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Brittle Fracture Analysis of Dissimilar Metal Weld Joints
Abstract
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.
... This work is based on many experimental results [1][2][3][4]. A mock-up representing the new generation of narrow gap weld used for the EPR TM , made of a 18MND5 (A533) pipe, a 316L pipe and a Ni base alloy 52 narrow gap weld, was used ( Figure 2). ...
... Many fracture tests were performed on NT, CT and SENT specimens at -125°C to identify the different parameters [1][2][3] of the criterion (Table 1). Those specimens were homogeneous (A533) or multi-material (A533/Ni base alloy/316L). ...
... The first parameter is σ th : the value was determined and is considered to be 1375 MPa [1][2][3]. The transferability of those parameters from one geometry to another was also evaluated [1-4, 6, 14]. ...
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. In addition, residual stresses are also considered and their impact on the fracture resistance is evaluated.
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.
... The probability of fracture of each case, considering the applied mechanical load, is compared. Characterization tests and finite elements analyses were presented in123. They were used to obtain the tensile curves of the different materials including the HAZ and to understand the involved mechanical phenomenon. ...
... The experimental part was presented in123. All the tensile curves of the materials, including the HAZ, were determined with tensile tests at -125°C (Table 1). ...
... All the experimental tests were simulated with CAST3M [10] using elastic-plastic models and introducing the experimental tensile curves of the materials123. From these calculations, the values of J and V th were determined for each test. ...
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.
... In this study, two pipes ( fig. 1 & 2) with welds are used for the experimental part [1]. Both of them are representing welds between the main coolant line and the pressure vessel of a nuclear plant. ...
... All of these results can be used to compare the fracture resistance of the ferritic steels to their HAZ. It would be interesting to know which one has the best properties and which one would break first for an equivalent mechanical loading [1]. In order to study this aspect, a stress-based criterion mock-up, using a threshold stress, developed by Chapuliot [4,5,6] will be used and the probability of brittle fracture occurrence calculated. ...
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.
... In order to determine the tensile curves of all the materials, and the HAZ, special flat specimens of 2 mm thickness have been designed. A detailed presentation of this test campaign is presented in [17,18]. With such specimens, the characterized part of the material is where the notch is machined. ...
This paper deals with a fracture mechanics analysis of a narrow-gap Dissimilar Metal Weld in the brittle fracture domain. The considered case is a Ni base alloy weld joint between a ferritic component and an austenitic pipe and 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 nearby ferritic part of the component: two positions of a postulated crack are compared, showing that the benefit in terms of fracture resistance for a crack in the ferritic Heat Affected Zone is mainly due to the loss of constraint induced by the plasticity in the Nickel
base alloy weld metal.
... In order to determine the tensile curves of all the materials, and the HAZ, special flat specimens of 2 mm thickness have been designed. A detailed presentation of this test campaign is presented in [17,18]. With such specimens, the characterized part of the material is where the notch is machined. ...
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.
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.
This paper deals with the brittle fracture risk evaluation for a C-Mn weld joint in the upper shelf of the brittle to ductile fracture transition temperature range, with the main objective to develop a predictive criteria, able to demonstrate the complete absence of brittle fracture risk. The question was investigated in the frame of two PhDs. In the first one (V. Le Corre PhD), a critical stress based fracture criterion was proposed, qualibrated and validated against experimental data for the base metal. This work gave promising results with, in particular, the capability of the model to predict non fracture for a cracked pipe submitted to bending at low temperature. In the second one (T.H. N’Guyen PhD), the model was calibrated and applied to the weld joint. The work showed that material heterogeneity of the weld metal must be taken into account in order to obtain a good representation of the fracture behavior. Again, the model was confronted to different specimen geometries and showed its capability to reasonably predict constraint and geometrical effects on the brittle fracture appearance risk. The paper gives a synthesis of the main results obtained during these two PhDs, questions still to be solved and perspectives for the continuation of this work.
The aluminum alloy 6061-T6 is a material used for some part of experimental reactor for its interesting physical and mechanical proprieties. To respect the European rules on pressure equipments, a material must present some minimal characteristics for toughness and ductility. In this frame, a new measurement system for tensile testing was developed in the LISN laboratory 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 determinate 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. For example, the application of this new measurement system on tensile test allow to determine the behavior of the Mod.9Cr-1Mo until a strain of 130% while the conventional method can not determine the behavior up to 10% corresponding to the initiation of necking.
This paper evaluates the brittle fracture risk for a C–Mn weld in the upper shelf of the brittle-to-ductile transition: the criterion considered is based on a critical stress σth, with the failure probability related to the volume around the crack where the maximum principal stress exceeds σth.The weld shows a complex microstructure with two types of melted zone. SEM observations showed that the main cleavage sites are located in the coarse grain zone, near an inclusion. In test analyses, the material’s heterogeneity in the weld metal is considered to estimate local fields at the weld pass scale.
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 the K /SUB IC/ temperature dependence.
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.
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.
« Etude de la compétition déchirure ductile / rupture fragile : application à la tenue mecanique des tubes en acier C-Mn et de leurs joints soudés
- Le Corre
Le Corre V., 2006, « Etude de la compétition déchirure
ductile / rupture fragile : application à la tenue mecanique
des tubes en acier C-Mn et de leurs joints soudés », PhD
Thesis.
« Prédiction de la non-rupture fragile dans un joint soudé en acier C-Mn dans le domaine de la transition fragile/ductile
- T H N'guyen
N'Guyen T.H., 2009, « Prédiction de la non-rupture
fragile dans un joint soudé en acier C-Mn dans le domaine
de la transition fragile/ductile », PhD Thesis.