Effects of Vanadium Carbide Precipitation and Dislocation Density on Hydrogen Absorption and Retention in 1300 MPa Steel Fasteners

High-strength steel fasteners such as screws, bolts, and clamps with high strength and good hydrogen embrittlement (HE) resistance are desirable for many industrial applications. However, HE susceptibility generally increases with increasing strength. Introducing hydrogen trapping sites such as alloy carbides has been proposed to reduce the amount of diffusible hydrogen that is able to accumulate at stress concentrators and lead to HE. While it is well‑established that alloy carbide formation can increase the density of hydrogen traps, it is also acknowledged that alloy carbides can increase the amount of hydrogen absorbed during cathodic charging. In this study, the effects of dislocation density and vanadium microalloying on hydrogen absorption and retention from cathodic charging are investigated in 4140-type commercial alloys. After quenching and tempering to precipitate vanadium carbides, the bulk hydrogen concentration was measured in pre-charged coupons using melt extraction analysis. The effect of dislocations introduced through cold rolling before and after tempering was also investigated. Dislocation density and alloy carbide precipitation both increase the amount of hydrogen absorbed during cathodic charging and retained in the material after outgassing. Since alloy carbide formation increases hydrogen absorption and retention, the improvement in HE resistance achieved through trapping at carbides might be diminished.