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Role of Non-Metallic Inclusions and the Microstructure Constituents on HIC Performance

Product Number: MPWT19-14439
Author: Amro Al-Hattab1,Diaa Elsanosy2, Gaurav Tomer3, Abdullah Al-Jarbou4
Publication Date: 2019

With increasing oil & gas demand and depletion of sweet reserves, oil & gas companies in the regional
economies are focusing towards the exploitation of sour resources. This necessitates the use of pipelines
and down-hole tubing made from special steels with significant resistance to hydrogen-induced cracking
(HIC). These steels are produced through specific technologies for enhanced chemical composition control
and microstructural engineering to incorporate the required strength, weld ability and improved HIC
resistance. It is well established that the HIC initiates at sites with microstructural heterogeneities whether
due to presence of gross nonmetallic inclusions or the micro-structural constituents. The presence of central
segregation further aggravates the conditions particularly when the final pipe sizes require the longitudinal
slitting of the coils. Presence of non-metallic inclusions in the steel makes it vulnerable to hydrogen-induced
cracking under wet H2S environment. The mechanism of HIC begins with the generation of hydrogen atoms
by corrosion reaction of H2S and Fe in the presence of free water. The hydrogen atoms then diffuse into
steel and accumulate around the inclusions. The higher number of inclusions equates to the more sites
available for hydrogen adsorption. Recombination of hydrogen atoms to H2 molecules builds up a heavy
gas pressure in the interface between matrix and inclusions. Cracking initiates because of the tensile stress
field caused by hydrogen gas pressure and crack propagates in the surrounding steel matrix. The
longitudinal slitting exposes the internal microstructural abnormalities to the skelp edges which are then
incorporated in the thermally stressed weld zone. While the post-weld heat treatment (PWHT) mostly
homogenizes the weld zone microstructure, the presence of excessive central line features cannot be
completely removed thereby making this zone more prone to HIC attack