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NUCu-140 is a copper-precipitation strengthened steel that exhibits excellent mechanical properties with a relatively simple chemical composition and processing schedule. As a result, NUCu-140 is a candidate material for use in many naval and structural applications. Before NUCu-140 can be implemented as a replacement for currently utilized materials, a comprehensive welding strategy must be developed under a wide range of welding conditions. This research represents an initial step toward understanding the microstructural and mechanical property evolution that occurs during fusion welding of NUCu-140.
Actual cooling rates for Grade 91 welds were measured for the SMAW and GTAW processes and found to be in the range of 13oC/s and 43oC/s, correspondingly. Based on these measured cooling rates and on the developed ICHAZ CCT diagram, it has been concluded that formation of ferrite in the ICHAZ of Grade 91 steel welds is possible during shielded metal arc welding. Welding parameters such as heat input, preheat and interpass temperature can be selected to ensure faster cooling rates to reduce or potentially avoid formation of ferrite in the ICHAZ. Reducing or eliminating the presence of ferrite in the ICHAZ may reduce the susceptibility of Type IV cracking in Grade 91 welds. However, further investigations are needed to clarify the potential role of ferrite in the ICHAZ on Type IV cracking in welds of creep strength enhanced ferritic steels.
This research presents two possibilities to prepare and test the Fine Grain of a Heat Affected Zone, which, practically, could be considered as the weakest part of welded joints in the presence of any microdefect. It is a narrow zone located between the fusion zone and the unaffected base material; therefore, only a few methods are suitable to test its mechanical properties. The 18CrNiMo7-6 steel was used as the base material. As this steel is usually used for the production of dynamically loaded components, testing of its fatigue behaviour and fracture toughness was crucial, but also measurement of its hardness and impact toughness. To investigate the mechanical properties of a Fine-Grain Heat-Affected Zone (FG HAZ), two different methods for simulation of as-welded microstructures were used in this research: A weld thermal cycle simulator (WTCS) and austenitising in a laboratory furnace +.
Thermodynamic and kinetic modeling were used to determine appropriate heat treatment schedules for homogenization and second phase dissolution of welds in alloy 740H. Following these simulations, a two-step heat treatment process was applied to specimens from a single pass gas tungsten arc weld (GTAW). Scanning electron microscopy (SEM) has been used to assess the changes in the distribution of alloying elements as well as changes in the fraction of second phase particles within the fusion zone. Experimental results demonstrate that homogenization of alloy 740H weld metal can be achieved by an 1100°C/4hr treatment. Complete dissolution of second phase particles could not be completely achieved, even at exposure to temperatures near the alloy's solidus temperature. These results are in good agreement with thermodynamic and kinetic predictions.
Solute redistribution and microstructural evolution have been modeled for gas tungsten arc fusion welds in experimental Ni base superalloys. The multi-component alloys were modeled as a pseudo-ternary [gamma]-Nb-C system. The variation in fraction liquid and liquid composition during the primary L {r{underscore}arrow} [gamma] and eutectic type L {r{underscore}arrow} ([gamma] + NbC) stages of solidification were calculated for conditions of negligible Nb diffusion and infinitely rapid C diffusion in the solid phase. Input parameters were estimated by using the Thermo-Calc NiFe Alloy data base and compared to experimentally determined solidification parameters. The solidification model results provide useful information for qualitatively interpreting the influence of alloy composition on weld microstructure. The quantitative comparisons indicate that, for the alloy system evaluated, the thermodynamic database provides sufficiently accurate values for the distribution coefficients of Nb and C. The calculated position of the [gamma]-NbC two-fold saturation line produces inaccurate results when used as inputs for the model, indicating further refinement to the database is needed for quantitative estimates.
The last two decades have seen a steady and impressive development, and eventual industrial acceptance, of the high energy-rate manufact turing techniques based on the utilisation of energy available in an explo sive charge. Not only has it become economically viable to fabricate complex shapes and integrally bonded composites-which otherwise might not have been obtainable easily, if at all-but also a source of reasonably cheap energy and uniquely simple techniques, that often dispense with heavy equipment, have been made available to the engineer and applied scientist. The consolidation of theoretical knowledge and practical experience which we have witnessed in this area of activity in the last few years, combined with the growing industrial interest in the explosive forming, welding and compacting processes, makes it possible and also opportune to present, at this stage, an in-depth review of the state of the art. This book is a compendium of monographic contributions, each one of which represents a particular theoretical or industrial facet of the explosive operations. The contributions come from a number of practising engineers and scientists who seek to establish the present state of knowledge in the areas of the formation and propagation of shock and stress waves in metals, their metallurgical effects, and the methods of experimental assessment of these phenomena.
Grade 92 is a creep-strength enhanced ferritic (CSEF) steel widely used in the power generation industry. This steel shows a clear reduction in the cross-weld creep performance resulting in Type IV failure in the heat affected zone (HAZ). To study the creep behavior of the susceptible HAZ region responsible for reduction in cross-weld creep behavior, phase transformation analysis and microstructural characterization techniques are being utilized as part of an overall effort to develop a standardized procedure for creating representative and relevant synthetic HAZ microstructures and samples. Simulated and real weld HAZ microstructures are characterized using optical and electron microscopy techniques. Simulated Grade 92 HAZ samples were prepared using a GleebleTM 3800 Thermomechanical simulator. Heating rates for the HAZ simulations represented furnace heating and commonly used arc welding processes for component fabrication. Peak temperatures range from 880°C to 1250°C, representative of the partially transformed zone (PTZ) and completely transformed zone (CTZ), respectively. All samples were prepared using standard techniques, etched with Vilella’s reagent for optical microscopy, analyzed using SEM imaging, EBSD, and carbon replica extraction in the TEM for carbide analysis. Simulated samples were then compared to bead-on-plate samples created using representative heat inputs. Dilatometry results from GleebleTM HAZ simulations confirmed Ac1 and Ac3 transformation temperatures for each heating rate used in this study. Simulated samples were then created in the CTZ well above the Ac3 temperature, PTZ between the Ac1 and Ac3 temperatures, and PTZ above the Ac3 temperature. Bead on plate tests were conducted on 1” Grade 92 plates using 20, 35, and 50 kJ/in heat inputs to represent SMAW, SAW, and GTAW processes. BOP tests were cut and measured for thermocouple placement for thermal history acquisition. Previous studies found that increasing the heating rate for the simulated HAZ was found to increase hardness and the austenite phase transformation temperatures and decrease the martensite transformation temperature during free cooling. It is suspected that carbide dissolution and precipitation behavior is affected but more advanced characterization techniques were required to confirm. Simulated samples were analyzed using SEM imaging and EBSD to characterize prior austenite grain size, grain misorientation, carbide size, and distribution. Carbon replica extraction techniques were used for TEM analysis to identify and characterize carbides found in the HAZ. Microhardness mapping was also conducted. Analysis performed on real and simulated samples enabled the complete characterization of the region of interest and will lead to development of a standardized procedure for replication of relevant simulated microstructures using resistive and furnace heating methods. The characterization techniques employed in this study were used to define microstructural characteristics in real Grade 92 weld heat affected zones and compared to simulated samples to determine their efficacy. Carbide analysis resulted in a model to predict carbide behavior as a function of peak temperature and heating rate. Using advanced electron microscopy characterization techniques lead to standardizing a procedure for the development of relevant and representative simulated HAZ microstructures in 9%Cr CSEF steels.