117 research outputs found
Cracks susceptibility elimination in fiber laser cladding of Ni-based alloy with addition of tungsten carbides
No full text"In plastic injection molding the most critical component in
terms of wear and corrosion problem is represented bythe
barrel-screw system. This is due to the friction of plastic on
the inner part of the barrel and to contact between the screw
and the inner walls of the barrel. Fiber laser cladding is a
promising technique that, through deposition of powder onto
a substrate increases wearproprieties and consequently
lifetime of the component. In particular, cladding Ni-Cr
alloy with WC is an extremely promising combination of
elements for this purpose. Specifically, WC is able to
increase the hardness of the deposit so that it becomes more
suitable for severe wear application. On the other hand, high
Cr amount contained in the Ni-based alloy, guaranties high
corrosion resistance for harsh environments. Nevertheless,
this combination of materials is highly susceptible to cracks.
Through the optimization of process parameters and preheating
temperature (250-350-450-550°C) defect free
coatingsare obtained. This work presentsan investigation on
fiber laser cladding of Ni based alloy (Ni40, Ni60) with
addition of spherical tungsten carbides (-53+20 μm) on C60
steel substrate. Optimal conditions are evaluated through
macro and a micro analysis. In the first case penetrantliquids
are used to evaluate crack presence.For the second field,
micro hardness was measured and microstructure if crosssectioned
sampleswere investigated by optical microscopy.
Microstructural evolution after cellular precipitation in a high nitrogen austenitic steel
No full textGli acciai inossidabili austenitici presentano notevoli proprietà di resistenza alla corrosione e buona formabilità. Gli acciai inox tradizionali, però, presentano dei limiti: economici collegati al crescente costo del Nichel, meccanici per il basso carico di snervamento e di biocompatibilità a causa delle allergie legate alla presenza di Ni.
Gli acciai ad alto azoto (HNS) sono caratterizzati da proprietà corrosionistiche e meccaniche superiori a quelle di numerosi acciai commerciali.
L’azoto presente in soluzione solida interstiziale produce un incremento delle proprietà meccaniche, stabilizza la fase gamma (sostituendo il Nichel) e incrementa la resistenza al pitting.
Grazie a queste proprietà tali acciai possono avere applicazione in diversi settori quali: petrolchimico, generazione di energia, produzione della carta e nel campo biomedico.
La precipitazione di nitruri di cromo nell’intervallo di temperatura 550°C - 1050 °C, però, può limitare l’utilizzo di tali acciai nelle applicazioni ad alta temperatura.
Scopo della tesi è stato quello di analizzare l’ evoluzione microstrutturale di un acciaio austenitico ad alto tenore di azoto (0.8%) dopo trattamenti termici a due diverse temperature (800 e 850 °C) per tempi crescenti attraverso misure di microdurezza, di diffrazione dei raggi X ed osservazioni di microscopia ottica (MO), elettronica in scansione (SEM) e in trasmissione (TEM).
La sperimentazione ha permesso di descrivere la progressiva precipitazione dei nitruri di cromo che è accompagnata da variazioni locali di proprietà meccaniche e da una ridistribuzione asimmetrica del contenuto di azoto nella zona trasformata e non trasformata.
Per completare l’indagine e verificare la migrazione a lungo raggio dell’azoto sono state effettuate prove XPS (Spettroscopia di fotoemissione di raggi X) di precisione e di FI (Frizione Interna).Austenitic stainless steels (ASS) have excellent corrosion resistance and good formability. The traditional ASS have constrains connected to the increasing price of Nichel, low yield strength, as well as irritability to human organs.
High nitrogen austenitic stainless (HNAS) steels provide improved mechanical properties and corrosion resistance compared to the large part of conventional austenitic grades. Nitrogen in solid solution, in fact, is a beneficial alloying element to increase strength level, it is also a strong austenite-stabilizing element and improves the resistance to localized corrosion.
As a result, the high nitrogen steels have found broader applications in engineering particularly where advantage is taken of their excellent combination of high strength and corrosion resistance.
The property deterioration in the aging temperature range (550 to 1050°C) of the HNS steels, however, is a serious aspect that could limit using of these steels as heat-resistant materials.
This thesis had the purpose to investigate the microstructural changes of as-prepared and heat treated HNS austenitic steel (0.8 wt%), in order to characterize its thermal evolution. After each step of heating (at 800°C and 850 °C) the material has been examined by X-ray diffraction (XRD), optical microscopy (OM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and microhardness tests. It was observed that the discontinuous precipitation leads to an unequal redistribution of N between transformed and untransformed zones and it induces to a local variation of mechanical properties. To investigate better the phenomenon of the nitride precipitation IF (Internal Friction) measurements and XPS (X-ray photoelectron spectroscopy) spatially resolved analysis were carried out
Replication casting of open-cell AlSi7Mg0.3 foams
No full textA typical aluminum alloy for casting (AlSi7Mg0.3) was used to produce open-cell foams by replication of a salt precursor. The process was set to minimize complexity and costs of the casting operations: the preform sintering was avoided and mold temperature lower than the eutectic temperature of the alloy was used. Open-cell foams with a relative density about 35% and high compressive strength resulted. Material analyses showed that, in replication casting, the material response to the process is optimal and a homogeneous and fine grain size distribution is visible in the foams. (C) 2011 Elsevier B.V. All rights reserved
Effect of pre-heating temperature on microstructure and wear resistance of a laser cladded Ni-based alloy reinforced with tungsten carbides
No full textInterrelation between macroscopic, microscopic and chemical dilution in hardfacing alloys
No full textA common way to extend service life of steel tools under heavy duty service conditions is the use of hardfacing coatings. Coating an economically feasible basematerial like carbon steelwith a hardfacing alloy by any processes based on welding, casting or cladding improves significantly its wear resistance. All these processes involve high heat inputs in order to partially melt the substrate material to create a sound bonding between the substrate and the wear resistant alloy. The intermixing and the elemental diffusion from the hardfacing material into the substrate and vice versa cause a change in microstructure, hardness and wear properties of the diluted alloy. It is not trivial to categorize the amount of dilution and its effects on material properties since composition and microstructure change discontinuously, especially in casting processes, due to different diffusion rates of elements and due to phase transitions. This paper presents a simplified model to correlate the amount of dilution in hardfacing alloys after a casting process considering several reference parameters. Three coefficients are derived to describe the degree of dilution based on crucible casting experiments and are numerically correlated by linear regressions. They specify distinct issues about macroscopic and microscopic dilution, aswell as changes in chemical composition. Themodel is then applied to a spin casting case studywhich can be taken as a reference example for industrial hardfacing processes where dilution effects are clearly observable
Tuning the Composition and Microstructure of Fe-Based Alloy Thick Coatings for Heavy Duty Applications
No full textThick coatings for heavy duty applications in many ndustrial fields are produced by generating hard layers of Fe-based complex alloys on the critical surfaces. The composition and structure of these coatings have to be carefully tuned in order to generate a defect-free layer after
solidification and to meet the expected service conditions.
The present study is aimed at investigating and developing improved Fe-based alloys for the above described thick coatings by a careful definition of the microstructural features required to bear service conditions and by defining the optimal compositions to achieve the desired properties.
Primary carbide distribution and size as well as steel matrix composition are the main microstructural features that will be evaluated in order to fine tune coating performance in terms of toughness and hardness. The experimental part of the investigation here presented will be mainly concerned to laboratory casting the above alloys and to assess their microstructure and properties
Jon Keune, Shared Devotion, Shared Food: Equality and the Bhakti-Caste Question in Western India [Ludovica Tozzi], p. 391
No full text"Shared Devotion, Shared Food: Equality and the Bhakti-Caste Question in Western India" by Jon Keune deals with an in-depth analysis of Marathi literature from 1854 to 1950, in order to find an answer to the question: "Can the idea that people are equal before God inspire them to treat each other as equals? Can theological egalitarianism lead to social equality?”. While trying to answer such a complex question, the author reveals the strengths and flaws of the so-called "most liberal Hindu religious movement"
High-Temperature Behavior of High-Pressure Diecast Alloys Based on the Al-Si-Cu System: The Role Played by Chemical Composition
No full textAl-Si-Cu foundry alloys are widely applied in the form of high-pressure diecast components.
They feature hypo- or nearly eutectic compositions, such as AlSi9Cu3(Fe), AlSi11Cu2(Fe),
and AlSi12Cu1(Fe) alloys, which are used in the present study. Diecast specimens, with a thickness
of 3 mm, were used for tension tests. The short-term mechanical behavior was characterized at
temperatures from 25 up to 450 C. At temperatures above 200 C, the tensile strength properties
(YS and UTS) of the investigated alloys were severely affected by temperature, and less by
chemical differences. Material hardness and ductility indexes better highlighted the differences
in the mechanical behavior of these age-hardenable alloys and allowed us to relate them to the
microstructure and its changes that took place at test temperatures. Thermodynamic calculations
were found to be useful tools to predict phases formed during solidification, as well as those related
to precipitation strengthening. By means of the performed comprehensive material characterization,
deeper knowledge of the microstructural changes of Al-Si-Cu foundry alloys during short-term
mechanical behavior was obtained. The gained knowledge can be used as input data for constitutive
modeling of the investigated alloys
Tribological and mechanical behaviour of Cr3C2–NiCr thermally sprayed coatings after prolonged aging
No full textCr3C2–NiCr HVOF thermally sprayed coatings have been extensively applied on industrial components working at high temperature to improve their erosion, corrosion and oxidation resistance. In order to monitor the coating microstructural stability, a systematic investigation in the intermediate range of temperature 250–400 °C was carried out. Improved hardness and sliding wear resistance of the Cr3C2-based coatings was observed after long-term aging at temperature higher than 250 °C. However, the residual stress relaxation and a strengthening effect contributed to reduce the cracking resistance of the aged coatings, anticipating the onset of damage and crack opening to lower deflection levels during four-point bending tests
Life events and benign paroxysmal positional vertigo: A case-controlled study
No full textCONCLUSIONS: Within the poorly understood mechanisms implicated in the aetiology of benign paroxysmal positional vertigo (BPPV), the results of this trial provide clinical evidence of a potential role of emotional stress connected to adverse life events as a trigger of otoconial dysfunction. High levels of anxiety, depression and somatization were recorded and considered psychogenic precursors of BPPV, thus emphasizing the role of psychological distress in precipitating peripheral vestibular disorders. Therefore, appraisal of life stress and psychological attitudes may have potential implications in the clinical assessment of this labyrinthine vertigo and its frequent relapses. OBJECTIVES: BPPV is one of the most common peripheral vestibular disorders, and although it has been the subject of several studies and debates, its aetiology still remains unknown in most cases. Because it has been shown that emotional stress is related to the onset or worsening of other inner ear dysfunctions such as Ménière's disease and sudden hearing loss, this study investigated the hypothesis that life events, mood and psychological attitudes may have a causal relationship with BPPV. PATIENTS AND METHODS: Fifty patients (40 females and 10 men; mean age 43.5+/-10.1 years, range 30-65 years) were recruited and compared with 50 healthy volunteers matched for sex, age and socio-demographic variables. Patients were selected among dizzy patients who were referred to the ENT Clinic of the University of Modena and Reggio Emilia from the emergency unit with a primary diagnosis of 'positional vertigo' and enrolled in the study only if they had a paroxysmal positional nystagmus as diagnosed by Dix-Hallpike and Semont's manoeuvres. Patients with a history of recurrent vertigo and additional otoneurological diseases were excluded. The Paykel Life Events Scale, Spielberger State-Trait Anxiety Inventory, Symptom Check List-90 Revisited and Hamilton Depression Scale were the psychometric questionnaires used to complete the audiological and vestibular examinations. RESULTS: Patients with BPPV reported significantly more life events than control subjects in the year preceding the onset of vertigo (p0.05). Psychometric questionnaires recorded significantly higher levels of anxiety, depression and somatization in the pathological sample (p<0.005), as well as an increased obsessive-compulsive attitude (p<0.05)
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