A Study of Thermal Aging Effects on Microstructures of Dissimilar Metal Weldment

2013 ◽  
Author(s):  
Kyoung Joon Choi ◽  
Jong Jin Kim ◽  
Ji Hyun Kim
Keyword(s):  
Weld Metal ◽  
Alloy Steel ◽  
Thermal Aging ◽  
Base Alloy ◽  
Boundary Region ◽  
Low Alloy Steel ◽  
Aging Effects ◽  
Fusion Boundary ◽  
Dissimilar Weld ◽  
Dissimilar Metal

Characterizations of microstructure and mechanical property were conducted to investigate the effect of long-term thermal aging on the microstructural evolution in the fusion boundary region between weld metal and low alloy steel in dissimilar metal welds. A representative dissimilar weld mock-up made of Alloy 690-Alloy 152-A533 Gr. B was fabricated and aged at 450°C for 5,500 hours. It was observed that the region near the fusion boundary was generally divided into several regions including dilution zone in the Ni-base alloy weld metal, fusion boundary, and heat-affected zone in the low alloy steel. A steep gradient was shown in the chemical composition profile across the interface between A533 Gr. B and Alloy 152. The precipitation of carbides was also observed along and near the fusion boundary of aged dissimilar metal joints. It was also found that the precipitation of Cr carbides was enhanced by the thermal aging near the fusion boundary.

Investigation of Microstructural Changes due to Thermal Aging in Dissimilar Metal Welds

2014 ◽  
Author(s):  
Kyoung Joon Choi ◽  
Seung Chang Yoo ◽  
Chi Bum Bahn ◽  
Ji Hyun Kim
Keyword(s):  
Electron Microscopy ◽  
Heat Treatment ◽  
Alloy Steel ◽  
Thermal Aging ◽  
Boundary Region ◽  
Low Alloy Steel ◽  
Chemical Gradient ◽  
Fusion Boundary ◽  
Dissimilar Metal

To investigate the effect of long-term thermal aging on the fusion boundary region between low-alloy steel A533 Gr. B and weld metal Alloy 152, a representative dissimilar weld mockup composed of Alloy 690/Alloy 152/A533 Gr. B was aged in laboratory furnaces under accelerated temperature conditions. The aging time was determined using the diffusion equation. The heat treatment was performed at 450°C for 60-y equivalent time (5,500 h) to simulate thermal aging effects. An additional aging heat treatment was also performed at 400°C for 15- and 30-y equivalent times (6,450 and 12,911 h, respectively) to determine the effects of temperature on aged microstructures. The characterization was mainly conducted in the microstructure of the fusion boundary region in the weld root region using scanning electron microscopy, transmission electron microscopy, and three-dimensional atom probe tomography. It was determined that the region near the fusion boundary was generally divided into several regions, such as a dilution zone (that included a chemical gradient in the weld side), fusion boundary, and heat-affected zone in the low-alloy steel. The results of this study showed that heat treatment increased Cr content in the dilution zone, but the chemical gradient in the weld side near the fusion boundary persisted. For the microstructure, it was observed that treatment induced the formation and growth of Cr precipitates in the fusion boundary region of the dissimilar metal joints due to the thermodynamic driving force. At two heat treatment conditions (400 and 450°C), although the extent of the results described above differed, the trend in the results appeared to be the same. This microstructure information can improve the understanding of cracking-resistant change when structural changes occur. Furthermore, such data will be important for assessing the effects of aging on structural components and for evaluating the long-term operation of nuclear power plants.

Effects of thermal aging on microstructures of low alloy steel–Ni base alloy dissimilar metal weld interfaces

2013 ◽  
Vol 441 (1-3) ◽  
pp. 493-502 ◽  
Author(s):  
Kyoung Joon Choi ◽  
Jong Jin Kim ◽  
Bong Ho Lee ◽  
Chi Bum Bahn ◽  
Ji Hyun Kim
Keyword(s):  
Alloy Steel ◽  
Thermal Aging ◽  
Base Alloy ◽  
Low Alloy Steel ◽  
Dissimilar Metal ◽  
Dissimilar Metal Weld ◽  
Metal Weld

Thermal Aging Assessment and Microstructural Investigations of Alloy 52 Dissimilar Metal Welds for Nuclear Components

2019 ◽  
Author(s):  
Miguel Yescas ◽  
Pierre Joly ◽  
François Roch
Keyword(s):  
Fracture Toughness ◽  
Alloy Steel ◽  
Thermal Aging ◽  
Base Alloy ◽  
Narrow Gap ◽  
Low Alloy Steel ◽  
Nickel Base Alloy ◽  
Pressurized Water ◽  
Dissimilar Metal ◽  
Nickel Base

Abstract Dissimilar Metal Welds (DMW) are commonly found between the ferritic low alloy steel heavy section components and the austenitic stainless steel piping sections in nuclear power plants. In the EPR™ design which is the latest FRAMATOME Pressurized water reactor (PWR) these DMW involve a narrow gap technology with no buttering, and only one bead per layer of a nickel base alloy weld filler metal (Alloy 52). In order to assess the thermal aging performance of this relatively new narrow gap DMW design, a significant internal R&D program was launched some years ago. Several representative mock-ups were thoroughly characterized in the initial condition as well as in the thermal aged condition, up to 50,000 hours aging at 350°C. The characterisations were focused on the fusion line between the ferritic low alloy steel (LAS) and the nickel base alloy since a particular microstructure is present in this area, especially in the carbon depleted area of the Heat Affected Zone (HAZ) which is often regarded as the weak zone of the weld joint. Metallography, hardness, nanohardness, chemical analyses, and Atom Probe Tomography, as well as fracture toughness tests were carried out on different specimens in different thermal aging conditions. The results show that the fracture toughness behaviour in the ductile-brittle domain of the low alloy steel carbon depleted HAZ at the interface with the alloy 52 weld metal of the DMWs is excellent, even for a thermal ageing equivalent to 60 years at service temperature. This was found in spite of the carbon depleted zone of the HAZ, the variations of hardness, chemical composition, particularly the carbon gradients, and the thermal aging effect induced by phosphorous segregation at grain boundaries.

Thermal Aging Effects on Microstructures of Type-II Boundary in Dissimilar Metal Weld Joints

2014 ◽  
Author(s):  
Seung Chang Yoo ◽  
Kyoung Joon Choi ◽  
Ji Hyun Kim
Keyword(s):  
Heat Treatment ◽  
Weld Metal ◽  
Thermal Aging ◽  
Chromium Content ◽  
Boundary Region ◽  
Nanoindentation Test ◽  
Type Ii ◽  
Aging Effects ◽  
Fusion Boundary ◽  
Narrow Zone

In order to investigate the long-term thermal aging effects on the type-II boundary region in Alloy 152 weld metal, a representative dissimilar weld mock-up made of Alloy 690–Alloy 152–A533 Gr. B has been fabricated and heat treated under accelerated temperature conditions. To simulate the thermal aging effects, the heat treatment was performed at 450°C for 15, 30 and 60-yr equivalent times (1,375, 2,750 and 5,500 h). The aging time was determined by the diffusion equation based on the activation energy for chromium diffusion. The microstructure characterization was primarily conducted in the type-II boundary region of the weld root, which is a boundary parallel to fusion boundary existing within 100um from the fusion boundary and is known to be less resistant to stress corrosion cracking than other regions in the weld. The investigations were performed by scanning electron microscope, electron backscatter diffraction, and nanoindentation test. In this study, the dilution zone of the chromium content was observed at the weld metal region within a 1.5-mm range from the fusion boundary. Ferrites and high angle grain boundaries are found at the type-II boundary region of weld metal. In the narrow zone between the type-II boundary and fusion boundary, the hardness is relatively higher than that of other regions. These results show that the chromium content in the dilution zone increases with heat treatment, but the stiff chemical gradient still exists in the weld region at the narrow zone between the type-II boundary and fusion boundary.

Fracture Toughness in the Ductile-Brittle Transition and Thermal Ageing Behavior of Decarburized Heat Affected Zone of Alloy 52 Dissimilar Metal Welds of Nuclear Components

2014 ◽  
Author(s):  
Pierre Joly ◽  
Miguel Yescas ◽  
Elisabeth Keim
Keyword(s):  
Fracture Toughness ◽  
Stainless Steel ◽  
Alloy Steel ◽  
Heat Affected Zone ◽  
Power Plants ◽  
Base Alloy ◽  
Thermal Ageing ◽  
Low Alloy Steel ◽  
Dissimilar Metal ◽  
Brittle Transition

Dissimilar metal welds (DMW) are used in nuclear power plants between the nozzles of main components in low alloy steel and stainless steel pipes, or safe-ends connected to the main coolant line pipes. AREVA proposes for EPR™ an improved design of DMW involving narrow gap welding without buttering between the low alloy steel nozzles and the stainless steel safe-ends, and the use of a corrosion resistant weld filler metal (Alloy 52). AREVA performed a thorough characterization of this type of welds, which shows a particular microstructure close to the fusion line between the low alloy steel and the nickel base alloy, where the heat affected zone of the low alloy steel is decarburized. This paper presents results of fracture toughness tests performed with the crack tip located in this area, in the ductile to brittle transition in the as post-welded heat treated condition and after thermal ageing. The results show an excellent fracture toughness behavior of this particular area, compared to that of low alloy steel parent metal.

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