In hot forming processes, steel tools are subjected to rapid temperature transients and cyclic oxidation. AnT(2)COK, a new analytical model based on diffusion-controlled oxide growth in cyclic conditions, is proposed to predict the parabolic oxidation kinetics in spallation-free conditions. Compared to cyclic oxidation models of the literature (COREST, COSP, DICOSM, etc.), it is applicable for any shape of thermal cycle, fully transient or with dwell time. The model is applied to thermal fatigue tests on X38CrMoV5 steel, for T-max between 500 and 685 degrees C. Its validity is demonstrated above 550 degrees C, using frequency factor and activation energy calculated from isothermal tests.

The interaction between thermal fatigue and aluminizing and/or oxidation is investigated using an experimental approach based on decoupling of mechanisms. Virgin and pre-aluminized steel specimens are tested in air and nitrogen between 100 and 650 degrees C. Homogeneous uniaxial micro-crack network forms on the oxidised or pre-aluminized surface in air, with a better resistance to micro-cracking for the intermetallic coating. The propagation of the micro-cracks is delayed in nitrogen, whilst no evidence of micro-cracking is observed on the virgin specimen. The premature cracking of the steel depends on the formation of the superficial micro-crack network, and the crack propagation is assisted by oxidation.

FeNiCr samples (800HT) were exposed at 570 degrees C, 1 bar to a 47.25CO-47.25H(2)-5.5H(2)O atmosphere (a(c) = 33) flowing at 18 mu m/s. Pitting corrosion was observed. Pits showed a flattened morphology and a constant pit diameter growth rate. Corrosion rings appeared successively at the surface during pit growth. A four step mechanism is proposed which includes internal oxidation of carbides, graphitisation and localised enhanced graphitisation. Gas velocity and thermal cycling play key roles in pit morphology. Thermal cycling induces circular cracks. Low gas velocity induces the gas to evolve in crevices, due to local oxygen consumption. (C) 2016 Elsevier Ltd. All rights reserved.

The present article focuses on the influence of machining on the fatigue life of a titanium alloy: Ti6Al4V. An experimental design was adopted in order to highlight the effects of machining parameters on surface integrity while generating very different surfaces with a view to subsequent fatigue testing (four point bending tests). Firstly, the impact of machining parameters on surface integrity was demonstrated. Then, the influence of surface integrity on fatigue lifetime was observed: no influence of the geometric and metallurgical parameters was observed. However, the mechanical parameter (e.g., residual stress) seemed to have a preponderant influence. To conclude, a machining plan of procedure was proposed to significantly improve the fatigue lifetime as compared with a reference industrial plan of procedure.

The PER72® grade is used as a wrought engine turbine disk, which is a critical high temperature component. During the heat treatment process, residual stresses are generated during the quench, which may lead to irreversible damages on the workpiece. The aim of this study is to better understand the mechanisms involved in the residual stress generation. Therefore, the influence of quenching conditions on the high temperature tensile properties and the multi-scale microstructure evolutions are investigated after cooling. PER72® specimens are annealed above the solvus temperature, directly on the servo-hydraulic testing machine. Three quenching rates are used: 30 ∘C/min, 120 ∘C/min, and 300 ∘C/min. For each condition, the cooling is interrupted at 1000 ∘C, 850 ∘C, 600 ∘C and 20 ∘C to perform isothermal tensile test. Specimens are post-mortem analysed. On one hand the fracture surface is investigated using SEM. On the other hand the microstructure evolution was observed and quantified at different scales using SEM directly on the bulk or after the chemical extraction of precipitation. The precipitation size and volume fraction statistics, X-Ray diffraction for the crystallography and composition of the different phases are investigated. It was shown that the testing temperature does not significantly influence the γ′ distribution of particles. Conversely, the γ′ precipitation is strongly influenced by the cooling rate. Notably, the average size, the distance between particles as well as the number density of γ′ precipitates are significantly modified by the cooling rate. Changes in tensile properties are related to microstructural.

A technique for the microstructural study of steels, based on the use of matrix dissolution to collect the very low number density precipitates formed in martensitic steels, has been considerably improved. This technique was applied to two different grades of alloy, characterized by high nickel and cobalt contents and varying chromium, molybdenum and vanadium contents. The technique was implemented at temperatures ranging between 900 degrees C and 1000 degrees C, in order to accurately determine experimental data including the crystallographic structure and chemical composition of the carbides, the carbide solvus temperatures, and variations in the chemical composition of the matrix. These experimental investigations reveal that the solubility of molybdenum in FCC carbides can be very high. These results have been compared with the behavior predicted by computational thermodynamics, and used to evaluate and improve the thermodynamic Matcalc steel database. This upgraded database has been validated on three other steels with different chemical compositions, characterized by the same Fe-Cr-Mo-V-C system. (C) 2013 Elsevier B.V. All rights reserved.

Surface integrity of machined components has a significant impact on their functional performance. Modification of surface integrity, which includes roughness, residual stress and microstructure may limit product performance such as fatigue life. In this study, the influence of machining conditions in turning on surface integrity of a wrought Mg-Zn-Zr-RE alloy was investigated. First, turned surfaces were obtained through a design of experiments, where input parameters are cutting speed, feed, depth of cut and nose radius. Second, modifications of surface integrity such as tensile/compressive residual stress, microhardness, twinning and surface roughness were correlated with cutting parameters. This study suggests optimal cutting conditions to achieve a given surface integrity and improve fatigue life.

A technique for the microstructural study of superalloys, based on the use of matrix dissolution to collect the very fine precipitates formed in superalloys during heat treatments, has been considerably improved. This technique was applied to two different superalloys, characterized by different titanium, aluminum and niobium contents. The technique was implemented in order to accurately determine experimental data including the crystallographic structure, chemical composition of gamma prime precipitates and the chemical composition of the matrix after precipitation. Following a special treatment, it was possible to analyze the size and morphology of the precipitates using high-resolution SEM, for a large range of dimensions down to 10 nm. From the chemical compositions determined using ICP-OES, gamma prime molar fractions were determined for both studied superalloys. Finally, the Nb/(Ti + Al + Nb) and Ti/(Ti + Al + Nb) ratios were found to be very similar in the grade and in the precipitates. (C) 2013 Elsevier B.V. All rights reserved.

In order to improve the knowledge of the precipitation mechanism in martensitic steels containing carbon, XRD synchrotron experiments were performed. Firstly, the influence of Ni, Co and Al were studied and it was found that the precipitation of iron carbides occurs in same way as in Fe-C steel. However, with the addition of molybdenum and chromium in same steels, XRD synchrotron investigations clearly showed alloyed carbides directly precipitate, thereby preventing the iron carbides formation.

Aluminium was added to a 0.2% C–2.5% Cr–1.4% Mo–11% Ni steel to modify the precipitation sequence during tempering treatment. The main goal was to obtain fine co-precipitation of an intermetallic phase and M2C carbides (where M is a combination of Cr, Mo and small amounts of Fe). Small angle neutron scattering, synchrotron X-ray diffraction, transmission electron microscopy and atom probe tomography were performed to characterize the nanometric precipitation. The tempering response of samples austenitized at 900 °C revealed a strong interaction between the two types of precipitation, leading to a significant modification of both the precipitation sequence of carbides and the arrangement of carbide nucleation sites compared with these sites in a single precipitation steel. Indeed, a microstructural investigation clearly showed that iron carbide precipitation was either delayed or did not occur during the tempering process, depending of the alloying elements added. Moreover, double precipitation directly influenced the mechanical resistance, as well as the toughness, leading to an ultrahigh-strength, high toughness steel.

Les assemblages stratifies par procede de brasage sont largement utilises dans le secteur de l’outillage rapide applique a la fonderie, la plasturgie. La modelisation du comportement thermique transitoire de ce type d’assemblage stratifie en tenant compte de la presence des joints brases est proposee. Le modele est obtenu par la methode des elements finis etendus (X-FEM) couplee avec la methode des developpements asymptotiques raccordes (DAR), le couplage pouvant etre generalise a de meme types d’applications.

Thermal Fatigue (TF) is a life-limiting factor in die-casting dies. The effect of the maximum temperature of thermal cycle, Tmax, and the heating (or heat flux density) of thermal fatigue (X38CrMoV5 AISI H11) at 47 HRC is investigated. Two microscopic and macroscopic heat-checking cells are identified. The microscopic heat checking is limited to the double-layer oxide scales formed on the external surface of TF specimens. An engineering Paris-type law using Kmax, can describe TF surface crack propagation of all tests examined. TF life based on σmax is used to rationalise TF micro- and macroscopic cracking.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not.The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Fatigue-oxidation interaction models for life prediction of hot forming tools steels under transient thermomechanical loadingsChristophe Daffos, Pascal Lamesle, Farhad Rezai-Aria

Modelling of the displacement field of a brazed assembly taking account the presence of a brazed joint which is considered as a singularity is proposed. The model is based on the eXtended Finite Element Method (X-FEM) coupled with the matching asymptotic development (DAR). The fundamentals of our approach is given and illustrated in a 1D example elastic deformations. The DAR solution of that problem is given, as well as the expressions of the DAR inspired enriched X-FEM functions. The implementation in the X-FEM framework is detailed and a case study result is shown and compared with fine grid FEM calculation.

Owing to the growing interest in high-pressure gas quenching, a complete mathematical modelling of this process has been launched with the aim of predicting the microstructural and mechanical state of steel bodies from the quenching operating conditions. The comprehensive numerical modelling combines the calculation of fluid flow and heat transport for the gaseous phase (using Fluent software) and calculation of heat conduction, solid-state phase transformations and residual stresses and distortions in the solid body (using Sysweld software). Our numerical results are compared to the experimental data obtained from a laboratory gas-quenching device set up at the Ecole des Mines d'Albi. Helium- and nitrogen-quenching trials were performed for nickel and 27MnCr5 steel cylinders. The measured gas flow velocities are compared to gas velocities calculated with the Fluent-Sysweld coupling, as well as the temperature evolutions at different locations in the cylinders. On the other hand, the ability of the model for predicting microstructure, residual stresses and deformations is illustrated with the gas quenching of a 60NiCrMo11 steel cylinder.