Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes For Hoop Creep Enhancement
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 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. The objectives of this program are to (1) increase creep-strength at temperature in ODS-alloy tube and liner components by 100% via, (2) preferential cross-roll flow forming and grain/particle fibering in the critical hoop direction. The research program outlined here is iterative in nature and is intended to systematically (a) examine and identify post-extrusion forming methodologies to create hoop strengthened tubes, which will be (b) evaluated at ''in-service'' loads at service temperatures and environments. In this 12th quarter of performance, program activities are concluded for Task 2 and continuing for Tasks 3, 4 and reported herein. Two sets of MA956 tubes rotary cross-rolled at rolling angles of {Beta}=2{sup o} and 8{sup o} and two sets of flow formed and recrystallized tubes are being evaluated under hoop creep conditions in air. While, the rotary cross-rolled samples exhibit improvement over the baseline performance, it is he flow formed tubes that exhibit superior performance. Current test exposure for flow formed materials exceeds 6000 hours for a creep test operating at 950 C at 2Ksi stress. The Larsen-Miller parameter for these improvised flow-formed tubes now exceeds 52.4, i.e., better than ever recorded previously. These samples are at various stages of creep testing and evaluation in Task 4. The creep performance enhancement in cross-rolled MA956 material samples versus the base creep property is elucidated. At least 2-3 orders of magnitude of improvement in creep rates/day and concomitant increases in creep-life are demonstrated for the flow formed tubes versus the base reference tests.
- Research Organization:
- Univ. of California (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FC26-03NT41985
- OSTI ID:
- 894894
- Country of Publication:
- United States
- Language:
- English
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Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes for Hoop Creep Enhancement
Cross-Roll Flow Forming of ODS Alloy Heat Exchanger Tubes For Hoop Creep Enhancement