Hydrogen-embrittlement behavior of a V+Nb-microalloyed high-strength bolt steel subjected to ausforming

Abstract

The potential significance of ausforming, i.e., controlled forging with finish-forging temperature as low as ∼625 °C followed by direct quenching and tempering at 600 °C (CF-T600 sample), in enhancing the hydrogen embrittlement (HE) resistance of a novel V + Nb-microalloyed high-strength bolt steel was investigated by slow strain rate tensile (SSRT) tests using pre-hydrogen-charged notched round bar specimens. Conventionally hot-forged and re-austenitized and tempered sample (HF-T600 sample) was used for comparison. The results show the ausformed CF-T600 sample exhibited a significantly enhanced HE resistance by ∼83 % and a decreased HE susceptibility by ∼66 % compared with the HF-T600 sample, although the strength level of the former was higher than that of the latter. SSRT fracture surface observation revealed that the adopted ausforming suppressed the brittle intergranular fracture along the prior austenite grain (PAG) boundaries and reduced the area fraction of brittle crack initiation region. The enhanced HE resistance of the CF-T600 is mainly attributed to the PAG refining, the polygonal ferrite formed along PAG boundaires, the enhanced precipitation of nano-sized V-rich MC as well as the characteristic banded microstructure. It is suggested that ausforming coupled with microalloying with V + Nb is a cost effective and environment friendly means to further considerably enhance the HE resistance of high-strength bolt steel.