The paper compares Hydrogen Induced Cracking (HIC) resistance and Hydrogen Permeation (HP) results for two API X65 microalloyed steels, with different contents of Mn and Nb: one containing low Mn and high Nb (L-Mn) and the other, high Mn and low Nb (H-Mn). The main objective is to correlate the microstructural differences between these steels with hydrogen diffusion and trapping behavior and hydrogen-induced cracking resistance.

Both steel plates were characterized with optical and scanning electron microscopy in their transverse sections, in relation to the rolling direction. HIC resistance tests were made in accordance with the NACE TM0284-11 standard; samples obtained from the transverse section were also submitted to Hydrogen Permeation tests, based on the ASTM G148-97 standard. NACE solution A saturated with H2S was used in the two procedures. Besides, Thermal Desorption Spectroscopy measurements were made, in order to show which steel trapped more hydrogen atoms, and carbides/carbonitrides volume fractions were estimated with ThermoCalc software. The L-Mn steel presents a homogeneous microstructure through the plate thickness, composed of refined ferrite and small pearlite islands. The H-Mn steel has a heterogeneous microstructure through the plate thickness, composed of ferrite and pearlite bands, and presents centerline segregation. Hydrogen permeation tests showed that, despite all the microstructural differences, the hydrogen effective diffusion coefficient (Deff) was almost the same for both steels – the Deff obtained for the L-Mn steel is slightly higher than for the H-Mn steel. Contrary to expectations, the L-Mn steel presented higher hydrogen subsurface concentration (C0) and number of trapping sites per unit volume (Nt) values. Thermal Desorption Spectroscopy analysis confirmed that the L-Mn steel traps more H atoms than the H-Mn one. These results, along with the similar Deff values, can be explained by the presence of nanoprecipitates of microalloying elements, which, according to ThermoCalc simulations, appear in higher volume fraction in the L-Mn steel. Finally, the HIC tests results showed that the L-Mn steel has a better performance in sour environments; this behaviour is related with its special microstructural features.

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