Effect of post-processing heat treatment on hydrogen embrittlement susceptibility of API 5L X70 pipeline steel

Three different heat treatments were applied on X70 pipeline steel: below transformation temperature (X70-720), in the intercritical temperature range (X70-780), and in the intercritical temperature range + tempering (X70-780/720). Then, cathodic hydrogen charging was conducted on these heat-treated samples to study the effect of hydrogen ingress on the degradation of their mechanical properties in relation to their microstructural characteristics. The mechanical test results showed a reduction in elongation and toughness of all hydrogen -charged samples, as well as a strong correlation between hardness and hydrogen embrittlement (HE) suscepti-bility. However, the treatments, which resulted in the formation of cementite precipitates within the ferritic matrix (X70-780/720), showed improved resistance to HE. Also, the crystallographic texture analysis revealed that a high volume fraction ratio of {111} || ND to {100}< 011 > contributed to the significant improvement of HE resistance in X70-780/720, while a low volume fraction ratio resulted in the least HE resistance in X70-720 specimens.

Automated summary

Three different heat treatments were applied on X70 pipeline steel to study the effect of hydrogen ingress on the degradation of mechanical properties. The results showed a reduction in elongation and toughness of all hydrogen-charged samples, as well as a strong correlation between hardness and hydrogen embrittlement susceptibility. The treatments that resulted in the formation of cementite precipitates within the ferritic matrix showed improved resistance to hydrogen embrittlement.