| Title |
Microstructure and Hydrogen Embrittlement of High-Strength API X65, X70, X80 Linepipe Steels |
| Authors |
정민섭(Min-Seop Jeong) ; 백관우(Kwan-Woo Paek) ; 오동규(Dong-Kyu Oh) ; 황병철(Byoungchul Hwang) |
| DOI |
https://doi.org/10.3365/KJMM.2026.64.5.411 |
| ISSN |
1738-8228(ISSN), 2288-8241(eISSN) |
| Keywords |
Linepipe steel; Thermo-mechanical control processing (TMCP); Hydrogen embrittlement; Thermal desorption analysis (TDA) |
| Abstract |
This study clarifies how microstructural features govern hydrogen embrittlement resistance in
high-strength API X65, X70, and X80 linepipe steels produced under different chemical compositions and
thermo-mechanical control processing (TMCP) conditions. Microstructures were quantitatively characterized
by electron backscatter diffraction (EBSD). Hydrogen embrittlement resistance was evaluated by
electrochemically hydrogen-charged slow strain-rate testing (SSRT) using round-notched tensile specimens
and the results were expressed as the relative notch tensile strength (RNTS). The hydrogen distribution and
trapping behavior were examined by silver decoration and a thermal desorption analysis (TDA). The RNTS
values were 0.84 (X65), 0.88 (X70), and 0.86 (X80), indicating that X70 steel offered the highest resistance
while X80 steel provided comparable resistance despite having the highest strength. Fractography and crosssectional
observations indicated that hydrogen increased cleavage-like fracture near the notch root in all
steels. The X65 steel additionally showed quasi-cleavage in the central region, consistent with its large grain
size. By contrast, hydrogen-assisted crack propagation was suppressed in the X80 steel. This is attributed to
its small grain size and a high fraction of high-angle grain boundaries, together with enhanced trapping at
pearlite ferrite?cementite interfaces that reduced diffusible hydrogen. Interrupted SSRT results suggested
cracks preferentially initiated at martensite?austenite (M/A) constituents. These findings underscore the roles
of microstructural refinement and hydrogen trapping in achieving high strength with improved embrittlement
resistance. |