Fine-particle slurry wear resistance of selected tungsten carbide thermal spray coatings

The surface degradation of tungsten carbide based thermal spray coatings when exposed to fine-particle slurry abrasion has been investigated. The coatings that were studied contain binder-phase constituents consisting of either nickel or cobalt. The coatings were deposited onto test cylinders using a detonation gun device. After applying approximately 0.15 mm thickness of thermal spray coating, the coatings were ground, then diamond polished to achieve surface roughnesses of 0.03 μm Ra or less. The coatings were exposed to a three-body abrasive wear test involving zirconia particles (less than 3 μm diameter) in a water-based slurry. Results show that preferential binder wear plays a significant role in the wear of these tungsten carbide coatings by fine abrasives. In the comparison presented here, the coating containing nickel-based binder with a dense packing of primary carbides was superior in terms of retaining its surface finish upon exposure to abrasion. The coating containing a cobalt binder showed severe surface degradation.     

Machining of sintered steel under different lubrication conditions

Cutting experiments of sintered steel Fe–2Cu–0.8C under different lubrication/cooling conditions and with different tool materials were performed in order to clarify the reasons for the low machinability of the sintered steel, and investigate the applicability of minimum quantity of lubricant (MQL) to the machining of sintered steel. As a theoretical approach to the short tool life in machining sintered steel, the impacts to the cutting edge caused by micro-voids were theoretically modeled, and it was revealed that the cutting edge suffered from impacts at a very short interval causing high cycle fatigue. Extremely short tool life in dry machining of sintered steel already used for wet machining proved that coolant definitely increased the stress amplitude in high cycle fatigue. Application of the air jet to a tool showed the positive effect in reducing tool wear. This indicated the possibility that micro-cracks nucleated by high cycle fatigue were healed by oxidation. MQL cutting with small amount of oil and air jet is proved to be acceptable for machining sintered steel from the results of cutting experiments using uncoated and coated carbide and cermet tools. Finally, it is confirmed that thick coating of a carbide tool is effective for dramatically improving the machinability.     

钴基合金-碳化钨复合涂层材料耐磨性能的研究

采用真空熔烧法制得钴基合金—碳化钨复合涂层材料,借助扫描电子显微镜、X射线衍射仪等先进的测试手段对涂层的组织结构和表面形貌进行观察分析。应用盘销式摩擦磨损试验机对不同碳化钨质量分数的复合涂层材料和淬火态45钢进行了磨损试验。结果表明:在相同试验条件下,复合涂层的耐磨性显著高于淬火钢,且其耐磨性随碳化钨质量分数的增加而提高:淬火钢的耐磨性随着载荷的增加迅速降低,而复合涂层的耐磨性则变化不大。     

Friction and wear behaviour of sintered steels submitted to sliding and abrasion tests

Fe–C–Mo and Fe–C–Cr steels were sintered by PM processes carried out using different values of temperature and pressure, leading to different microstructures and density values. Flat specimens were submitted to tribological tests in order to evaluate their behaviour under both dry sliding and abrasive wear conditions. A flat-on-cylinder tribometer was used for the sliding tests, while a micro-scale ball cratering device was used for the abrasion tests. The dry sliding wear resistance of the PM steels was mainly influenced by the composition and sintering conditions. In this regard, the best behavior was observed for the more hardenable Fe–C–Mo steels with higher Mo content, sintered under conditions giving rise to bainitic microstructures. A determining role was also played by the porosity content and pore shape: reduction in porosity (obtained by increasing the sintering temperature and the compacting pressure), as well as an increase in pore roundness, led to a significant improvement in the resistance to sliding wear. A mild oxidative wear regime were observed for all the sintered steels under relatively low values of the applied load, while an increase of the applied load led to a delamination wear regime. The resistance to abrasive wear was low for all the tested steels, irrespective of composition and sintering cycle.     

Abrasion of cemented carbides by small grits

The abrasion properties of a series of cemented carbides with different carbide grain sizes, different amounts and types of binder phases have been investigated under varied conditions. Abrasion results from other works are also incorporated for comparison reasons. The results are interpreted in the light of a previously published model for the abrasion properties of multiphase materials, although this is the first time this model is applied to materials with very high amounts of hardphase. It is confirmed that the abrasion resistance of tungsten carbide–cobalt materials may vary considerably, also for fixed amounts of metal and hardphase. Not only the wear resistance level but also the ranking depends both on the test conditions and on the microstructure. It is further showed that some nano-crystalline materials posses a wear resistance superior to those of the pure carbide material.     

Early stages of wear of copper steel, and electroless nickel against

The early stages of wear of copper, mild steel, an alloy steel, and electroless nickel in the as-deposited and heat-treated conditions, with a load of 50 g against a dry nitrided steel wheel with a surface speed of 0.28 m s⁻¹, were measured using a Talysurf and a precision relocation technique, and the worn surface was examined using scanning electron microscopy. Wear occurred mainly by abrasion. Extensive ploughing with cooper resulted in metal removal and redeposition on the surface. Rapid and severe removal of patches of material occurred witb mild steel, and fragments of the metal were redeposited on the surface. As-plated electroless nickel formed many small cracks in the wear tracks. These were absent in the heat-treated coating when surface damage occurred by brittle fracture of the Ni₃P. Profilometry results taken during the wear tests were analysed and related to the microscopical results and the wear processes.     

超音速火焰喷涂WC-Co涂层耐磨性研究

利用超音速火焰喷涂（HVOF）工艺制备了WC-Co涂层，测定了涂层孔隙率、显微硬度及干摩擦磨损过程中涂层材料失重，得出涂层干摩擦因数随时间的变化关系，分析了涂层摩擦磨损机制。结果表明，WC-Co涂层致密，平均孔隙率为1．29％，显微硬度达1140HV（测试载荷2．94N），干摩擦条件下材料失重低于电镀Cr镀层2个数量级；摩擦初期，干摩擦因数迅速增加，主要磨损特征是粘结相富Co区的犁沟切削，摩擦中后期，摩擦副间实际接触面积增大，摩擦因数变化较小，磨损趋于稳定。WC-Co涂层的主要磨损机制是疲劳磨损和犁沟切削。     

Abrasive wear resistance of starch consolidated and sintered high speed steel

The abrasive wear resistance of starch consolidated (SC) and super solidus liquid phase sintered (SLPS) M3/2 high speed steel (HSS) samples have been evaluated by a two-body micro-abrasion test (low stress abrasion), using 6 μm diamond abrasive particles, and a three-body abrasion test (high stress abrasion), using significantly larger abrasive particles of blast furnace slag (600 HV) and silicon carbide (2400 HV), respectively. In the tests a commercial powder metallurgical (PM) HSS was used as a reference material.The results show that the microstructure of the SC and SLPS HSS samples is strongly dependent on the sintering temperature used. With increasing temperature the microstructure ranges from a porous (5% porosity) relatively fine grained low temperature sintered microstructure to a fully dense relatively coarse grained high temperature sintered microstructure with eutectic carbides/carbide networks. However, despite the pronounced microstructural differences displayed by the as-sintered HSS microstructures these show a relatively high abrasive wear resistance, comparable with that of a HIPed HSS reference, both under low and high stress abrasion contact conditions. The characteristic features of the low and high temperature sintered microstructures, i.e. the pores and coarse eutectic carbides/carbide networks, only show a limited impact on the wear rate and the wear mode (dominant wear mechanism). The results obtained imply that near net shaped components manufactured by starch consolidation and super solidus liquid phase sintering might be of interest in tribological applications.     

Assessment of the erosion resistance of steels used for slurry handling and transport in mineral processing applications

The wear environment of steels used for containing, transporting and processing erosive mineral slurries is often such that fluid borne particles form a layer moving at high speed across the wearing surface. Information on the performance ranking of such materials is limited, particularly with respect to the influence of steel hardness and microstructure on the resistance to erosion. This is particularly important for the oil sands industry of Northern Alberta where handling and processing of essentially silica-based solids results in extremely severe wear conditions. This paper presents slurry erosion data obtained on 11 commercially available wear resistant plate and pipeline steels with hardness values up to 750 HV. These data were obtained using a Coriolis erosion tester operated at 5000 rpm with an aqueous slurry containing 10 wt.% of 200–300 μm silica sand particles.

The Coriolis erosion tester was selected because it provides a low-angle scouring action that simulates the erosive conditions encountered in oil sands and tailings pipeline transport and in some related processing operations. Results show that this test method is able to discriminate clearly between the erosion resistance of these steels, expressed in terms of specific energy (the energy necessary to remove unit volume of test material), with the most erosion resistant steel being more than five times superior to the least resistant. A graphical relation between steel hardness and erosion resistance is given. A comparison is also made between slurry erosion data and the performance of the materials in the ASTM G65 dry sand rubber wheel (DSRW) sliding abrasion test. Comments on the influence of the macro- and microstructures of the steels on their wear behaviour are included.     

Erosion–corrosion assessment of materials for use in the resources industry

The erosion–corrosion properties of a range of ferrous-based materials that are currently being used or have potential for use in the resources industry have been assessed using a slurry pot erosion–corrosion (SPEC) test rig that has the capability of establishing the separate components of erosion, corrosion and synergy.Testing was performed, at 30 °C, in an aqueous slurry containing 35 wt% AFS 50–70 silica sand and a 3.5 wt% NaCl solution. Erosive action was supplied through high-speed rotation of a rubber-lined impeller.Erosion–corrosion performance of materials evaluated was related to composition/microstructure and hardness. Test data correlated with available service experience.The results showed that the cast Cr white irons with (i) a structure that was essentially a duplex stainless steel containing a distribution of hard carbides and (ii) a near eutectic Cr white iron exhibited the highest erosion–corrosion resistance of the materials tested. The evaluation of the Cr white irons also highlighted the influence of Cr and C levels on the E–C properties of these materials.E–C assessment of selected carbon steels confirmed that the erosion-only rates and synergistic levels showed a general decline with increasing carbon content and hardness. As expected, a low C steel pipe product displayed very mediocre erosion–corrosion behaviour as a consequence of its very low intrinsic corrosion resistance and inferior wear properties. This reflected service experience, however, such products are still being used, due to the comparatively low initial cost.A TiC particle-reinforced AISI 316 stainless steel exhibited an almost 45% improvement in the E–C resistance, when compared with an AISI 316L stainless steel.