Mechanisms of high-temperature fatigue failure in alloy 800H
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F U L L Y austenitic, iron-based, INCOLOY* 800H is *INCOLOY is a trademark of INCO Alloys International, Inc., Huntington, WV.
used as a structural material in the petrochemical industry and for heat exchanger components in various energy conversion systems. In these applications, the components are often subjected to repeated thermal stresses as a result of temperature gradients which occur on heating and cooling during start-up and shut-down operations. Therefore, resistance to low-cycle fatigue (LCF) is an essential requirement. As a result, specific attention has been directed to the behavior of alloy 800H in recent years.U-~2] These investigations were conducted with the following objectives: (1) to ascertain the phenomenological effects of strain rate, strain range, and temperature on cyclic stress response and life under balanced cycling conditions; v,2] (2) to evaluate environmental effects on LCF behavior;t3m (3) to assess the creep-fatigue interaction behavior by suitably designing and performing strain hold and asymmetrical slow-fast cycling tests;t5 8] (4) to evaluate the effects of hold time and frequency on crack growth and to arrive at suitable parameters for describing the crack growth behavior under pure fatigue
K. BHANU SANKARA RAO, formerly USA National Research Council Associate, NASA-Lewis Research Center, is Senior Scientist, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. H. SCHUSTER, Section Head, is with the Institute for Materials in Energy Systems Research Centre, Juelich D-52425, Germany. G.R. HALFORD, Senior Scientific Technologist, is with the Structures Division, NASALewis Research Center, Cleveland, OH 44135. This article is based on a presentation made at the "High Temperature Fracture Mechanisms in Advanced Materials" symposium as a part of the 1994 Fall meeting of T.S., October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
and creep-fatigue interaction;t9,1~ (5) to determine the deformation and damage mechanisms under pure fatigue t~.2j and creep-fatigue interaction loading conditions; tH,j21 and (6) establishing suitable methods for predicting LCF lives based on smooth specimen data. r~2J In the LCF tests conducted on alloy 800H employing either total or plastic strain as the control parameter, cyclic lives were found to be a strong function of temperature and strain rate. Under balanced cycle conditions (equal tension and compression loading rates in a cycle), in the temperature range between 600 ~ to 900 ~ fatigue lives were dramatically reduced by both increasing temperature and decreasing strain rate. ~,21Under balanced cycling conditions at 800 ~ a vacuum environment produced a nearly fivefold increase in life over that in air.t8] The shortened life in air was primarily attributed to oxidation-enhanced microcrack nucleation and microcrack growth.t3] At elevated temperatures, the loading waveform also had a large influence on fatigue life and crack grow
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