Hydrogen trapping in AISI 4340 steel was investigated by thermal analysis and hydrogen solubility experiments.
Below 400℃, total hydrogen solubility of AISI 4340 steel is greater than lattice hydrogen equilibrium solubility extrapolated from high temperature because of the interaction between the hydrogen and MnS interface.
Total hydrogen solubility at 80℃ decrease in the order of martensitic structure, spheroidized structure, pearlitic structure, and the reason that the total hydrogen solubility of spheroidized structure is greater than that of pearlitic structure is related to the high ferrite-carbide interface area in spheroidized structure.
Lattice hydrogen solubility equation at 1 atm. in AISI 4340 steel above 400℃ is C=9.1 exp (-4041/RT).
Evolution rate peaks of thermal analysis were observed at 115℃, 272℃, 338℃, 405℃, 495℃ in AISI 4340 steel, when the hydrogen charged specimen with various defects were heated at constant heating rate of 3.4℃/min. Analysis suggests that in AISI 4340 steel, the evolution rate peak at 115℃ corresponds to hydrogen release from ferrite-carbide interface, 272℃, dislocation, 338℃, microvoid, 495℃, MnS interface respectively.
The activation energy for evolution of trapped hydrogen from microvoid and MnS interface were found to be 11500cal/mole and 17200cal/mole, respectively. They were determined experimentally from the change of measured peak temperature with different heating rate.
The peak height of dislocation in 45% cold rolled AISI 4340 steel is ten times higher than that in 45% cold rolled pure iron.
It was observed that the microvoids were formed at MnS interface during the themomechanical process.
Experimental results showed that hydrogen exists as molecules in microvoid.
Hydrogen energy level around microvoid is suggested from trap activation energy and heat of solution.
