The diffusivity of hydrogen in Nb stabilized stainless steel
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I.
INTRODUCTION
ALTHOUGHthere
are extensive diffusion studies of hydrogen in other austenitic stainless steels, 1'2'3 very little information exists on the diffusivity of hydrogen in 347 stainless steel. 4'5 This particular metal is characterized by the addition of niobium (Nb + Ta ~ 10 times the carbon concentration) to prevent chromium carbide precipitation at the grain boundaries. The stabilization preserves good corrosion resistance following welding or exposure at temperatures up to 900 ~ thus making it a highly functional material for many applications in the space program. This paper presents measurements of the hydrogen diffusivity in 347 stainless steel as determined by a real time dynamic technique applied under ultrahigh vacuum (UHV) conditions. The UHV furnace and the dynamic technique employed have been reported elsewhere. 6 Data are also presented on the changing surface composition of the metal as a function of time and temperature.
II.
EXPERIMENTAL
Cylindrical samples 12.5 mm in diameter and 25 turn in length were cut from an annealed rod with ASTM grain size no. 8 obtained from A1-Tech Specialty Steel Company, Durkirk, New York. A weight percent analysis of this rod showed Cr = 16.47, Ni = 9.73, Mn = 1.33, Nb = 0.90, Si = 0.17, S = 0.009 and C = 0.01. Sufficient hydrogen existed in the as-received samples to obviate the need for charging. The samples were electropolished with a methanol 6 pct by volume perchloric acid solution at - 8 0 ~ for five minutes, rinsed with methanol, and then quickly hot air dried before insertion into the UHV furnace
R.A. OUTLAW is Research Scientist, NASA, Langley Research Center, Hampton, VA 23665. D, T. PETERSON is Metallurgist, 222 Metals Development, Ames Laboratory, Ames, 1A 50011. Manuscript submitted March 21, 1983.
METALLURGICAL TRANSACTIONS A
or surface analysis system. Electropolishing lowered the surface roughness to near one, as indicated by the high surface reflectivity, and minimized the amount of surface contaminants and the thickness of the oxide layer. Samples were loaded into a 347 stainless steel sample holder (that had been previously vacuum degassed for two hours at 1200 ~ and then inserted in the UHV furnace, shown schematically in Figure 1. After lowering the sample to point "A" where it was kept at room temperature by external forced air cooling, the sample exchange region was then baked for 36 hours at 150 ~ Following cool down, the isolation valve was opened and the furnace heated to the experiment temperature. Sufficient time was allowed for the furnace components to degas and the pressure to reach a stable value of approximately 1 • 10 -7 Pa in the measurement volume. The sample and holder were then quickly lowered into the hot zone where the sample temperature increased at a rate of 60 ~ per minute initially and then decreased to average of about 20 ~ per minute. The desorbed gas was channeled by the furnace tube up into the measurement volume where it was simultaneously pumped through a calibrated restricting annular conductance in
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