1,720,962 research outputs found
Evaluation of hydrogen trapping and diffusion in two cold worked CrMo(V) steel grades by means of the electrochemical hydrogen permeation technique
Get PDFHydrogen diffusion kinetics, which is influenced by the hydrogen trapping and de-trapping phenomena within the steel microstructure, plays an important role on the behaviour of steel components under hydrogen environments. Hence, the complex interaction between hydrogen atoms and steel microstructure must be analyzed in order to discuss the impact of hydrogen on the structural damage. Quenched and tempered low-alloy ferritic steels from the Cr-Mo family, with and without vanadium, have been subjected to different plastic deformation ratios by cold rolling. Dislocation densities have been determined by the analysis of the peak broadening on X-Ray diffractograms. Hydrogen diffusion kinetics was characterized by means of hydrogen permeation transients. In addition, binding energies between hydrogen atoms and microstructure were also determined using thermal desorption analysis (TDA). The analysis of the results highlights the influence of dislocations density and vanadium carbides on the hydrogen diffusion kinetics. In the 2.25Cr1Mo steel grade, hydrogen apparent diffusion coefficient decreased after the cold-work due to the increase in the density of traps (mainly related to dislocation core, ΔETL = 55–60 kJ/mol). Nevertheless, after 10% of plastic deformation, apparent diffusion coefficient ‘saturates’ according to the ‘plateau’ determined in the dislocation density evolution at higher deformation levels. Due to the vanadium addition (+0.31%), hydrogen apparent diffusion coefficient was notably reduced (compared to that obtained in the V-free steel grade). Hydrogen trapping and diffusion are the result of the interplay between vanadium carbides (ΔETL = 35 kJ/mol) and dislocation core
Sensitivity to hydrogen embrittlement of AISI 4140 steel: A numerical study on fracture toughness
Get PDFContamination of steel structures, working in environments where the presence of hydrogen cannot be neglected, can lead to significant degradation of mechanical properties, in particular, fracture toughness. In order to estimate the local hydrogen concentration at the crack tip and to understand the embrittlement mechanism, numerical models are important tools to support experimental tests that are quite complex to perform. This paper presents the application of a cohesive zone model, which couples diffusion and mechanical fields, to study the hydrogen embrittlement on AISI 4140 steel. The total hydrogen concentration, sum of the contents of hydrogen present in the lattice and in dislocation traps, is the quantity governing the embrittling effect. The input parameters of the model were calibrated using experimental tests performed on steel samples; then, initial lattice concentration was calibrated based on hydrogen pre-charged tests. A sensitivity analysis was proposed, discussing the effects of material, environmental and testing input parameters. The analysis confirms the capability of this numerical tool in predicting the mechanical response in presence of hydrogen, highlighting its potential to be used for practical design and assessment
Effect of warm shot peening treatments on surface properties and corrosion behavior of AZ31 magnesium alloy
No full textMagnesium alloys are considered as a suitable choice for temporary biodegradable implants due to their biocompatible and biodegradable properties, able to avoid a second surgery when implant removal is needed. Nevertheless, nowadays one of the shortcomings of magnesium-based materials is their poor corrosion resistance and the associated high corrosion rate. This fact considerably hinders their application in biomedical field. The aim of this work is to induce a severe plastic deformation on the upper layer of the AZ31 Mg alloy in order to modulate its surface properties to slow down the kinetics of the corrosion damage. Specimens were submitted to conventional and severe shot peening treatments at room temperature, 240 °C (near recrystallization temperature) and 360 °C (above recrystallization temperature); the specimens were then analyzed in terms of grain refinement, surface roughness, work hardening, and residual stresses. Potentiodynamic polarization tests were also performed to evaluate the influence of the shot peening treatments on the specimens' corrosion resistance. The results evidenced surface roughness as the most influential factor in corrosion behavior, although for the specimens with similar roughness, also the effect of grain size is notable
The positive role of nanometric molybdenum–vanadium carbides in mitigating hydrogen embrittlement in structural steels
Get PDFThe influence of hydrogen on the fracture toughness and fatigue crack propagation rate of two structural steel grades, with and without vanadium, was evaluated by means of tests per-formed on thermally precharged samples in a hydrogen reactor at 195 bar and 450 °C for 21 h. The degradation of the mechanical properties was directly correlated with the interaction between hydrogen atoms and the steel microstructure. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural trapping sites, and also to study the hydrogen eggresion kinetics at room temperature. The electrochemical hydrogen permeation technique was employed to estimate the apparent hydrogen diffusion coefficient. Under the mentioned hydrogen precharging conditions, a very high hydrogen concentration was introduced within the V-added steel (4.3 ppm). The V-added grade had stronger trapping sites and much lower apparent diffusion coefficient. Hydrogen embrittlement susceptibility increased significantly due to the presence of internal hydrogen in the V-free steel in comparison with tests carried out in the uncharged condi-tion. However, the V-added steel grade (+0.31%V) was less sensitive to hydrogen embrittlement. This fact was ascribed to the positive effect of the precipitated nanometric (Mo,V)C to alleviate hydrogen embrittlement. Mixed nanometric (Mo,V)C might be considered to be nondiffusible hydro-gen-trapping sites, in view of their strong hydrogen-trapping capability (~35 kJ/mol). Hence, mechanical behavior of the V-added grade in the presence of internal hydrogen was notably improved
Effect of Shot Peening on Oxidation and Precipitation in Inconel 718
Get PDFIn this study, the effect of the surface state on the behaviour of Inconel 718 alloy exposed to 640 ∘ C and 700 ∘ C environments for times varying between one and one hundred hours was investigated. In particular, the focus was set on the evolution of oxidation and precipitation phenomena during thermal exposure. Three surface states were considered: two generated through shot peening treatments featuring different coverage levels, while the third condition is a non-peened one. Shot peening treatments modify the surface condition and introduce higher residual stresses and microhardness values than in the non-treated condition. The morphology of the oxides appears to be different depending on the condition observed. Regarding the kinetics, over time the oxidation process follows a parabolic trend and appears to be influenced by the surface state; in particular, severe shot peening treatment is characterized by the highest intensity of the phenomenon. However, the order of magnitude of the weight gains measured suggests that the observed variations can be neglected, and that the positive effect of shot peening can be exploited without introducing oxidation problems. From the point of view of the microstructural evolution, an increase in the coarsening kinetics of γ ” phase was observed in the shot peened layer
Critical hydrogen concentration for crack propagation in a CrMo steel: Targeted experiments for accurate numerical modelling
Get PDFThis study focuses on CrMo steel experiencing decohesion mechanism in presence of hydrogen. A tailored experimental characterization is performed with tensile, permeation and toughness experimental tests to obtain all the inputs for the numerical simulations of a propagating crack in a C(T) specimen. The used finite element framework is based on the cohesive zone modelling. The aim of the numerical model and of the work is the identification of a critical hydrogen concentration inducing crack tip propagation. Given the tailored inputs, these models accurately estimate the hydrogen concentrations in the lattice and the reversible traps, and follow their redistribution along the ligament during the time. From the obtained results, we could quantify that a decrease of two orders of magnitude in the test speed reduces the critical hydrogen concentration at the crack tip, necessary to activate the failure of the first cohesive element and therefore the propagation, from 0.994 to 0.784 wppm, that is −21%
Obtaining tailored surface characteristics by combining shot peening and electropolishing on 316L stainless steel
Get PDFShot peening is a well-established surface treatment commonly used to improve mechanical properties of material's surfaces. Studies have shown notable enhancement of fatigue strength, wear, scratch and corrosion resistance and fretting properties as a result of an intensive shot peening treatment, the so-called severe shot peening. Nevertheless, this process has some drawbacks including quite high surface roughness and possible embedment of shot residuals at the treated surface, which can have adverse effects depending on the final application. In this paper, electropolishing has been evaluated as a cleaning post-treatment for shot peened surfaces of 316L stainless steel. For this purpose, electropolishing voltage has been varied with the aim of obtaining clean and smoother surfaces without sacrificing the mechanical improvements produced by shot peening. The treated surfaces have been characterized considering surface roughness, morphology, wettability, residual stresses and microhardness evolution after various shot peening treatments combined with electropolishing using different parameter set-ups to identify the most promising combinations
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