A water atomized 4130 steel powder was processed by Laser powder bed fusion and investigated both in as-built condition and after quench and tempering thermal treatment. Analyses were focused on the ...different microstructures developed and on steel fracture behavior in terms of tensile fracture elongation, Charpy impact properties, and linear elastic fracture toughness. Comparisons were also drawn by testing a reference 4130 steel fabricated from a gas atomized powder. The slightly higher oxygen content found in the water atomized powder led to the formation of finely dispersed nano-size oxide particles in the steel matrix. It was found that these inclusions have a minor effect on the tensile properties, but a significant influence on the impact toughness response. The fracture toughness tests showed that the orientation leading to propagation of cracks along the inter-layer planes represented the most critical situation, and the steel toughness could be significantly improved after the quench and tempering treatment owing to the achievement of a more homogeneous microstructure. The results suggest that the investigated water atomized low-alloy steel powder feedstock can be considered as a suitable and cheaper alternative for structural parts produced by additive manufacturing, which could replace the more popular gas atomized steel grades.
Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD, and recently also electrical nonvolatile memory, named phase change ...memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.
•Severe shot peening prior to nitriding, caused deeper compound and compressed layer.•Severe shot peening after nitriding, caused remarkable increment of surface residual stress.•Notwithstanding the ...high surface roughness, severe shot peening improved the fatigue limit by 11.6%.•Nitriding improved the fatigue limit by 51.3%.•Combination did improve surface characteristics. It did not succeed to improve fatigue limit.
Fatigue strength of mechanical components can be greatly enhanced by generating compressive residual stress, increasing the hardness and reducing the grain size. It is well known that while the use of mechanical treatments is able to generate an effective field of compressive residual stresses and, if severe parameters are used, to cause grain refinement, thermochemical treatments are able to increase the surface hardness. This justifies the interest in developing combined treatments, able to achieve all the just mentioned factors.
In this study, the effect of combination of severe shot peening and nitriding on the fatigue limit of a low-alloy steel is investigated. Severe shot peening was conducted by using particular processing parameters to obtain ultra-fine/nano-structured surface layers. Micro-structural observation, micro-hardness, surface roughness and XRD measurement of residual stress were performed on single and hybrid surface treated specimens including nitrided, severely shot peened, nitrided plus severely shot peened and severely shot peened plus nitrided specimens. The fatigue limit of all series was experimentally determined and compared with the as-received specimens. Severe shot peening and Nitriding improved the fatigue limit by 11.6% and 51.3% respectively. Combination of severe shot peening and nitriding improved hardening, residual stress and nitrogen diffusion with respect to the single treatment. Nevertheless, it could not guarantee further improvement in the fatigue limit as compared with the nitrided smooth specimens. The results are critically assessed by considering the local fatigue limit concept.
In this work, fire simulations were carried out on zinc-coated steel sheets usually employed for the structural parts of furniture, transformer rooms and aeration pipes. The thermal alteration ...induced by the fire was simulated by heating samples at 13 different temperatures (from 20° C to 1050° C) and for different exposure times (1 h and 6 h) in a laboratory furnace. The metallurgical investigation concerned the surface coloring by photographic analysis, the morphology and chemical composition of the surface oxides by SEM and EDS and the microstructure modifications as a function of the temperature by LOM. The influence of the different parameters was discussed by a statistical approach. The obtained results allowed to associate the temperature to one or more alteration of the steel microstructure and of the color, the morphology and the chemical composition of the surface oxides.
This study shows a reliable procedure to prescribe the preferential use of a material for food contact. Release tests with optimized parameters were performed on six different stainless steel ...accepted for the use in food contact: AISI 420, AISI 430, AISI 202, AISI 303, AISI 304, and AISI 316. Total reflection X-ray fluorescence spectroscopy was used to measure the concentration of Cr, Mn and Ni in contact solutions from release tests. Results show that AISI 202 and 430 release the lowest amount of Mn, Cr and Ni. While, AISI 420 is the worst material, exceeding the limit set in the Italian regulation for all the three metals of interest. One sample was selected to test the reproducibility of TXRF measurements performed in three different laboratories around the world. Results show that quantitative analyses by means of TXRF satisfy the requirements of this field of application.
•A release test to simulate food contact with stainless steel was optimized.•Quantitative analysis of release tests water solutions was performed by TXRF.•TXRF has the advantage of simultaneous identification of all elements present in solutions.•The developed procedure is useful to prescribe the preferential use of a material for food utensils.
Failure analysis of a steam turbine rotor Barella, S.; Bellogini, M.; Boniardi, M. ...
Engineering failure analysis,
09/2011, Letnik:
18, Številka:
6
Journal Article
Recenzirano
► We investigates the rotor turbine failure of a 60
MW unit of a thermal power plant. ► Several different analyses were carried out in order to identify the failure’s root cause. ► The observed ...fracture mechanism was traced back to high cycle fatigue damage. ► The origin of the damage correspond to anomalous high stresses. ► High stresses on the surface are due to start-up cycles and the blade fixing method.
A large and growing portion of electricity is being produced by aging thermal power plants, and although steam turbines are being constructed with excellent high quality materials such as CrMoNiV steel, varying forms of metallurgical degradation due to creep and/or fatigue could still affect the parts and components during long-term operation at high temperatures
1. Moreover, the de-regulated electricity market, which has existed for approximately 15
years, has led to energy companies operating their power plants in a flexible manner, as opposed to continuous operation, in order to maintain profitability in a very competitive commercial environment
2.
This paper investigates the rotor turbine failure of a 60
MW unit of a thermal power plant. The rotor was made of CrMoNiV steel, and the failure occurred after approximately 10
years in operation. Several different analyses were carried out in order to identify the failure’s root cause: visual examination, SEM fractography, micro-hardness measurement, and microstructural characterization.
The fracture of the shaft was located at the first stage
2 and the fatigue fracture extended over roughly 75% of the initial cross section. Primary failure causes were identified by the analyses performed, and the observed fracture mechanism was traced back to high cycle fatigue damage. The origin of the fatigue phenomenon can be traced to the stress field generated on the rotor surface by both the frequent startup cycles and the blade fixing method.
Data retention in phase-change memory (PCM) devices is the result of two physical phenomena: structural relaxation (SR) and crystallization of the amorphous chalcogenide material. Although the two ...processes are rather different, we show in this paper that a common physical interpretation for SR and crystallization kinetics is possible. Activation energies and Arrhenius pre-factors for SR and crystallization are found to coherently obey a Meyer–Neldel rule and are explained by many-body thermal excitation of weakly-bonded and normally-bonded clusters, respectively.
The surgical treatment of the intermediate-risk DTC (1–4 cm) remains still controversial. We analyzed the current practice in Italy regarding the surgical management of intermediate-risk unilateral ...DTC to evaluate risk factors for recurrence and to identify a group of patients to whom propose a total thyroidectomy (TT) vs. hemithyroidectomy (HT). Among 1896 patients operated for thyroid cancer between January 2017 and December 2019, we evaluated 564 (29.7%) patients with unilateral intermediate-risk DTC (1–4 cm) without contralateral nodular lesions on the preoperative exams, chronic autoimmune thyroiditis, familiarity or radiance exposure. Data were collected retrospectively from the clinical register from 16 referral centers. The patients were followed for at least 14 months (median time 29.21 months). In our cohort 499 patients (88.4%) underwent total thyroidectomy whereas 65 patients (11.6%) underwent hemithyroidectomy. 151 (26.8%) patients had a multifocal DTC of whom 57 (10.1%) were bilateral. 21/66 (32.3%) patients were reoperated within 2 months from the first intervention (completion thyroidectomy). Three patients (3/564) developed regional lymph node recurrence 2 years after surgery and required a lymph nodal neck dissection. The single factor related to the risk of reoperation was the histological diameter (HR = 1.05 (1.00–1-09),
p
= 0.026). Risk stratification is the key to differentiating treatment options and achieving better outcomes. According to the present study, tumor diameter is a strong predictive risk factor to proper choose initial surgical management for intermediate‐risk DTC.
Purpose
Hypoparathyroidism is one of the most common and most feared complications of total thyroidectomy (TT). The aim of this study is to detect possible markers that may facilitate early tracing ...of hypocalcaemia-prone patients in order to reduce clinical cost by optimizing patient discharge and to avoid unnecessary treatment.
Methods
Over an 18-month period, 995 patients, 23 % male and 77 % female, aged 52.9 ± 13.4 years, underwent TT in ten Lombardy hospitals. The following parameters were analyzed: calcaemia before and 12–24 and 48 h after surgery, pre- and post-operative parathyroid hormone (PTH) at 24 h and pre-operative 25OH vitamin D.
Results
Mortality was nil and morbidity was 22.4 %. Mean 24-h calcaemia and PTH were 2.17 ± 0.15 mmol/l and 31.81 ± 20.35 pg/ml, respectively; mean 24-h PTH decay was 36.7 ± 34.12 %. Four hundred seventy-three (47.5 %) patients were hypocalcaemic at discharge; 142 of whom had transient hypoparathyroidism that became permanent in 27. Patients developing hypocalcaemia had significantly higher values of PTH and calcium decay. At multiple logistic regression, only 24-h calcium decay, PTH drop and the presence of symptoms and parathyroid auto-grafting were significantly related to hypoparathyroidism. The association of these factors had a 99.2 % negative predictive value (NPV) for the development of hypoparathyroidism. A 70 % PTH drop had a 93.75 NPV for transient hypoparathyroidism. A 12 % calcaemia decay had a 95.7 NPV for hypoparathyroidism.
Conclusions
Hypocalcaemic asymptomatic patients with less than 70 % PTH and 12 % calcaemia decay may be safely discharged without treatment. Symptomatic patients and those with parathyroid grafting should receive calcium and vitamin D.
PDF
