The controlled, _ne-grained microstructure of thermomechanically processed Nb microalloyed linepipe steels is destroyed in the vicinity of welds used in fabricating pipelines. There are conflicting views on the influence of niobium in the `heat-affected zone', particularly in the region closest to the weld fusion line which is most dramatically impacted by the thermal cycling that occurs during welding. Consequently, there is a need to fully characterise the influence of niobium on the evolution of structures and properties in this zone. The aim of the work presented in this thesis was to quantify and characterise precipitates of niobium and dissolved niobium across sub-zones of the weld heat-affected zone, in order to develop a better understanding of the effects of niobium across the region. In order to achieve this, heat treatments were undertaken for the first time to simulate each sub-zone of the heat affected zone such that unique states of dissolved niobium and precipitated niobium was developed. A novel technique as designed and applied for the first time to measure and quantify the precipitate sizes and size distributions in bulk samples of Nb micro-alloyed steels. In addition, measurements of the dissolved niobium across the heat-affected zone were completed in order to ensure that the discrete effects of all states of niobium were subjected to analysis. Weld simulations of the coarse-grained heat-affected zone, the region closest to the weld fusion line, were conducted and assessed against the measured states of niobium. This was followed by the manufacture of commercial welds in order to assess the variation of structures and properties across the heat-affected zone for different plate conditions generated by heat treatment prior to welding. This work established that that a wide range of niobium carbide precipitate sizes were crucial in assuring the excellent mechanical properties in the line-pipe steel, coarser precipitates were found to control the austenite grain size that evolved in the coarse-grain heat affected zone, while fine precipitates dissolved in the thermal cycles close to the weld fusion line, and produced finer microstructures.

No description available.

Super Duplex Stainless Steels have superior corrosion resistance properties and strength compared to conventional steels. However, these properties are influenced by the different phases that precipitate during the heat treatment process. The conventional way of studying the time and temperature effects on the properties and micro-structure of SDSS is to prepare many samples at different temperatures and holding times. The welding research group at Production Technology Center, Trollhättan, Sweden, has recently developed a unique heat treatment method to produce a wide range of temperature by using a stationary TIG arc heat source. It results in a graded micro-structure in a single sample at a specific time period. The accuracy of the results ob-ained from this process is highly related to the accuracy of the temperature field model next to weld pool. In this work, a model was developed by using OpenFOAM CDF code, to predict the temperature field of the super duplex stainless steel samples that have been subjected to this novel TIG arc heat treatment process. The developed model was able to capture the trend in the overall temperature field in the heat affected zone. However, there was some mismatch between the modelled and experimental temperature profiles in certain locations in the heat affected zone. Further improvements have to be done to the developed model in order to take the phase transformation effect into account. A preliminary investigation has been carried out on how to implement this in the current model and reported in the thesis.

With increasing miniaturization in microelectronics the wirebonds used in IC packages are witnessing a thrust towards fine pitch wirebonding. To have a precise control over loop height of the wirebond for fine pitch wirebonding, it is imperative to do mechanical characterization of the wirebond. The present work studies the mechanical properties of gold wire and wirebond using nanoindentation. The wirebond specimen surface was planarized using mechanical polishing. The loop height of the gold wirebond is directly proportional to the length of the heat affected zone (HAZ) above the ball of gold wirebond. Metallographic preparation of gold wirebond cross section reveals the presence of undesirable coarse grain structure in HAZ due to recrystallization and grain growth in the gold wire adjacent to the ball. The recrystallization temperature of our gold wire was found using D.S.C. to be 340.66°C. The doping elements present in the gold wire used, were identified using TOF-SIMS. Nanoindentation of the gold wire was done at different maximum loads to observe the hardness variation with load. The nanoindentation of gold wirebond has confirmed a v-shaped hardness profile in the HAZ. The hardness minima for the particular gold wire used with a ball size ratio of 2.4 was observed at distance of 160-170 µm from the neck of the ball. The elastic modulus was found to vary randomly and to be independent of the microstructure in the wirebond. A yield stress profile based on empirical hardness-yield strength correlation has been predicted for the gold wirebond. / Singapore-MIT Alliance (SMA)

The heat-affected zone (HAZ) is the most common region of weld failures. The weld failures are directly related to the microstructure. Microstructure control of the HAZ is crucial to weld quality and prevention of weld failures. However, publications on modeling the development of the HAZ are relatively limited. Moreover, no efforts have been made to predict the HAZ microstructures in real-time. The primary goal of this research is to present a methodology to enable real-time predictions of microstructure evolution in the HAZ and its mechanical properties. This goal was achieved by an approach based on materials science principles and real-time sensing techniques. In this study, the entire welding process was divided into a series of sub-processes. Real-time multiple measurements from multiple sensors were incorporated into the sub-processes. This resulted in an integrated welding system upon which the predictions for the final HAZ microstructure are based. As part of the integrated system, the microstructural model was used to predict the TTT curves, volume fractions of the decomposition products, and hardness numbers of the heat-affected zones of steel alloys. Actual welds were performed under two different sets of conditions, and the resulting experimental data were compared with predictions made using the microstructural model. The predicted and experimental microstructure and hardness are found to be in good agreement, indicating that the microstructural model can be used in real applications. This research can act as an important component of future research to enable physics-based flexible control of welding. / text

Haynes alloy 282 is a new gamma prime (γ’) precipitation strengthened nickel-base superalloy developed for high temperature applications in land-based and aero turbine engines. Joining is a crucial process both during the manufacturing of new components and repair of service-damaged turbine parts. Unfortunately, the new superalloy cracks during laser beam welding (LBW), which is an attractive technique for joining superalloys components due to its low heat input characteristic that preclude the geometrical distortion of welded components. This research is therefore initiated with the goal of studying and developing an effective approach for preventing or minimizing cracking during LBW of the new superalloy Haynes 282. Careful and detailed electron microscopy and spectroscopy study reveal, for the first time, the formation of sub-micron grain boundary M5B3 particles, in the material. Microstructural study of welded specimens coupled with Gleeble thermo-mechanical physical simulations shows that the primary cause of weld heat affected zone (HAZ) cracking in the alloy is the sub-solidus liquation reaction of intergranular M5B3 borides in the material. Further weldability study showed that the HAZ liquation cracking problem worsens with reduction in welding heat input, which is normally necessary to produce the desired weld geometry with minimum distortion. In order to minimize the HAZ cracking during low heat input laser welding, microstructural modification of the alloy by heat treatment at 1080 - 1100oC has been developed. The pre-weld heat treatment minimizes cracking in the alloy by reducing the volume fraction of the newly identified M5B3 borides, while also minimizing non-equilibrium grain boundary segregation of boron liberated during dissociation of the boride particles. Further improvement in resistance to cracking was produced by subjecting the material to thermo-mechanically induced grain refinement coupled with a pre-weld heat treatment at 1080oC. This approach produces, for the first time, crack-free welds in this superalloy, and the benefit of this procedure in preventing weld cracking in the new material is preserved after post-weld heat treatment (PWHT), as additional cracking was not observed in welded specimens subjected to PWHT.

Haynes alloy 282 is a new gamma prime (γ’) precipitation strengthened nickel-base superalloy developed for high temperature applications in land-based and aero turbine engines. Joining is a crucial process both during the manufacturing of new components and repair of service-damaged turbine parts. Unfortunately, the new superalloy cracks during laser beam welding (LBW), which is an attractive technique for joining superalloys components due to its low heat input characteristic that preclude the geometrical distortion of welded components. This research is therefore initiated with the goal of studying and developing an effective approach for preventing or minimizing cracking during LBW of the new superalloy Haynes 282. Careful and detailed electron microscopy and spectroscopy study reveal, for the first time, the formation of sub-micron grain boundary M5B3 particles, in the material. Microstructural study of welded specimens coupled with Gleeble thermo-mechanical physical simulations shows that the primary cause of weld heat affected zone (HAZ) cracking in the alloy is the sub-solidus liquation reaction of intergranular M5B3 borides in the material. Further weldability study showed that the HAZ liquation cracking problem worsens with reduction in welding heat input, which is normally necessary to produce the desired weld geometry with minimum distortion. In order to minimize the HAZ cracking during low heat input laser welding, microstructural modification of the alloy by heat treatment at 1080 - 1100oC has been developed. The pre-weld heat treatment minimizes cracking in the alloy by reducing the volume fraction of the newly identified M5B3 borides, while also minimizing non-equilibrium grain boundary segregation of boron liberated during dissociation of the boride particles. Further improvement in resistance to cracking was produced by subjecting the material to thermo-mechanically induced grain refinement coupled with a pre-weld heat treatment at 1080oC. This approach produces, for the first time, crack-free welds in this superalloy, and the benefit of this procedure in preventing weld cracking in the new material is preserved after post-weld heat treatment (PWHT), as additional cracking was not observed in welded specimens subjected to PWHT.

Developed thesis compares MAG welding and plasma welding, with the same input parameters. Based on the literature was reviewed weldability and welding of both methods. It was experimentally for both technologies specifically designed heat input, evaluation and macrostructure of the weld dilution, the dimensions of the heat affected zone and finally test the hardness in the transverse direction Vickers.

No description available.

No description available.