Surface wave studies of hydrogen damage incubation time
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I.
Table I.
INTRODUCTION
Ni-base superalloys and the most highly alloyed austenitic stainless steels are used in the aerospace, nuclear, and petroleum industries. In certain thermomechanical states and operating environments, these normally strong, ductile and inert alloys exhibit brittle failure in very short times. ~-4 One of the predominant forms of failure, especially in deep, sour-gas well and geothermal environments, is hydrogen embrittlement (HE). The effect of additional strengthening by cold-working and aging on susceptibility of these stainless alloys to HE has been investigated, and some hypotheses have been proposed2'4'5 to account for embrittlement. However, the investigations have been made 6'7'8on precracked samples or by tensile tests; consequently, the results obtained are relevant to crack propagation rather than initiation. The incubation of cracks on the surface can be expected to have a great influence on the attenuation of ultrasonic surface Rayleigh waves (RW). As has been s h o w n , 9'1~ the formation of cracks occurring during prolonged charging causes an increase in attenuation in Fe and Ni alloys that coincides with the beginning of cracking. It is the purpose of this study to determine if RW can provide a quantitative measure of crack incubation time and damage type in fcc stainless alloys undergoing cathodic H charging. In addition, the work is aimed at characterizing the effect of thermomechanical state and alloy type on surface cracking tendency during charging.
II.
EXPERIMENTAL
Five materials, including pure Ni, Ni-base HASTELLOY C-276, Fe-base CARPENTER 20Cb3, 316L stainless steel, and low-carbon steel were tested. Their compositions are shown in Table I. The materials were 50 pct cold-worked. ANDRZEJ ZIELINSKI. formerly a Senior Postdoctoral Fellow. Department of Metallurgical Engineering and Materials Science at Notre Dame. is now with the Ship Design and Research Center. 80-958 Gdansk. Poland. NICHOLAS F. FIORE. formerly Professor and Chairman. Department of Metallurgical Engineering and Materials Science. University of Notre Dame. Notre Dame. IN 46556. is now Corporate Director of Technology, Cabot Corporation. Boston. MA 02110. Manuscript submitted June 25. 1981. METALLURGICAL TRANSACTIONS A
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Chemical Composition of Materials Tested Material
Element Ni 200 C Mn P S O Si Cr Ni Mo W Cu Fe Cs Pb Nb N V
0.05 0.18 0.005 . 0.18 -bal.
0.15 0.1 ---
C-276
20 Cb3
0.02 1.00 ---
0.06 2.0 0.035 0.035 . . . 0.08 1.0 15.5 19.0-21.0 bal. 32.5-35.0 16.0 2.0- 3.0 3.8 --3.0- 4.0 4.5 bal. 2.5 ------0.35 --
316L 0.25 1.78 0.018 0.015 0.59 17.45 13.96 2.25 -0.15 bal. 0.13 0.002 0.0021 0.0050 --
Low-Carbon Steel 0.024 0.12 0.007 0.043
bal.
cut into strips 25 mm wide • 175 mm long x 1.5 mm thick and bent into 90 deg Vees. Some of the samples were annealed for 10 minutes (at 1125 ~ for C-276, 1010 ~ for 20 Cb3, 1120 ~ for 316L, and 930 ~ for low-carbon steel) and others were aged at 500 ~ for 100 hours. (This aging treatment greatly increases HE tendency in some fcc stainless alloy
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