Date: Tuesday, Mar 19th
Presenter: Leijun Li, Department of Mechanical and Aerospace Engineering, Utah State University
The effect of microstructural evolution, as affected by various heat-treatment conditions, on creep performance of heavy-section Grade 91 has been studied using a novel stress-relaxation test. Martensitic transformation and tempering are characterized by dilatometry, microscopy, and diffraction methods. The critical transformation temperature AC1 is identified to be the upper bound temperature for heat-treatment in order to prevent formation of fresh martensite. For an increased creep performance, a lower heat-treatment temperature should be selected, as long as the temperature is high enough to temper the fresh martensite to meet the impact toughness requirement.
Directionally solidified nickel based superalloy GTD-111 has been repaired using pulsed Nd:YAG laser powder deposition. Epitaxial solidification of the deposit has been achieved. The cellular dendritic grains are uniform in size throughout multiple layers of deposit. Hot crack formation is associated with the stray grains, which are controlled by the geometry and overlap of the deposits. With optimized process parameters, crack-free multiple-layered deposits have been achieved for tip repair of GTD-111 blades using RENE 80 and IN625 filler metals.
Future research directions and potential projects will be discussed. Planned research includes accelerated test techniques for microstructure aging and creep of Cr-Mo steels for the nuclear industry, materials design and fabrication for solar energy, repair of precision castings, fabrication of heavy-section structural steels, and process optimization of thermomechanical deformation and heat treating.