Hydrogen-entry properties of torsional prestrained carbon steels

Hydrogen-entry properties of torsional prestrained ferritic-pearlitic low-carbon JIS-S10C, JIS-S25C and JIS-S45C steels were investigated. A round bar specimen was prestrained by torsion test and then charged with hydrogen by cathodic charging or immersion charging. The torsion test enabled to introduce a large plastic deformation to the specimen without necking, which led to much higher level of hydrogen content, C_H, than tensile test. C_H was increased with an increase in torsional prestrain at the specimen outer surface, ε_pre,s. In the prestrain range, ε_pre,s < 30%, there was little influence of pearlite on C_H. By contrast, when ε_pre,s > 40%, there was a marked influence of pearlite on C_H in JIS-S10C and JIS-S25C, whereas, C_H in JIS-S45C was lower than that of JIS-S25C despite the fact that fraction of pearlite in JIS-S45C was much higher than that in JIS-S25C. It was presumed that such a complex hydrogen-entry property was determined by the competition between (I) the increase in dislocation density near the boundaries between ferrite and pearlite and (II) the inhibition against the increase in dislocation density by a stress redistribution in pearlite grains. The torsional strain at final rapture was the largest in JIS-S10C, which accordingly conduced to the largest hydrogen content in the broken specimen. The series of experimental results manifested that the torsion test enables the investigation of strength properties as well as the hydrogen-entry properties for BCC steels under an extremely large hydrogen content (~30 mass ppm).