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Brittle fracture analysis of a narrow gap dissimilar metal weld
Abstract
One important part of the integrity demonstration of large
ferritic components is based on the demonstration that they
could never undergo brittle fracture. Connections between a
ferritic component and an austenitic piping (Dissimilar Metal
Weld - DMW) have to respect these rules, in particular the Heat
Affected Zone (HAZ) created by the welding process and
which encounters a brittle-to-ductile transition.
Within that frame, the case considered in this article is a
Ni base alloy narrow gap weld joint between a ferritic pipe
(A533 steel) and an austenitic pipe (316L stainless steel). 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.
The bases of this study rely on a stress-based criterion
developed by Chapuliot et al., using a threshold stress (σth)
below which the cleavage cannot occur. This threshold stress
can be used to define the brittle crack occurrence probability,
which means it is possible to determine the highest loading
conditions without any brittle fracture risk.
Steam generation from hot reformer effluent in a hydrogen unit of a petroleum refinery increases the refinery margin. Upstream and downstream piping in hydrogen plant were constructed by SA335 P22 (AS) and type SS304L (SS) materials, respectively. This paper reports
failure of dissimilar pipe weld after six months of high pressure steam service. Visual inspection indicates several ratchet marks initiated from outer wall surface at heat affected zone (HAZ) of AS pipe accompanied with brittle fracture. Further, failed samples were examined for chemical analysis, micro-hardness, fractography & elemental analysis and microstructural analysis. Fractographic
results indicated that the dissimilar weld was failed due to thermal fatigue. Moreover, microstructural analysis revealed the presence of martensite in AS HAZ region with higher micro-hardness. In-spite of fixed operating temperature at 600 °C, there was a difference in thermal expansion that induced large strain gradient at interface and result in low cycle thermal fatigue failure. Type 308L filler metal was directly employed for dissimilar welding without buttering at interface layer was the main cause of failure. The principal factors that were responsible for Dissimilar Metal Weld (DMW) failure are discussed with respect to Schaeffler diagram and correct welding procedure for failure prevention is proposed. It is further recommended to employ 309/309L filler metal at interface layer according to ASME codes & standards to avoid DMW failure.
Experiments were performed on three heats of A508 class 3 steel in order to determine the mechanical conditions for cleavage
fracture. These tests were carried out on various geometries including 4-point bend specimens and axisymmetric notched tensile
bars with different notch radii which have been modelized using the finite element method. In one heat, the temperature range
investigated was from 77 K to 233 K. It is shown that the cleavage resistance is increased by tensile straining. Moreover,
the probability of fracture obeys the Weibull statistical distribution. All the results can be accounted for in terms of a
local criterion based on Weibull theory and which takes into account the effect of plastic strain. In this criterion, the
parameters which were experimentally determined are found to be temperature independent over the range 77 K to 170 K. The
applicability of the approach proposed for cleavage fracture at the crack tip is also examined. It is shown that the experimental
results published in the literature giving the variation of fracture toughness with temperature can be explained by the proposed
criterion which predicts reasonably well both the scatter in the experimental results and theK
ICtemperature dependence.
This paper deals with the brittle fracture risk evaluation for a C-Mn piping component in the upper shelf of the brittle to ductile fracture transition temperature range, with the main objective to validate a predictive criteria, able to demonstrate the complete absence of brittle fracture risk. The criteria is based one a critical stress and the volume around the crack were the maximum principal stress exceed this critical stress. The model is calibrated on notched tensile specimens and CT specimens. A four-points bending pipe test has then been designed using this criterion to insure that no brittle fracture will occurs at a temperature that all CT specimens failed by cleavage. The material is a French secondary loop Tu42C ferritic steel and the pipe dimensions for the test are the same than the size of the principal secondary loop pipes. The results of the pipe test confirm the prediction with the model and the interpretation lead to define an equivalence between the loading conditions (based on the J parameter) of the pipe and the loading condition of a CT specimen.
This article describes an analysis made to develop a simplified stress-based criterion for brittle fracture focussed on the lowest probability of failure. For that, on the basis of fine numerical interpretation of two series of fracture-tests on 16MND5 reactor vessel steel, a number of variables were proposed:•A stress threshold σth below which cleavage cannot occur. This stress is determined by testing on notch tensile specimen at low temperature.•A minimum toughness Kmin(T) required to make a crack unstable. The originality is here to consider this parameter depends on temperature.•For KJ>Kmin(T), a volume susceptible to cleavage, defined as the volume of material subjected to stress exceeding the threshold stress and noted Vth, representative of the fracture probability.These three variables are explained in the article then used to establish a tentative criterion for expressing the risk of brittle fracture, in the presence or absence of a crack.
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.
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.
Ce travail de thèse s'inscrit dans le contexte de la sûreté nucléaire, et plus précisément, de l'intégrité des circuits secondaires des Réacteurs à eau pressurisée (REP). L'étude porte donc sur le comportement à rupture de structures minces soudées dans le domaine haut de la transition fragile/ductile. Elle a pour objectif de développer le modèle en contrainte seuil initialement développé par Chapuliot, qui permet de prédire la non-rupture par clivage de cette structure soudée. Le modèle est identifié pour la soudure de l'acier au C-Mn de construction nucléaire, en s'intéressant plus particulièrement à la limite supérieure du domaine de transition.Une contrainte seuil, en-dessous de laquelle le clivage ne peut avoir lieu, est identifiée à partir d'essais de traction à basses températures sur éprouvettes axisymétriques entaillées prélevées dans le joint soudé. Cette contrainte seuil permet de définir le volume seuil, ou volume dans lequel les contraintes principales maximales dépassent la contrainte seuil au cours de l'essai.L'analyse au MEB des faciès des éprouvettes rompues montre que la zone fondue brute de solidification dans la ZAT est la zone la plus susceptible de cliver. La relation entre la probabilité de rupture fragile et le volume seuil dans cette zone est établie via une fonction de sensibilité, grâce à des essais sur éprouvettes CT et à leur simulation multi-matériaux. Le modèle ainsi identifié est testé pour prévoir la non rupture par clivage d'éprouvettes SENT prélevées dans le joint soudé et sollicitées en traction. Les résultats obtenus sont encourageants relativement à la transférabilité du modèle à la structure réelle
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