Investigating Through-Thickness Crack Propagation From Local Hard Zones in TMCP Steel Exposed to Wet Sour Service

Abstract Thermo-mechanically Controlled Process (TMCP) plates have been used in sour service line pipe applications for many years without any concern about local hard zones (LHZs). The reported failure in Kashagan pipelines in 2013 raised technical concerns about the potential of LHZs, produced on the surface of the line pipe during TMCP processes, leading to serious through thickness Sulfide Stress Cracking (SSC) failures. The aim of this research is to evaluate the possibility of propagation of a through-thickness crack under the LHZs when the steel is subjected to SSC testing at extreme sour service conditions. Steel Specimens were produced with surface LHZs and subjected to SSC at sour conditions under NACE region 3. Surface Micro-Hardness were measured for each specimen with all reading above 248 HV 0.1 to simulate LHZs condition. It was also ensured that the areas beneath LHZ are soft zones with hardness values lower than 248 HV0.1. The results of this research showed that TMCP steel with LHZs did not experience SSC failure or through-thickness crack propagation in a severe sour environment (NACE region 3). The research concluded that further comprehensive researches are required to evaluate if LHZs surrounded by soft zones (hardness lower than 248 HV) will result in through thickness crack initiation and prorogation when exposed to extreme sour service conditions.