Effect of Hydrogen on the Very High Cycle Fatigue Properties of Quenched and Tempered Steels Containing (Ti,Mo)C Precipitates

The effects of hydrogen trapping behavior of undissolved and temper-induced (Ti,Mo)C precipitates on the very high cycle fatigue properties of quenched and tempered Cr-Mo steel were investigated. Results reveal that spherical undissolved (Ti,Mo)C precipitates with a hydrogen desorption activation energy of 142.6 kJ/mol cannot trap hydrogen through electrochemical charging; fine, temper-induced (Ti,Mo)C precipitates are effective hydrogen trap sites. Hydrogen trapped by fine, temper-induced (Ti,Mo)C precipitates with a desorption activation energy of 17.0 kJ/mol cannot diffuse out from the sample even if exposed to atmosphere for 336 h, while it may desorb from the trap site under cyclic loading and then diffuse to the crack tip or stress concentration field, resulting in a decrease in fatigue strength. Diffusible hydrogen trapped by dislocations and grain boundaries with a desorption activation energy of 16.9 kJ/mol can rapidly diffuse to crack tip or stress concentration field and then reduce the threshold value of stress intensity factor (SIF) of crack growth remarkably, resulting in a decrease of fatigue strength; this portion of hydrogen can diffuse out from the sample after atmosphere exposure for 96 h; Considering that the hydrogen content in both hydrogen trapping site is equivalent, the deleterious effect of hydrogen trapped by fine, temper-induced (Ti,Mo)C precipitates on fatigue strength is relatively smaller than that of the diffusible hydrogen trapped by dislocations and grain boundaries. Non-metallic inclusions (Al2O3) with a hydrogen desorption activation energy of 70.9 kJ/mol also cannot trap hydrogen through electrochemical charging.