STRESS-ASSISTED HYDROGEN ATTACK CRACKING IN 2.25CR-1MO STEELS AT ELEVATED-TEMPERATURES

Crack growth in 2.25Cr-1Mo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 degrees C to 500 degrees C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K-l = 20 MPa root m (at 480 degrees C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K-l(6.5). Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K-l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement.