Determining the Directionally-Dependent Effective Hydrogen Diffusivity of High Strength Steel Strand

Abstract

<p class="1Body">An important factor in the Hydrogen Embrittlement (HE) of cold-worked carbon steel strand is the effective hydrogen diffusivity of the strand material. The speed at which HE can occur is determined by the transport of the hydrogen to locations of stress concentrations. A Scanning Kelvin Probe (SKP) was used to measure the flattened surface of strand wires before and after being charged with hydrogen to determine a relationship between the delta work function measured (and indirectly the potential of the specimen) and the presence of hydrogen near the surface of that area of the specimen. Numerous scans were performed to determine a change in delta work function between the probe and the specimen. This change in delta work function was related to the presence of hydrogen at the specimen surface over time, giving a rate of hydrogen movement within a specific area of the specimen (both in the radial and drawn directions). The change in delta work function over time was then related to the diffusion of hydrogen over time in a given area resulting in a directional effective diffusivity of hydrogen through the strand which includes the effects of hydrogen trapping and method of hydrogen transport.</p><p class="1Body">The results indicate that the effective or apparent hydrogen diffusivity in the drawn direction in pearlitic, cold worked, carbon steel strand is 5.24E-08 cm<sup>2</sup>/sec. The effective hydrogen diffusivity in the radial direction was faster (3.56E-07 cm<sup>2</sup>/sec) as compared to the wire drawn direction.</p>