Experimental analysis of temperature field and distortions in multi-pass welding of stainless cladded steel

The stainless clad steel materials (particularly A283-Gr-C hot-rolled to austenitic stainless steel plate SA240 TP 316L) have become widely used in the fabrication of heat exchangers, pressure vessels, and other components owing to their very interesting properties and low production cost. These cladded materials are mostly joined using multi-pass welding techniques. The temperature distribution that supervenes during welding affects the microstructures and the mechanical properties and may generate residual stresses in the heat-affected zone of the welded plates. Since limited experimental data are available in the literature regarding the complex cases of multi-pass welding of cladded steel materials, a thorough experimental study was performed in order to evaluate the temperature distribution on the welded plates. In fact, eight K-type thermocouples were fixed at different distances from the weld centerline in order to record the temperature evolution along longitudinal, transversal, and thickness directions during the welding process. Due to the unavoidable effects of the generated heat fluxes that always follow the welding, several dimensional changes were occurred on the welded plates. In this study, the longitudinal shrinkage and the angular distortion generated during the welding process by the effect of the heat fluxes were investigated. Then, tensile and bending tests were performed in order to check the welded plate reliability. It was found that (i) the welded joint presents a higher mechanical tensile strength than the parent metal and (ii) no separations, fractures, or tearing appear on the weld joint surface after the bending test

Damage tolerance issues peculiar to supersonic civil transport aircraft

International audienceThis paper presents results of experimental investigations on creep, fatigue and creep-fatigue crack growth behaviour of the 2650-T6 aluminium alloy in temperature, along with numerical simulation of stress distribution around the tip. The results will provide a preliminary database on the fatigue properties of the 2650 T6 alloy under loading representative of service conditions and predict the damage tolerance assessment of the future civil transport aircraft fuselage. Testing were carried out to evaluate the effect of creep-fatigue interaction and get insight into the damage processes. Numerical simulation of stress field around the crack tip were performed to account for the observed behaviour. The crack growth rates measured on CT specimens were correlated with the stress intensity factor K. In creep tests, an influence of the initial value of K on the low crack growth rates is shown. The behaviour is not deeply affected by temperature in the range 100-130°C. However, at 160°C, crack growth rates are faster than at 130°C due to an increase of creep contribution in local stress-strain response, that might be enhanced by ageing. In fatigue (triangular wave loading), no difference is noticed in crack growth rates at 20 and 130°C. In creep-fatigue, the crack growth rates (trapezoidal wave loading) at 130°C are faster than during creep or fatigue crack growth in a given domain of K. The fracture surfaces indicate that creepfatigue interaction is characterised by a higher portion of intergranular fracture. Thus, a detrimental creep-fatigue interaction at 130°C has to be taken into account in crack propagation law used in damage tolerance analysis. The constitutive law used in the calculations for numerical simulation was identified on the basis of cyclic relaxation tests. The computation results indicate that the von Mises stress at the beginning of dwell during creep-fatigue is higher than in the case of creep and fatigue

Failure analysis of shot–sleeves used in brass high pressure die–casting process

A failure investigation has been conducted on a two cases of shot-sleeve used in brass–die casting and made up of AISI H10 tool steels to study their failure mechanisms. The chemical composition of the shot–sleeves material and the hardness profiles were evaluated. A preliminary examination of shot–sleeves reveal the presence of cracks network on their inner surfaces which proves that thermal fatigue was probably the main cause of their failure. A Meticulous investigation of these damaged surfaces reveal the presence of an additional small zone cited in the vicinity of the plunger entry side. This zone presents several scratches sign of abrasive wear. Then, the measurement of the cracks length and their linear density along longitudinal and transversal cuts of the damaged shot–sleeve sample are carried out. Results show that the observed cracks network can be divided into two zones. The first one was the most damaged in terms of cracks density and length given that it is the first zone which enter in contact with the molten metal. However, the cracks network examined in the second zone appears to be superficial

Fatigue thermique d'un acier inoxydable austénitique 304L : simulation de l'amorçage et de la croissance des fissures courtes en fatigue isotherme et anisotherme

Membre du Jury: Fissolo, Antoine et Degallaix, Gérard et Petit, Jean et Amzallag, Claude et Strudel, Jean Loup et Rémy, LucLes canalisations coudées des circuits de refroidissement des centrales thermiques sont soumises à des fluctuations thermiques de faible amplitude et de fréquence variable. Ces fluctuations associées aux variations de température des fluides présentent un risque de fissuration et de fuites. Afin de prévenir de tels risques, EDF a lancé le programme CRECO RNE 808, « Fatigue thermique des aciers inoxydables austénitiques 304L » pour étudier expérimentalement sur élément de volume, lamorçage et le début de propagation des fissures en fatigue thermique dans les aciers inoxydables austénitiques. Lobjectif est de comparer le comportement et lendommagement du matériau en fatigue mécano-thermique (cyclage en température + cyclage en déformation) et en fatigue isotherme et est orienté sur le problème RRA, dans lequel les conditions extrêmes ont été déterminées par EDF pour le métal Tmax = 165°C et Tmin = 90°C et où la fréquence des fluctuations thermiques peut atteindre un Hertz. Une sollicitation thermomécanique sur élément de volume est nécessaire en effet pour simuler la fatigue thermique des zones critiques dune structure. Des essais de fatigue isotherme, aux deux températures extrêmes du cycle thermique, ont montré un comportement similaire du matériau et un effet du module sur la réponse mécanique du matériau. On na pas constaté de réelle différence de durée de vie entre les deux températures étudiées pour des amplitudes de déformation mécanique supérieures à 0,18% alors que pour des faibles amplitudes de déformations la durée de vie du matériau diminue quand la température augmente. Des essais de fatigue mécano-thermique réalisés sur des éprouvettes prélevées dans une bride de barrière thermique et dautres sur une tôle ne montrent pas de différence que ce soit de point de vue comportement ou durée de vie pour des amplitudes de déformations mécaniques supérieures à 0,18%. Dans le but détudier leffet de la contrainte moyenne, induite par pré-écrouissage cyclique, sur la durée de vie du matériau, des essais ont été réalisés sur des éprouvettes pré-écrouies cycliquement à lambiante à ±1%. Une première comparaison des deux essais réalisés sur matériau pré-écroui ne montre pas de différence entre le chargement mécano-thermique et isotherme à 165°C que ce soit en terme de comportement ou de durée de vie. Un effet de la contrainte moyenne (préécrouissage monotone à lambiante) a été mis en évidence en fatigue isotherme à 165°C, avec une réduction importante de la durée de vie sur le digramme Dep/2 = f(Nf). Sous sollicitations mécano-thermiques, la durée de vie sur matériau pré-écroui est plus importante que dans des conditions isothermes à 165°C. Une étude du comportement en relaxation du matériau entre 50 et 250°C a montré la présence de deux stades de relaxation : un premier stade pendant lequel le matériau se relaxe rapidement avec une viscosité indépendante de la température et de la déformation plastique cumulée, et un deuxième stade pendant lequel la contrainte macroscopique varie peu dans le temps. Létude a montré que la capacité de relaxation du matériau diminue quand la température augmente de 50 à 250°C : le matériau présente donc une anomalie de comportement marquée dans ce domaine de températures. Un pré-écrouissage cyclique à lambiante à ±1% diminue le taux de relaxation du matériau à 50°C et fait apparaître une troisième stade de relaxation du matériau avec une viscosité plus importante. Un pré-écrouissage monotone à lambiante jusquà une contrainte maximale de 500MPa, réduit encore plus le taux de relaxation du matériau à 50 quà 250°C. Les essais de fatigue mécano-thermique à grand nombre de cycles (106 cycles et au delà) conduisent à des durées dessais pouvant dépasser un an. Afin dexplorer, néanmoins, leffet de lamplitude de déformation dans ce domaine de durée de vie, et compte tenu du fait que sur site les premiers stades damorçage se produisent dans des zones superficielles perturbées, nous avons réalisé des essais de propagation de fissures courtes à partir dune entaille (longueur 2a = 0.5mm) en plasticité généralisée en contrôle de déformation mécanique, comme sur élément de volume. Un modèle de durée de vie est introduit au dernier chapitre. Le calcul est basé sur les résultats des essais de fissuration en plasticité généralisée décrite dans le chapitre 6. En premier lieu un modèle de comportement cyclique est identifié aux deux températures 90 et 165°C. La deuxième partie sintéresse au calcul de durées de vie. À travers une représentation énergétique des résultats, nous avons discuté la validité du modèle de durée de vie en nous basant sur une analyse assez simplifiée. Deux approches sont présentées, avec pour chaque cas une comparaison des durées de vie expérimentales aux durées de vie obtenus expérimentalement en fatigue isotherme à 90 et 165°C et en fatigue mécano-thermique. Le modèle de durée de vie retenu a été validé en fatigue isotherme à 90 et 165°C ainsi quen fatigue mécano-thermique

Fatigue thermique d'un acier inoxydable austénitique 304L (simulation de l'amorçage et de la croissance des fissures courtes en fatigue isotherme et anisotherme)

PARIS-MINES ParisTech (751062310) / SudocEVRY-MINES ParisTech (912282301) / SudocSudocFranceF

On the Origin of the Local Hardening Zone on Welded Stainless Clad Steel Plates

The joining of stainless clad steel plates (SCSPs) by welding processes is relatively difficult due to differences in the chemical compositions and the physical and mechanical properties between both the carbon and the stainless steels comprising the clad material. These welded structures often suffer from several structural integrity problems such as bulging phenomena that can appear after bending tests, in the welded zone, due to the presence of a local hardening zone (LHZ). The main purpose of this paper is to investigate the origin of the LHZ typically produced in the welded joint of SCSPs after the bending operation. Optical micrographs revealed the presence of a typical pearlitic-ferritic structure in the welded zone filled with E7018 metal and a dendritic δ-ferrite structure solidified under a skeletal form in the welded zone filled with ER316L metal. The microstructure of the weld metal transition zone (WMTZ) filled with ER309L metal shows the presence of martensitic laths as well as cellular and columnar structures. In addition, the WMTZ revealed the presence of three types of grain boundaries, which are formed during the gas tungsten arc welding process: solidification sub-grain boundary, solidification grain boundary, and migrated grain boundary. Vickers microhardness measurements performed along the thickness of the welded joint showed that the highest microhardness value (406 HV) was observed at the WMTZ. The significant increase of the microhardness value in this transition zone was attributed to the presence of martensitic laths as well as cellular and columnar structures

Modeling of strain rate effect on the pseudoelastic behavior of NiTi SMA using a simple thermomechanical coupling model

NiTi SMA represents a popular group of metals with interesting properties like pseudoelasticity and a high energy dissipative capacity. Indeed, NiTi alloys, presents an interesting damping capacity due to solid-solid phase transformation. Such characteristics allow them to be used in many civil engineering fields mainly as actuators and damping devices. However, because of their complex behavior, the design process of these applications seems to be complicated and consequently, needs to build accuracy numerical models. One of these already existing models is the micromechanical Likhachev model. It seems to be very attractive thanks to its general formulation, numerical efficiency and model parameters relatively easy to determine. This paper concerns the numerical simulation of the pseudoelasticity behavior of NiTi wires dedicated to the application of damping devices, at different stress and strain rates. A thermomechanical coupling is introduced in the Likhachev formulation in order to simulate the thermomechanical behavior of NiTi under strain control. In this paper the model is adapted to a strain-controlled formulation, and 1D simulation is achieved using FORTRAN code. Parameters model are identified thanks to SiDoLo software and the corresponding results are presented and discussed. A good agreement with experimental data is found for the two formulations

Numerical computation of the energetic criterion at isothermal and thermal-mechanical cyclic tests under generalized plasticity of the F17TNb stainless steel

International audienceThe aim of this work is to investigate the energetic integral J criterion in generalized plasticity of the F17TNb at isothermal and Thermal Mechanical Fatigue (TMF) tests using a numerical simulation. An optimization of the constitutive law based on mechanical results was carried out using ABAQUS standard solver. Isothermal tests showed that ΔJ evolution is higher at lower temperature (300 °C) and a has a linear dependence at log-log presentation with da/dN at 300 °C. TMF tests simulation showed that Out of Phase (OP) configuration is more damaging than In Phase (IP) tests. A comparison between OP and isothermal tests highlighted the greater value of ΔJ at isothermal tests

On the Origin of the Local Hardening Zone on Welded Stainless Clad Steel Plates

International audienceThe joining of stainless clad steel plates (SCSPs) by welding processes is relatively difficult due to differences in the chemical compositions and the physical and mechanical properties between both the carbon and the stainless steels comprising the clad material. These welded structures often suffer from several structural integrity problems such as bulging phenomena that can appear after bending tests, in the welded zone, due to the presence of a local hardening zone (LHZ). The main purpose of this paper is to investigate the origin of the LHZ typically produced in the welded joint of SCSPs after the bending operation. Opticalmicrographs revealed the presence of a typical pearlitic-ferritic structure in the welded zone filled with E7018 metal and a dendritic δ-ferrite structure solidified under a skeletal form in the welded zone filled with ER316L metal. The microstructure of the weld metal transition zone (WMTZ) filled with ER309L metal shows the presence of martensitic laths as well as cellular and columnar structures. In addition, the WMTZ revealed the presence of three types of grain boundaries, which are formed during the gas tungsten arc welding process: solidification sub-grain boundary, solidification grain boundary, and migrated grain boundary. Vickers microhardness measurements performed along the thickness of the welded joint showed that the highest microhardness value (406 HV) was observed at the WMTZ. The significant increase of the microhardness value in this transition zone was attributed to the presence of martensitic laths as well as cellular and columnar structures

Long term ageing of polyamide 6 and polyamide 6 reinforced with 30% of glass fibers: physicochemical, mechanical and morphological characterization

International audienceHygrothermal ageing of polyamide 6 (PA6) and polyamide 6 reinforced with 30 wt% of glass fibers (PA6GF30) was undertaken. Immersion was conducted in distilled water at 90 degrees C and 100% relative humidity (RH) for up to 80 days (1920 h). Results revealed a noteworthy decrease either in glass transition temperature T-g or in tensile properties, at early stage of ageing, for both studied materials. This decline was mainly caused by the plasticization effect of water and the weakness of the interfacial interactions leading as a consequence to a loss of adhesion between fiber and matrix. Afterwards, physical and mechanical properties decrease monotonically testifying the occurrence of exhaustive damages and chemical reaction phenomena. Such phenomena were yellowing and crazing formation which were observed for both materials after 1920 h of conditioning. The former is caused by the thermo-oxidation whereas the latter results from the release of internal stresses induced by water sorption. These chemical reactions were monitored by infrared spectroscopy. Thus, an increase of the free N-H stretch and the carbonyl groups (imides) was noted. Accordingly, it seems that long term immersion in distilled water at high temperature induces chemical reactions which indicate the severity of the damage