Effect of aluminum on the stress rupture properties of Cr-Mo-V steels
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replicas and X-ray identification of carbide residues. The extraction replicas were prepared by deposition of a carbon layer onto polished and etched samples, immersion in 10 pet Br-methanol solution, followed by floating the replicas and mounting them on grids • For carbide identification, portions of the various samples were anodically dissolved in a 10 pet HCl-methanol electrolyte .7 The insoluble carbide residue was collected and subjected to X-ray diffraction in a 2-radian Debye-Scherrer camera, using Cr K a radiation. The experimental conditions of extraction residues and of X-ray irradiation were identical for all samples. RESULTS Stress Rupture Tests Stress rupture tests were conducted under constant load at 950°F (510°C) and 1100°F (593°C). In addition to smooth bar specimens, notch-bar specimens with theoretical elastic stress concentration factors (KT) of 4 and 10 were also tested to assess the sensitivity of creep properties to the presence of a notch. Results of the tests are summarized in Table III. In steel A failure always occurred in the smooth bar prior to failure in the notch bar. In steel B, on the other hand, all failures at 1100°F (593°C) occurred in the notch bar first. Figs. 1 and 2 depict the variation of log tr (log time to rupture) with a(stress) for steels A and B respectively. Estimated values of rupture stress are identical for both steels at 950°F (510°C). At 1100°F (593°C), the rupture stresses of steel B are higher at 100 and 1000 h, but approach that of steel A at 10,000 h. Variation of log (log minimum creep rate) with Q at 1100°F (593°C) is depicted in Fig. 3. A linear relation existed between the two parameters, indicating that d varies exponentially with v. The linear plot for steel B lies consistently below that for steel A, indicating higher creep strength for steel B. Fig. 4 depicts the variation of smooth bar ductility (pct RA) with time-to-rupture for both steels. A marked contrast is observed in ductility at 1100°F (593°C), between steel A and steel B. While pet RA VOLUME 6A. NOVEMBER 1975-1997
are generally above 80 pet for the former, very low values in the range 3 to 20 pet are observed for steel B. Even at 950°F (510°C), the pct RA for steel B decreases steadily with log tr , while such a trend is not observed in steel A. Notch-bar test data for steels A and B are included with the smooth-bar data in Figs. 1 and 2. For steel A, the smooth-bar curve at 1100°F (593°C) lies consistently below the notch-bar curve. Based on extrapolation of the two plots, it appears that this steel would be free of notch sensitivity for times in excess of 10 5
h. In sharp contrast to steel A, the notch-bar curves at 1100°F (593°C) for steel B intersect the smooth-bar curve in very short times (-130 h), indicating onset of notch weakening. At 950°F (510° C), onset of notch sensitivity is indicated in about 10 4 h. Stress Rupture Tests on Reheat Treated Steels In view of the fact that the test materials for steels A and B were taken from large commercial forgings, the possibility that micr
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