Hydrogen-involved fracture toughness behavior of low-alloy primary piping steel

Abstract

The tensile and fracture toughness behavior of hydrogen-charged ASME SA508 Cl.1a low-alloy primary piping steel and the dependence of strain rate at both room and high temperatures were studied. Fracture toughness was estimated by using the modified critical fracture strain model. The test strain rates and temperatures were $8.3 x 10^{-5}, 2.08 x 10^{-4}, 1.25 x 10^{-3}, 0.83 x 10^{-2} s^{-1}$ and 25, 177, 250, 316 ℃ respectively. The electrolyte for hydrogen charging was made to be 1 N $H_2SO_4$ and charging time was 30 minute for each specimen. The effect of cathodic hydrogen charging on the microstructure has also been addressed to confirm that the charging method does not cause any surface damage. At room temperature, the presence of hydrogen led to an increase in yield strength as well as decrease in ductility in all strain rates. Ductility was greatly reduced by the internal atomic hydrogen at lower strain rate about $10^{-5} s^{-1}$ and fracture mode was cleavage type. At high temperatures (177, 250 and 316 ℃), hydrogen induced softening and reduction of ductility at strain rates about $10^{-5}, 10^{-4}$ and $10^{-3} s^{-1}$. Observable hydrogen effect on yield strength, ductility, serration behavior, and cleavage type fracture was appeared at 177 ℃, $10^{-5} s^{-1}$ and 250 ℃, $10^{-3} s^{-1}$. Negative strain rate sensitivity was observed at $250^\circ C$ from strain rate $10^{-5}$ to $10^{-3} s^{-1}$ for both as-received and charged specimens. For as-received condition, fracture toughness was reduced by dynamic strain aging (DSA) effect in $10^{-5} s^{-1}$, 177 ℃ and in $10^{-3} s^{-1}$, 250 ℃. Hydrogen itself didn’t produce DSA effect but at the DSA regions it might enhance the DSA effect by about 20% of as-received condition. Fracture toughness was also reduced by internal hydrogen at $10^{-5} s^{-1}$, 316 ℃ (~50%); at $10^{-4} s^{-1}$, 250 ℃ (~33%) ; and at $10^{-2} s^{-1}$,177 and 316 ℃ (~33%). The range of fracture toughness for as-receiv...