Hydrogen trapping in AISI 4340 steel

Hydrogen trapping in AISI 4340 steel was investigated by thermal analysis at a constant heating rate of 3·4 K min−1. Peaks were observed in the evolution rate v. temperature plots at 388, 545, 611, 678, and 768 K in AISI 4340 steel samples having either pearlitic or spheroidized carbide structure, and analysis suggested that the evolution rate peaks at 388, 545, 611, and 768 K corresponded to the hydrogen release from ferrite/carbide interfaces, dislocations, microvoids, and MnS interfaces respectively. The activation energies for the evolution of trapped hydrogen from microvoids and MnS interfaces were found to be 48·3 and 72·3 KJ mol−1, respectively. It is observed that microvoids were formed at MnS interfaces during the thermomechanical process and these were related to the hydrogen embrittlement phenomena in AISI 4340 steel. It is also found that, in cold worked AISI 4340 steel having a pearlitic structure, the main trapping site of hydrogen was dislocations, not microvoids, the interface area of ferrite/carbide being increased because of the breakdown of lamellar carbides. However, in AISI 4340 steel which was not cold worked, the main trapping site of hydrogen was the MnS interfaces. Experimental results indicated that hydrogen exists in molecular form in microvoids.