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I.

INTRODUCTION

III.

THEkinetics of hydrogen embrittlement generally depend upon parameters such as strain rate, temperature, loading mode, and specimen geometry, tl-9] In our previous study, I~~ both atomistic and macroscopic diffusion models, which take into account hydrogen trapping by newly generated dislocations and hydrogen transport by mobile dislocations, have been developed. A series of experiments has also been performed in order to analyze hydrogen transport during plastic deformation, t"] In this study, the hydrogen embrittlement process itself is examined in order to determine how the transport process can be applied to understand the kinetic aspects of hydrogen embrittlement. II.

EXPERIMENTAL PROCEDURES

The material used in this study was Armco iron. The chemical compositions are given in Table I. The material was centerless-ground to the specimen dimension for the slow strain rate tensile test: the gage length was 1.27 cm and the reduced section diameter 0.203 cm. Thermal treatment was subsequently performed on each specimen. Specimens were annealed at a temperature of 800 ~ for 1 hour in Ar gas followed by furnace cooling down to about 200 ~ and then air cooling. The slow strain rate tensile test was used to measure the susceptibility of Armco iron to hydrogen embrittlement. The specimens were fractured in an electrolyte while simultaneously charging the specimen with electrolytic hydrogen. The electrolyte was deaerated by purging with N 2 gas for more than one day. Hydrogen was charged for about 18 hours before straining the specimen to assure an initially uniform hydrogen concentration. All slow strain rate tests were performed at an ambient temperature about 25 ~

A. Experimental Results In order to study the kinetics of hydrogen embrittlement, slow strain rate tensile tests were performed using Armco iron at various strain rates. Figure 1 shows the stress-strain curves at various hydrogen concentrations. The low hydrogen concentration was achieved by using 0.1 N NaOH as a charging solution and the charging current density of 0.263 m A / c m 2. The high concentration was achieved by using 0.1 N H 2 S O 4 at 1 mA/cm 2. Hydrogen causes a slight hardening up to the maximum load. However, the most significant effect of hydrogen on the stress-strain curve occurs after the maximum load or after macroscopic necking begins. Therefore, it appears that the crucial condition in the embrittlement process occurs near the point of maximum load. The hydrogen concentration at this strain termed as C* will be estimated by the method described in our previous paper, tul and applied to describe the kinetic aspects of hydrogen embrittlement. Figure 2 shows the effect of strain rate on the ductility at high hydrogen concentration. In Figure 2, it can be seen that the reduction of area at fracture is strongly affected by the strain rate. At the higher strain rates, the hydrogen effect disappears and the reduction of area in air is very close to that in a hydrogen environment. The strong strain rate dependence is