라인파이프 강재의 SOHIC에 미치는 금속학적 인자의 영향 및 HIC와 SOHIC의 상관 관계 연구

Abstract

HIC and SOHIC are the typical hydrogen embrittlement phenomena of linepipe steels during service in sour environment containing H2S gas. SOHIC is other name of type I SSC. Considering hydrogen diffusion and trapping phenomena related to the metallurgical parameters in steels, the effect of metallurgical trapping sites on HIC and SOHIC should be clearly understood to prevent HIC and SOHIC of linepipe steels. However, although many researchers have reported the effects of metallurgical factors on HIC and SOHIC, the metallurgical factors based on crack initiation and propagation of SOHIC is not clear unlike HIC which is comparably evident. Also, HIC and HIBC as the initiation crack of SOHIC have two similarities despite the different conditions related to the applied stress for cracking. One is that the cracking propagates in the rolling direction of steel plate and the other one is that the cracking can be explained by internal hydrogen pressure theory. Because of the similarities, it has been considered that HIC resistant steels may be effective for prevention of SOHIC. However, even recently, some researchers have been reported that HIC resistant steels may be susceptible to SOHIC. Thus, there exists uncertainty in the relationship between HIC and SOHIC. This dissertation is composed of two kinds of topics

the effect of metallurgical factors on SOHIC and the relationship between HIC and SOHIC of linepipe steels.In the first topic of this dissertation, sulfide stress cracking (SSC) resistance of three API X70 grade steel plates with different microstructures is evaluated using NACE TM0177A method. SSC property is discussed in terms of metallurgical parameters such as: primary microstructure, second phases and inclusions. An empirical failure mechanism of type I SSC, stress oriented hydrogen induced cracking (SOHIC), is suggested in terms of metallurgical factors affecting crack initiation and propagation. Results show that in case of steels having the same oxide inclusion level, the soft microstructure is more sensitive to SSC than hard microstructure because of formation of more cracks in steels. An oxide cluster acts as the nucleation site of the initial crack of SOHIC, hydrogen induced internal blister crack (HIBC), parallel to applied stress. The primary nucleation sites of the secondary cracks among HIBCs in steels are enlarged TiNb(C,N) precipitate, oxide inclusions and martensite austenite (M/A) constituents. TiNb(C,N) is more sensitive than oxide inclusions for nucleation of the secondary crack among HIBCs, and M/A constituents acts as nucleation sites of the secondary crack in final stage before failure. SOHIC failure mechanism is summarized as follows

Formation of HIBCs parallel to applied stress → Bending or branching of HIBCs → Formation of the secondary cracks perpendicular to applied stress among HIBCs → Failure of steels by connection of the HIBCs and the secondary cracks.In the second topic of this dissertation, the relationship between hydrogen induced cracking (HIC) and SOHIC of API X65 grade steel plates in sour environment containing H2S gas, is clarified. HIC and SSC tests are performed in reference with NACE (National Association Corrosion Engineers) standards for evaluation of cracking resistance of the tested steels. Particularly, HIC tests are conducted using cathodic hydrogen charging method to increase diffusible hydrogen content of tested steels. Diffusible hydrogen content of tested steels is measured using the modified JIS Z3113 method. Results show clearly the evidence that HIC is the initial crack of SOHIC with the sameness of crack nucleation sites such as MnS and TiNb(C,N) cluster. HIC resistance by cathodic hydrogen charging condition is proportional to SSC resistance. The results are discussed in terms of diffusible hydrogen content and metallurgical factors such as microstructure, inclusion and hard 2nd phase for clarifying the relationship between HIC and SOHIC.