The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part I. Deformatio
- PDF / 2,286,649 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 41 Downloads / 195 Views
INTRODUCTION
ENERGY shortages and environmental considerations mean there is an ever-increasing demand for increases in the efficiency of energy conversion in fossil energy and biomass power generation systems.[1] This, in turn, leads to a demand for materials capable of operating at higher temperatures and stresses, often in corrosive environments. For example, in Sweden, there is particular interest in advanced, indirect combined cycle gas turbine (CCGT) power generation systems for renewable energy generation, since these may offer overall energy conversion efficiencies for biomass of 45 pct and above. However, this requires heat exchangers with tubing capable of operating at metal temperatures up to 1150 ⬚C. Ferritic oxide dispersion strengthened (ODS) alloys formed by mechanical alloying are promising candidate materials for such high-temperature components operating in aggressive environments.[2,3] Among these, the coarse-grained ferritic ODS alloy PM2000 offers combinations of high-temperature creep and oxidation resistance, which, while the most attractive among competing materials, currently fall short of requirements for the demanding creep duty, which would be placed on pressurized tubing in a high-temperature heat exchanger.[4] PM2000 is, essentially, a Cr-Al ferritic alloy containing a dispersion of added Y2O3 particles mainly in the size range 20 to 50 nm in diameter. The presence of aluminum allows the formation of an extremely adherent surface layer of ␣ -alumina, which provides excellent oxidation, sulfidation, and carburization resistance at high temperatures,[5] while the Y2O3 dispersion improves high-temperature creep and Y.L. CHEN, formerly Research Associate, Department of Engineering, University of Liverpool, Liverpool, L69 3GH England, is Experimental Officer, MicroStructural Studies Unit (MSSU), School of Engineering, The University of Surrey, Guildford, GU 2 7XH England. Contact e-mail: [email protected] A.R. JONES, Senior Lecturer, is with the Department of Engineering, The University of Liverpool. Manuscript submitted September 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
stress rupture life.[6] High-temperature creep strength is further improved by the formation of an extremely coarsegrained structure with high grain aspect ratio (GAR) through high-temperature recrystallization annealing of the alloy prior to service.[7] The principal axis of the coarse grain structure is generally parallel to the principal working direction imposed during consolidation. Conventional Fe-based ODS alloy tubing processed by unidirectional extrusion followed by recrystallization annealing results in tubes with a highly anisotropic, coarse-grained structure, which is axially aligned. This exhibits excellent axial creep properties. However, for application as pressurized tubing in CCGT systems, the maximum principal creep stress is in the hoop rather than the axial direction and Fe-based ODS alloy tubing currently exhibits substantially poorer hoop than axial creep resistance. It would be expected
Data Loading...
