Corrosion Behavior of Spherical Chromium Carbide Reinforced NiCrBSi Hardmetal Coatings in Sulphuric Acid Solution

In the present work, four groups of spherical chromium carbide reinforced NiCrBSi hardmetal coatings were prepared on AISI 4145 steel by plasma transferred arc (PTA) technique. The corrosion behavior of the four as-received hardmetal coatings in 0.5 mol/L H2SO4 solution was investigated by polarization curve and electrochemical impedance spectroscopy (EIS). The results revealed that more Cr-rich carbides (Cr3C2, Cr7C3 and M23(C, B)6) are formed in the chromium carbide reinforced coatings, while for the NiCrBSi hardmetal coating only Cr7C3 carbide was detected by XRD. The polarization results show that the chromium carbide reinforced NiCrBSi hardmetal coatings have positive corrosion potential and lower corrosion current, providing a better protective effect to the substrate metal. The combined effects of Cr-rich carbide ceramic phases and a more stable passive film of Cr2O3 greatly improved the corrosion resistances of the chromium carbide reinforced NiCrBSi hardmetal coatings. The coating with the highest spherical chromium carbide addition has more pores because of the thermal stress due to the difference of thermal expansion coefficient between the NiCrBSi bonding phase and chromium carbide reinforced phase. The negative effects of the pores weaken the corrosion resistance, and the coating with the 30% chromium carbide content shows the best corrosion resistance. For NiCrBSi hardmetal coatings with higher reinforced chromium carbide content, the repeatability of the corrosion current obtained by polarization fitting is not as good as that of coatings with lower chromium carbide content. The repeatability of polarization results becomes worse when the specimens keep in a more stable passive state.

Effect of SiCw Whiskers on the indentation thermal shock resistance of ZrB2-MoSi2 Composites

A comparative evaluation of the thermal shock resistance (TSR) of ZrB2-20 MoSi2-20 SiCw (ZMSw20) and ZrB2-20 MoSi2 − 5SiCw (ZMSw5) composites were studied using the indentation quench method. High-dense ZrB2 based composites were prepared by multi-stage spark plasma sintering at 1700°C. The results show that the ZMSw20 composite expressed superior crack shielding and TSR under quenching circumstances. The critical temperature differential ΔTc of ZMSw20 ceramic (ΔTc= 800°C) was higher than that of ZMSw5 ceramic (ΔTc=600°C). The significant enhancement in TSR was imposed residual stresses that improved the resistance to crack progression during thermal shock. Furthermore, an increment in silicon carbide content reduces the crack growth, and increases the TSR of the composites.

Experimental Investigations on the Mechanical Properties, Microstructure and Corrosion Resistance of Cu-20Al-4Ni/SiC Composites Synthesized Using Powder Metallurgy

The present investigation pertains to synthesize aluminium bronze silicon carbide composite by powder metallurgy route. Three various weight percentages of silicon carbide (0, 2, 4 & 6) were reinforced with aluminium bronze matrix (Cu-20%Al-4%Ni) and the hydraulic press was used to prepare the green compact. This compact was heated at two different temperatures such as 6500C and 7500C using tubular furnace. The effect of silicon carbide on density, sinterability, compression test and hardness test was analyzed. The scanning electron microscope and energy dispersive spectroscopy were used to confirm the presence of alloying elements. The results showed that the sinterability and density were reduced with an increase in silicon carbide content. The composite reinforced with 6%SiC exhibited lowest compressive strength among other composites. The 4 wt.% SiC reinforced composites sintered at 7500C has highest corrosion resistance.

Preparation of single-layer coating polyester cotton composites with electric loss of their electromagnetic properties

In this paper, single-layer coated polyester–cotton composites were prepared using PU-2540 waterborne polyurethane resin as the adhesive, graphite and silicon carbide as functional particles, and adopting a coating technology on the plain polyester–cotton fabric. First, the single-layer graphite-coated polyester cotton composite was prepared with graphite as the functional particle, and the influence of graphite content on the reflection loss and shielding effectiveness was studied. When the applied electric field frequency is 1610 MHz and the graphite content is 40 wt%, the minimum reflection loss is −26 dB; when the applied electric field frequency is 39.9 MHz and the graphite content is 50 wt%, the maximum shielding effectiveness is 12 dB. Then the single-layer silicon carbide-coated polyester–cotton composite was prepared with silicon carbide as the functional particle, and the influence of silicon carbide content on the reflection loss and shielding effectiveness was studied. With the applied electric field in the range 500∼3000 MHz, the greater the content of silicon carbide, the smaller the reflection loss, the better the wave-absorbing ability, the larger the shielding effectiveness, and the better the shielding performance. Finally, the single-layer graphite/silicon carbide-coated polyester–cotton composites were prepared by doping graphite and silicon carbide in different proportions, and the influence of doping ratio on dielectric properties, reflection loss, and shielding effectiveness was investigated. The real part of the dielectric constant of the material was highest – that is, the polarization ability of the material was best when there were only graphite particles in the doping medium and the silicon carbide content was 0. The imaginary part of the dielectric constant and the tangent of loss angle of the material were the highest – that is, the loss and attenuation ability of the material were best – when the doping ratio of graphite to silicon carbide is 4:1. With the applied electric field in the range 500∼3000 MHz, and with increasing graphite content, the reflection loss of the material became smaller, showing an enhanced wave-absorbing property, and the shielding effectiveness of the material increased, showing an enhanced shielding performance.

The Effect of Laser Scanning Speed on Microstructure and Performance of Cr3C2-NiCr Cermet Fabricated by in-situ Laser Cladding

To explore the effect of laser scanning speed on the microstructure and performance of Cr3C2-NiCr cermet layers fabricated by in-situ laser cladding, Cr3C2-NiCr cermet layers were laser cladded from Ni/Cr/Graphite (25:65:10 wt.%) elemental powder mixtures. The microstructures of the laser cladded cermet layers and the formation mechanism were investigated. In addition, the effect of laser scanning speed on the microstructure, friction and corrosion performance of the Cr3C2-NiCr cermet layers was studied. The results indicated that the in-situ laser cladded Cr3C2-NiCr cermet layers were composed of NiCr binder and Cr3C2. The laser scanning speed had a significant influence on the carbide content, composition and size. Furthermore, it affected the in-situ laser cladded cermet layer’s hardness and wear resistance. The corrosion resistance of the in-situ laser cladded cermet layer was superior to that of laser cladded nickel-based alloy and was improved with decreasing laser scanning speed.

Effect of Initial Microstructure on Soft Annealing of a Low-Carbon Bainitic Steel

A low-carbon bainitic tool steel exhibiting high hardness after hot rolling typically has poor machinability. To soften this type of steel and to accelerate the soft annealing process, an austenitizing step was designed based on thermodynamic calculations of phase stability and introduced prior to the annealing step. Different initial microstructures were prepared by three austenitizing temperatures (680 °C, 850 °C, 1000 °C) and three cooling methods (water quenching, oil quenching, and air cooling). The effect of initial microstructure on microstructures and hardness was studied. Softening equations, a function of annealing temperature and time, were established for different initial microstructures, and the relationships between annealing temperature, annealing time, activation energy, and hardness were explored. The predicted hardness was consistent with the measured values. Martensitic structure has a low activation energy for diffusion and a higher softening rate compared to that of the bainitic structure. In addition, the higher the carbide content in the bainitic structure, the smaller the activation energy tended to be.

TiC – Cr3C2 – WC – NiCr – Mo – C cermet plasma coatings

Two bulk cermets TiC – WC – Cr3C2 – Ni 20 % Cr – Mo – 2.8% C after liquid-phase sintering at 1400 °C for 1 hour were used to manufacture powders for plasma spraying of coatings. Cermets were obtained with limited time of mechanical alloying at the stage of mixing. Plasma coatings were sprayed on a setup with a nozzle attached to a plasmatron for local protection of the sprayed particles from the air atmosphere. The content of WC – Cr3C2 – C in the cermets provided compensation for carbon losses at all stages of coating production and the formation of an annular zone, the volume of which determines the increase in the TiC content in the coatings by 20 % and the formation of additional carbides in the matrix. The microhardness of cermet with an initial carbide content of 60% is 15.26 – 16.83 GPa with a load on the indenter of 200 G and 20.91 – 24.68 GPa with a load on the indenter of 20 G, the difference was explained by a scale factor. The contribution of the microhardness of carbides to the microhardness of cermet with an initial carbide content of 60% was estimated according to the rule of mixtures, proceeding from their volume fraction and microhardness of cermet under a load on the indenter of 20 G. In the initial powder for spraying, this contribution is high, 33.19 GPa, close to Hardness TiC. The contribution of microhardness of carbides in the coating is lower, 28.09 GPa.

Formation of Cemented Tungsten Carbide Layer with Compositional Gradient Processed by Directed Energy Deposition

In order to partially improve the wear resistance for dies and jigs, layer-forming technologies for adding cemented tungsten carbide is effective way as an additive manufacturing technique. As one of the layer-forming technologies, directed energy deposition (DED) has attracted much attention from industry. However, many cracks are usually observed inside a formed layer processed by the DED due to the difference of material properties between an iron-based substrate and a cladded cemented tungsten carbide layer. In terms of durability of the formed layer, the crack formation is not preferable as well. In this study, as an attempt to suppress the crack initiation and propagation in cladded layers processed by the DED, formation of compositionally graded cemented tungsten carbide layer was performed by inserting a layer with low to medium tungsten carbide content between the substrate and cemented tungsten carbide layer. At first, single layers of cemented tungsten carbide having various tungsten carbide content were formed on iron-based substrates by the DED processing to understand the relationships between the tungsten carbide content and the number of formed cracks. By considering these experimental results, the compositionally graded cemented tungsten carbide layers were optimized. It was revealed that the crack initiation and propagation could be suppressed by compositionally graded cemented tungsten carbide layers.

As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy

The microstructures of an electron beam melted (EBM) nickel-based superalloy (Alloy 718) were comprehensively investigated in as-built and post-treated conditions, with particular focus individually on the contour (outer periphery) and hatch (core) regions of the build. The hatch region exhibited columnar grains with strong 〈001〉 texture in the build direction, while the contour region had a mix of columnar and equiaxed grains, with no preferred crystallographic texture. Both regions exhibited nearly identical hardness and carbide content. However, the contour region showed a higher number density of fine carbides compared to the hatch. The as-built material was subjected to two distinct post-treatments: (1) hot isostatic pressing (HIP) and (2) HIP plus heat treatment (HIP + HT), with the latter carried out as a single cycle inside the HIP vessel. Both post-treatments resulted in nearly an order of magnitude decrease in defect content in hatch and contour regions. HIP + HT led to grain coarsening in the contour, but did not alter the microstructure in the hatch region. Different factors that may be responsible for grain growth, such as grain size, grain orientation, grain boundary curvature and secondary phase particles, are discussed. The differences in carbide sizes in the hatch and contour regions appeared to decrease after post-treatment. After HIP + HT, similar higher hardness was observed in both the hatch and contour regions compared to the as-built material.