The static creep behavior of pure iron has been studied with cathodic charging of hydrogen at room temperature.
Pure iron was machined into specimens with gage dimension of 25×4x1mm, which were annealed for 3 hours at 1223 K and furnace cooled, after which yield stress was 140 MPa and grain size was 0.06-0.08 mm.
Stress and current density were kept constant during creep test using the constant stress creep tester and the constant current power supply. All specimens were electrolytically precharged for 20 minutes at test conditions, and the fracture surface was observed with optical microscope and SEM.
Creep strain increased with cathodic charging of hydrogen and creep rate decreased to zero after a certain time of charging in spite of the continued charging. As the current density increased, the creep strain increased. When the cathodic charging was cut off, creep rate decreased rapidly. Creep strain caused by cathodic charging of hydrogen was relatively larger at lower stress as compared with high stress. It is believed that the critical value of charging time and current density exists in order to affect the creep behavior of pure iron.
Diffused hydrogen into the specimen causes surface blistering and internal damage due to hydrogen precipitation. When fracture occurred, the specimen was embrittled. Hydrogen-induced softening may be explained by the increase of mobile dislocation density due to hydrogen precipitation not by the increase of mobile dislocation velocity due to hydrogen atom.
At the content of the precharged hydrogen increased, the initial strain decreased. This phenomenon is considered to indicate the lattice hardening due to hydrogen.
The results of this study may support the planar pressure theory of hydrogen embrittlement.
