Hydrogen Permeation Of High Strength Steels

ABSTRACT The mobility of hydrogen has been compared in high-strength steels using an electrochemical hydrogen permeation technique. The relative susceptibility of high strength steels to hydrogen embrittlement, and possibly to stress corrosion cracking and corrosion fatigue, has been related to the mobility of hydrogen within the metal. Differences in hydrogen mobility are thought to be related to the presence of hydrogen "traps" within the metal. Results of comparison tests between conventionally-prepared and electroslag remelted steels are presented. BACKGROUND Extensive metallurgical research over the past years has led to modern high-strength alloys which give the designer and project manager increased options on ways to reduce weight, increase armor performance, and improve reliability. Translation of these metallurgical improvements from laboratory scale ingots into production quantities has required introduction of improved foundry practices. Electroslag remelting (ESR) of steels has been introduced in recent years as a less expensive means of producing commercial quantities of "cleaner" steels with improved mechanical properties.l. The improved mechanical properties of ESR steels have been attributed, among other things, to reduced content! of sulphur, phosphorus, arsenic, antimony, and tin 1–2 and to reduced inclusion levels. Steels having lower levels of these elements are generally less susceptible to temper embrittlement4–5,7–11 although the reasons-for this remain controversial. Many authors have tried to relate susceptability to stress corrosion cracking and hydrogen embrittlement to the mobility of hydrogen in the stressed metal near a propagating crack tip3, 8, 12–14 This mobility is related to alloy chemistry, thermal history, microstructure, and mechanical properties in ways which have been subject to speculation and research for a number of years15–17. Many of the metallurgical parameters which seem to affect susceptibility to hydrogen embrittlement and stress corrosion cracking are also known to affect susceptibility to temper embrittlement. While the precise mechanisms for the apparent interrelationships between temper embrittlement, stress corrosion cracking, and hydrogen embrittlement remain unresolved, most authorities agree that reduced hydrogen trapping, i.e. increased hydrogen mobility, in steels can be related to lessened susceptibility to these forms of metal failure4–7. Thus, if electroslag remelted steels can be shown to have increased hydrogen mobility characteristics when compared to similar steels having normal levels of sulfur, phosphorus, etc., they may also be expected to be less susceptible to stress corrosion cracking and hydrogen embrittlement. Electroslag-remelted steels are becoming available from a number of suppliers.18–21 ELECTROCHEMICAL HYDROGEN PERMEATION TECHNIQUE The electrochemical hydrogen permeation technical, first introduced by Devanathan and Stachurski22 was used in this study to measure the hydrogen permeation characteristics of a number of ESR 4340 steel alloys. Figure 1 shows a block diagram for the apparatus used in this investigation. One side of the sample (the right, or exit, side in Figure 1) is exposed to an aqueous solution and maintained at a constant anodic potential by means of a potentiostat. The current necessary to hold this sample at the preset potential is allowed to equilibrate to a minimum current value.