Effect of Hold time on low-cycle fatigue behavior of AISI 4140 steel at 303-473K was investigated in air, argon and high pressure hydrogen gas. Each of total strain amplitudes was controlled to be 0.5, 0.65, 1.0, 1.3 and 1.6% with zero mean level. To see the effect of hydrogen, high pressure hydrogen gas (12 MPa) was maintained and the results obtained under hydrogen were compared with the equivalent data in air and argon gas.
The experimental results in air and argon gas show that dynamic strain aging is occurring during hold time period and the piled-up dislocations near the carbides are locked by the Cottrell atmosphere. Formation of the Cottrell atmosphere around the dislocations near the carbides induces the redistribution of carbon or carbides in the matrix. These randomly dispersed grown-up carbides behave as the increased number of dislocation pile-up sites from which fatigue cracks may initiate. Therefore, as the hold time is applied on fatigue cycles under constant strain amplitude in air and argon gas the dislocation density at each site will be lowered to retard the fatigue damage or improve the fatigue life.
However, the experimental results in hydrogen clearly show that hydrogen reduces the fatigue life regardless the test conditions.
And a new method of fatigue life measurement using magnetic property changes is proposed in the appendix, and this method is more sensitive than the peak stresses change method or 4-point D.C. potential drop method in detecting fatigue damage.
