Hydrogen resistance of austenitic stainless steels and the influence of the nickel equivalent on properties
For the ramp-up of the hydrogen economy, complex material and tubing solutions are required. In certain applications, seamless stainless steel tubes offer a viable option. In this context, Tim Wallbaum from DMV explains the fundamental mechanisms of hydrogen embrittlement and how the hydrogen resistance of seamless stainless steel tubes can be influenced by their chemical composition.
By Tim Wallbaum, DMV GmbH
Hydrogen embrittlement and key influencing factors Hydrogen embrittlement is a phenomenon where metallic materials experience a degradation in mechanical properties due to exposure to hydrogen. This effect can significantly weaken properties like tensile strength, elongation at break, reduction in area, fatigue strength, and fracture toughness. Under certain conditions, hydrogen embrittlement can diminish load-bearing capacity or even cause failure at stress levels below the material’ s yield or tensile strength. In some cases, failure can occur without any external load, solely from internal stresses 1, 2. For hydrogen embrittlement to take place, three conditions must converge: sufficient hydrogen concentration, a certain level of applied or residual stress, and a material that is susceptible to embrittlement. Each of these factors is further influenced by conditions such as temperature and pressure, the type of stress( tensile, compressive, static, or dynamic), and material characteristics like safety factors, microstructure, and chemical composition 3.
Mechanism of hydrogen diffusion Due to its small atomic size, hydrogen can penetrate the crystal lattice of stainless steel, migrating through interstitial sites between metal atoms. Concentration gradients primarily drive this diffusion process, but it is also influenced by external factors such as pressure and temperature. When stainless steel is exposed to hydrogen gas, molecular hydrogen( H 2) dissociates on the metal surface into atomic
Figure 1. The prerequisites for hydrogen embrittlement 3
hydrogen. This dissociation process is facilitated by higher temperatures and surface imperfections, which provide the energy required to break hydrogenhydrogen bonds. In its atomic form, hydrogen can enter the stainless steel matrix, diffusing through the interstitial spaces within the lattice 4. The rate and extent of hydrogen diffusion are strongly influenced by the microstructure, alloy composition, and environmental conditions of the stainless steel. For instance, austenitic stainless steels, which have a face-centered cubic( FCC) lattice structure, generally offer greater resistance to hydrogen diffusion than ferritic or martensitic stainless steels due to fewer direct
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