Doctor of Philosophy, The Ohio State University, 2022, Welding Engineering

Otherwise ductile metals can experience catastrophic brittle failure in the presence of liquid metals. Although this phenomenon was first noted in the early 1900s, it has gained recent attention as the cause of failure in novel advanced high strength steels during welding. With the right combination of stress, temperature, and microstructure liquid metal can percolate through the grain boundaries of the solid substrate and reduce ductility to near zero. Although many mechanisms for embrittlement have been put forth suggesting brittle failure, reduction in surface energy, dislocation emission due to liquid metal adsorption, and dissolution, no consensus has been reached. The effects of test temperature and hold time are explored for both zinc and copper embrittlement of type 304L stainless steel through the use of hot tensile testing. Copper embrittlement peaks slightly above the melting temperature of copper (1085°C), while zinc embrittlement is most severe at 800°C. Holding the samples at the testing temperature before applying strain allowed the zinc plated samples to regain ductility, but only had slight effects on copper embrittlement. The reasons for these trends in embrittlement are explored in reference to the pseudo-binary phase diagrams between steel and zinc and copper. iv Characterization of zinc-steel interactions via electron microscopy revealed the presence of an α ferrite layer which forms as the zinc diffuses into the solid steel, which causes nickel to be ejected into the remaining liquid. At high enough concentrations of nickel the liquid layer isothermally solidifies as γ Zn (Ni). Thermo-Calc is also used to perform diffusion simulations of the zinc-steel interface both to the bulk and at a grain boundary. The simulations provide a chain of events which lead to the formation of the characterized structures. The results gleaned during characterization in combination with the diffusion simulations are then used to formulate a mechanism for embr (open full item for complete abstract)

Committee: Carolin Fink (Advisor); Boyd Panton (Committee Member); Antonio Ramirez (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2016, Welding Engineering

Pipes of solid solution strengthened Ni-based alloys, as Alloy 826 and Alloy 800H, have been used for high temperature service in once through steam generators (OSTGs) on off-shore platforms. The oil and gas industry is seeking to increase service temperature, improve service reliability, and extend service life to 40 years of such installations. Alloy 230 has better-high temperature stability and mechanical properties, and higher service temperature than Alloys 825 and 800H, and is therefore considered as a potential replacement of these alloys in newly built OTSGs. However, the weldability and the high temperature service behavior in welds of Alloy 230 have not been thoroughly investigated yet. This study is a comprehensive comparative research focused on susceptibility to solidification cracking and stress relief cracking in Alloys 800H, 825, and 230. To evaluate the solidification behavior and solidification cracking susceptibility in these alloys, the Cast Pin Tear Test (CPTT), thermodynamic simulations with Thermo-Calc, and the technique of Single-Sensor Differential Analysis (SS-DTA) were used. The results revealed that Alloy 230 and Alloy 825 were more resistant to solidification cracking than Alloy 800H, due to narrower solidification temperature range and crack back filling with eutectic constituents. The OSU Stress Relief Cracking (SRC) Test was applied to evaluate the susceptibility to stress relief cracking in autogenous gas tungsten arc welds of the investigated alloys. None of the three alloys failed by stress relief cracking mechanism while loaded at stress equal to 90% of the high temperature yield strength at 650 oC for 8 hours.. Alloys 825 and 800H showed significant amount of stress relief, while Alloy 230 sustained the applied load at 650C with almost no stress relief. Tensile testing at 650 oC after the 8 hours SRC test showed that the autogenous weld in Alloy 230 had significantly higher yield and tensile strength and slightly lower el (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Avraham Benatar (Advisor) Subjects: Materials Science; Metallurgy

Master of Science, The Ohio State University, 2014, Welding Engineering

Cracking has been reported in newly constructed water wall panels of fossil power plants during startup testing. Both high hardness (exceeding 350 HV) and high levels of welding residual stress have been reported in welds of waterwall panels made of T23 and T24 steels. Stress-relief cracking (SRC) is being considered as a possible failure mechanism during high temperature exposure such as PWHT. High temperature exposure of non PWHT-ed welds of Grade T23 and T24 steels leads to hardening in the weld and coarse-grained heat-affected zone (CGHAZ). It has been suggested that such a hardening mechanism can lead to stress-relief cracking (SRC). One of the objectives in this study was to investigate the phase transformation behavior and develop continuous cooling transformation (CCT) diagrams in the CGHAZ of Grade T12, T22, T23, and T24 steels. The GleebleTM thermo-mechanical simulator and a dilatometer were utilized in this study. The CGHAZ microstructure in Grade T23 and T24 steels was a mixture of bainite and martensite with hardness higher than 340 HV in the studied range of t8/5 cooling time from 2 to 50 seconds. The CGHAZ microstructure in Grade T22 gradually changed from a mixture of martensite and bainite to predominantly bainitic with allotriomorphic ferrite. This corresponded to a moderate reduction in hardness from 340 to 300 HV. In Grade T12 steel, the microstructure of the CGHAZ gradually changed from predominantly martensitic with hardness of 340 HV to bainitic and a mixture of bainite with idiomorphic and allotriomorphic ferrite with hardness lower than 230 HV. The other objective of this study was to evaluate the susceptibility to SRC in the CGHAZ of T24 steel and in 3-pass welds of Grade T12, T22, T23, and T24 steel tubes. A GleebleTM-based strain-age cracking test developed at The Ohio State University was modified to better replicate the conditions of PWHT in highly restrained welds and quantify the stress-relief cracking susceptibility in creep (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); John Lippold (Committee Member) Subjects: Materials Science; Metallurgy

Master of Science, The Ohio State University, 2011, Welding Engineering

The application of friction stir processing (FSP) on three Ni-base alloys, Alloy 625, Alloy 718, and Hastelloy X utilizing a tungsten-rhenium tool was investigated. A processing window for each alloy was defined in terms of travel speed and tool rotation rate. Process forces and thermal histories were successfully recorded for each alloy. Peak process temperatures were recorded at 1150°C for Alloy 625 and Hastelloy X and 1100°C for Alloy 718 using embedded type K thermocouples. FSP microstructures were investigated using optical and electron microscopy. Significant grain refinement was experienced by each alloy with stir zone grain sizes of 6µm, 5µm, and 4µm for Hastelloy X, Alloy 625, and Alloy 718, respectively, relative to starting grain sizes of 88µm, 26µm, and 44µm. Optical microscopy revealed streaking on the advancing side of the stir zone for each alloy. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis showed these bands to be a result of tool wear. Hardness traverses show that all three alloys experience peak hardness outside the stir zone in the thermomechanically-affected zone. Additionally, it was observed that Alloy 718 and Hastelloy X experienced hardening in the stir zone in comparison to the base metal whereas Alloy 625 underwent softening. Longitudinal sub-size tensile specimens extracted from stir zones showed that Hastelloy X experienced significant strengthening due to FSP relative to the base material whereas Alloy 625 and Alloy 718 experienced virtually no improvement in strength in the FSP region relative to base material. FSP was further investigated as a means to reduce the susceptibility of these alloys to heat-affected zone (HAZ) liquation cracking. Spot-varestraint testing was used to evaluate the effects of FSP on HAZ liquation in terms of total crack length (TCL) and maximum crack length (MCL). Testing showed a reduction in HAZ liquation susceptibility due to the reduction in HAZ grain size. Optical (open full item for complete abstract)

Committee: John Lippold PhD (Advisor); Sudarsanam Babu PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2007, Industrial and Systems Engineering

The use of numerical methods to analyze the design and performance of mechanical components has been widely used in industry for many years. The results obtained have been used to improve the design of the products by providing useful insights into the critical areas of the component during operation. However, the numerical analysis of the manufacturing process that integrates machine, tooling and products has not been widely done due to the greater complexity of the physical phenomena involved. This dissertation work presents a computer modeling methodology developed to predict the final dimensions and residual stresses in a die casting. The determination of the mechanical properties of the casting material needed for the computer model is also presented here. Furthermore, to validate the adequacy of the modeling methodology computer model predictions are compared against experimental measurements taken on productions castings. The methodology uses the finite element method to analyze the solidification and cooling conditions of a casting during the die casting process. An innovative method that uses a shell mesh is presented that allows tracking the elastic deflections in the die cavity resulting from the die casting process loads. A fully coupled thermal-mechanical analysis was done to model the die casting process. The finite element model was solved using the finite element package Abaqus. The determination of the casting constitutive model was done as part of this dissertation work. Tensile bars made of die casting aluminum alloy A380.0 in compliance with industrial testing standards were produced. A series of tensile tests at different combinations of temperatures and strain rates were conducted to determine the casting constitutive behavior using a Gleeble 1500 thermo-mechanical simulator. A Design of Experiments was done to validate the adequacy of the computer model predictions and to study the effect of process variables on casting dimensions. Castings we (open full item for complete abstract)

Committee: Richard Miller (Advisor) Subjects: Engineering, Industrial

Doctor of Philosophy, The Ohio State University, 2006, Welding Engineering

The ductility-dip cracking (DDC) susceptibility of NiCrFe filler metals was evaluated using the strain-to-fracture (STF) Gleeble(R)-based testing technique. These high chromium Ni-base filler metals are frequently used in nuclear power plant applications for welding Ni-base Alloy 690 and included INCONEL(R) Filler Metal 52 and 52M (FM-52 and FM-52M), and a number of FM-52M-type experimental alloys including two with additions of molybdenum and niobium. A wide range in DDC susceptibilities was observed in the tested alloys including significant variations in susceptibility with only small compositional changes. The spot pre-weld downslope time was found to significantly affect the DDC resistance when the current downslope time was altered. Faster cooling rates resulted in finer solidification substructure, fewer metastable intragranular precipitates, and a reduced DDC susceptibility. Subsequent solutionizing and precipitation heat treatments improved the DDC resistance and were attributed to microstructure homogenization and M23C6 precipitation that resulted in improved grain boundary strength. A significant decrease in DDC susceptibility was observed in an alloy with 4% Mo and 2.5% Nb and was attributed to the skeletal precipitate morphology whose large surface area and dense distribution were highly effective at pinning grain boundaries. Elongated intergranular M23C6 precipitates that formed during the heat treatment of one alloy resulted in dynamic recrystallization during STF testing and were attributed to particle stimulated nucleation (PSN). The resulting recrystallized microstructure had a smaller grain size and sigma 3 twin boundaries (special boundaries) that were effective at blunting DDC crack propagation; both increasing the resistance to DDC. The observed DDC cracking susceptibility was compared with thermodynamic calculations and single sensor differential thermal analysis (SS-DTA) to develop a better understanding of how carbide precipitation affects g (open full item for complete abstract)

Committee: John Lippold (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2006, Welding Engineering

Friction stir welding an application which has the potential to make full thickness welds in a single pass, while eliminating fume, reducing distortion, and eliminating solidification defects. Interest in the process by industries which rely on iron and its alloys for structural material is increasing. While friction stir welding has been shown to be feasible with iron alloys, the understanding of friction stir welding process effects on these materials is in its infancy. Friction stir weld material tracer experiments utilizing stainless steel markers were conducted with plates of ingot iron and HSLA-65. The markers showed that material is moved in a curved path around the tool and deposited behind the tool. Material near the surface is moved a greater distance as it is acted upon by the tool shoulder. A friction stir weld was made on a plate of HSLA-65 with Inconel sheathed thermocouples embedded in the tool path. Heating rates calculated from the slope of the acquired temperature data show that the peak heating rate occurs at temperatures between 350°C and 500°C. An increase in the heating rate occurring at elevated temperature was associated with the transformation from ferrite to austenite. Peak temperatures on the top of the plate exceeded 1200°C and peak temperatures acquired on the bottom exceeded 1000°C. Hot torsion tests with non-uniform temperature profiles were conducted on both ingot iron and HSLA-65 samples. An annular sample geometry with internal and external gas quench achieved cooling rates in the hot torsion samples similar to those observed in friction stir welding. Localization of strain in the intercritical temperature region was determined to be caused by differences in the activation energy for deformation for ferrite and austenite. Adiabatic heating due to shear strain was shown to be related to the Zener-Holloman parameter. Microstructures created in both the ingot iron and HSLA-65 were very similar to those observed in friction stir welds m (open full item for complete abstract)

Committee: John Lippold (Advisor) Subjects:

Master of Science, The Ohio State University, 2003, Welding Engineering

The purpose of this work was to use a thermo-mechanical simulator to develop a simple test that will quantify susceptibility to the various forms of postweld heat treatment cracking, including stress-relief cracking and strain-age cracking. Materials evaluated include Waspaloy and Alloy 718. Samples are initially given an elevated temperature thermal exposure that simulates the weld HAZ. Upon cooling from elevated temperature, the sample is loaded such that yield strength magnitude stresses are present at room temperature. The sample is then heated to a selected PWHT temperature and held for up to 8 hours. Acquisition of tensile force data during PWHT shows an initial relaxation of stresses, followed by a rise in stress as precipitation reactions proceed. Hot ductility tests were performed at various PWHT temperatures and times. From the reduction in area measurements a mathematical model was developed to relate ductility to PWHT temperature and time. The alloys tested show a significant dip in ductility at elevated temperature. Waspaloy samples exhibited lower ductility at elevated temperatures than did Alloy 718, confirming the higher susceptibility of Waspaloy to strain-age cracking. SEM fractography and optical microscopy were used to analyze the failed samples. Minimum ductility samples revealed ductile intergranular fracture paths in Waspaloy and a tendency for transgranular cracking in Alloy 718. Intergranular micro-cracks were evident in a coarse grained heat affected zone in both alloys. Grain size was shown to effect ductility and fracture mode. Smaller grained simulated HAZ samples exhibited more ductility and a tendency for transgranular ductile fracture. Larger grains resulted in an increase in intergranular fracture and a reduction in elevated temperature ductility. Alloy 718 showed evidence of bending of crystal lattices to accommodate stresses. Both alloys showed indications of an effect of grain boundary orientation on cracking susceptibility.

Committee: John Lippold (Advisor) Subjects: