Fig 2
Difference of principle between BWR and PWR. In BWR, the water heated in the RPV directly enters the turbine. In PWR, it is used to heat a secondary circuit. (Aalto University, 2012)
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Dissimilar metal welds (DMWs) between low-alloy steels (LAS), stainless steels (SS) and nickel-base alloys are very important in the design of conventional and nuclear power plants (NPPs). They help to reach better performances for high temperature environment but they can promote premature failure of components. Failure is often related to crackin...
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... shown in Figure 2, the main difference is that in BWRs, the steam is produced directly in the reactor pressure vessel and goes to the turbine. In PWRs, a first circuit of water is heated in the reactor pressure vessel. ...
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... both BWR and PWR, the failure mechanism will be mainly SCC and irradiation-assisted stress corrosion cracking (IASCC), the main failure locations being the heat-affected zones (HAZ) of the welds. (Hänninen, 2009) 2. Dissimilar metal welds ...
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... in welds usually occurs along grain boundaries. This includes weld solidification cracking, weld metal liquation cracking, HAZ liquation cracking, stress relief heat treatment and strain-age cracking, and ductility dip cracking (Lippold et al. 2000). ...
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... stresses in DMWs are also mainly due to the difference in the coefficient of thermal expansion of the materials. If these are very different, internal stresses will appear in the intermetallic zone during any temperature change across the different regions of the weldment (Srinivasan et al. 2006). This is of particular importance during temperature fluctuations ( Kotecki et al. 1997) that will cause thermal fatigue cracking to happen, especially if brittle phases have appeared in the transition zones. ...
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... metal welds between austenitic stainless steel and ferritic steel containing ). An optical view of such a weld is given in Fig. ...
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... II boundaries are grain boundaries that run about parallel to the weld interface at a very short distance into the weld metal (less than 100 m away from the fusion line) (see Fig. 29 ...
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... its similar thermal expansion coefficient and melting point compared to ferritic and austenitic steels make it the usual choice of buttering alloy (Sudha et al. 2008). Figure 32 shows the effect of a simulated Ni interlayer between two Cr-Mo steels of different chromium contents (2,25Cr-1Mo and 9Cr-1Mo). ...
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... has been noticed, however, that Inconel 52 weld microstructure can contain dynamically recrystallized zones, which may improve SCC growth through a dendritic structure. Figure 33, continuous NbC (shiny precipitates) and distributed TiC precipitates (dark particles) form in the interdendritic regions (Naffakh et al. 2009). In Inconel 52, precipitates are distributed in the interdendritic regions and also at the GBs. ...
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... partially melted zone on the Inconel 657 side of the joint is wider than that on the other side. The tendency of dendritic boundaries to melt in Inconel 657 is attributed to the enrichment of Nb at GBs. (Naffakh et al. 2009) ...
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... mechanical damage such as fretting or fatigue is likely to occur more than pure corrosion damage. (Hong et al. 2005) ...
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... if operating experience and testing continue to show excellent behavior of Alloy 690/52/152 materials in resistance to PWSCC under normal PWR conditions (Fyfitch et al. 2012), they are not immune to SCC, and cracks can grow at moderate or high rates in PWR primary water under certain conditions (see Fig. 42 ...
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... is related to GB segregation during cooling, which is mostly influenced by the content of S and P of the weld metal. (Nishimoto et al. 2006) ...
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... point is that it slightly decreases the grain size, thus, reducing the intensity of segregation. (Nishimoto et al. 2006) ...
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... first cut from the NG-GTAW weld has not been post-weld heat treated and is called "As-welded" (AW). The other has been Heat- treated (HT) following the cycle presented in Figure 52. It consists of heating and maintaining the sample at 550°C for 20 h, then increasing shortly the temperature to 610°C for 10 h. ...
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... sample is a cross-section of the weldment (see Fig. 61). Figure 62 shows the details of the welding parameters used for this weld. Considering here a 50% peak + 50% background pulse for the layers 9-12, and an GTAW efficiency of 0,7, four different heat inputs have been used for this weld: 102 -layers 9-12: Q= 7 kJ/cm -layers 13-14: Q= 9,5 kJ/cm -layers 15-20: Q=10,9 kJ/cm -layers 21-26: Q= 9,3 kJ/cm ...
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... the same weld configuration as the previous sample, it presents a weld between two Alloy 690 plates with Inconel 152 weld metal. Figure 92 shows a global view of the Inconel 152 weld passes. They are thicker (about 5 mm) than in the previous CIEMAT sample because manual arc welding has been used here. ...
Similar publications
Nb-stabilized TP347H stainless steel is widely used as reheater and superheater components in a power plant setting. Because of the extreme environment of high temperature and high pressure due to steam in power generation, material degradation and failure can occur, especially in the heat-affected zone (HAZ). This presents an urgent problem that r...
Citations
... These Type I boundaries are oriented nearly perpendicular to the fusion line. Researchers have substantiated that both Type I and Type II boundaries, primarily characterized by high-angle boundaries, are susceptible to stress corrosion cracking (SCC) [4,[45][46][47]. Surprisingly, discussions regarding their presence in DWJ are rare. ...
Unfavourable operating conditions of equipment in the energy industry resulting from high-temperature loads determine the need to use special materials and technological solutions, including welding procedures. In this article, buttering using IN82 (ERNiCr-3) consumables was proposed as a method to improve the weldability of grade 92 steel joined by the gas tungsten arc welding (GTAW) process with AISI 304L (IN617 filler). The microstructural characterization of samples was carried out using an optical microscope, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy. The welded joint was further characterized by hardness, tensile (room temperature and at 620 °C temperature) and impact tests. Additionally, the fracture surfaces of tensile and impact tests were studied by SEM. Despite the confirmation of the diffusion of alloying elements and significant changes in their concentration, which indicates the formation of Ti and Nb-rich phases, no welding imperfections were detected and favourable joint structures and acceptable properties were obtained. In particular, this concerns the limitation of the formation of brittle structures and the elimination of the untempered martensitic layer. At the same time, there was a significant decrease in the maximum hardness of heat-affected zone (HAZ) on the grade 92 steel side to a relatively low value of 310 HV, and a minimum tensile strength criterion of 600 MPa was achieved with a simultaneous increase in ductility (35% elongation) of the joint. Comparatively, when compared to a non-buttered welded joint, the joint produced with a buttering layer exhibited an increase in the elongation and impact toughness of the welded joint without any compromise in ultimate tensile strength (Sut). The fracture surface of tensile and impact-tested specimens was also characterized using SEM/EDS. Summarizing all the results, it can be concluded that the proposed GTAW procedure of grade 92 and 304L steels can be used in extreme working conditions, in ultra-supercritical power units or the petrochemical and chemical industries.
... The RPV specimen used in this study is of SA302B grade steel. The majority of reactor pressure vessels (53 out of 73 PWRs) operating in 1994 are of this steel type and remaining with SA533B1 steel (Mouginot and Hänninen, 2013). The chemical composition is shown in Table 1. ...
Spray cooling is a versatile technology for various cooling applications involving high surface heat fluxes. Experimental facility was built to study heat transfer performance of an upward multi-nozzle array of water sprays impacting a surface of heated plate made of reactor vessel grade steel. The effect of inclination angles of the steel surface on the cooling performance was investigated to assess heat transfer in complex semispherical/ semielliptical geometry of large reactor lower head and to address possible application of spray cooling in severe accident management (SAM) of light water reactors (LWRs) based on In-vessel melt retention with external reactor vessel cooling (IVR-ERVC). Joule heating of SA302B steel foil of 0.15 mm thickness and surface area of 96 cm^2 enabled prototypic heat fluxes to be evacuated during reactor accident. A 2 × 3 array of full jet narrow-coned pressure-swirl spray nozzles was used to reproduce multi-nozzle cooling. The tests were conducted as a series of consequent steady states realized at stepwise increasing power and surface heat fluxes up to the maximum values of 29 kW and 2.97 MW/m^2 limited in the specific facility design. Seven surface inclinations, between 0 degrees and 90 degrees were tested and no significant variations in spray cooling performance with the inclination of the heated surface was found. The results indicated a promising prospect of using a multi-nozzle array system for cooling of large surface area of reactor lower head. Much higher heat fluxes can be safely extracted by spray cooling in comparison with the critical heat fluxes that appeared at RPV water pool cooling at natural convection. The maximum value of heat flux at direct spray impact zones and its drop-off slightly from the center to the periphery of the spray cone was detected in the tests. The water flow rate and liquid subcooling significantly influenced maximum steel surface temperature but had no noticeable effects on surface temperature uniformity. The spray-to-spray interaction had no observable effects on local surface temperatures, however, the colliding zones where four spray cones have visible effects on local surface temperatures due to poor liquid momentum. The results also showed that more uniform liquid film distribution could be obtained for some inclinations because of improved liquid drainage, which in turn leads to maintaining low surface temperatures.
... Table 1. Chemical composition of the SA302B steel used in the study [14]. ...
... Observation of microstructure in the welding area shows that epitaxial grain growth has been seen in the fusion line area, because the welding process carried out by the Incoloy 825 base metal uses the autogenous welding method [18,19]. This epitaxial mechanism can be seen from the results of the microstructure in all A-TIG variable processes in various variables of addition of isopropanol solvent. ...
The purpose of this study was to investigate the effect of flux solvent on the depth-to-width ratio (DWR), microstructure, and the hardness of Incoloy 825 with activated tungsten inert gas (A-TIG) welding process. Effect of solvents on active flux of TIG welding for geometry characteristics were investigated by metallographic and defense tests. The arc constriction and Marangoni convection are considered as the two main factors for increasing the penetration of the A-TIG weld pool. Marangoni convection provided change to fluid flow in the anode area of welding it formed a thermionic electron condition because of the large positive ion collisions in the welding arc. This study investigated the effect of acetone/isopropanol as a TiO 2 active flux solvents on A-TIG incoloy 825 autogenous welding. Metallography observations carried out give the best results on the addition solvents of 40% isopropanol can provide DWR 0.549 while 100% acetone solvent has DWR 0.321 and for 100% isopropanol solvent has a much smaller DWR which is 0.276. In addition to providing penetration rates, this isopropanol have an effect increasing of hardness especially in the fusion line at 40% isopropanol 198.79 HV and 100% isopropanol 202.52 HV.
... In a case of low-alloy steels, which crystal structure is BCC, this is possible due to the high welding temperature when the base metal undergoes an allotropic delta-ferrite (δ-Fe) to austenite (γ-Fe) transformation. Then the fusion line becomes an austenite (FCC) boundary between the base metal (FCC) and the weld metal (FCC) in the austenitic temperature range (Mouginot & Hänninen 2013). Type II boundaries may be sites for carbide precipitation, particularly if significant carbon migration from the base metal has occurred. ...
... Changing base metal dilution, which affects the composition gradient in the weld metal at the fusion line, supports the existence of this transition region (Nelson et al. 2000). Type II boundaries have generally lower Ni and Cr contents due to dilution from the base metal to the weld metal (Chung et al. 2011, Mouginot & Hänninen 2013. Yoo et al. (2015) show in their study that Type II boundaries shift away from the fusion line during thermal aging. ...
Dissimilar metal welds (DMW) are used in nuclear power plants, in particular for the constitution of the primary circuit, for joining primary circuit pipes, made of austenitic stainless steel, with the nozzles of the primary component vessels made of low-alloy steel (LAS). This safe-end is a part of the primary loop of reactor pressure boundary, and it is one of the key safety related components of nuclear reactor. Ni-base alloys are typically used as a weld metal in DMW joints. Narrow-gap welding (NGW) technique is used, since the amount of weld metal is small, heat input is low and the method is more economical than conventional V-grooved weld. Thermal aging of materials used in pressurized water reactors (PWR) is a time and temperature dependent degradation mechanism that typically results in a decreased toughness of the material. In general, structures constructed from alloys that exhibit ductile-to-brittle behaviour should thus be used only at temperatures above the transition region. Accelerated thermal aging at a temperature higher than the nuclear power plant operating temperature is used to simulate the thermal aging effects during the NPPs service time. Segregation of phosphorus to grain boundaries in pressure vessel steels during service at elevated temperatures is a relatively common occurrence. The segregation of P promotes a change in the brittle fracture mode from transgranular to intergranular, and degradation in mechanical properties. This is considered as a major degradation mechanism for thermal aging.
In this work, the impact toughness of as-received and 5000 hours aged conditions of the safe-end DMW joint, and in particular the heat-affected zone (HAZ), is characterized. The used test method is the instrumented Charpy-V impact toughness test. Ductile-to-brittle transition region was characterized and crack arrest toughness estimated for the test material. Scanning electron microscopy (SEM) was used to examine fracture surfaces and electron probe microanalyzer (EPMA) was used to reveal the elemental distribution and composition evolution over the HAZ and fusion line. As a result of thermal aging, the transition temperature T42J and the maximum absorbed energy increased significantly. In this work, the EPMA analyses were shown to provide a good overview of the chemical composition over the fusion line. Diffusion of elements occurred during the thermal aging is clearly visible in analysed data.
... C diffuses from the HAZ to the fusion zone during welding due to this gradient [7] which can result in formation of martensite according to the Schaeffler diagram [8]. The tendency for C to migrate from HAZ into the weld metal during post-weld heat treatment (PWHT) is especially strong in high-Cr alloys, such as Alloy 52 [9,10]. ...
Dissimilar metal welds (DMWs) have several different microstructural zones in the vicinity of the low-alloy steel – Ni-based weld metal fusion boundary that have an effect on the mechanical and fracture mechanical behaviour of the weld joint. These microstructural zones are especially small when modern narrow-gap (NG) gas tungsten arc welding (GTAW) technique is utilized. Determination of the fracture toughness properties of the microstructural zones of a DMW joint is of utmost importance for successful structural integrity and lifetime analyses. This paper presents the results from fracture resistance (J-R) tests, micro-hardness measurements and microstructural characterization performed for a DMW mock-up manufactured by Mitsubishi Heavy Industries (MHI). The studied material is fully representative to that of OL3 EPR pressurized water reactor safe-end weld. The results show that the DMW joint is tough at the SA 508 low-alloy steel – Alloy 52 weld metal interface, which typically is the weakest area of a DMW. Fracture surfaces of the J-R test specimens tested in T-L orientation show a wavy appearance that follows the weld bead boundaries. Crack propagation path was studied also using optical 3D profilometry. Microhardness results show that there is a steep hardness gradient at the fusion boundary where the carbon-depleted zone (CDZ) of the SA 508 exhibits the lowest hardness and the narrow zone exhibiting the highest hardness is located right next to the fusion boundary at the Alloy 52 side.
... C diffuses from the HAZ to the fusion zone during the welding or post-weld heat treatment (PWHT) due to this gradient (Lundin, 1982) which can result in formation of some martensite according to the Schaeffler diagram (DuPont et al., 2009). The tendency for C to migrate from HAZ into the weld metal during PWHT is especially strong in high-Cr alloys, such as Alloy 52 (Mouginot & Hänninen, 2013;Hänninen et al., 2014). ...
... According to a survey of the literature, recent research has focused on DMWJs that employ Ni-based filler metals [26], i.e. Alloy 82 and Alloy 182, or Alloy 52 and Alloy 152, ignoring the fact that SS safe-end to LAS pipenozzle DMWJs that use 309L/308L stainless steel as filler metals are still common in operating PWRs. ...
The microstructure of an SA508–309L/308L–316L domestic dissimilar metal welded safe-end joint was characterized in this work by optical microscopy, scanning electron microscopy (with electron back scattering diffraction) and micro-hardness testing. Epitaxial growth and competitive growth are evident in the 308L–316L fusion boundary regions. A martensite layer, carbon-depleted zones, and type-II and type-I boundaries are found in the SA508–309L fusion boundary regions, while only martensite and austenite mixed zones are observed in the SA508–308L fusion boundary regions. The microstructure near the fusion boundary and the microstructure transition in the SA508 heat affected zone are quite complex. Both for SA508–309L/308L and 308L–316L, the highest residual strain is located on the outside of the weldment. The residual strain and the grain boundary character distribution change with increasing distance from the fusion boundary in the heat affected zone of 316L. Micro-hardness measurements also reveal non-uniform mechanical properties across the weldment.
... According to a survey of the literature, recent research has focused on DMWJs that employ Ni-based filler metals [26], i.e. Alloy 82 and Alloy 182, or Alloy 52 and Alloy 152, ignoring the fact that SS safe-end to LAS pipenozzle DMWJs that use 309L/308L stainless steel as filler metals are still common in operating PWRs. ...
... The dimensions and configuration of the NG-GTAW weld (NG-DMW) are presented in Figure 3. The experimental program is described in more detail in Ref. [13]. Cross-sections were extracted from the NG-DMW. ...
... The microhardness profiles for the PWR type NG-DMW in the as-welded (AW) and PWHT conditions are shown in Figures 6-7 and 8, respectively. The results are presented and discussed in more detail in Ref. [13]. ...
... Ongoing metallographic investigations and awaited miniature specimen tensile tests are expected to provide further illumination on the characteristics of the discovered hardness profile changes and their underlying mechanisms in the NG-DMW; for instance, whether the steep increase in hardness in the Alloy 52 weld metal side can be attributed to precipitation of carbides or other metallic phases during complex stages of the PWHT. [13]. Note that these measurements were made in a slightly different location of the weld interface compared to Figure 6. ...
A characteristic feature of dissimilar metal weld (DMW) is the metallurgical and material property mis-match resulting from a steep gradient of microstructures with significantly different strength and toughness properties. This mis-match inevitably affects the entire failure behaviour of DMWs under external operational loads, not only from crack initiation standpoint, but especially regarding the development of crack driving force accentuating crack path deviation and subsequent crack growth. The determination of relevant fracture toughness properties for all the different microstructural regions of the DMW is of utmost importance for successful structural integrity and lifetime analyses. This paper deals with experimental material characterisation of two configurations of ferrite (SA508)–austenite (Type 304/316) DMWs made using (i) a beveled V-groove and Alloy 82/182 filler metal with a buttering layer and (ii) a narrow-gap weld and Alloy 52 filler metal without any buttering. Results of hardness surveys and fracture mechanical (J-R curve) tests are presented and analysed. The role of mis-match (i.e. metallurgical constraint) in e.g. promoting sudden crack path deviations is discussed in the light of detailed post-test sectioning metallography and specimen fractography identifying the actual crack initiation and propagation path.