Near-threshold fatigue crack growth properties at elevated temperature for 1Cr-1Mo-0.25V steel and 12Cr stainless steel
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I. I N T R O D U C T I O N N E A R - t h r e s h o l d fatigue crack growth properties are important in the design of structures subjected to low stresses and long-term cycling. If service at elevated temperatures is anticipated, the design problem becomes more complex than at room temperature, tl-7] For example, it has been found that creep damage, tit oxidation, t3,< and a decrease in Young's modulus t4,7~ can contribute to an acceleration of the fatigue crack growth rate, whereas oxide-induced crack closure ta-Tt can lead to a decelera, tion. The designer's problems are further complicated because the amount of information available on the threshold levels and fatigue crack growth properties at elevated temperatures is limited. In this paper, the threshold and low-rate fatigue crack growth properties are investigated for a Cr-Mo-V low alloy steel and a Type 403 stainless steel at temperatures between 25 and 550 ~ and at frequencies of 0.5, 5, and 50 Hz. In order to avoid the complications of crack closure, tests were conducted under a AK-decreasing condition where the minimum load is increased with crack extension while maintaining the m a x i m u m load constant, t6,8~ In order to clarify the mechanism of fatigue crack growth at elevated temperatures, the thickness and the composition of the oxide layer on the fracture surfaces were evaluated by Auger electron spectroscopy, scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction techniques.
SABURO MATSUOKA, Senior Researcher, ETSUO TAKEUCHI, Researcher, and SATOSHI NISHIJIMA, Director, are with the National Research Institute for Metals, Fatigue Testing Division, 2-3-12 Nakameguro, Meguroku, Tokyo 153, Japan. ARTHUR J. McEVILY, Professor, is with the Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, CT 06268. Manuscript submitted February 22, 1988. METALLURGICAL TRANSACTIONS A
II. E X P E R I M E N T A L P R O C E D U R E
A. Materials The materials investigated were a Cr-Mo-V low-alloy steel and a SUS403 stainless steel. Their chemical compositions are given in Table 1; mechanical properties and heat-treatment conditions are given in Table II. The m a x i m u m and minimum values of the mechanical properties of these alloys as determined from nine heats and cited in N R I M (National Research Institute for Metals in Japan) Creep Data Sheets Nos. 9A and 13A t91 are shown for comparison in parentheses in Table II.
B. Fatigue Crack Growth Testing Fatigue crack growth tests were conducted with 50 mmwide and 10 mm-thick Compact Type (CT) specimens in a 10 kN servo-hydraulic type machine. A sinusoidal w a v e f o r m was used with frequencies of 0.5, 5, and 50 Hz. The test environment was air, and the test temperatures ranged from room temperature (--25 ~ to 550 ~ T w o AK-decreasing test methods were employed in this study (Figure 1). One was a new test method wherein the minimum load, Pmin, was increased with increasing crack length, a, while the m a x i m u m load, P m a x , maintained con
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