Effect of Internal Hydrogen on the Fatigue Crack Growth Rate in the Coarse-Grain Heat-Affected Zone of a CrMo Steel

The effect of internal hydrogen in the fatigue crack growth rate of the coarse grain region of a 2.25Cr1Mo steel welded joint was analyzed in this work. The microstructure of the coarse grain region was simulated by means of a heat treatment able to provide the same microstructure with a similar prior austenite grain size and hardness to the one in a real welded joint. The fatigue crack growth rate was measured under standard laboratory conditions using compact tensile (CT) specimens that were (i) uncharged and hydrogen pre-charged in a hydrogen pressure reactor (under 19.5 MPa and 450 degrees C for 21 h). The influence of fatigue frequency was assessed using frequencies of 10 Hz, 0.1 Hz, and 0.05 Hz. Additionally, two load ratios (R = 0.1 and R = 0.5) were applied to analyze their influence in the da/dN vs. Delta K curves and therefore in the fatigue crack growth rate. The embrittlement produced by the presence of internal hydrogen was clearly noticed at the beginning of the fatigue crack growth rate test (Delta K = 30 MP root m), obtaining significant higher values than without hydrogen. This effect became more notorious as the test frequency decreased and the load ratio increased. At the same time, the failure mechanism changed from ductile (striations) to brittle (hydrogen decohesion) with intergranular fracture (IG) becoming the predominant failure mechanism under the highest loads (R = 0.5).