The Consequence of Stress Intensity on Fatigue Crack Propagation in High-Strength Steels in Sour Environment

Abstract

The fatigue crack propagation behavior of high-strength carbon steel in 5% NaCl buffered with Na2CO3/ CO2 and in contact with 0.83 H2S at 20°C was studied using frequency scans and microscopic surface analyses. The solution pH was 7. A fatigue crack growth rate (FCGR) was measured as a function of frequency for stress intensity factors (ΔK) of 315.4 and 153.5 MPa√cm, respectively. The FCGR versus frequency plot showed a linear dependence with a gradient of -0.5 for of 315.4 MPa√cm, whereas at 153.5 MPa√cm linear dependence was not observed. Samples failed catastrophically after the cyclic stress testing in the autoclave for 315.4 and 153.5 MPa√cm. Microscopic investigations of the fracture surface for the higher ΔK revealed the presence of pits, secondary cracks and characteristics suggesting overall brittle fracture. The fractograph after cyclic testing at lower ΔK revealed areas characteristic of ductile fracture near the outer surfaces of the compact tension specimen and indications of brittle fracture in the central region away from the outer surfaces. The frequency scans and the microscopic investigations confirmed different failure mechanisms of the steel for 315.4 and 153.5 MPa√cm.