Increasing wear resistance of the protective sleeve of the centrifugal pump made of gray cast iron by surface hardening

One of the weak spots that reduce the guaranteed operating time of centrifugal pumps is the assembly that includes the seal and protective sleeves. The main purpose of the bushings in the rotor kit is to protect the shaft from corrosion, erosion and wear. The sleeve operates under abrasive wear conditions by packing elements and abrasive particles that get into the liquid pumped by the pump. The protective sleeves made of gray cast iron do not meet the service life in connection with accelerated surface wear. Goal. The purpose of the work is to ensure high wear resistance of cast iron pump parts by heat treatment. Method. The chemical composition was determined on a portable laser analyzer Laser Z200 C +. The structure was studied using an optical microscope. The wear was investigated by the roller-block method on a friction machine. Surface quenching was carried out with a high-frequency lamp generator in a single-turn inductor. Results. The microstructure of gray cast iron as cast consists of pearlite, ferite, double phosphide eutectic and inclusions of lamellar graphite. Microhardness of small-lamellar pearlite is Н50 = 1550–2220 MPa, microhardness of phosphide eutectic is Н50 = 6500–8000 MPa. Surface induction quenching by high-frequency currents followed by low tempering is an effective way to increase the wear resistance of cast iron products of small cross-section and does not cause warping. Cast iron for surface quenching should have a pearlite structure, and graphite should be contained in the form of small inclusions. The optimal heating temperature for induction hardening was determined as 900 °С, which made it possible to obtain the microstructure of the hardened layer – martensite, double phosphide eutectic and graphite. Scientific novelty. The developed modes of surface quenching make it possible to preserve a double phosphide eutectic in the structure of the surface layer, which makes it possible to obtain high hardness and wear resistance of the hardened layer. Practical significance. Surface induction quenching with low tempering of cast gray iron increased its wear resistance by 2,4 times. The previous normalization had practically no effect on the durability of cast iron, since its structure contains less than 10% ferrite.

NaYF4-based upconverting nanoparticles with optimized phosphonate coatings for chemical stability and viability of human endothelial cells

The increasing interest in upconverting nanoparticles (UCNPs) in biodiagnostics and therapy fuels the development of biocompatible UCNPs platforms. UCNPs are typically nanocrystallites of rare-earth fluorides codoped with Yb3+ and Er3+ or Tm3+. The most studied UCNPs are based on NaYF4 but are not chemically stable in water. They dissolve significantly in the presence of phosphates. To prevent any adverse effects on the UCNPs induced by cellular phosphates, the surfaces of UCNPs must be made chemically inert and stable by suitable coatings. We studied the effect of various phosphonate coatings on chemical stability and in vitro cytotoxicity of the Yb3+,Er3+-codoped NaYF4 UCNPs in human endothelial cells obtained from cellular line Ea.hy926. Cell viability of endothelial cells was determined using the resazurin-based assay after the short-term (15 min), and long-term (24 h and 48 h) incubations with UCNPs dispersed in the cell-culture medium. The coatings were obtained from tertaphosphonic acid (EDTMP), sodium alendronate, and poly(ethylene glycol)-neridronate. Regardless of the coating conditions, 1−2 nm-thick amorphous surface layers were observed on the UCNPs with transmission electron microscopy. The upconversion fluorescence was measured in the dispersions of all synthesized UCNPs. Surface quenching in aqueous suspensions of the UCNPs was reduced by the coatings. The dissolution degree of the UCNPs was determined from the concentration of dissolved fluoride measured with ion-selective electrode after the aging of UCNPs in water, physiological buffer (i.e., phosphate-buffered saline – PBS), and cell-culture medium. The phosphonate coatings prepared at 80 °C significantly suppressed the dissolution of UCNPs in PBS, while only minor dissolution of bare and coated UCNPs was measured in water and cell-culture medium. The viability of human endothelial cells was significantly reduced when incubated with UCNPs, but it increased with the improved chemical stability of UCNPs by the phosphonate coatings with negligible cytotoxicity when coated with EDTMP at 80 °C.

Effects of Laser Energy Density On Carbide Dissolution, Element Distribution and Microstructure Evolution of AISI P20 Steel During Laser Surface Quenching

Laser surface quenching (LSQ) was performed on AISI P20 mould and hot-working die steel with an objective to improve surface characteristics. The steel was treated under three different process parameter conditions. The microstructure, element distribution and residual stresses were investigated through SEM, EDS and XRD analyses. The effect of laser energy density on carbide dissolution/ablation, microstructure evolution were thoroughly investigated. The dissolution/ablation of carbides significantly affected the formation of martensite and retained austenite, and the distribution of elements and phase in the microstructure. The results of the study and analyses of treated surface revealed that the LSQ treatment significantly improved the microstructure, eliminated the pores or other defects. Furthermore, the degree of carbide dissolution/ablation was closely related to the laser energy density. Comparing to Cr7C3, Cr3C2 was more difficult to dissolve at lower laser energy density. Thus, those incompletely dissolved Cr3C2 would hinder the growth of austenite and reduce the carbon content in austenite and lead to the formation of low-carbon martensite. The highest laser energy density (150 J/mm2), was able to produce finer microstructure and significantly reduced the inhomogeneity in distribution of Cr between the poor and the rich Cr areas.

Increasing the service life of the equipment by electromechanical surface quenching

The paper analyzes traditional methods of hardening. The results of comparative wear tests of the samples made of 20KhNZA steel after nitrocementation and electromechanical surface quenching are presented. The results of the wear tests of the samples made of steel 20KHN3A after nitrocementation and electromechanical surface quenching indicate the high efficiency of the electromechanical processing technology. A competitive feature of the technology of electromechanical surface quenching of parts made of 20KHNZA steel is the possibility of flexible control of the parameters of high-speed electric heating and simultaneous thermoplastic deformation of the material of the surface layer of the most loaded parts in order to form gradient fast-quenched structures at a depth of up to 0.05…0.1 mm in the manufacture and repair of parts. Conclusions and recommendations on the method of using these types of hardening are providedю English version of the article is available at URL: https://panor.ru/articles/increasing-the-equipment-service-life-by-electromechanical-surface-quenching/64911.html

Engineering Red-Enhanced and Biocompatible Upconversion Nanoparticles

The exceptional optical properties of lanthanide-doped upconversion nanoparticles (UCNPs) make them among the best fluorescent markers for many promising bioapplications. To fully utilize the unique advantages of the UCNPs for bioapplications, recent significant efforts have been put into improving the brightness of small UCNPs crystals by optimizing dopant concentrations and utilizing the addition of inert shells to avoid surface quenching effects. In this work, we engineered bright and small size upconversion nanoparticles in a core–shell–shell (CSS) structure. The emission of the synthesized CSS UCNPs is enhanced in the biological transparency window under biocompatible excitation wavelength by optimizing dopant ion concentrations. We also investigated the biosafety of the synthesized CSS UCNP particles in living cell models to ensure bright and non-toxic fluorescent probes for promising bioapplications.

Influence of surface quenching on morphology and phase composition of ferritic-pearlitic steel

The study was carried out by means of transmission electron microscopy on thin foils to investigate the changes in matrix morphology and phase composition occurring in ferritic-pearlitic steel of St2 grade (Russian) under plasma electrolytic surface quenching. In the original state St2 steel is a material which underwent quenching under the temperature of 890 °C (2 – 2.5 h) with cooling into warm water (30 – 60 °C) and further tempering under the temperature of 580 °С (2.5 – 3 h). Surface quenching was conducted in aqueous salt solution during 4 seconds under the temperature of 850 – 900 °C, voltage of 320 V, and current rate of 40 A. In the original state morphological components of the steel matrix were lamellar pearlite and non-fragmented and fragmented ferrite. Surface quenching resulted in the following transformations of morphology and phase composition: 1 – to martensitic transformation (morphological components are lath martensite, lamellar low-temperature and high temperature martensite), 2 – to steel self-tempering (inside all martensite crystals there are thin plate-like precipitations of cementite), 3 – to diffusion transformation γ → α and precipitation of retained austenite (γ-phase) given as thin layers along the boundaries of laths and plates of low-temperature martensite and inside all the crystals of lamellar martensite in the shape of “needles” like in twin type colonies. Surface quenching led to precipitation of special carbides of Мe23С6 phase. It was revealed that carbide precipitation is attributed primarily to decomposition of retained austenite and martensite and also to partial dissipation of cementite and, moreover, it is due to carbon removal from dislocations and the boundaries of α-phase crystals. That means that in all cases carbon from retained austenite, α-solid solution, cementite particles and defects of crystal lattice is used for the formation of special carbides.

Residual Stress Distribution of Non-Uniform Quenching in Al-Zn-Mg-Cu Aluminum Alloy Thick Plate

Effect of multi-section linear non-uniform heat transfer coefficient on quenching residual stress distribution in 27mm-thick Al-Zn-Mg-Cu aluminum alloy plate was simulation studied by using the finite element method, and the surface quenching residual stress distribution was measured by the X-ray diffraction method and hole-drilling method. The results show that the surface quenching residual stress represents the same distribution with non-uniform heat transfer coefficient in the transverse direction and the stress level maintains initial stress level of the heat transfer coefficient at each location. The distribution of the quenching residual stress in the center of the plate is approximately uniform and the stress level is approximately equal to average of maximum and minimum initial stress level. The measured surface quenching residual stress shows a wavy distribution in the transverse direction, which is similar to the simulated surface stress distribution without considering the stress level. The measurement results can be explained by the multi-section linear non-uniform quenching model.

Surface Electric Arc Hardening of Low-Carbon Steels

Examined geometric characteristics, microhardness and features of structure formation in the heat affected zone of steels 09G2, 20L, 20FL. These studies were carried out after surface quenching by a magnetically controlled (scanning) DC electric arc in a protective argon atmosphere. It is shown that electric arc hardening forms on the treated surface of the steel a thin layer of martensitic-austenitic structure with varying composition and hardness. A ferrite-austenitic structure is formed in the region of transition from the base metal to the heat-strengthened metal. This structure contains crushed ferrite grain and winding boundaries between the structural components. On the periphery of austenitic grains martensitic layer is observed. Repeated heating, occurring during heat treatment of the adjacent surface area, is accompanied by a partial decay of martensite and austenite of a pre-hardened structure with the formation of bainite-and sorbitol-like tempering structures. On the surface, experienced repeated heating, the volume fraction of austenite increases. The dependences allowing to control the structural state and depth of the hardening zone are established.