Ratcheting Fatigue Behavior of Modified 9Cr–1Mo Steel at Room Temperature

Abstract

Fusion reactor components undergo asymmetric cyclic loading,that leads to a progressive increase in plastic strain and causes deterioration in life of engineering components. This investigation deals with fatigue behavior of modified 9Cr–1Mo steel at room temperature and presents the effect of mean stress (σ_m), stress amplitude (σ_a) and stress rate ($\dot{\sigma }$) on fatigue life, deformation and fracture behavior under asymmetric cyclic loading. A series of fatigue tests were conducted under asymmetric stress-controlled loading with different combinations of σ_m (190–210 MPa), σ_a (400–420 MPa) and $\dot{\sigma }$ (50–450 MPa/s). The plastic strain increased with increase in σ_m and σ_a and the fatigue life was reduced, whereas increase in $\dot{\sigma }$ reduced the accumulated plastic strain and the cyclic life was increased. The deformation behavior and microstructural changes under the influence of the three parameters (σ_m, σ_a and $\dot{\sigma }$) were examined by tranmission electron microscope (TEM). With increase in the three parameters (σ_m, σ_a, $\dot{\sigma }$), the lath martensitic structure changed to subgrain structure along with dislocation cells and forest dislocations. Scanning electron microscopy revealed unique features of fractured specimens, with progressive reduction in diameter towards the fracture-end, fatigue striations on the tapered circumferential surface and dimples on the fracture surface.

Graphic Abstract