Influence of Environmental Conditions and Metallurgical Factors on Hydrogen Induced Cracking of Line Pipe Steel

Presently, it is considered that the failure of natural gas pipe line by hydrogen induced cracking (HIC) in wet hydrogen sulfide (H₂S) is one of the most important environmental embrittlement phenomena of materials. For the past few years, through a number of investigations into HIC phenomena, mechanism, fractography and so on [1] ~ [19], the essential factors to HIC have been determined and the line pipe steels less susceptible to HIC have been developed commercially [7] [14] [17] [18]. From the view point of the protection of material from HIC in the wet H₂S environment, the problems are able to be reduced into two factors to be discussed. They are the hydrogen content absorbed from environment (C₀) and the threshold hydrogen content for HIC of materials (Cth). In the case of C₀ ≧ Cth, cracking occurs and in the case of C₀ < Cth, no cracking occurs. For the prevention of HIC, a decrease of C₀ value or an increase of Cth value is necessary. C₀ value depends on the environmental factors such as existence of water vapour, injection of chemical inhibitors and on the metallurgical factors, especially alloying element. On the other hand, Cth value depends on only the metallurgical factors such as the existence of segregation, shape control of inclusions, the microstructure and so on.