Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes For Hoop Creep Enhancement
C Cr ro os ss s--R Ro ol ll l F Fl lo ow w F Fo or rm mi in ng g o of f O OD DS S A Al ll lo oy y H He ea at t E Ex xc ch ha an ng ge er r T Tu ub be es s F Fo or r H Ho oo op p C Cr re ee ep p E En nh ha an nc ce em me en nt t Quarterly Technical Progress Report C Cr ro os ss s--R Ro ol ll l F Fl lo ow w F Fo or rm mi in ng g o of f O OD DS S A Al ll lo oy y H He ea at t E Ex xc ch ha an ng ge er r T Tu ub be es s F Fo or r H Ho oo op p C Cr re ee ep p E En nh ha an nc ce em me en nt t
... Mechanically alloyed oxide dispersion strengthened (ODS) Fe-Cr-Al alloy thin walled tubes and sheets, produced via powder processing and consolidation methodologies, are promising materials for eventual use at temperatures up to 1200 o C in the power generation industry, far above the temperature capabilities of conventional alloys. Target end-uses range from gas turbine combustor liners to high aspect ratio (L/D) heat exchanger tubes. Grain boundary creep processes at service temperatures, particularly those acting in the hoop direction, are the dominant failure mechanisms for such components. The processed microstructure of ODS alloys consists of high aspect ratio grains aligned parallel to the tube axis, a result of dominant axial metal flow which aligns the dispersoid particles and other impurities in the longitudinal direction. The dispersion distribution is unaltered on a micro scale by recrystallization thermal treatments, but the high aspect ratio grain shape typically obtained limits transverse grain spacing and consequently the hoop creep response. Improving hoop creep in ODS-alloy components will require understanding and manipulating the factors that control the recrystallization behavior, and represents a critical materials design and development challenge that must be overcome in order to fully exploit the potential of ODS alloys.