Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

Tribological properties of pulsed laser deposited Mo-S-Te composite films at moderate high temperatures

Tribological investigations of Mo-S-Te composite films were conducted on films grown at room temperature by pulsed laser deposition. The chemistry and microstructure of the films were characterized by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, and micro Raman spectroscopy. The films showed a granular morphology and a preferred basal plane growth of 2H-MoS₂ parallel to the substrate after annealing at high temperatures. The friction coefficients of the films were 0.05 at 300 °C and 0.10 at 450 °C for more than 10,000 cycles in air. Smeared hexagonal MoS₂ lubricant films were observed inside wear tracks while the tribochemical formation of wear debris occurred both inside and outside the wear tracks. The Te additives for increasing the film durability were proposed to slow oxidation of the lubricants at elevated temperatures by thermally-induced tellurium migration to the surface and the subsequent formation of the Te diffusion barrier. This mechanism could be significantly effective in high-temperature tribotests because of the increased tellurium mobility at high temperatures.     

Tribological behaviour of titanium carbide/amorphous carbon nanocomposite coatings: From macro to the micro-scale

The tribological behaviour of nanocomposite coatings made of nanocrystalline metal carbides and amorphous carbon (a-C) prepared by PVD/CVD techniques is found to be very dependant on the film deposition technique, synthesis conditions and testing parameters. Focusing in the TiC/amorphous carbon-based nanostructured system, this paper is devoted to an assessment of the factors governing the tribological performance of this family of nanocomposites using a series of TiC/a-C films prepared by magnetron sputtering technique varying the power applied to each target (titanium or graphite) as model system to establish correlations between film microstructure and chemical compositions and tribological properties measured by a pin-on-disk tribometer. The film microstructure goes from a quasi-polycrystalline TiC to a nanocomposite formed by nanocrystals of TiC embedded in an amorphous carbon matrix as observed by transmission electron microscopy (TEM). The nanocrystalline/amorphous ratio appears to be the key-parameter to control the tribological properties and its quantification has been done by electron energy-loss spectroscopy (EELS). A significant change in the tribological performance is observed for nanocomposites with amorphous carbon phase contents above 60–65%. The friction coefficient decreases from 0.3 to 0.1 and the film wear rates by a factor of 10. Examination of the wear scars on ball and film surfaces by laser micro-Raman spectroscopy has allowed to determine the presence of metallic oxides and carbonaceous compounds responsible of the observed friction behaviour. The revision of the literature results in view of the conclusions obtained enabled to explain their apparent dispersion in the tribological performance.     

含硫高速钢的高温摩擦性能(英文)

引入适量的硫可以明显改善铁基材料的摩擦学性能。采用粉末冶金技术制备了含硫高速钢样品材料,采用销盘高温摩擦试验机研究了该样品材料与不同配副(自对偶和GT35钢结硬质合金)摩擦时的高温摩擦性能,通过SEM、EDS和XPS检测了摩擦面,并分析了其摩擦机理。结果表明:与自对偶盘和GT35盘这2种对偶材料摩擦时,摩擦因数的变化趋势是随着温度的升高而逐渐降低的,磨损率则是随着温度的升高而逐渐变大的;在400℃和700℃时的摩擦因数曲线比室温时更加平稳,与GT35摩擦时样品材料的综合高温摩擦性能最佳;在400℃以下时,由于样品材料中硫化物的软化会使得材料的摩擦因数随着温度的升高而降低,在400℃以上时样品材料摩擦因数的降低主要是由于摩擦面的氧化物作用而引起的。     

Development of in-situ composite surface on mild steel by laser surface alloying with silicon and its remelting

In the present study, an attempt has been made to develop in-situ iron silicide dispersed surface on mild steel substrate by laser surface alloying with silicon using a high-power continuous wave CO₂ laser. The effect of laser surface remelting of the alloyed surface using argon and nitrogen (with and without graphite coating) as shrouding environment has also been studied. The microstructure of laser surface alloyed mild steel with silicon consists of uniformly dispersed iron silicide in grain refined α-iron matrix with an improved microhardness to 575 VHN as compared to 150 VHN of as-received mild steel substrate. Surface remelting in Ar atmosphere coarsened the microstructure and reduced the area fraction of silicide and hence, reduction in the microhardness to 450 VHN. Surface remelting in nitrogen increased the microhardness to 740 VHN due to the formation of iron nitrides in addition to the presence of silicides. Graphite coating prior to remelting improved the microhardness to 800 VHN due to the presence of martensites along with nitrides and silicides. A maximum enhancement in wear resistance was achieved when remelting was done in nitrogen environment with carbon deposition. The mechanism of wear was found to be predominantly abrasive in nature as compared to adhesive and oxidative in as-received mild steel.     

Modification of nitride layer on cold-work tool steel by laser melting and friction stir processing

The microstructural control of the nitrided case on a cold-work tool steel (SKD11) plate by laser melting and friction stir processing (FSP) was studied. The laser melting and the FSP were used as a pretreatment for the nitriding to refine the microstructure of the SKD11. The diffusion zone of the nitrided case on the SKD11 plate contained thick brittle boundary lines consisting of local formed nitride particles. On the other hand, the microstructure of the diffusion zone was very uniform and the thick brittle boundary lines disappeared as a result of the combined treatment of the laser melting and the FSP before the nitriding process.     

Friction surfaced tool steel (H13) coatings on low carbon steel: A study on the effects of process parameters on coating characteristics and integrity

Tool steel H13 was friction surfaced on low carbon steel substrates. Mechtrode (consumable rod) rotational speed and substrate traverse speed were varied, keeping the axial force constant. The effects of process parameters on coating characteristics and integrity were evaluated. A process parameter window was developed for satisfactory deposition of tool steel coatings. Coating microstructures were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy. Microhardness tests, shear tests, and bend tests were conducted on coatings. The results show that coating width is a strong function of mechtrode rotational speed, while coating thickness is mainly dependent on substrate traverse speed. Lower mechtrode rotational speeds results in wider coatings, while higher substrate traverse speeds produce thinner coatings. Thinner coatings exhibit higher bond strength than thicker coatings. Coatings show no carbide particles, yet exhibit excellent hardness (above 600 HV) in as-deposited condition due to their martensitic microstructure.     

Impact of tool wear on joint strength in friction stir spot welding of DP 980 steel

Abstract

Friction stir spot welding has been shown to be a viable method of joining ultra high strength steel, both in terms of joint strength and process cycle time. However, the cost of tooling must be reasonable in order for this method to be adopted as an industrial process. Recently a new tool alloy has been developed, using a blend of polycrystalline cubic boron nitride (PCBN) and tungsten rhenium (W–Re) in order to improve the toughness of the tool. Wear testing results are presented for two of these alloys: one with a composition of 60% PCBN and 40% W–Re, and one with 70% PCBN and 30% W–Re. The sheet material used for all wear testing was 1·4 mm DP 980. Lap shear testing was used to show the relationship between tool wear and joint strength. The Q70 tool provided the best combination of wear resistance and joint strength.     

High cycle fatigue behaviour of Ti–6Al–4V alloy at elevated temperatures

High cycle fatigue behaviour of Ti–6Al–4V alloy was studied at 623 K and 723 K. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. In the short life regime, fatigue strength was lower at 723 K than at 623 K, but in the long life regime it was nearly the same at both temperatures. At elevated temperatures, cracks were generated earlier at applied stresses below the fatigue limit at ambient temperature, indicating lowered crack initiation resistance. Small cracks grew faster at elevated temperatures than at ambient temperature, which became more noticeable with increasing temperature. After allowing for the elastic modulus, small cracks still grew faster at elevated temperatures.     

Friction stir welding of 304 stainless steel using Ir based alloy tool

Abstract

In order to investigate the durability of an Ir based alloy tool, friction stir welding (FSW) of 304 stainless steel plates was performed under various welding conditions, and the mechanical properties of the joints were measured. Defect free joints are possible under certain conditions, and the mechanical properties of the joints are the same as the base material. When the rotation pitch is ～0·3, the degree of wear of the Ir based alloy tool with 1·4 mm probe height, which was used for the FSW of a 304 stainless steel plate with 2 mm thickness, was less than or equal to that of the polycrystalline cubic boron nitride tool with 2 mm probe height, which was used for the FSW of a 304 stainless steel plate with 9 mm thickness. The result of the tool life test in this study showed that the Ir based alloy tool enabled the FSW of 304 stainless steel over a 75 m length.     

热喷涂合成Fe3Al基涂层的高温摩擦学特性

采用自制的新型热喷涂粉芯丝材和高速电弧喷涂技术成功制备出Fe3Al金属化合物基合金涂层，对涂层的组织、成分和相结构进行了初步分析，并测试了涂层在室温至650℃的摩擦学特性。结果表明：Fe3Al涂层的基体相由含29％Al(摩尔分数)的Fe3Al和FeAl二相混合物组成，Fe3Al／WC涂层基体为Fe-26％Al(摩尔分数)合金；添加少量WC硬质相的Fe3Al／WC涂层具有良好的高温减摩特性和耐磨性，450℃和650℃其相对耐磨性分别比20g钢高1．92倍和9．23倍；WC／W2C硬质相对Fe3Al基体的强化作用，以及涂层表面形成具有自保护作用的高硬度致密氧化膜是Fe3Al／WC涂层高温耐磨性提高的主要原因。     

Influence of the metallic matrix ratio on the wear resistance (dry and slurry abrasion) of plasma sprayed cermet (chromia/stainless steel) coatings

This work is related to the manufacturing and tribological testing of plasma sprayed cermet coatings of chromium oxide and stainless steel in order to obtain wear resistant coatings to dry and slurry abrasion. Raw materials were fused and crushed powders of chromium oxide (Cr₂O₃) with a particle size ranging from 20 to 45 μm and gas atomized stainless steel (iron base with 17 wt.% of Cr and 12 wt.% of Ni) with a particle size distribution between 20 and 53 μm. Both powders were simultaneously injected with two separated injectors in a direct current (DC) plasma jet (Ar-H₂ (25 vol.%) at atmospheric pressure (APS).The influence, on the coating micro-structural and tribological properties, of various stainless steel weight percentages in chromium oxide has been studied. All coatings exhibited a lamellar structure with a random distribution of the two materials. The effect of the percentage of stainless steel on the microstructure of the coating, studied by scanning electron microscopy (SEM), has shown that increasing the stainless steel percentage increased the coating cohesion. The increase of Cr₂O₃ in the coatings resulted in higher hardness and in lower weight losses during wear tests in dry abrasion. The study has also shown that the optimum stainless steel percentages in coatings were not identical to reach their maximum resistance to slurry or dry abrasion.     

Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate

The principle aim of this study is to investigate the wear behaviour of FeCr coatings on Ni-based bond deposited plain carbon steel substrate for several applications in power generation plants. For this purpose, FeCr and Ni-based powders were sprayed on plain carbon steel substrates using a thermal flame spray technique. Fabricated layers were characterized by using a X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), microhardness and surface roughness testers. FeCr coatings were subjected to sliding wear against AISI 303 stainless steel counter bodies under dry and acidic environments. A pin-on-plate type of apparatus was used with normal loads of 49 and 101 N and sliding speed of 1 Hz. XRD results revealed that FeCr, Fe, Cr, Fe–Cr–Ni, γ-Fe₂O₃ and Fe₃O₄ phases are exist in the coating. In addition, some inhomogenities such as oxides, porosity, cracks, unmelted particles and inclusions were observed by SEM. The surface morphologies of FeCr samples after wear experiments were examined by SEM and EDS. It was found that friction coefficients of the coatings in dry condition are higher than that in acidic environment.     

Wear behaviour of steel coatings produced by friction surfacing

Friction surfacing was performed to produce multi-layer coatings of AISI 1024, AISI 1045 and AISI H13 over mild steel substrates where a continuous joining was achieved between adjacent layers and between the clad and the substrate. Microscopic and hardness characterization revealed the presence of bainitic and martensitic microstructures which influenced the hardness of the coatings. The study aimed to determine which material combination was more wear-resistant. The analysis suggested that AISI 1024 presents the least wear, both in terms of friction coefficient and wear rate. This is due to the formation of adherent protective oxide layer which is not present in both the AISI 1045 and AISI H13 steels.     

Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures

Thermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma nitriding does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe₃O₄ and Fe₂O₃ scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine, equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling.     

Cyclic softening properties of 30Cr2MoV steel at elevated temperatures

Low cycle fatigue tests for 30Cr2MoV steel are done at both room and elevated temperatures and their softening properties are investigated. Based on the experimental results, a quantitative relationship between hardness and remaining life is established, allowing prediction of the remaining life of metals. A modified Mason-Coffin equation including the aging effect is also derived. Both the experimental and theoretical results show that it is useful to be able to predict the remaining life of a steam turbine rotor through the measurement of hardness. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Deformation of oxide scale on steel surface during hot rolling

Although oxide scale has significant influence on surface quality of hot-worked products, deformation of the scale during hot working has not been understood sufficiently. The authors propose an experimental technique to observe the scale as hot-worked using glass coating, in this study. Immediately after hot rolling at 1273 K, glass powder was sprinkled over a mild steel sheet. The glass coating suppresses further oxidation and separation of the scale from the steel. Scanning electron microscopy revealed that the scale deforms uniformly if the reduction is small (r < 20%). If the reduction is high (r > 30%), steel is extruded through cracks in the scale to the outermost surfaces. The friction between the rolls and the sheet decreases with increasing scale thickness when the scale does not deform uniformly.     

Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment

A nanocrystalline surface layer was fabricated on a 38CrMoAl steel plate by means of a surface mechanical attrition treatment (SMAT). The average grain size in the top surface layer (10 μm thick) is about 10 nm, and the grain size stability can be maintained up to 450 °C. The effect of the surface nanocrystalline layer on the gas nitriding process at a lower temperature was investigated by using structural analysis and wear property measurements. The surface nanocrystallization evidently enhances nitriding kinetics and promotes the formation of an ultrafine polycrystalline compound layer. The results of the investigation showed that this new gas nitriding technique can effectively increase the hardness and wear resistance of the resulting surface layer in comparison with conventional nitriding, demonstrating a significant advancement for materials processing.     

高温后水泥基复合材料的力学性能

研究了高温后ECC(经设计的水泥基复合材料)的残余力学性能。将ECC试件分别加热到不同的温度(200、400、600和800℃),保持不同的恒温时间(0.5、1和2 h)。结果表明,试件自然冷却后受压,200℃除外,发现ECC的强度和刚度随温度和恒温时间增加而降低。采用MIP(压汞)试验测试试件的孔结构,发现试件总孔隙率和平均孔径随温度和恒温时间的增加而增加(200℃除外),这很好地解释了火后ECC试件力学性能的变化。     

Effect of simultaneous surface modification process on wear resistance of martensitic stainless steel

In order to improve the wear resistance of martensitic stainless steel, a surface treatment system was developed that combines high-frequency induction heating (IH) with fine particle peening (FPP). In this system, a compressed air spray from the FPP nozzle rapidly cools the specimen surface, which is heated by the IH system. The specimen surface can be simultaneously modified by work hardening and quenching. Vickers hardness and retained austenite measurements were conducted to characterize the surface-modified layer generated by the developed process. Surface microstructures were also observed by scanning electron microscopy (SEM) and optical microscopy. The process created a surface with a high hardness and an extremely fine-grained microstructure. The fine-grained microstructure was generated by dynamic recrystallization. The process reduced the amount of retained austenite in the surface layer because it increased the precipitated chromium carbide content. Reciprocating sliding wear tests were conducted to evaluate the wear resistance of the surface. The specimen modified by the developed process exhibited higher wear resistance than specimens that had only been quenched. This implies that the developed simultaneous process can significantly improve the wear resistance of steel surfaces.