Session: MF-02-04 Materials for Hydrogen Service (Joint with C&S)-4

Paper Number: 154665

154665 - A Comparison of Hydrogen Embrittlement Evaluation Methods for 1100 MPa Steels for Fastener Applications 

Abstract: 

High strength steels used in fastener applications can fail unexpectedly due to hydrogen introduced into the material through processing or during service. Several different hydrogen embrittlement evaluation methods are used in literature, and the experimental outcomes can vary widely between these test methods.  In this study, slow strain rate tests of smooth specimens, rising displacement experiments of notched specimens, and constant load tests of notched specimens were compared for three 1100 MPa 0.4 wt pct C steels: a 1Mn-1Cr Base condition, a condition with 0.15 wt pct V addition, and a condition with 0.15 wt pct V and 0.30 wt pct Mo additions. The Base condition was tempered at 500 °C for 1 h while both V-added conditions were tempered at 600 °C for 1 h to precipitate alloy carbides. Two direct comparisons were made through this approach: notched and smooth specimen geometries were directly compared for a slow, continuous strain rate, and rising displacement and constant mechanical loading techniques were compared for notched specimens. Hydrogen was introduced into the gauge section of the specimens through cathodic hydrogen charging in 0.5 M H₂SO₄ + 100 ppm As₂O₃ with an approximate 0.2 mA∙cm^-2 current density before hydrogen embrittlement testing. Melt extraction analysis was used to estimate the hydrogen concentration in hydrogen pre-charged specimens after cathodic hydrogen charging, and residual hydrogen concentration was measured near the fracture surface in mechanical test specimens following failure. The results indicate that notched specimens are more sensitive to hydrogen charging compared to smooth specimens, because residual hydrogen concentration measurements indicate that hydrogen accumulates at the stress concentration during loading, leading to hydrogen embrittlement at lower hydrogen charging levels compared to smooth slow strain rate tests. While a general trend between the degree of hydrogen embrittlement and pre-charging time was observed for smooth specimens, fracture often occurred at inclusions, causing significant variability in the results. A static applied load emphasizes the time dependency of hydrogen diffusion to the notch. The V- and Mo-added condition absorbed a high hydrogen concentration during pre‑charging and exhibited little time dependence for hydrogen induced fracture, with similar performance between rising displacement and constant load tests. The V-added condition, by contrast, absorbed less hydrogen during pre-charging but is expected to have similar or higher hydrogen diffusivity. Time dependency was more evident in the V-added condition compared to the V- and Mo-added condition, with failure occurring at much lower stress levels under constant load compared to rising displacement tests.

Presenting Author: Michelle Kent Colorado School of Mines

Presenting Author Biography: Michelle Kent is a PhD candidate in Metallurgical and Materials Engineering at Colorado School of Mines in Golden, CO, USA. The title of her PhD thesis is “Hydrogen Embrittlement in High Strength Steel Fasteners”.

Authors:

Michelle Kent Colorado School of Mines
Emmanuel De Moor Colorado School of Mines
Kip Findley Colorado School of Mines