shape, globular second phase particle could reduce the stress concentration during rolling and heat treatment process obviously and lower its harmfulness to steel toughness. A theoretical model to predict the morphology of the secondary phase particle in steel has been established by introducing a dimensionless Jackson α factor, and the morphology of the secondary phase particle is determined by its dissolved entropy, growth direction and temperature or undercooling. Non-aqueous solution electrolysis extraction and room temperature organic (RTO) technique were applied to detect the 3D morphology of the secondary phase particle and its inner morphology combining with SEM. The morphologies of particles observed in four different types of steels are in good agreement with the theoretical predictions. Theoretical predictions and experimental observation were both confirmed that the secondary phase particle is faceted in morphology when its Jackson α factor is more than 3 and non-faceted when its Jackson α factor less than 2.
Pipe is the main mode of transportation of oil and gas contemporary, and its security and reliability has an important influence on the smooth development of regional economy and even the security situation. For decades, quite a number of researches have been mainly focusing on various factors on the stress corrosion cracking (SCC) of both high and middle strength pipeline steels in soil or underground water conditions, but the division of the sensitive potential ranges which determines the different SCC mechanisms was rarely reported. Soil environmental stress corrosion cracking (SCC) of pipeline steel in the process of service operation is one of the biggest security hidden dangers. The external environment SCC of pipeline steel mainly includes two modes, high pH SCC and close to neutral pH SCC. Between them, the high pH SCC occurred mainly in CO32-/HCO3- under the coating of liquid, the mechanism of cracking is widely regarded as membrane rupture, crack tip anodic dissolution mechanism; near neutral pH SCC occurred mainly in the coating containing low concentration of HCO3- resident fluid or groundwater environment. Due to pipe in the process of serving for a long time, pipeline external coating damage and strip defects are common, under the joint action of the applied potential and soil medium, SCC will generally occur in nearly neutral pH environment, which lead to a serious risk in nearly neutral pH SCC. As a new generation of high strength pipeline steel, the X90 steel probes into its SCC sensitivity at different applied potentials in a certain pH environment is of great significance. In this work, the SCC behavior as well as its mechanism of X90 pipeline steel and its weld joint in an simulated solution of the near neutral soil environment (NS4 solution) were studied by slow strain rate tensile tests (SSRT), potentiodynamic polarization tests and SEM observation of fracture surfaces. The results showed that both the as received X90 pipeline steel and its weld joint have obvious SCC susceptibilities, which initiated and extended in transgranular cracking mode under different applied potentials. Within the potential ranges from OCP to -1000 mV, the SCC mechanism of both X90 steel and its weld joint microstructures are a combined mechanisms of anodic dissolution (AD) and hydrogen embrittlement (HE), i.e. the AD+HE mechanism. The SCC susceptibility is apparent under the OCP due to a strong AD effect. At -800 mV, the SCC susceptibility comes to a minimum due to AD and HE being weaker, and it presents the highest SCC susceptibility at -900 mV because the HE effect was greatly enhanced. The SCC susceptibility of the weld organization is higher than that of the base metal, which may be related to organization phase transformation in the welds and metallurgical reaction.
As the main corrosion form of coal- or heavy oil-fired boilers, dew point corrosion occurs when corrosive gases (SO3, HCl, NO2, et al) are cooled and converted to condensed acids. The condensed acids (H2SO4, HCl and HNO3) are much corrosive to steel, causing corrosion damage to plant materials. The service temperature is designed lower and lower to improve energy efficiency recently, which makes dew point corrosion more and more serious. Q315NS steel produced by appropriate alloy design is much suitable for those parts vulnerable to dew point corrosion in power and petrochemical industry due to its excellent corrosion resistance in H2SO4 solution. As an efficient and low-cost process, welding is an essential process in the utilization of Q315NS. The corrosion mechanism of the heat affected zone is much complex due to the presence of microstructure gradients, which is largely determined by the welding thermal cycle. However, there is little research elucidating the effect of welding thermal cycle on corrosion behavior of Q315NS steel in H2SO4 solution. In this work, the microstructure evolution and corrosion behaviour in the 50%H2SO4 (mass fraction) solution of welding heat affected zones of Q315NS was investigated by comparison with base metal using welding thermal simulation technique, scanning electron microscope and electrochemical measurements. The results show that the microstructures of ferrite and pearlite are observed in base metal, fine-grained region and incomplete recrystallization region, while coarse-grained region consists of granular bainite. All the equivalent circuits of Q315NS with or without welding thermal cycle contain a resistor of corrosion product and a capacitor of electric double layer, and all specimens have passivation behavior. The base metal and the incomplete recrystallization region have the lowest corrosion current density and the largest charge-transfer resistance, which means the best corrosion resistance, while the coarse-grained region has the highest corrosion current density and the least charge-transfer resistance. Rod-like shaped corrosion product was formed by deposition on the surface of the coarse-grained region specimen while a porous-structured corrosion product was formed on the surface of other specimens.
Uranium is a valuable nuclear fuel material, but this application is unavoidably handicapped by the easy creep behavior of the metal caused by the combination of stress and irradiation in nuclear reactor. Uranium-based amorphous alloys, as a kind of potential new materials in the nuclear industry, would be challenged by this issue when used in such situation. However, creep properties of these materials have not been reported in the previous studies. In order to preliminarily investigate the creep phenomenon derived from stress function, this work is performed to study the ambient creep behavior of a new amorphous alloy U65Fe30Al5. This alloy was tested by using a nanoindentation technique under different peak loads and loading rates. The results indicate that the creep displacement gradually increases with either the peak load or the loading rate in equal creeping time, but this tendency vanishes when exceeding a critical loading rate. The fitting based on an empirical creep equation reveals that the stress exponent of the alloy ascends when raising the peak load, and firstly declines with the loading rate and then keeps constant above the critical rate. Compared with conventional crystalline alloys, the U-Co-Al alloy shows a larger stress exponent, reflecting the possible existence of rich free volume in the amorphous alloy.
Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr55Cu30Al10Ni5 (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al5Ni3Zr2, CuZr2 and Al2Zr3 phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr2 nanocrystal, CuZr2+ZrCu dendrite-like eutectic, CuZr2+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75~106 μm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106~150 μm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the Zr55 deposited layers when the content of Al5Ni3Zr2 phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the Zr55 deposits fabricated by different powder sizes.
The development of nanocrystalline Fe-Si-B-Nb-Cu alloys, commercially known as Finemet, has established a new approach to obtain soft-magnetic materials with high magnetic flux density. The material consists of α-Fe(Si) nanocrystals embedded in an amorphous matrix, which is made by means of partial crystallization. The composition and local structure of the precursor amorphous alloys are crucial for the formation of the unique nanocrystalline structure. The present study is devoted to understanding the composition characteristics and developing new compositions of Finemet alloys. Using the “cluster-plus-glue-atom” model and noticing the crystallization characteristic of Finemet alloy, a “dual-cluster” amorphous structure model is proposed. In this model, the precursor amorphous structure of Finemet alloy is considered to contain a mixture of the [(Si, B)-B2(Fe, Nb)8]Fe cluster derived from the Fe-B-Si-Nb bulk glassy alloys, and the [Si-Fe14](Cu1/13Si12/13)3 cluster from Fe3Si phase. A series of new Finemet nanocrystalline alloy compositions are designed by mixing [(Si, B)-B2(Fe, Nb)8]Fe and [Si-Fe14](Cu1/13Si12/13)3 cluster formulas with a ratio of 1∶1. Thermal analysis results show that [(Si0.8B0.2)-B2Fe7.2Nb0.8]Fe+[Si-Fe14](Cu1/13Si12/13)3 (alloy composition: Fe74B7.33Si15.23Nb2.67Cu0.77) amorphous alloy exhibits a maximal temperature interval of about 192 K between the first and second crystallization peaks. Magnetic measurement results show that the Fe74B7.33Si15.23Nb2.67Cu0.77 nanocrystalline alloy exhibits optimal soft magnetic properties with a saturation magnetization Bs about 1.26 T, a coercive force Hc about 0.5 A/m and an effective permeability μe about 8.5×105 at 1 kHz after isothermal annealing at 813 K for 60 min. The soft magnetic properties of the new composition nanocrystalline alloys are better than that of the typical Finemet nanocrystalline alloy (Fe73.5Si13.5B9Cu1Nb3).
2A14 aluminum alloy is the important raw materials of aerospace, which belongs to the heat treatment aluminum alloy. Friction stir welding (FSW) can weld aluminum alloy with high quality, and can avoid the pores and cracks of fusion welding effectively. In order to obtain better mechanical properties of FSW joints, the surface nanocrystallization method is introduced into FSW technology. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a smooth gradient nanostructured (GNS) layer was formed on the surface of 2A14 aluminum alloy before FSW. The FSW joints microstructure and fracture morphology of the original and HSNC specimens were researched by OM, SEM and TEM. The results showed that nanostructure layer zone (NLZ) was formed when GNS with shape similar to the "S" line was distributed in the thermal-mechanical affected zone (TMAZ) and the nugget zone (NZ) of the HSNC specimen. The lowest micro-hardness and fracture position of the original specimen occurred on the TMAZ of advancing side (AS). The lowest micro-hardness and fracture position of the HSNC specimen occurred on the NZ. The tensile strength of HSNC specimen was 6.4% higher than the original sample. The elongation of HSNC specimen was 14.1% more than the original specimen. The fracture mode of both specimens was toughness fracture. The fracture morphology of the HSNC was isometric dimple when the fracture morphology of original specimen were non-isometric dimple and avulsion dimple. Analysis showed that the NLZ of the FSW joints was beneficial to improving the strength and the plastic deformation capability simultaneously.
As one of the highest temperature magnesium alloys, Mg-Zn-Cu ternary alloys are draw much attention in recent years. However, the previous investigations were mainly focused on their microstructure and mechanical properties. The investigations on their solidification and hot tearing behaviors are barely discussed. In order to improve the industrial applications of Mg-Zn-Cu alloy, it is necessary to better understand the solidification pathways, phase constituent and hot tearing susceptibility (HTS) of these alloys. In this work, the effect of Cu additions on the hot tearing behaviors of Mg-7Zn-xCu-0.6Zr (x=0, 1, 2, 3) alloys was studied using with T type hot tearing mold. The microstructure and the morphology of cracking zone of Mg-7Zn-xCu-0.6Zr (x=0, 1, 2, 3) alloys were observed by XRD and SEM, respectively. The hot cracking susceptibility of Mg-7Zn-xCu-0.6Zr (x=0, 1, 2, 3) alloys were characterized by the measurement of crack volume. The experimental results show that the grain sizes of Mg-7Zn-0.6Zr alloys were refined by addition of Cu element. Meanwhile, the amount of eutectic phase was increased with increasing the Cu content and the separated dendritic was refilled by the eutectic phase. Hot tearing susceptibility of Mg-7Zn-0.6Zr was decreased with increasing of Cu content. The Mg-7Zn-0.6Zr and Mg-7Zn-1Cu-0.6Zr alloys were completely broken. The hot cracks of Mg-7Zn-xCu-0.6Zr (x=0, 1, 2, 3) alloys were formed by liquid film, solidification shrinkage and interdendritic bridge.
The effect of Cu content on the evolution of intermetallic compounds (IMCs) in Ni/Sn-xCu/Ni (x= 0.3, 0.7, 1.5, mass fraction, %) micro solder joints during soldering at 240 ℃ under a temperature gradient of 1045 ℃/cm was investigated. Asymmetrical growth and transformation of interfacial IMCs and asymmetrical dissolution of Ni substrate were clearly observed. In Ni/Sn-0.3Cu/Ni micro solder joints, though the interfacial IMC remained as the initial (Ni, Cu)3Sn4, asymmetrical IMC growth between cold and hot ends occurred, i.e., the (Ni, Cu)3Sn4 IMC at the cold end was obviously thicker than that at the hot end. In Ni/Sn-0.7Cu/Ni and Ni/Sn-1.5Cu/Ni micro solder joints, the interfacial IMC gradually transformed from the initial (Cu, Ni)6Sn5 into (Ni,Cu)3Sn4. Meanwhile, the transformation at the cold end lagged behind the hot end, namely asymmetrical transformation phenomenon occurred. Moreover, the transformations at the cold and hot ends in the Ni/Sn-1.5Cu/Ni micro solder joints both lagged behind those in the Ni/Sn-0.7Cu/Ni micro solder joints. Based on the analysis of the Cu and Ni atomic fluxes for the IMC growth at both cold and hot ends, the thermomigration (TM) direction was confirmed to be from the hot end towards the cold end. The Cu concentration in the micro solder joints had a significant effect on the main TM element, and thus affected the growth and transformation behavior of the interfacial IMCs at the two ends. In addition, TM promoted the diffusion of Ni atoms into solder at the hot end, which accelerated the dissolution of the hot end Ni substrate. Most of the dissolved Ni atoms migrated to the cold end and participated in interfacial reaction locally. On the contrary, TM inhibited the diffusion of Ni atoms at the hot end, resulting in no obvious dissolution of the cold end Ni substrate.
Graphite flakes reinforced Al matrix composites (Gf /Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this work, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50%Gf /Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three Gf /Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al alloy matrix without cracks and pores. The (001)Gf basal plane of the graphite flakes were almost parallel to the circular plane (xy plane) of the composites ingot. However, for the small graphite flakes, their (001)Gf basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the Gf /Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf /Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/(mK). The interfacial thermal conductance (hc) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The hc of 300 and 500 μm graphite flake Gf /Al alloy composites were slightly lower than the theoretical value (calculated by the acoustic mismatch model). However, the hc of the 150 μm graphite flake Gf /Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.
Amorphous carbon coatings mainly composed of sp3 and sp2 bonds have a great potential to be widely used in modern industry for their attractive properties, such as high hardness, high wear resistance and low friction coefficient. However, the high internal stress and poor adhesion of amorphous carbon coatings limit the range of industrial applications. In order to reduce the internal stress and improve the tribological performance, a series of carbon-based coatings with different atomic fraction of Cr were prepared by magnetron sputtering. The microstructure of coatings was characterized by XRD, SEM, TEM, XPS and Raman spectra. The mechanical and tribological properties of coatings were analyzed. The results showed that with the increase of atomic fraction of Cr, the carbon-based coatings changed from amorphous structure to nano-crystalline/amorphous composite structure, the ratio of sp2 bond increased and the ratio of sp3 bond decreased gradually. Also, the hardness and the internal stress showed a decreasing trend with the increase of atomic fraction of Cr. In addition, a small amount of Cr doping could effectively reduce the friction coefficient and specific wear rates of coatings. Excessive Cr doping is beneficial to the increase of the ratio of sp2 bond, however, the dispersion distribution of the metal phase leads to the increase of the friction coefficient and specific wear rates, so that the tribological properties were deteriorated.
It has been recognized that low temperature martensitic transformation can reduce harmful tensile stress and generate beneficial compressive stress in weld zone of single pass welded joints. The influence of martensitic transformation is even greater in 9%Cr steel because of its high hardenability and low transformation temperature (about 100~400 ℃). However, compressive stress was confined in certain parts of weld zone in multi-pass butt-welded 9%Cr steel pipes. In this work, stress evolution in a multi-pass butt-welded 9%Cr steel pipe was predicted using Abaqus software, and the effect of martensitic transformation was further investigated. The simulated results show that the overall pattern for the axial and hoop stresses appears to be similar, despite the lower magnitudes for axial stress. The maximum compressive stress was found in the final weld pass, and the maximum tensile stress was formed in the weld pass adjacent to the final weld pass. Stress in weld passes adjacent to weld root is relatively low. Tensile stress due to thermal contraction in the final weld pass was relieved by martensitic transformation and clear compressive stress was formed. However, little effect of martensitic transformation was found on the significant tensile residual stress in weld passes adjacent to the final weld pass. The final weld pass has the primary effect on the formation of residual stress. Compressive stress was indeed generated by martensitic transformation in former weld pass, however it was relieved by weld thermal cycle of latter weld pass. As a result, the effect of martensitic transformation appears to be confined to the final weld pass. The influence of martensitic transformation is greater around outer surface than that around inner surface.