Effect of Multiwalled Carbon Nanotubes on the Thermal Conductivity and Porosity Characteristics of Blast Furnace Carbon Refractories

Different amounts of carbon nanotubes (CNTs) (0–5 mass pct) containing carbon refractory specimens for a blast furnace were prepared and coked for 3 hours at 1473 K (1200 °C) and 1673 K (1400 °C). The thermal conductivity and porosity characteristics of the coked specimens were evaluated using the flash diffusivity technique and mercury porosimetry, respectively. It was found that CNTs acted as carbon source, and most of them were consumed during coking. With the increase of CNT content, the aggregation of CNTs became more severe, the amount of SiC whiskers formed increased and their aspect ratio became larger, and the SiC whiskers tended to be distributed nonhomogeneously. The thermal conductivity of a 4 mass pct CNT containing a carbon specimen was highest because of the contributions of SiC and residual CNTs. The porosity characteristics of a 0.5 mass pct CNT containing a carbon specimen was best because of the uniform filling of SiC whiskers. The excessive addition of CNTs degraded the porosity characteristics because of the severe aggregation of CNTs.     

Friction and abrasion resistance of cast aluminum alloy-fly ash composites

The abrasive wear properties of stir-cast A356 aluminum alloy-5 vol pct fly ash composite were tested against hard SiC _p abrasive paper and compared to those of the A356 base alloy. The results indicate that the abrasive wear resistance of aluminum-fly ash composite is similar to that of aluminum-alumina fiber composite and is superior to that of the matrix alloy for low loads up to 8 N (transition load) on a pin. At loads greater than 8 N, the wear resistance of aluminum-fly ash composite is reduced by debonding and fracture of fly ash particles. Microscopic examination of the worn surfaces, wear debris, and subsurface shows that the base alloy wears primarily by microcutting, but the composite wears by microcutting and delamination caused by crack propagation below the rubbing surface through interfaces between fly ash and silicon particles and the matrix. The decreasing specific wear rates and friction during abrasion wear with increasing load have been attributed to the accumulation of wear debris in the spaces between the abrading particles, resulting in reduced effective depth of penetration and eventually changing the mechanism from two-body to three-body wear, which is further indicated by the magnitude of wear coefficient.     

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.     

Microstructure and Mechanical Properties of Oxide-Dispersion Strengthened Al6063 Alloy with Ultra-Fine Grain Structure

The microstructure and mechanical properties of the ultra-fine grained (UFG) Al6063 alloy reinforced with nanometric aluminum oxide nanoparticles (25 nm) were investigated and compared with the coarse-grained (CG) Al6063 alloy (~2 μm). The UFG materials were prepared by mechanical alloying (MA) under high-purity Ar and Ar-5 vol pct O₂ atmospheres followed by hot powder extrusion (HPE). The CG alloy was produced by HPE of the gas-atomized Al6063 powder without applying MA. Electron backscatter diffraction under scanning electron microscopy together with transmission electron microscopy studies revealed that the microstructure of the milled powders after HPE consisted of ultra-fine grains (>100 nm) surrounded by nanostructured grains (<100 nm), revealing the formation of a bimodal grain structure. The grain size distribution was in the range of 20 to 850 nm with an average of 360 and 300 nm for Ar and Ar-5 pct O₂ atmospheres, respectively. The amount of oxide particles formed by reactive mechanical alloying under the Ar/O₂ atmosphere was ~0.8 vol pct, whereas the particles were almost uniformly distributed throughout the aluminum matrix. The UFG materials exhibited significant improvement in the hardness and yield strength with an absence of strain hardening behavior compared with CG material. The fracture surfaces showed a ductile fracture mode for both CG and UFG Al6063, in which the dimple size was related to the grain structure. A mixture of ductile–brittle fracture mode was observed for the UFG alloy containing 0.8 vol pct Al₂O₃ particles. The tensile behavior was described based on the formation of nonequilibrium grain boundaries with high internal stress and dislocation-based models.     

Optimizing fracture toughness and abrasion resistance in white cast irons

A series of twelve Cr-Mo white irons varying in carbide volume from 7 to 45 pct were tested for dynamic fracture toughness and wet sand abrasion resistance. Carbon content was varied from 1.4 to 3.9 pct. Two matrix microstructures were employed, and the compositions (copper and chromium content) were varied to assure constant matrix compositions. Chromium was varied from 11.6 to 25.7 pct. In addition, one composition of white iron was subjected to thirty different heat treatments to define the effect of matrix microstructure on dynamic fracture toughness and abrasion resistance. It was shown that for the abrasive wear system used, a carbide volume of about 30 pct represented an optimum quantity, above which abrasion resistance decreased. Martensitic irons provided consistently better abrasion resistance than austenitic irons. Dynamic fracture toughness decreased with carbide volume, as expected. Higher toughness values were obtained with predominantly austenitic matrix microstructures than with predominantly martensitic matrix microstructures. Considering both abrasion resistance and fracture toughness, it was shown that heat treated irons could provide an optimal combination of these properties. Formerly Visiting Research Metallurgist, Climax Molybdenum Co. Research Laboratory.     

Microstructure and Mechanical Properties of CNTs/A356 Nanocomposites Fabricated by High-Intensity Ultrasonic Processing

Carbon nanotube (CNT)-reinforced A356 alloy nanocomposites were successfully fabricated by introducing a method of CNT predispersion and high-intensity ultrasonic treatment. The scanning electron microscope and energy-dispersive spectrometer results showed that high-intensity ultrasonic treatment was able to disperse the CNTs into the melt. When the ultrasonic power was less than 2.1 kW, the microhardness and tensile properties (ultimate tensile strength (UTS), yield strength (YS), and elongation) of the nanocomposites improved as the ultrasonic power increased. Further, the microhardness, UTS, and YS improved as the CNT content increased while elongation decreased. The microhardness, UTS, and YS of the 0.8 wt pct CNTs/A356 nanocomposites fabricated by high-intensity ultrasonic processing at an ultrasonic power of 2.1 kW were increased, respectively, by 27.8, 17, and 29.2 pct compared to the A356 alloy without CNT addition, and the ductility remained. The fracture analysis confirmed that CNTs were homogeneously distributed in the matrix, and strong interfacial bonding formed between CNTs and the matrix. Also, transmission electron microscope results confirmed that CNTs were stale embedded in the matrix and the formation of brittle Al₄C₃ was suppressed.

     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M₂C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.     

Wear Behavior of the Lead-Free Tin Bronze Matrix Composite Reinforced by Carbon Nanotubes

Copper-coated carbon nanotubes were prepared by the electroless plating route. The structure and component of copper/carbon tubes were characterized using a transmission electron microscope and energy dispersive spectrometer. The results show that the surface of the carbon tubes was covered by the copper particles. Copper/carbon tubes were used as the substitute of part of tin and all of lead in the tin bronze matrix, and the tribological properties of carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composites were studied. The effects of the carbon nanotube volume fraction and sliding distance in unlubricated ball-on-disc wear test were investigated. The 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite shows the Vickers hardness of 126.9, which is approximately 1.6 times higher than that of Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze. The wear rate and average friction coefficients of 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite were lower than those of the Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze, respectively.     

On the Prospects of Using Nanoindentation and Wear Test to Study the Mechanical Behavior of Fe-Based Metallic Glass Coating Reinforced by B4C Nanoparticles

In this study, Fe-based metallic glass was served as the matrix in which various ratios of hard B₄C nanoparticles as reinforcing agents were prepared using a high-energy mechanical milling. The feedstock nanocomposite powders were transferred to the coatings using a high-velocity oxygen fuel process. The results showed that the microstructure of the nanocomposite coating was divided into two regions, namely a full amorphous phase region and homogeneous dispersion of B₄C nanoparticles with a scale of 10 to 50 nm in a residual amorphous matrix. As the B₄C content is increased, the hardness of the composite coatings is increased too, but the fracture toughness begins to be decreased at the B₄C content higher than 20 vol pct. The optimal mechanical properties are obtained with 15 vol pct B₄C due to the suitable content and uniform distribution of nanoparticles. The addition of 15 vol pct B₄C to the Fe-based metallic glass matrix reduced the friction coefficient from 0.49 to 0.28. The average specific wear rate of the nanocomposite coating (0.48 × 10⁻⁵ mm³ Nm⁻¹) was much less than that for the single-phase amorphous coating (1.23 × 10⁻⁵ mm³Nm⁻¹). Consequently, the changes in wear resistance between both coatings were attributed to the changes in the brittle to ductile transition by adding B₄C reinforcing nanoparticles.

     

Effect of Aluminum Content on Wear Resistance of Hot-Forged Multiphase Steel

A medium-carbon steel was alloyed with Mn, Cr, Si, and Al to obtain carbide-free bainite steel. The thermomechanics and chemistry of steel were used to produce medium carbon containing four phases: ferrite, pearlite, bainite, and chromium carbide. The morphologies of different phases were characterized and analyzed by using optical and scanning electron microscopes. An abrasive dry sliding wear (pin on ring) of two types of medium-carbon, hot-forged steels containing different aluminum contents was investigated at different pressures and sliding velocities. The sliding duration time was 30 minutes under dry sliding conditions. The wear rate of Alloy 1 and 2 revealed negligible wear rates at low velocity and pressure. On the other hand, the wear rate highly increased to maximum at maximum velocity and pressure for Alloy 1 and 2. Alloy steel 2 of 2 pct Al revealed a maximum wear rate of 720 mg/min compared with 160.8 mg/min for Alloy 1 contains 1 pct Al. Experimental results showed that increased aluminum content is directly proportional to the ferrite volume fraction, which greatly influences the wear resistance performance and mechanical properties of the two types of steel.

     

Effects of Alloying Elements on Microstructure,Hardness, Wear Resistance,and Surface Roughness of Centrifugally Cast High-Speed Steel Rolls

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M₂C, and M₇C₃ carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.     

Mechanical Properties and Corrosion–Abrasion Wear Behavior of Low-Alloy MnSiCrB Cast Steels Containing Cu

Two medium carbon low-alloy MnSiCrB cast steels containing different Cu contents (0.01 wt pct and 0.62 wt pct) were designed, and the effect of Cu on the mechanical properties and corrosion–abrasion wear behavior of the cast steels was studied. The results showed that the low-alloy MnSiCrB cast steels obtained excellent hardenability by a cheap alloying scheme. The microstructure of the MnSiCrB cast steels after water quenching from 1123 K (850 °C) consists of lath martensite and retained austenite. After tempering at 503 K (230 °C), carbides precipitated, and the hardness of the cast steels reached 51 to 52 HRC. The addition of Cu was detrimental to the ductility and impact toughness but was beneficial to the wear resistance in a corrosion–abrasion wear test. The MnSiCrB cast steel with Cu by the simple alloying scheme and heat treatment has the advantages of being high performance, low cost, and environmentally friendly. It is a potential, advanced wear-resistant cast steel for corrosion–abrasion wear conditions.     

超音速火焰喷涂技术制备的双峰WC–CoCr涂层磨粒磨损特性

采用超音速火焰喷涂(high velocity oxy-fuel,HVOF)工艺分别制备了双峰结构和常规结构的WC–CoCr复合涂层。比较了不同结构WC–CoCr涂层的组织结构、显微硬度和断裂韧性;在涂层磨粒磨损实验的基础上,探讨了双峰结构WC–CoCr涂层的磨损机理。结果表明:与常规结构的WC–CoCr复合涂层相比,在由含质量分数30%超细WC粉末制备的双峰结构涂层中,WC在黏结相中溶解最多,断裂韧性最低;由含质量分数50%超细WC粉末制备的双峰结构涂层最致密,显微硬度与断裂韧性最高,耐磨粒磨损性能最优良。     

Microstructure,Mechanical and Abrasive Wear Behavior of 8.0 wt pct Cr White Iron Subjected to Continuous and Cyclic Annealing Treatment

Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV.

     

Correlation of Microstructure,Hardness, and Fracture Toughness of Fe-Based Surface Composites Fabricated by High-Energy Electron Beam Irradiation with Fe-Based Metamorphic Alloy Powders and VC Powders

Iron-based surface composites were fabricated with Fe-based metamorphic alloy powders and VC powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and fracture toughness was investigated. Mixtures of metamorphic powders and VC powders were deposited on a plain carbon steel substrate, and then the electron beam was irradiated on these powders without flux, to fabricate surface composites. The composite layers 1.3 to 1.8 mm in thickness contained a large amount (up to 47 vol pct) of hard Cr₂B and V₈C₇ particles formed in eutectic colony regions and inside colonies, respectively. The hardness of the surface composites was approximately 2 to 4 times greater than that of the substrate because of Cr₂B and V₈C₇ particles. According to the microfracture observation of the composite fabricated with mixing 30 wt pct VC powders, microcracks initiated at coarse V₈C₇ particles ins inside colonies as well as at Cr₂B particles in colony regions, and were connected with other microcracks in a zigzag shape. Thus, it showed a higher fracture toughness and hardness twice as high as the composite fabricated without mixing VC powders.     

Compressive and Tensile Properties of STS304-Continuous-Fiber-Reinforced Zr-Based Amorphous Alloy Matrix Composite Fabricated by Liquid Pressing Process

An STS304-continuous-fiber-reinforced Zr-based amorphous alloy matrix composite with excellent fiber/matrix interfaces was fabricated without pores and misinfiltration by liquid pressing process. Approximately 60 vol pct of continuous fibers were homogeneously distributed in the matrix, in which considerable amounts of polygonal and dendritic crystalline phases were formed by the diffusion of metallic elements from the fibers. The ductility of the composite under compressive or tensile loading was drastically improved over that of the monolithic amorphous alloy. According to the compressive test results, a strength of 700 to 830 MPa was sustained until reaching a strain of 40 pct, because fibers interrupted the propagation of shear bands initiated in the matrix and took over a considerable amount of load. Under tensile loading, the deformation and fracture occurred by crack formation and opening at matrices, necking of fibers, fiber/matrix interfacial separation, and cup-and-cone–type fracture of fibers, thereby resulting in a high tensile elongation of 27 pct.     

Role of Silicon Carbide in Phase-Evolution and Oxidation Behaviors of Pulse Electrodeposited Nickel-Tungsten Coating

Silicon carbide (SiC) was reinforced in the pulse electrodeposited nickel-tungsten (Ni-W) coatings deposited on the steel substrate, and isothermal oxidation test was performed at 1273 K (1000 °C) for 24 hours. Addition of just 2 vol pct of SiC showed 26 pct increase in the relative oxidation resistance of Ni-W coating. The increased oxidation resistance was attributed to the phase evolution (SiO₂, Cr₂O₃, CrSi₂, Ni₂SiO₄, Cr₇C₃, Cr₃C₂, and Cr₃Si), which suppressed the spallation of the oxide scale in Ni-W-2 vol pct SiC. The presence of Fe₂O₃ phase in the oxidized Ni-W coating was mainly responsible for the major multiple spallations at the interface and in the bulk, which resulted in the degradation of oxidation resistance.

     

Dry sliding wear behavior of A356-15 Pct SiC p composites under controlled atmospheric conditions

The present investigation was carried out to provide a deeper insight into the mechanism of wear behavior of A356-15 vol pct SiC _p composite under controlled argon and oxygen atmospheres through a detailed characterization of worn surfaces and subsurfaces. Dry sliding wear tests were performed for both as-cast and T6-treated specimens using a pin-on-disc machine with three sliding velocities (0.5, 1, and 2 ms⁻¹) and three loads (1, 2, and 3 MPa). The wear rate of A356-15 vol pct SiC _p composite was lower by nearly one order of magnitude under argon atmosphere compared to the specimens tested under oxygen atmosphere for all experimental conditions. Under argon atmosphere, the mechanism of material removal was by delamination wear and did not change within the parametric regime. In the case of the specimen tested under oxygen atmosphere, the wear behavior of the composite depended on the experimental conditions. At low load and low sliding velocity, the material removal was by abrasion. While at high load and high sliding velocity, the material removal mechanism was by delamination wear. Further, the mechanical mixed layer (MML) formed under argon atmosphere was more stable and homogenous compared to that formed under oxygen atmosphere. The MML formed under both atmospheres revealed much less in Fe content.