Doctor of Philosophy

dissertationThe objective of this work is to understand the mechanisms of high temperature failures in dissimilar metal welds between modified 9Cr-1Mo steel (P91) and austenitic stainless steel (AISI 304). It is proposed to apply functionally graded multilayers between these metals by friction-based solid state welding methods to obtain smooth and gradual transition. The study examined the effect of the smooth and gradual transition in physical and metallurgical properties imparted by the weld interlayers on the stress rupture behavior of weld transition joints between P91 and AISI 304. Two types of transitions joints were developed: welds with three interlayers (P91/IN625/IN600/IN800H/AISI304) and welds with a single interlayer (P91/IN600/AISI304). The experimental study involved the evaluation of microstructural and mechanical properties of weld interlayers and base metals. The mechanical property evaluation included hardness profiles across weld interfaces and stress rupture behavior. The evolution of microstructure in friction welds was studied using cellular automata modeling technique. Dynamically recrystallized grain sizes predicted by the Cellular automata method were found to be comparable with the experimental results. The strain rates predicted by the model at the center and edge of the weld fabricated with 1500 RPM rotational speed were found to be 1850 s-1 and 290 s-1, respectively. A soft zone in the heat affected zone of P91 was observed, 2mm away from the weld interface which was found to be due to the carbon diffusion. Three interlayer weld transition joints showed better rupture life than the single interlayer welds. The increased rupture life of three interlayer welds is attributed to the gradual transition in coefficient of thermal expansion between P91 steel and AISI 304. A stress exponent of 3 was obtained suggesting creep mechanism as viscous glide due to solute drag effect. Creep damage tolerance factor (λ) of 1.75 was obtained which indicates the damage mechanism is cavity growth by the combined effect of power law creep and diffusional creep