Abstract
The current work examines hydrogen sensitivity in different pipeline steels (X65, X70 and X80 HSLA grades) from four productions. Hydrogen Induced Cracking (HIC) experiments were performed and then the welds were characterized via optical and scanning electron microscopy techniques. The optical micrographs revealed cracks only in one of the four welds. Transverse cracks were found along bainitic-ferrite/carbide islands within the heat affected zone and the base metal of production B. Found inclusions e.g. MnS inside the cracks acted as initiation points for the HIC. However, the weld zones in all productions consisting of acicular ferrite and grain boundary ferrite were found to be resistant in hydrogen embrittlement. Therefore, the presence of bainitic ferrite with carbides at the grain boundaries in the microstructures and the intense presence of MnS inclusions caused HIC in pipeline steel from production B. The manufacturing process, the forming and welding conditions in the examined case seem not to have negatively influenced the pipeline steel in terms of HIC.
Highlights
The increase in demand of oil and gas leads manufacturers to use High Strength Low Alloyed steels (HSLA) which have an ideal combination of formability, weldability, yield strength and low cost
The aim of the present study is to examine the susceptibility of hydrogen induced cracking (HIC) in the weld zones of steel pipes welded according to the Longitudinal Submerged Arc Welding (LSAW) method in usual LSAW pipeline productions that are not designed to resist in sour environments
Blocks of Bainitic Ferrite (BF) laths appeared in the heat affected zones (HAZ)
Summary
The increase in demand of oil and gas leads manufacturers to use High Strength Low Alloyed steels (HSLA) which have an ideal combination of formability, weldability, yield strength and low cost. Oil and gas contain small amount of hydrogen sulfide (H2S), which may cause hydrogen embrittlement through hydrogen induced cracking (HIC) or other embrittlement mechanisms. It is well known that HIC is one of the major reasons of pipeline steel failures under sour service conditions. A corrosion reaction on the steel’s surface causes the diffusion of atomic hydrogen into the steel lattice, where it can be trapped into reversible (e.g. dislocations or grain boundaries) or irreversible traps (e.g. inclusions). The atomic hydrogen recombines into molecular at inclusions and imperfections in the steel matrix. The formation of H2 gas is accompanied by substantial pressure of thousand megapascals, which cause significant damage mechanisms [1], [2]
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