Thermal analysis of trapped hydrogen in pure iron
- PDF / 843,899 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 68 Downloads / 221 Views
I.
INTRODUCTION
T o understand hydrogen embrittlement of steel, many studies on the role and effect of hydrogen on properties of steel have been carried out recently. 1,2,3The Fe-H system has been the subject of many thermodynamic and kinetic researches on interstitial solute because of the high hydrogen diffusivity in iron. However, the diffusion mechanism of hydrogen in iron is not clearly understood. As hydrogen embrittlement is closely related to the hydrogen movement in metal, it is very important to verify the exact physical nature of hydrogen behavior in iron, both from an academic viewpoint as well as for practical applications. Figure 1 shows that the diffusivity of hydrogen in iron deviates markedly from that predicted by the extrapolation of high temperature data taken below 573 K. 4-t~The apparent activation energy in this low temperature region is in the range of 33 to 38 KJ/mol, while in a higher temperature region it is 7.6 KJ/mol. Many researchers ]]-17 believe that this phenomenon takes place due to hydrogen being trapped in lattice defects of iron (dislocation, microvoid, and grain boundary) at low temperatures, in as much as the trapping site has a lower energy level than a normal site. Pressouyre H has classified types of trap sites by their physical nature for hydrogen in iron, and suggested vacancies, alloying elements, dislocations, interfaces, and microvoids as possible trap sites. Analyzing the hydrogen diffusivity data in iron, Oriani ]2 has estimated trap density and trap-hydrogen interaction energy, and has claimed that interfaces and microvoids are the major trap sites in steels which are not cold worked and cold worked, respectively. Kumnick et all3 in their research of hydrogen permeation in cold worked steel have concluded that dislocation and dislocation debris are trap sites for hydrogen. Riecke ~4has also found that dislocations and dislocation pile-ups act as traps in the measurement of hydrogen diffusivity in hydrogen charged, cold worked, and recrystallized iron specimen. Pressouyre and Bernstein 15have found that TiC traps hydrogen irreversibly. In this study they have analyzed permeation transients. As a basic work, Wada et a116 observed W.Y. CHOO, Graduate Student, and JAI YOUNG LEE, Professor, are both with the Department of Materials Science at the Korea Advanced Institute of Science and Technology, Seoul, Korea. Manuscript submitted January 27, 1981. METALLURGICAL TRANSACTIONS A
-I( -I1 A
t.)
E
-20 -22 -24
Z .J
-26 -28 -30
I
I
I
I
2
3
I / T (* K)X IOs Fig. 1 - Representative data for the apparent diffusivity of hydrogen in a-iron and in ferritic steels.
the change of specific heat through a first order phase transformation at 13.7 K in the hydrogen-iron system and verified that hydrogen is trapped in microvoids as gas molecules. Hargi 17compared the diffusivity of hydrogen in single crystal and polycrystal iron and concluded that the hydrogen trapping site was at a grain boundary. In order to explain the anomalous behavior of hydrogen in iron, several mode
Data Loading...
