Visualization of the hydrogen desorption process from ferrite, pearlite, and graphite by secondary ion mass spectrometry

The distribution and desorption processes of hydrogen and deuterium have been visualized by secondary ion mass spectrometry (SIMS). The present article deals with four principal points: (1) visualizing the hydrogen distribution, (2) visualizing the hydrogen desorption process from each metallurgical microstructure under various holding times at 25 °C, (3) visualizing the hydrogen desorption process during heating, and (4) determining the correspondence between desorption profiles and desorption sites. A spheroidal graphite cast iron specimen was prepared for visualizing hydrogen, since it consists of basic microstructures of steels such as ferrite and pearlite. Hydrogen and deuterium were occluded into the cast iron. The amount of hydrogen and the existing states of hydrogen in the cast iron were analyzed by thermal desorption spectrometry (TDS). The TDS analyses show that the hydrogen desorption has two peaks, namely, the low- and high-temperature peaks corresponding to trap activation energies of 21.6 and 105.8 kJ/mol, respectively. The SIMS analyses of the specimen cooled after heating to 100 °C, 200 °C, and 300 °C reveal that the hydrogen desorbs from the ferrite after heating to 100 °C, from the pearlite and the interfaces between the ferrite and the graphite after heating to 200 °C, and from the pearlite after heating to 300 °C. The graphite remains, trapping hydrogen after heating to 300 °C. On the basis of TDS and SIMS results, the relationship between the desorption profile and desorption sites was identified; that is, the low-temperature peak corresponds to ferrite, pearlite, and graphite/ferrite interfaces, while the high-temperature peak corresponds to graphite.