Microstructural Influence on Hydrogen Absorption in Steels

Abstract

The presence of hydrogen in structural metals and alloys especially steels has proven to be a problem warranting a significant amount of attention for over a century. In steels presence of hydrogen severely degrades its mechanical properties such as ductility and fracture toughness, commonly known as hydrogen embrittlement. Hydrogen can be introduced into materials during processing or in-service application under aggressive environments. Hydrogen embrittlement is of particular concern under hydrogen rich aqueous environments. Electrochemical charging environments have been shown to have a more deleterious effect on mechanical properties of steels than gaseous environments. For a given environment the ingress of hydrogen into a material is also influenced by its microstructural features. Following this, in this thesis, a comprehensive analyses of the effect of microstructural features on hydrogen absorption in steels have been carried. The focus is confined to pure iron, AISI 1018 low-carbon steel and AISI 4340 high strength alloy steel. The hydrogen is introduced into cylindrical billets of the three materials through electrochemical charging. The total hydrogen content of the charged billets are then quantified following gas fusion analysis principle commonly termed as melt extraction. This allows the quantification of absorbed hydrogen in a given material as a function of charging time. The results reveal that hydrogen content in pure iron increases with charging time and tends to saturate. On the other hand in 1018 and 4340 following initial saturation, the hydrogen content tends to increase again with continued charging. The mechanical properties of the hydrogen charged billets of the three materials are also characterized using nanoindentation technique. Lastly, thermo-kinetic modeling of hydrogen absorption in these materials has been carried out. The modeling results aid in better understating the correlations between hydrogen absorption and microstructural trapping sites present in these materials.