Date of Defense
27-11-2025 2:00 PM
Location
F1 - 1043
Document Type
Thesis Defense
Degree Name
Master of Science in Mechanical Engineering (MSME)
College
College of Engineering
Department
Mechanical and Aerospace Engineering
First Advisor
Prof. Abdel-Hamid. I. Mourad
Keywords
Expansion percentage, Minimum leak path, Pull-out load, Titanium, Tube, Tubesheet, Tungsten inert gas welding.
Abstract
Tube-to-tubesheet joints, an integral component of shell and tube heat exchangers, are known for their vulnerability to leakage problems at the joint region due to mechanical and chemical factors coupled with fabrication techniques involved in creating these joints. An ideal condition of manufacturing process parameters and geometrical variables required to produce tube-to-tubesheet joints is highly desired. One of the most promising candidate materials to fabricate tube-to-tubesheet joints is titanium or titanium alloys due to their excellent strength and resistance to corrosion. The present study investigates the structural integrity of three categories of Titanium Grade 2 based tube-to-tubesheet joints comprising of welded joints, expanded joints, and welded-expanded joints. The influence of Tungsten inert gas welding process and three expansion percentages (4%, 6%, and 8%) on the mechanical and metallurgical characteristics are studied with the presence of double grooves on the inner wall of the tubesheet. The study focuses on three distinct zones of the tube: the weld zone (WZ), the heat affected zone (HAZ), and the unaffected base material (UBM), which refers to the portion of the tube that was not influenced by thermal or mechanical processes. The results showed that α′-phase was dominant in the weld zone and the heat affected zone with no noticeable signs for β-phase remnants in the mentioned zones. The microstructural analysis revealed a coarsening in the grain structure when moving from the UBM to WZ through the HAZ. It was observed that HCP α-phase crystals transformed into α′-phase crystals post rapid cooling in WZ and HAZ. Additionally, the results of the XRD analysis confirmed the absence of β-phase at the HAZ and WZ due to the indistinguishable diffraction pattern between the base material (α-phase dominant) and the weld (α′-phase dominant). Microstructural analysis on the expanded joints showed a direct relationship between the expansion percentage and the reduction of grain size near the roller expander in the expanded zone, while grains in transition and unexpanded zones experienced an insignificant impact due to expansion at both tube’s surfaces. The macroscopic studies revealed that the minimum leak path of the weld was satisfactorily greater than two-thirds of the tube wall thickness, or 1.02 mm. The tube pull-out test showed that the tube pull-out loads were greater than the maximum allowableaxial load for all the cases considered, verifying that every joint manufactured for this project was mechanically sound under axial loading conditions. Moreover, the weld region was found to be the hardest region in Vicker's hardness test due to the dominance of the α′-phase, with a maximum hardness of 173.6 HV, and all measured hardness values in the weld region, tubesheet base material, heat affected zones, and tube materials were found to be below the maximum allowable limit. In conclusion, the integration of welding and expansion techniques for Titanium Grade 2 tube-to-tubesheet joints led to a reliable joint design exhibiting superior mechanical performance, refined microstructure, and safe leak-tight characteristics suitable for critical heat exchanger applications.
Included in
INVESTIGATION OF MECHANICAL PERFORMANCE AND METALLURGICAL CHARACTERISTICS OF TITANIUM GRADE 2 TUBE-TO-TUBESHEET JOINTS
F1 - 1043
Tube-to-tubesheet joints, an integral component of shell and tube heat exchangers, are known for their vulnerability to leakage problems at the joint region due to mechanical and chemical factors coupled with fabrication techniques involved in creating these joints. An ideal condition of manufacturing process parameters and geometrical variables required to produce tube-to-tubesheet joints is highly desired. One of the most promising candidate materials to fabricate tube-to-tubesheet joints is titanium or titanium alloys due to their excellent strength and resistance to corrosion. The present study investigates the structural integrity of three categories of Titanium Grade 2 based tube-to-tubesheet joints comprising of welded joints, expanded joints, and welded-expanded joints. The influence of Tungsten inert gas welding process and three expansion percentages (4%, 6%, and 8%) on the mechanical and metallurgical characteristics are studied with the presence of double grooves on the inner wall of the tubesheet. The study focuses on three distinct zones of the tube: the weld zone (WZ), the heat affected zone (HAZ), and the unaffected base material (UBM), which refers to the portion of the tube that was not influenced by thermal or mechanical processes. The results showed that α′-phase was dominant in the weld zone and the heat affected zone with no noticeable signs for β-phase remnants in the mentioned zones. The microstructural analysis revealed a coarsening in the grain structure when moving from the UBM to WZ through the HAZ. It was observed that HCP α-phase crystals transformed into α′-phase crystals post rapid cooling in WZ and HAZ. Additionally, the results of the XRD analysis confirmed the absence of β-phase at the HAZ and WZ due to the indistinguishable diffraction pattern between the base material (α-phase dominant) and the weld (α′-phase dominant). Microstructural analysis on the expanded joints showed a direct relationship between the expansion percentage and the reduction of grain size near the roller expander in the expanded zone, while grains in transition and unexpanded zones experienced an insignificant impact due to expansion at both tube’s surfaces. The macroscopic studies revealed that the minimum leak path of the weld was satisfactorily greater than two-thirds of the tube wall thickness, or 1.02 mm. The tube pull-out test showed that the tube pull-out loads were greater than the maximum allowableaxial load for all the cases considered, verifying that every joint manufactured for this project was mechanically sound under axial loading conditions. Moreover, the weld region was found to be the hardest region in Vicker's hardness test due to the dominance of the α′-phase, with a maximum hardness of 173.6 HV, and all measured hardness values in the weld region, tubesheet base material, heat affected zones, and tube materials were found to be below the maximum allowable limit. In conclusion, the integration of welding and expansion techniques for Titanium Grade 2 tube-to-tubesheet joints led to a reliable joint design exhibiting superior mechanical performance, refined microstructure, and safe leak-tight characteristics suitable for critical heat exchanger applications.