Effect of cold working on the hydrogen trapping phenomena in pure iron
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INTRODUCTION
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lot of hydrogen can be dissolved in iron and steel in high temperature heat treating processes because hydrogen solubility in iron and steel increases with temperature. It is easy to reach a supersaturated state of hydrogen at room temperature by fast cooling which causes hydrogen embrittlement of iron and steel. Specially high strength low alloy steel (HSLA), which is in great demand in modem industry, is susceptible to hydrogen embrittlement. Many researchers L2'3have studied the Fe-H system and found that hydrogen movement toward the crack tip area is important to the understanding of hydrogen embrittlement phenomena in iron and steel. Figure 1 shows that measured apparent hydrogen diffusivity in iron at low temperature (below about 300 ~ deviates markedly from that predicted by extrapolation of high temperature data to low temperature. 4-m The apparent activation energy in this low temperature region is in the range of 36 to 48 U/mole, while in the higher temperature region it is 12 to 20 kJ/mole. Many researchers ~-~7 believe that this phenomenon takes place due to the hydrogen trapping in the lattice defects of iron (dislocation, microvoid, and grain boundary) at low temperature, as the trapping site, has a lower energy level than a normal lattice site, and thermal energy of hydrogen is small. Pressouyre" has classified trapping sites by their physical nature for hydrogen in iron and suggested vacancy, alloying element, dislocation, interface, and microvoid are possible trapping sites. Analyzing the apparent hydrogen diffusivity data in iron, Oriani ~2has estimated trap density and trapping site-hydrogen interaction energy and claimed that interfaces and microvoids are major trapping sites in steels which are not cold worked and cold worked, respectively. Kumnick et a113 in their research of hydrogen permeation in cold worked steel have concluded that dislocation and dislocation debris are trapping sites for hydrogen. Riecke ~4 has also found dislocations and dislocation pile-ups act as trapping sites in measurement of apparent hydrogen diffusivity of a
W. Y. CHOO, formerly a Graduate Student at Korea Advanced Institute of Science and Technology, is now a Postdoctoral Research Fellow, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. JAI YOUNG LEE is a Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 150 Cheongryang, Seoul, Korea. Manuscript submitted October 7, 1981.
METALLURGICALTRANSACTIONS A
hydrogen charged, cold worked, and recrystallized iron specimen. Pressouyre and Bemstein 15 have found that TiC particles trap hydrogen irreversibly. In their study, they used permeation transients method. As a basic work, Wada et a116 observed the change of specific heat through first order phase transformation in the hydrogen-iron system and verified that hydrogen is trapped in microvoid as gas molecules. As stated above, many researchers believed that small apparent hydroge
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