Henry Granjon Prize Competition 2008 Joint Winner — Category B: “Materials Behaviour and Weldability” Evolution of Microstructure and Mechanical Properties of the Heat-Affected Zone in 9Cr Steels
“…The ferritic steels of interest can experience phase transformations during the thermal cycles associated with fusion welding. As a basic exemplar of the phase transformations, Figure 6(A) shows the microstructural effects with peak temperature while Figure 6(B) on the right side provides the calculated equilibrium phase diagram for a composition consistent with Grade 91 steel. Figure 6. Schematic of the sub-zones of the heat affected zone in relation to the calculated equilibrium phase diagram of a 9Cr–1Mo steel [47]. …”
Section: Microstructure Of Ferritic Steelsmentioning
confidence: 99%
“…Schematic of the sub-zones of the heat affected zone in relation to the calculated equilibrium phase diagram of a 9Cr–1Mo steel [47]. …”
Section: Microstructure Of Ferritic Steelsmentioning
Dissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep.
“…The ferritic steels of interest can experience phase transformations during the thermal cycles associated with fusion welding. As a basic exemplar of the phase transformations, Figure 6(A) shows the microstructural effects with peak temperature while Figure 6(B) on the right side provides the calculated equilibrium phase diagram for a composition consistent with Grade 91 steel. Figure 6. Schematic of the sub-zones of the heat affected zone in relation to the calculated equilibrium phase diagram of a 9Cr–1Mo steel [47]. …”
Section: Microstructure Of Ferritic Steelsmentioning
confidence: 99%
“…Schematic of the sub-zones of the heat affected zone in relation to the calculated equilibrium phase diagram of a 9Cr–1Mo steel [47]. …”
Section: Microstructure Of Ferritic Steelsmentioning
Dissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep.
“…A relatively great heat input of fusion welding technologies (including GTAW) leads to a formation of significant heat-affected zone (HAZ) within the regions of welded base materials located close to the weld fusion zone. In general, the HAZ region with its strongly heterogeneous microstructure, so-called "HAZ microstructural gradient", is considered to be the critical region of welded joints because of its high propensity for several types of cracking and final failure occurrence [9,10]. The findings of our several former studies about dissimilar T92/TP316H weldments [11][12][13][14] have clearly indicated that the HAZ microstructural heterogeneity can be efficiently suppressed by the application of unconventional "quenching-and-tempering" post-welding heat treatment (QT PWHT).…”
The influence of isothermal aging at 620 °C in combination with subsequent electrochemical hydrogen charging at room-temperature was studied on quenched-and-tempered T92/TP316H martensitic/austenitic weldments in terms of their room-temperature tensile properties and fracture behavior. Hydrogen charging of the weldments did not significantly affect their strength properties; however, it resulted in considerable deterioration of their plastic properties along with significant impact on their fracture characteristics and failure localization. The hydrogen embrittlement plays a dominant role in degradation of the plastic properties of the weldments already in their initial material state, i.e., before thermal aging. After thermal aging and subsequent hydrogen charging, mutual superposition of thermal and hydrogen embrittlement phenomena had led to clearly observable effects on the welds deformation and fracture processes. The measure of hydrogen embrittlement was clearly lowered for thermally aged material state, since the contribution of thermal embrittlement to overall degradation of the weldments has dominated. The majority of failures of the weldments after hydrogen charging occurred in the vicinity of T92 BM/Ni weld metal (WM) fusion zone; mostly along the Type-II boundary in Ni-based weld metal. Thus, regardless of aging exposure, the most critical failure regions of the investigated weldments after hydrogen charging and tensile straining at room temperature are the T92 BM/Ni WM fusion boundary and Type-II boundary acting like preferential microstructural sites for hydrogen embrittling effects accumulation.
“…Cracking is governed by creep cavitation, which evolves to micro-and macrocracks and which can often only be detected in the final stages of component life. 6,8,9,[13][14][15][16] In recent years, an advanced 9Cr steel with controlled addition of boron and very low amounts of nitrogen was developed. 17,18 The results revealed that addition of boron can significantly decrease the minimum creep rate by retarding the onset of the tertiary creep region.…”
Section: Introductionmentioning
confidence: 99%
“…Multipass welds showed that the original prior austenite grain size in the heat affected zone (HAZ) is retained after exposing to multiple thermal cycles. 10,14 Based on these results, Mayr 14 investigated an advanced 9%Cr steel strengthened by boron and nitrogen. This so called MARBN concept combines the effect of boron strengthening (solid solution strengthening) with nitride strengthening by forming finely distributed vanadium/ niobium rich MX carbonitrides.…”
With the aim to increase base material creep strength and overcome the type IV cracking problem, a new design concept was developed. This so called martensitic boron-nitrogen strengthened steel (MARBN) combines boron strengthening through solid solution with precipitation strengthening by finely dispersed nitrides. In this work, uniaxial creep tests of the MARBN base material and welded joints have been carried out. The creep strength of the welded joints was analysed, and the evolution of creep damage was investigated. The creep tests of MARBN revealed increased strength of the base material of about z20% compared to the best commercially available 9Cr steel grade. At higher stress levels, the creep strength of crosswelds is between that of the MARBN base material and the conventional 9Cr base materials. Nevertheless, long term creep tests revealed a drop in creep strength of the MARBN welded joints. The underlying phenomena of crossweld creep behaviour are discussed in detail.
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