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Tensile Characterization of a GTAW Bimetallic Weld Mod 9Cr-1Mo–316L(N) With a New Measurement System for Tensile Testing
Abstract
Mod 9Cr-1Mo steel (T91) is a candidate material for steam generator of SFR (Sodium Fast Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr-1Mo steel has been undertaken at CEA in order to characterize the behavior of this material and of its welded junctions. In this frame, a new measurement system for tensile testing was developed in the laboratory of structural integrity and standards (LISN) of the CEA (French atomic commission), in order to characterize the local behavior of the material during a whole tensile testing. Indeed, with the conventional measurement system (typically an extensometer), the local behavior of the material can only be determined during the stable step of the testing. So, usually the behavior of the material during the necking step of the step is unknown. This new measurement is based on the use of some laser micrometers which allow measuring the minimum diameter of the specimen and the curvature radius during the necking phase with a great precision. Thanks to the Bridgman formula, we can evaluate the local behavior of the material until the failure of the specimen. This new system was used to characterize the tensile propriety of a bimetallic welded junction of Mod 9Cr-1Mo steel and austenitic stainless steel 316L(N) realized with GTAW process and inconel filler metal. These works lead to propose a tensile curve for each materials of the welded junction.
... To valid the method and the determined constitutive behavior, we propose to perform a finite element modeling of the tensile test with the Cast3m [2] software. The mesh used for this modeling and the associated boundary conditions are presented in Figure 10. ...
Structures containing large cracks and made in ductile materials may experience two types of failure mechanisms: ductile tearing or plastic collapse. Under displacement controlled loading ductile tearing is a stable crack growth mechanism. Plastic collapse leads to a much faster damage evolution.
Ductile tearing is the result of void growth and coalescence ahead of the crack front under the high strain concentration. This mechanism is slowed down by a high material hardening and under a high constraint. The global deformation of the structure is limited.
Plastic collapse is induced by plastic strain accumulation along slip lines. Slip lines depend on the geometry of the cracked structures and of the type of loading. Therefore plastic collapse produces large deformations of the structure.
Several studies (Nicak, 2009; Gilles, 2010; Le Delliou, 2017) on large ductile crack growth have been performed by Framatome, and EDF on deeply surface cracked plates made in Nickel base alloy 600. The tests were performed on Centre Cracked Tension specimens with a semi-elliptical surface crack. In this very tough material, the crack grew in its plane, but for large load levels, the plate was extremely deformed and a collapse mechanism appeared.
More recently, Tests on Fixed Grip SE(T) specimens in Nickel base alloy 600 were performed by CEA showing a different type of transition between ductile tearing to collapse in function of the crack length. The paper analyzes these experiments and simulates the ductile tearing using node release methodology. The prediction of the apparition of the collapse is obtained by determining collapse load with large displacement modeling.
From these results, a reconciliation of curves J-R of the CCT et SE(T) specimen will be done.
... Eq. 6 où a 0 et a sont respectivement le rayon de section minimale initial et courant obtenus par la mesure de contraction diamétrale assurée par un système de mesure par nappe laser [56,57]. Ce système de mesure est schématisé sur la Figure Au delà de l'instabilité, le chargement devient multiaxial du fait de l'effet d'entaille induit par la striction. ...
... Eq. 6 où a 0 et a sont respectivement le rayon de section minimale initial et courant obtenus par la mesure de contraction diamétrale assurée par un système de mesure par nappe laser [56,57]. Ce système de mesure est schématisé sur la Figure Au delà de l'instabilité, le chargement devient multiaxial du fait de l'effet d'entaille induit par la striction. ...
The AA6061-T6 aluminum alloy was chosen as the material for the core vessel of the future Jules Horowitz testing reactor (JHR). The objective of this thesis is to understand and model the tensile and fracture behavior of the material, as well as the origin of damage anisotropy. A micromechanical approach was used to link the microstructure and mechanical behavior. The microstructure of the alloy was characterized on the surface via Scanning Electron Microscopy and in the 3D volume via synchrotron X-ray tomography and laminography. The damage mechanism was identified by in-situ SEM tensile testing, ex-situ X-ray tomography and in-situ laminography on different levels of triaxiality. The observations have shown that damage nucleated at lower strains on Mg2Si coarse precipitates than on iron rich intermetallics. The identified scenario and the in-situ measurements were then used to develop a coupled GTN damage model incorporating nucleation, growth and coalescence of cavities formed by coarse precipitates. The relationship between the damage and the microstructure anisotropies was explained and simulated.
... The innovative testing device [8,9] using laser sensors to determine the tensile curves of the materials and the procedure are detailed in [3]. The Stress-Strain curves of all the materials have been obtained at the considered temperatures [3]. ...
Demonstration of large components integrity is based on the demonstration that they could never undergo brittle fracture. In parallel, considering a conventional defect somewhere in the component, failure must prevent ductile crack propagation.
Connections between a ferritic component and an austenitic one have to respect these rules. The considered case is a Ni base alloy weld joint between a ferritic pipe and an austenitic one. For brittle fracture exclusion, the aim of the present study is to show that in the same loading conditions, the weld joint is less sensitive to the brittle fracture than the surrounding ferritic part of the component. That is to say that the demonstration should be focused on the ferritic base metal which is the weakest material.
For that purpose, Chapuliot developed a stress-based criterion model, using a threshold stress (σth) below which the cleavage cannot occur. It can be used to define the brittle crack occurrence probability, which means that it is possible to determine the highest loading conditions without any brittle fracture risk.
For the experimental part of this demonstration, two different mock-ups with DMW are used. The first one is composed of a 16MND5 (A508 cl. 3) steel pipe welded to a 316L stainless steel pipe with Ni base alloy 82. As for the second one, the materials are the 18MND5 (A533) steel, the 316L stainless steel and the Ni base alloy 52. Conventional defects have been considered in the ferritic part, close to the weld joint in the heat affected zone, and far away from the weld joint in the ferritic part. Two hundred specimens have been taken from the mock-ups: special tensile specimens, compact tensile specimens, single edge notch tension specimens, notched tensile specimens, smooth tensile specimens. All the materials have been characterized at −125°C/−130°C and −170°C/−175°C, even the heat affected zones. Finite element calculations have been done to complete the experimental investigations. The first results show that, due to the mismatch of the materials, the brittle fracture risk is much lower in the HAZ. Thus, DMW HAZ could not be a weak part concerning brittle fracture. This paper presents the criterion, the experimental work and the analyses made to evaluate the conservatism of the homogeneous ferritic case compared to the DMW.
... Tests are performed on a 100 kN INSTRON tensile test machine [2] [3] equipped with a thermal chamber ( fig. 9 & 10with four windows so that the specimen shape can be measured with laser sensors in both directions x and y which are perpendicular to the loading direction. Also, in the loading direction, the laser sensors are moving on the z axis according to defined frequency and displacement so that they can scan the desired length of the specimen ( fig. ...
Characterization of the fracture resistance of weld joints, and in particular dissimilar metal welds (DMW), is a huge and difficult work where no standard currently exists. As a consequence, characterizations of materials have to be done prior to the fracture tests in order to consider the mechanical aspects of the material mismatch via F.E. analysis in the fracture toughness determination. However, performing these characterization tests may imply using a lot of material, which is sometimes not available, and can be expensive. Most studies are usually realized on reduced sized mock-ups.
The considered experimental mock-ups are pipes composed of a ferritic pipe welded to an austenitic one. The weld joints are made of Ni base alloys. A few small tensile samples have been extracted so that each material, even the heat affected zone (HAZ), can be characterized at low temperature. A fast method using laser sensors and a specific specimen shape has been developed and is used to identify these materials strain-stress curves at −125°C. Afterwards, these data have been used to simulate multi-material compact specimen tensile tests and single edge notch tension specimen, representing a conventional defect in the HAZ. Also, these two kinds of specimen have been extracted and the tests performed in order to compare the experimental results to the F.E. analysis. This paper presents the experimental work, the related specific devices, the F.E. analyses and the experimental analysis.
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