The corrosion behaviour of WC-VC-Co hardmetals in acidic media

The effect of increasing vanadium carbide (VC) content on the corrosion behaviour of tungsten carbide - 10 wt% cobalt hardmetals was investigated in 1 M hydrochloric (HCl), and sulphuric (H₂SO₄) acids solutions. Increasing VC content makes the open circuit potential (OCP) in the test solutions more negative than the base alloy. Specimens exhibited pseudo passivation in all the test solutions. Increasing VC led to decreasing corrosion current density. However, the corrosion current densities during chronoamperometric tests were lower for 0 wt% VC. XRD and Raman spectroscopy showed that hydrated WO₃ formed in the surface films of all specimens in hydrochloric acid (HCl), while hydrated vanadyl sulphate also formed for higher VC content specimens in sulphuric acid (H₂SO₄).     

Carbide size refinement by controlling the heating rate during induction tempering in a low alloy steel

The application of high heating rates in tempering treatments can provide a valuable tool for refining carbide sizes, mainly those located at high angle grain boundaries. This work analyses the influence of heating rates ranging from 1 to 300 °C/s during the tempering treatment of a 0.42%C low alloy steel. The results indicate that when high heating rates are combined with short holding times, predicting hardness will require the inclusion of the heating up and cooling down cycles in addition to the holding time and temperature used in the definition of the conventional Hollomon–Jaffe tempering parameter (TP). The effect of heating rate on carbide size distribution has been quantified, distinguishing between particles located at high (HAB) and low (LAB) misorientation angle boundaries. The former correspond to those carbides nucleated at prior γ grain, martensite block or packet boundaries whereas the latter refer to those nucleated within martensite laths and at lath boundaries. The refinement obtained has been evaluated from the point of view of hardness behaviour.     

Rapid diffusion bonding of WC-Co cemented carbide to 40Cr steel with Ni interlayer: Effect of surface roughness and interlayer thickness

WC-Co cemented carbides were rapidly diffusion bonded to 40Cr steels with pure Ni as interlayers by utilizing plasma activated sintering (PAS). The bonding was carried out at 750 °C for 13 min under a pressure of 40 MPa. It was found that the roughness of the initial surfaces still plays an important effect on the microstructure and mechanical behavior of the joints diffusion bonded by PAS irrespective of the electric current applied during bonding. The adoption of smoother original surfaces was significantly favorable to eliminate the interfacial interstices and microvoids. Correspondingly, the shear strength of the diffusion bonded joints increased with decreasing surface roughness. Additionally, the effect of interlayer thickness on the shear strength of the joints was also evaluated, and the results showed that the strength decreased sharply when thicker interlayer was employed. A maximum value of shear strength, 293.07 MPa, was obtained when the original surfaces was ground with P1200 grit SiC paper and at the same time 50 µm thick interlayer was used. In this case, the fracture initiated and run predominantly along the bonding interfaces instead of in the WC-Co substrate.     

The effects of fine WC contents and temperature on the microstructure and mechanical properties of inhomogeneous WC-(fine WC-Co) cemented carbides

Inhomogeneous WC-(fine WC-Co) cemented carbides with improved hardness and toughness were successfully prepared through the addition of fine WC using planetary ball milling combined with sinter isostatic hot pressing (SHIP) technology. The inhomogeneous microstructure of the alloys consisted of coarsened WC grains and WC-Co consisting of fine WC dispersoids and Co binder phase. The increase of temperature and the addition of fine WC enhanced the sintering process. The morphologies of the coarsened WC and of the fine WC consisted of triangular and near-hexangular prisms, respectively. Due to crack path deflection and crack bridging, the prism-like coarsened WC crystals efficiently hindered cracks propagation. Intergranular fracture became predominant when adding fine WC. However, the excessively coarsened WC and some pores in alloys with 20 wt% fine WC could decrease the mechanical properties. The inhomogeneous WC-(fine WC-Co) cemented carbides with 10 wt% fine WC, sintered at 1430 °C for 40 min, could provide a combination of superior hardness and toughness.     

Application of titanium machining chips in welding consumables for wear-resistant hardfacing

Equipment of sugar cane plants and mineral extraction are submitted to severe abrasive wear conditions. Welded hardfacings are usually applied to repair this kind of damage, where commercial chromium/carbon-rich welding consumables have usually been employed. In the present work we investigated the microstructure of experimental hardfacings made by addition of residues (chips) collected from the machining of ASTM F67 (unalloyed Ti, grade 4) alloy. Mixtures with different carbide-formers (Cr/Nb ferro-alloys) were also tested. Two layers of ‘pure’ chips (Ti), chips plus Fe–Cr (Ti–Cr), and chips plus Fe–Nb (Ti–Nb) were applied on low-carbon steel specimens by the GTAW/TIG process. The microstructure of hardfacing layers was observed by optical and scanning electron microscopy (SEM) equipped with EDS microanalysis. The microstructural characterization has determined that carbide distributions change significantly with the chemical nature of the hardfacing. SEM observations coupled with EDS microanalysis have confirmed the formation of complex carbides within the metal weld, whose stoichiometry was determined by X-ray diffraction (XRD) analysis. Mixed carbides of MC type and some cementite have been found. As a result it was suggested that use of ASTM F67 chips as carbide formers for composition of welding consumables can contribute to improved wear resistance of hardfacings, if compared with traditional chromium-based hardfacings.     

Evaluation of mass loss of weld-deposited hardfacing with cored wire Ti-FeCrC alloy

In this article, we have studied the mass loss of hardfacing applied by flux-cored arc welding. Heat input, shielding gas and number of layers of coating were changed to application of Fe–Cr–Ti–C self-shielded tubular wire. Overlapping beads were deposited on plates of carbon steel AISI 1020 to analyse the mass loss, hardness and microstructure. For the analysis of mass loss, beads were deposited forming overlapping layers of coatings. Hardness measurements were performed on the surface of the specimens of wear. A rubber wheel abrasion tester was used to access the mass loss of coatings. The coatings had retained austenite and martensite microstructure with carbides finely dispersed in the matrix. The main factors that contributed to the increase in mass loss were the cracks due to higher cooling rate of the samples deposited with low heat input, the dilution in the first layer of all samples also contributed to the increase in mass loss. The smallest mass losses were those deposited coatings with high heat input, the second and fourth layers, the samples AC2, AC4 and AS2. The volume fraction of titanium carbides contributed to the decrease in the mean free path between the particles of carbides and increased the wear resistance of the coatings.     

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350 nm) were obtained at a relatively low temperature (1550 °C) using a HfOCl₂·8H₂O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97 wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700 °C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700 °C.     

Tool life and wear mechanism of WC–5TiC–0.5VC–8Co cemented carbides inserts when machining HT250 gray cast iron

Cutter development has drawn a lot of attention for cast iron machining in recent years. In this study, a special cemented carbide of WC–5TiC–0.5VC–8Co (WTVC8) was used for a comprehensive HT250 gray cast iron machining test. Compared with the baseline plain WC–8Co(WC8) carbides, WTVC8 shows significantly higher tool life under the same cutting conditions due to significantly higher hardness and red hardness. The worn flank face observation shows that adhesion wear and oxidation are the main wear mechanisms and there is no apparent chipping/breakage and abrasion wear for both WTVC8 and WC8. Based on Taylor's equation, the accurate tool life models for both WTVC8 and WC8 have been constructed, which shows clearly that cutting speed has the most significant effect on tool life, followed by depth of cut and feed rate. The tool life models can serve as a quantified guidance for cutting performance optimization.     

Effect of Heat Treatment on Microstructure and Stress Rupture Properties of a Ni-Mo-Cr-Fe Base Corrosion-Resistant Superalloy

The influences of heat treatment and test condition on the microstructure and stress rupture properties of a Ni-Mo-Cr-Fe base corrosion-resistant superalloy have been investigated in this paper. Optical microscope and scanning electron microscope were employed for the microstructure observation, and X-ray diffraction, electron probe micro-analyzer, and transmission electron microscope were used for phase determination. It was found that the grain size increased and the volume fractions of initial M₆C carbides decreased along with the increase in solution treatment temperature. When tested at 650 °C/320 MPa, the stress rupture lives decreased with the increase in solution treatment temperature, but the stress rupture lives increased slightly at first and then decreased for the samples solution heat treated at 1220 °C when tested at 700 °C/240 MPa. The elongations showed the descendent trends under these two conditions. The stress rupture life and elongation for the aged samples all showed a noticeable improvement at 650 °C/320 MPa, but there was no noticeable improvement at 700 °C/240 MPa. The reasons can be attributed to the grain size, test conditions, and the initial and secondary carbides.     

高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。