Formation of Vacancy-type Defects in Cold-drawn Pearlitic Steel during Tensile Deformation with Hydrogen Charging

Abstract

The effects of hydrogen charge on the formation of vacancy-type defects in cold-drawn pearlitic steel were investigated by using positron lifetime spectroscopy, and the correlation between vacancy-type defects and susceptibility to hydrogen embrittlement was discussed. Pearlitic steels aged at 300°C and 450°C after cold-drawing were used. Both tensile deformed and subsequently fractured samples were prepared by slow strain rate tests (SSRT) with and without cathodic hydrogen charge. Average positron lifetime increased with increasing tensile strain both in the case of the 300°C-aged and 450°C-aged steels. There is no remarkable effect of hydrogen charge on the average positron lifetime of both steels. The results mean applying tensile strain increases the amount of lattice defects such as dislocation or vacancy, although hydrogen has little effect. On the contrary, positron lifetime of vacancy cluster component was increased by hydrogen-charged SSRT, and an increase in hydrogen concentration in both steels promoted vacancy clustering furthermore. The 450°C-aged steel showed more remarkable vacancy clustering than the 300°C-aged steel, implying that the lamellar structure in the 300°C-aged steel prevented vacancy clustering. Fracture strains after SSRT were decreased by hydrogen charge due to hydrogen embrittlement, and an increase in hydrogen concentration decreased the fracture strain in both steels. Fracture strains of all tested samples with and without hydrogen charge showed a strong dependence on the degree of vacancy clustering. That means vacancy clustering has a big effect on fracturing of steels, and a detrimental effect of hydrogen is attributable to the promotion of the vacancy clustering.