In-Situ Investigation of Initiation and Propagation Behavior of Bainitic Steels in Sour Environment

Hydrogen induced cracking (HIC) occurs by the poisoning effect of hydrogen sulfide (H₂S) which promotes hydrogen absorption and entry at steel surface. Therefore, it is important for linepipe steels to have sufficient HIC resistance in sour environments. The HIC resistance is usually evaluated by measuring cracks after the standardized immersion test such as NACE TM0284. However, the general evaluation method cannot investigate HIC initiation and propagation behavior separately. It is necessary to understand the effect of metallurgical factors on the cracking behavior of sour service linepipe. In this study, in-situ ultrasonic inspection equipment was applied to the HIC test for the several linepipe steels with bainitic microstructure in order to clarify crack initiation and propagation behavior quantitatively. The three dimensional (3-D) distribution of cracks in the specimen was successfully captured as time sequence, and the temporal change of the crack area ratio (CAR) was investigated. It was revealed that the CAR-time curves are consist of four stages with different CAR increment rate. The first stage is the incubation of crack initiation. In the second stage, cracks occur and grow, and adjacent cracks coalesced rapidly. Regarding the first and second stages, sensitivity for the HIC initiation was well correlated with the hydrogen diffusion coefficient and the density of crack initiation site, such as MnS and Nb inclusions. In the third stage, the coalesced cracks propagate along the center segregation region. From the investigation of individual crack behavior, the crack along harder region showed higher propagation rate. In the fourth stage, the crack propagation rate was decreased to be in stasis. It can be stated that crack growth in the final stage is strongly affected by the hardness of base material and the crack easily propagate when HIC occurs in high strength steels.