ENVIRONMENTAL AND METALLURGICAL PARAMETERS AFFECTING SULFIDE STRESS CRACKING RESISTANCE OF HIGH STRENGTH OCTG STEELS

Crack propagation in the presence of sour gas is the conjunction of electrochemical reactions and metallurgical processes that increase hydrogen absorption at the crack tip and therefore produces embrittlement. To evaluate Oil Country Tubular Goods (OCTG) susceptibility to Sulfide Stress Cracking (SSC) in wet H₂S environments NACE International adopted different testing methods. Particularly, Method D is used for design/fitness-for-service, material qualification and specification purposes. The material resistance to crack propagation in an aggressive environment is expressed in terms of a critical stress intensity factor, K_ISSC. Although this test is standardized, it is widely known that several variables can affect the obtained K_ISSC values: geometry of the specimen, initial applied intensity factor as well as the electrochemical (pH, temperature, H₂S content) and metallurgical variables that modify the amount of absorbed hydrogen. In this work basic research regarding hydrogen insertion and transport in NACE solutions is used to explain K_ISSC of 1Cr-0.7Mo steel tested in different H⁺/H₂S environments. Experimental evidence relating K_ISSC to the concentration of absorbed hydrogen is provided. Concentration of absorbed hydrogen is related to proton and hydrogen sulfide concentrations as well as dislocation density and stress. A mechanism of hydrogen insertion in the presence of hydrogen sulfide is also presented.